Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 02, 2010, 08:02:23 am
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I'm Looking for a good ( old or not ) tube mic preamp schematic (with layout if possible)
Have you a link or only a name for me ?
have you councils about that ?
Thanks
Kagliostro
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I've been collecting parts to make a couple of these pre's:
http://www.twin-x.com/groupdiy/displayimage.php?album=44&pos=1
If I can only get done with repairs and build something for myself!
Or you could look through this page and find dozens of ideas:
http://www.prodigy-pro.com/diy/index.php?action=printpage;topic=16909.0
Lots of good info over there.
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google Altec 1566
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Do you need bells and whistles? Or just gain for a microphone?
I have a pair of very nice custom made mic preamps, and the actual amplifier portion of the circuit is quite simple. Adding phantom power for condenser microphones is a minor additional complication (if you actually have condenser mics; not needed for dynamic mics).
The real labor involved in design and building for the maker of my preamps was the high pass and low pass filters to provide minor equalization to the mic signal. The EQ is only a bass roll-off and treble roll-off, but each filter uses one rotary switch to select the frequency and a second switch to select the amount of boost or cut. Some very nice switches were used inside, as well as a somewhat complicated arrangement to mount the caps on the switches.
The is also a high-gain range and low-gain range, which is switch selectable, but what that really does is to select between 2 stages of 12AY7/6072 (low gain) or 2 stages of 12AX7 plus the 2 stages of 12AY7 (high gain). From memory, I don't think any of the stages had a bypassed cathode resistor, as the full potential gain of the circuit is not wanted or needed.
An additional nice touch, which is very functional, is to have a power supply in a seperate chassis, which is connected via a 4- or 5-wire umbilical to the main preamp chassis. The power supplies happen to be off-the-shelf regulated d.c. supplies for both high voltage and the filaments, and the seperation is only to insure that any radiated noise for the rectification and regulation is as far from the input and output transformers as possible.
Last cool bit that was included were vintage UTC transformers were used for the input and output. My recollection is that it took quite some time for the builder to get ahold of those for use in the preamps, which delayed the building and delivery. I do wish that I knew where he got the chassis components, because they're very nice modular aluminum parts that are excellent for building something like this. In the past 10 years I haven't seen anything quite as nice for building up your own chassis. The four vertical sides are aluminum plates with countersunk holes to form a box, with a top and bottom plate held in place by counter sunk screws that feed into clips that hold onto an internal rail along the top and bottom edges. Inside is a piece of aluminum angle that bolts to the side and is used for mounting the glass/epoxy circuit board.
The circuit is built on aviation-style mounting stakes with teflon insultors holding the stakes to the board. Yeah, component leads are wrapped around the stakes, but this isn't the type of circuit that needs a lot of maintenance or parts-replacing.
If I could find a source for the chassis and the UTC transformers (or a comparable-quality brand like Sowter, Lundhal, etc), I could easily build up these preamps. The circuit is basically 2 channels of the old Universal Audio 610, not unlike the reissue they're making now, but with a remote power supply. If anyone is from Nashville or familiar withe Fred Cameron (who usually worked on microphones), he's the one who built my preamps.
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People have been scavenging the preamps of vinatge tube Akai & Roberts reel-to-reel tape recorders, and converting them to mic pre's. These have been availbale pretty cheap on ebay. Somweone had an internet site on this mod. It seems to have gone missing, but checkout this: http://www.audiokarma.org/forums/archive/index.php/t-79384.html
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There is a guy named Tablebeast that has a mod for the Akai Roberts pre's. I've seen him over at tapeop.com
http://messageboard.tapeop.com/viewtopic.php?t=67578&highlight=akai
This one is a combination mic pre amp and guitar amp.
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http://www.sotxampco.com/Schematics/STAC/SC4a.pdf
The link above is a stereo mic preamp I was developing but aborted due to financial reasons. I did have 1 channel breadboarded and it was likable. Probably not the most hifi thang in the world but iirc, it was quiet and usable. The James tone stack may not be the best idea, but I wanted something that could add some color. the schematic is sorta hard to read, especially the relay symbol. I never got to the power supply design, but make it 10X stiffer than you'd expect. The relay is controlled off board. It's a simple 555 time delay to keep the outputs disconnected until the filaments warm up and the PUS has a chance to charge.
J1 is supposed to be one of those nifty 1/4"/XLR combo jacks.
T1 is a Cinemag CM-QEE-3440-A but any ol 600:50K mic transformer will do.
U1 is the output protection relay.
The output shown is hiZ but I was planning on having a switchable balanced output transformer such as the Cinemag CM-2810. Now that I look at it again....... a 12AU7 cathode follower would be a HUGE plus on the ass end of this thang. IIRC, I never got a 2nd opinion on this design. I think I got frustrated by the cost of the iron, trouble sourcing pots, oh yeah.... I laid the PCB out upside down. :BangHead:
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Thanks to all
The pre is for my son, he asked me to look for a good tube schematic
don't know if he would like to have a complicated pre or only a good pre
I think that functions like those HBP named are welcome
As I'm not experienced with mic pre, so I' was looking to something that has a layout and/or detailed photos as to be "cloned"
Again thanks
Kagliostro
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Check out the JT-115K-E cascode mic pre from here:
http://www.jensen-transformers.com/apps_sc.html
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I was looking for the Universal Audio 610 schematic HBP mentioned
but I'm not able to find it anywere
does someone has a link to that schematic ??
Thanks
Kagliostro
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Someone give me this link
http://www.gyraf.dk/gy_pd/g9/g9pd.htm
It seems to be a good project but I think is far from my abilities
they use a PCB and I'm far from be able do obtain one by myself
also the transformers are very good ones (and expensives I think)
so I put here the link only for those that are more experienced and can have some interest in this project
Kagliostro
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Jacob is a brilliant young man. His stuff is excellent. Somebody at the Prodigy-Pro forum may have a copy of that PCB laying around. Expect to
bleed hemorrhage green for the transformers.
If you can wait about a week (a LOT on my plate right now), I bread-board something that will work, be fairly simple, and try to keep it within a certain budget. I'll do a P2P layout so you can turret board or tag strip it..... that is if you wanna?
-Richard
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What can I say
MANY THANKS RICHARD :grin:
Kagliostro
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>google Altec 1566
http://www.tangible-technology.com/tubes/1566/1566.html
We're thinking sorta along the same lines. The 1566 is a nice simple 2 bottle per side mic pre with just a level control... end of story. I was thinking more along the lines of carving a preamp circuit out for the 1567a mixer. Now you get B M T controls and the juicy sound of a 6FQ7. I've attached the 1567 original schematic, and a pair of copies of a preamp sliced out with a couple of mods. This isn't complete and in fact it should be reviewed for errors, but it's simply a starting point.
You need to answer these questions:
How many channels?
What sort of tone controls?
Rough budget?
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MANY THANKS Butterylicious
I think a 1 channel (as to begin) will be fine
the budget at the moment is not fixed but sure I don't want a poor project, only a not very expensive one
and, most important of all is that I need a simply construction one
to have tone controls or other functions is of sure a good thing, only I'm not able to manage a PCB project and need to have, at least, a layout or good photos to clone/realize the project
in an italian forum I find also the idea that is shown in the schematic attached here, but to use it in a mic pre (also if I like 6sn7 tubes) is very far from my abilities
there is a layout of this altec pre ?
http://www.tangible-technology.com/tubes/1566/1566.html
Kagliostro
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There is no reason to use a PCB. I can easily do a point to point turret board layout. The tricky part will be sourcing some of the parts, namely the transformers. The Altec 1566 will be just about as easy of a tube project as it can get. The 1567 will be a bit more complicated because of the tone stack, but it would be more colorful. Regardless, what you need to be looking for are the input transformer and the output transformer. We'll figure out the power transformer once you've decided which path you want to go down.
The input transformer is a "microphone to grid" transformer. Altec used a Peerless 4722 with a 600:50K impedance ratio.
The output transformer is a "line matching" transformer. Altec used a Peerless 15095 with a 15k:600/150 impedance ratio rated at 2 watts.
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Hi Butterylicious
Oh ....
to have a tone control is fine
also function like those HBP named are interesting
gain and input impedence selectors are fine
phantom power is required as my son has many different mic and more than one need phantom power
any council and help is appreciated
about transformers
do you see something of usable here ?
http://it.rs-online.com/web/search/searchBrowseAction.html?method=retrieveTfg&binCount=10&Ne=4294958127&Ntt=trasformatore+linea&Ntk=I18NAll&Nr=AND%28avl%3ait%2csearchDiscon_it%3aN%29&Ntx=mode%2bmatchallpartial&N=4294955874&Nty=1
Thanks
Kagliostro
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Today I try to understand better mic preamp and then I asked to my son what really he would like to have
the result was
wanted features:
. input gain control
. phantom power
. high pass filter
one step (someting like75hz or 80hz)
or
two steps (someting like 75hz/100hz or 80hz/150hz)
. output level control
welcome features but not essentials:
. compressor
. tone controls
. phase inverter
he has both Dynamic and Condenser mics
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a friend also give to me a project that seems (to me) interesting
this project is transformers-less
you can see it here attached
Many thanks for any help
Kagliostro
p.s.: another link for transformers
http://www.tube-town.net/ttstore/index.php/cat/c251_Miniatur-Transformatoren.html
do you think that they are usables ?
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I didn't even think about phantom power. Absolutely a necessary feature and easily added to any circuit. This is your project. I am happy to bread-board and create a layout for whatever circuit you want to build. I'm still leaning towards the 1567 hack but it's your call. As for transformers, Edcor makes mighty fine stuff at affordable prices and I'm sure they'll ship across the pond. Old mic to grid input transformers are pretty easy to find in old junked out tube PA's. The 2 watt output transformers are not nearly as plentiful in old junk PAs.
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I found a UA 610-A schematic (http://www.recordingjunkie.com/Documents/TechSpecs/Pre-Amps/UniversalAudio/610-A.pdf) by trolling around the place for such things... PRR and Buttery can tell you about Prodigy-Pro.
Anyway, it looks about right to me. I can't directly compare the schematic to my preamps, because I'm a little ways away from them at the moment. I do know my UTC output transformer probably wasn't rated for the standing d.c., and so there was a largish ~47uF cap coupling the plate to the primary, and therefore some kind of plate load resistor (this is a "shunt-feed" or "parallel-feed" arrangement, just like you see with SE power amps).
In my preamp, the external power supply is one IHB250-0.1 (International Power Linear supply, 215-265v, 0.1A), and (I think) one IHB12-1.7 (International Power Linear supply, 12v, 1.7A). In all, these are feeding 2 12AX7's and 2 12AY7/6072. The tubes are wired for 12.6v operation.
The 610 has several layers of feedback going on, to keep things relatively clean. But you'll still want to use "low gain" and/or the pad switch because it *will* distort on too-hot signals.
Also, the phantom power isn't shown, but hopefully someone will chime in with that. It's not terribly complicated, as I recall. Anyhow...
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Don't khow haw to thank you Butterylicious & HotBluePlates
Many thanks for all the help
as I'm not able to understand well
can you explain which can be the difference
using a schematic with transformers
(I see that the most mic pre uses transformers)
and a transformersless schematic
which can be the waste in the signal using a schematic without the transformers ?
other question
which Edcor transformers can be used, have you the models numbers ?
MANY THANKS again
Kagliostro
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Sorry I forgot to ask
I've some TL600 input transformers they are in RCF amps (AM20 - AM30) I got it to get components
the pre in this amps is transistor and not tube
I asked to RCF about info on that TL600 transformers but they were sorry to tell me they don't have infos about
how can I find the impedance of that transformers ? can I use the way to test a OT ?
Thanks
Kagliostro
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Just thought I'd chime in here as well...
The transformers from RS components that are usable are the OEP models. This is what the gyraf G9 diy kit uses and no they aren't that expensive. But you will need to get the shields with them. They are known to be a little bit dark so you might want to adjust for that.
You will most definitely need transformers for a tube mic preamp- transformerless could be done but it just isn't worth the hassle with tubes.
If you are looking for a simple solid state project look for the THAT Corporation 1512 chip. There are some places that sell these with PCB boards and components for quite cheap-- i.e. you could build four or more of these for the cost of one channel of tube.
Check out Rod Elliot's ESP website for an excellent phantom power schematic and mic pre schematic albeit transistor.
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One more option that will save you some time would be microphone preamp kits.
I recommend http://www.seventhcircleaudio.com/ They have a Neve 1073 clone(n72), a Millennia clone (c84), an API clone etc. They sell an 8RU chassis with a power supply. Then you are free to choose between 4 or 5 preamp designs. I have two n72's and two Millennias in my box right now. They are easy to assemble and still save you some money as compared to purchasing another high quality preamp.
The other preamp kits I have built are the Hamptone's. I sold the tube pre and kept the jfet preamp. http://hamptone.com/
http://www.glacieramps.com/Glacier_Amplification/Hamptone.html
http://www.glacieramps.com/Glacier_Amplification/Seventh_Circle_Preamps.html
I totally get that you may want to roll your own pre's but I thought I'd mention the kits as an option. I myself was getting into building my own pre's when I decided to be lazy and order the kits instead.
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Thanks bibi
for the info about phantom and transformers
I'm looking for tube schematics
Thanks also to TubeGeek
I was answering to bibi
about SS my son has find more than one kit for a rebuild of a neve
but he would like a tube one
and you post your answer
thanks any way
Kagliostro
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Hi Kagliostro
I don't know if you checked out that Jensen site on the link I posted earlier - but if has a couple of tube mic preamps and line/mic input isolation etc. The cascode pre has two cascode stages, which is interesting.
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Thanks tubeswell
When I've read your post I was at job and had no very much time, so I forgot to look well there
in this moment I try to open the link in that page, but I've a problem with acrobat
I'll try tomorrow (now here is 0.00 of the 14/03/2010)
and this morning I've to get up early as I and a radio amateur friend go to a flea market some Km far from here
Thanks again
Kagliostro
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The 1567a hack is a bust. I built the audio circuit last week but didn't get a chance to design and build the power supply until this morning. I have quite a list of reasons I do not feel this is the right circuit to proceed with. First of all, it's quite harsh sounding. This is probably because it has a huge amount of output. Second the tone stack not only uses somewhat obscure parts, but the the controlled frequencies are IMO less than ideal. The circuit itself is a bit more complex than necessary and again and again there are some funky parts. I'm sure this circuit could be tweaked out to become acceptable, but I'd rather start from scratch with something less complex and expensive using parts with more traditional values. Sorry it has taken me so long to come to this conclusions. I certainly hope you haven't been rounding up parts yet. I am going to look through my library of schematics for a "better" idea. I am free most of this weekend + I've been really itching to work in the lab. I'll post something later on this evening.
-Richard
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I decided to NOT copy something and design from scratch. Attached is what I've breadboarded. Seems to work respectfully well. I have added DC filaments (not shown on schematic yet) and plan to add phantom power later on this evening or tomorrow. I tried to use only components that are readily available. The input transformer is a standard mic to grid transformer. The output transformer is a standard 1:1 600 ohm transformer. The power transformer may not be the best choice for this project. I'm blowing off about 125V which seems wasteful. I am going to look at the circuit and see if it'll perform safely at a higher voltage. Tubes are your everyday 12AX7 and 12AU7. The tone stack is a typical James 200hZ/1kHz. I made V1a a plate follower for less gain and better input threshold. V1b is a standard grounded grid. V2a is a cathode follower to deliver a decently low output impedance which allows the use of a common 1:1 balancing line output transformer. You can get a lot more gain out of this circuit by removing R3. I'd suggest changing R2 to a 27k if you go this route. The working schematic is attached. I'll compile the design data and post it later on. Once all the "T's" are crossed and the eyes dotted, I'll crank out a layout.
-Richard
Note:
The 1.0 schematic below has errors. The final coupling cap shown has the wrong value and the James tone stack has been removed.
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V1b is a standard grounded grid.
huh???
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Hi
MANY THANKS RICHARD
no I didn't yet bought parts (I'm waiting for your project)
do you think it will be possible
to put a raw control in the ToneStack as to have the max available gain
(or to put a pot in series with R23 to have a progressive High Gain imput)
and also to insert a 2 step High Pass filter ??
MANY MANY THANKS AGAIN
Kagliostro
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V1b is a standard grounded grid.
huh???
Doh! Grounded Cathode.
>will be possible to put a raw control in the Tone Stack
Yes, but gain will jump 22dB. It's gonna go THUD (not pop) when you flip that switch.
>put a pot in series with R23 to have a progressive High Gain imput
You lost me... there is no R23...
>insert a 2 step High Pass filter ??
Sure can do. what frequencies?
Here's a link to the design data. (file too large to attach)
http://www.sotxampco.com/Temp/Kagliostro1.0-Data.pdf
Edit:
Now I'm leaning towards dumping the James tone stack all together. It's not hifi. After bypassing it, I saw the waveform JUMP in amplitude and become more symmetrical. That may be a first for me. Normally when an output wavefor jumps like that, it's usually clipped or distorted. How important are tone controls?
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Attached is the 1.1 schematic which is where I am stopping for the evening. I've yanked out the James tone stack and added the DC filaments.
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Hi Richard
I was thinking R3 and I wrote R23 - SORRY
you told that if R3 is not present the gain raises - ok you told also to change R2 to 27k instead of 33k
so I've think a way to obtain a variable gain control inserting a pot in series with R3 ( feasible thing ?? )
(did you estimated the actual input gain ?)
OK for the ToneStack :wink: (that wasn't a primary necessity)
about High Pass
two steps someting like 75hz/100hz or 80hz/150hz those will be fine
THANKS :smiley:
Kagliostro
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R3... ah yes, I understand. R3 is a feedback resistor. The ratio of R3/R2 sets the gain for the tube. Counting on thumbs, 1M/33k=a gain of 30. Increasing R3 to 2M effective makes the plate follower a grounded cathode amp since this configuration can only hit a gain of 59. The trade off is less input headroom and greater distortion. BTW, since I ditched the James tone stack, I made V1b a matching plate follower. Putting a 1Vp-p sine wave in, I can get slightly more than 25Vp-p out undistorted with the gain set at 50%. Sorry, I haven't measured max signal output yet, but IMO it's ample. Since this is a "studio mic preamp" and not a "geetar amp", distortion is bad, input headroom is good.
I'll crunch the number on the lo cut switch. 20Hz & 75Hz might be more beneficial than 75Hz and 80Hz.
I'm heading back out to the lab. Updates forthcoming.
-Richard
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Version 1.2 is attached. The phantom power supply bothers me a bit. I think there has to be a better way to do this, but it's not coming to me. The hi pass filtering has been added. I selected a .01uF cap value so that the various resistor values do not load down the previous stage. Shoot! I just realized R11b should be a 330k. 470k gets you a cut frequency of 34hZ. A 180k get you a cut frequency of 89Hz but 180k also starts to load down V1b across the entire audio frequency band.
Hopefully some folks will chime in and help me come up with a better phantom power supply design. Other than that, I think it's ready for a layout. Once again, there is 1/2 an unused bottle. This thang could easily go stereo with 1 more 12AX7...... and of course the I/O transformers. :rolleyes:
-Richard
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I just grabbed a little more telemetry. These measurements were taken at 1kHz w/o the input and out put transformers (all hiZ). I'm going to call these measurements conservative given the nature of my poor old test equipment.
Max signal in = 4Vp-p
Max signal out = 30Vp-p
Max gain = 100 or 40dB
Frequency response is flat from 30Hz to 17kHz.
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phantom power supply should be a stand alone - PS. not part of the mains.
http://sound.westhost.com/project96.htm
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>phantom power supply should be a stand alone - PS. not part of the mains.
Agreed. I guess I'm just being frugal cheap.
What about hanging a voltage quadrupler off the 12VAC? Even if this becomes a stereo unit, I still have >200mA at 12VAC to spare. That gives me 50mA at 48V. Phantom power is typically 48VDC but most condenser mics are fine with 12VDC so it seems regulation isn't necessary. It's too late to go out to the lab right now, but I've drawn my next experiment. Rev 1.3 is attached.
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Hi Richard
I've just seen your latest hard work (Rev 1.3)
About transformer / transformers
I understand you were using how much is available
Instead I've to get all the materials
so the question is about the 330v-0-330v PT
you told you have a drop of 125v that isn't so little
can I use a different PT with a lower voltage rate ?
about Phantom Power I'll try to find an adapted transformer
only need to have the correct schematic and layout to connect the Phantom Power Supply to the pre
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about input (150/50k) and output (600/600) transformers have you councils about particular brand/models ?
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about construction I'll try to put all the PS (anodic - filament - phantom power) in a separate chassis/box
so my question is about which cable is better to use ( multiple wire each shielded ? )
I was thinking to the long wires "walking" together ( both filament and anodic )
but you put a rectified filament supply so, I think, the problem is bypassed
also I would like to have indications about the ground connection between the two chassis
MANY THANKS for the given help
Kagliostro
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>can I use a different PT with a lower voltage rate ?
Of course. I selected this PT because: it is very good, I have one in stock, & it is available with a universal primary. Sure it's B+ is a little excessive but that is easily dealt with using a couple of dropping resistors. This is a mic preamp and not a guitar amp so a lot of power supply filtering is a plus. I am comfortable with this transformer but if you find something you like better, we can simply recalculate the secondary.
>about Phantom Power I'll try to find an adapted transformer
Not necessary. The voltage quadrupler works just fine. It's putting out 39VDC unloaded and 36VDC loaded using a cheap Audix F15 condenser mic. I measured ripple at approximately 0.2Vp-p It's not 48V but it should be fine. The only question now is to regulate or not.
>about input (150/50k) and output (600/600) transformers have you councils about particular brand/models ?
Going with the bargain route, Edcor MX8 will work for the mic input and the WSM 600/600 will work for the balanced output. Right now I'm using an old UTC mic to grid transformer which is probably not a wise choice for experimentation, but it's what I have. I am running w/o an output transformer since my power amp is hiZ.
>about construction I'll try to put all the PS (anodic - filament - phantom power) in a separate chassis/box
Personally, I'd avoid a split chassis design. Right now, everything will fit inside of a 2U rack chassis. The PT is about 1/2" too tall to squeeze it into a 1U rack chassis. This brings up a couple of layout questions:
1. Do you want the input and output jacks on the front or back?
2. I assume you want the power switch, gain control, lo cut, phantom switch, polarity switch, and ground lift switch on the front.
3. Right now I'm using a LED for the pilot light. Do you want any other indicators such as a LED to indicate phantom power is on?
As noted above, the voltage quadrupler does work. I added a bleeder resistor to the B+ supply which changed the value of R16. Rev 1.4 is attached.
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I've started the layout. It's far from complete. This is just the audio path. Imagine the tubes being right angle sockets or mounted on a "L" bracket. They will mount horizontal inside the 2U rack chassis.
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Hi Richard :smiley:
>> This is a mic preamp and not a guitar amp so a lot of power supply filtering is a plus.
excuse me please, you know, English isn't my language
do you mean that the voltage drop help in filtering :rolleyes:
and that with a lower HV PT the filtering will be of a less quality?
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Ok about the 2U rack (if I can find a lower transformer also a 1U rack will be fine)
inputs and outputs all in the rear panel
controls obviously in front panel
as final look do you mean something like the job of oatmeal769 (see attached image)
only with a turret board instead of montage strips ?
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About Phantom Power do you think it will be fine to regulate it ?
>> The only question now is to regulate or not.
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>> Do you want any other indicators such as a LED to indicate phantom power is on ?
a phantom power led is very fine
a plus will be 3 PPM led or at least 1 PPM led to detect visually signal presence/level
(But this is really a NOT strictly necessary feature)
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I forgot a very important feature
an output control level pot
my son told me that is a very important thing (for him)
do you think it will be possible to insert one ?
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Thanks for the Edcor models indications
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MANY THANKS for all this effort
hope the interest is also for other members of the forum
your hard work deserves
Kagliostro
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>excuse me please, you know, English isn't my language
do you mean that the voltage drop help in filtering
I'll try to clear this up. We want a lot of power supply filtering. Since we have plenty of B+, we can divide our voltage drop across 2 resistors and add more filtering along the way.
>Ok about the 2U rack
I did the layout using turret boards. I figured this was the easiest way to do it point to point. I did put a pair of montage strips in there for a couple of tie points. I am at a good stopping point with the layout. It's all done except for the I/O transformers. Please have a look. This is the inside view from the bottom. The footprint is 16 inches by 9 inches which I hope is OK for you. I have been resistant to work in Metric units.
>About Phantom Power
Once again I am stumped. I'm hoping PRR might chime in. I cannot find a fixed 48V regulator. I did find a 0 -120V variable regulator but it's $40.00 US.
>I forgot a very important feature, an output control level pot
Easily done. Simply make R15 a pot.
Rev 1.5 schematic and layout attached.
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LM317HV - about 3USD ea. work the eq. to get 48V - takes up to 60V Vin. if you don't mind working with SMT, LM3012 will take up to 80V Vin. BOTH are adjustable, i have never seen a 48V fixed reg. alternatively, you could use the discrete reg. in the link i posted.
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Hi :smiley:
filtering and PT voltages = OK
inches = OK ! :grin:
Turret Board = they are my preferred :wink:
Phantom Power = I think I've in my stock a little 220v / 15v - 16v - 18v toroidal transformer (don't remember well), If I've enough space in the box can I use that instead to use the 12.6v winding of the PT as to rise with the voltage ??
Output control level pot = :grin: very happy about that :grin:
THANKS Richard
Kagliostro
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>I think I've in my stock a little 220v / 15v - 16v - 18v
You can probably make that fit. Find it, measure it, and post the dimensions. I'll slide the audio board over and make room for it on the right next to the other PT. We wanna keep all that noisy AC stuff in 1 area as far as possible from the input.
I am going to order a LM317HV regulator and experiment with it. I think keeping the voltage quadrupler I have drawn but using the other PT will get us up over 48V, then we can regulate it down to the desired voltage with the LM317HV.
The only other thing we need to finalize are the input and output transformers. I think there's still plenty of romm for whatever you decide to use.
-Richard
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Hi Richard
here there is a little problem
I don't remember where is the transformer
it is sure in one hidden angle of my cavern
I'll find as soon as I can and than I'll give you dimensions
(as I can remember it is a 30va - large 80mm x tall 45mm - but I'm not sure - sorry for the mm indications if I'm correct in inch it is large 3.15 x tall 1.78)
also the 3 of april I and a friend (radio amateur) will go to a flea market (my friend it exchanges and sells there), so I can also think to exchange something with an adequate transformer if mine is not or I don't find it)
as always I thanks you for all the disturbance that I am giving to you
Kagliostro
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Hi
tubeswell give me this jensen link
http://www.jensen-transformers.com/apps_sc.html
carrying my attention here
http://www.jensen-transformers.com/as/as054.pdf
so I give a look there and to the other link in that page
but don't put my attention very much to the first schematic
today I've seen in an italian forum a question about a schematic that comes from jensen page
http://www.jensen-transformers.com/as/as016.pdf
can someone explain to me the functions ?
or better
I would like to know which function is PAD (0dB / -20dB)
and to know if there is a particular reason they put the polarity selection in the input instead of in the output
-------
Richard
what do you think about the Mic Z selector ? can that be a useful feature ??? :rolleyes:
about Phantom PS I find this link
http://www.elecfree.com/electronic/48v-phantom-power-supply-with-tl783c/
------
MANY Thanks :smiley:
Kagliostro
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interesting V regulator. up to 125V. about 6USD ea.
http://www.icbase.com/pdf/TI/TI17280106.pdf
http://focus.ti.com.cn/cn/lit/ds/symlink/tl783.pdf
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Hi Richard
I've not yet find the toroidal transformer
BUT I've find this
primary 0-115v / 0-115v
secondary 0-12v (1.25A) + 0-12v (1.25A)
they are PCB montage transformers
I didn't remember about those
they where given to me as a present from a friend
(I give him a pattern for the coat of his dog - may be this is unusual but I like to make by myself the coats for my dog :laugh:)
the intention, at that time was to use that as an isolating transformer
for my "laboratory", for instrument use
using it connected backward (115+115) / (12+12) then (12+12) / (115+115)
the dimensions are long 68mm x large 57mm x tall 40mm (mounting feet included)
from this I can have both 12v for filament and 24v for the phantom power (with a doubler 48v)
may be 12v for filament can be done but I think 230v HV is not enough for the tubes
or I can use it with primary (115v+115v) (115v+115v) connected in parallel to 230v
than put the 12v in series (12v+12v) + (12v+12v) and have 48v (unrectified) to be used for the Phantom power
what do you think about ?
Thanks
Kagliostro
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A separate PT for the phantom power is probably the best bet. I have not had a chance to order the regulator yet, but if we keep the phantom supply it's own deal, then we can play around with it separately.
I had a chat with ISO last night. I am going to experiment with some different values in the audio circuit. First of all, I am increasing B+ to 360V. I am lowering the input series resistor, feedback resistor, cathode resistor, as well as also lower the Rl pot. I am moving the output level control to in front of the cathode follower which will be better since a cathode follower has unity gain and V1b needs more load. My goal is to get a little better frequency response and better input headroom. I'm sacrificing a tiny amount of gain, but since I'm over 100AV, there's room to play. These changes look to be improving PSRR too. Hopefully I can load and measure the circuit tonight.
-Richard
Edit:
Telemetry on this schematic is erroneous. Some is left from version 1.5 and others are estimates. See Rev1.61 for correct DC measurements.
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:grin: WOW Richard
you're a machine
every day you put on a new quality
I'm waiting the results of your experiment
if i can chose the transformer for the phantom which unrectified voltage do you think is better to have ?
Many Thanks for all
Kagliostro
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>you're a machine
I'm a hack who can't leave well enough alone. :wink:
Version 1.6 is an improvement. Better input headroom, better frequency response. Attache is some telemetry. Please note my test equipment is almost as old as I am, except for my VTVM which has a solid 20 years on me.
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Rev1.61 is attached with updated DC telemetry.
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HI Richard
I would like you and the other friends in the forum will Have a happy Easter
NO I didn't find the toroidal transformer I was looking for (in my garage)
anyway there I've find 3 x 220v / 12v 5A - 12v 5A toroidal transformers
also I got (today at a flea market) one 40VA 220v-380v / 48v transformer (not toroidal but looks as new)
I'm sad because I was not able to get a beautiful box I've seen there, it was an RTTY demodulator, the box was all in aluminum and the measures were very close to a 2U rack (also if it was not a rack mount unit)
Patience soon there are other flea markets of electronic materials so I don't despair
(if I must go on with the pre construction I can also buy a new rack instead to look for a cheap one)
but I'll prefer to spent money more for good components instead than for the chassis (also if I must say that the RTTY unit was a VERY nice box for a pre at my eyes)
Kagliostro
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You have a happy Easter as well. We are having the in-laws as well as my wife's brother's family over for dinner. I am in the process of making Lentil Soup and Spanakopetes from scratch.
Version 1.6 is the one as far as I can tell. I am going to swap out V1 and run the response test again. I think I may have bad data at 20Hz. It'll be at least a week before I can rework the phantom power supply. I have not ordered the regulator as of yet.
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Hi
I understand very well about lentil soup (my wife do it very good)
but I'm very curious about Spanakopetes
What is that ??
don't know why but thinking to those Spanakopetes
makes my mouth water (hope you can understand what I mean)
----
these are days to spend with joy among their friends and loved ones
phantom power and the whole pre can wait the next weeks
----
remember I'm waiting about to know all about Spanakopetes
Have a nice Easter
Franco
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It's a traditional Greek appetizer typically called Spinach Pie in here in the states. Basically it's sauteed onions in olive oil, fresh spinach, dill, parsley, & bread crumbs. You let that part cool and add it to a mixture of eggs, feta cheese, & cottage cheese. You mix it all up and roll it into triangles using phyllo pastry. Bake it and it's mighty delicious. My daughters are vegetarian. My mother-in-law is bringing a ham. I always try to make something good w/o meat.
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OH I understand
Also here in Italy we use phyllo pastry
often with ricotta
(is a type of cheese you obtain from milk whey after you have got cheese from milk) and Spinach but your Spanakopetes are a bit differents
other times we have the inside different
Also my wife has cooked phyllo pastry this afternoon but the inside is a surprise
I'll tell you about what was inside tomorrow :wink:
Franco
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HAPPY EASTER
The inside of phyllo pastry was really a surprise
inside were:
Dried figs
Dried prunes
Blueberries jam
Figs jam
Crated bread
Butter
Walnuts
Sugar from cane
:grin: :grin: :grin: :grin:
Kagliostro
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Franco,
When you get a chance, ask you son how much gain he is looking for. Right now I feel that there is more than enough. AV is approximately 200 or 46dB. That's plenty to amplify even a very old weak ribbon mic. Using the cheap modern mics I have, I cannot turn the gain up past 25%. The overall frequency response is acceptable, but I want to see flat response from 20Hz to 20kHz.
I am going to try another experiment using only 12AU7's. Total gain calculated is just under 100 or 39dB. This should still be plenty. Often times back in my studio days, I was only looking for 3 to 12dB out of a mic pre. Although we're looking at less gain, we are getting wider bandwidth, tremendously improved input headroom, and a lower output impedance. Any one of these benefits is worth the cost of gain since I believe we have plenty to spare.
I still haven't worked on the phantom power supply. The cheap side of me has relented and it's going to be a separate power transformer & a regulated DC supply.
EDIT:
Of course it just dawned upon me that the PA211 (PA522 Europe) is only 15mA. :BangHead:
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Hi Richard
I'll ask to my son as soon as I can about haw much gain he need
(he has some old Sony ecm33f mic)
http://www.coutant.org/ecm33f/index.html
BUT not only this
so I must ask
about Phantom Power I find out a 50VA 220v/48v transformer amd I think this will be fine for that (or not ?)
yesterday I was looking to my scematic archive (for VVR and related) an I find out this
http://www.duncanamps.com/technical/mosfet.html
do you think that this circuit can increase the performance of the pre (with an adequate output voltage) ?
or this will only add noise ?
excuse me I don't understand
Of course it just dawned upon me that the PA211 (PA522 Europe) is only 15mA
can you explain ?
MANY THANKS FOR ALL
Franco
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Last moment add
Richard, what about the use of a led instead of a resistor+cap for cathode ?
in an italian HI-FI forum I often read about the use of led (different colors for different ratings) but don't know if that will increase the linearity of the circuit or only boost the signal :rolleyes:
Franco
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>>Of course it just dawned upon me that the PA211 (PA522 Europe) is only 15mA
The 12AU7 typically draws much more current than the 12AX7. Version 1.61 measured roughly 7mA total. 1 mA for each 1/2 of the 12AX7 and 5mA for the 12AU7. The 12AU7 circuit calculates to 15mA, 5 mA per stage, which is the max rating of the PT. Although acceptable to push a transformer to its limit with a guitar amp, it's not a good idea with a "hi fi" circuit.
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OK PA211 and PA522 are the PT (in italy TA) models :sad1:
SORRY
Kagliostro
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Right now I'm personally wanting a stereo mic pre with a gain structure more along the lines of AV=100, 30dB.... give or take. Using a total of 3 12AU7's (4 plate followers & 2 cathode followers), I'm looking at roughly 30mA. The Hammond 273AZ is 650VCT @ 46mA, 50V tap, 5VAC @ 2A and 6.3V @ 2A. That's ample. The 373AZ has the universal primary but unfortunately it looks like Hammond wants almost double for that feature.
That 50V tap sure is smelling like an AC source for the phantom power. Since there's 5V to play with too, I'm thinking about adding a time delayed relay to keep the outputs disconnected until the filaments warm up. First thangs first though. I need to bread-board a single channel and see what it does. Right now it looks good on paper my computer screen.
Edit:
The 273AZ does not have a bias tap.
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Hi Richard
I asked to my son about the desired gain
he told me that the higher is the gain the better it is (he has some old, pardon, vintage mic)
but a compromise is acceptable because a flat response is a must to have option
so you are the engineer, and I must remit to you to decide about best compromise
about transformer I think I'll wait your chose, only, as my opinion, the phantom power will have a separate transformer
THANKS
Franco
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>he told me that the higher is the gain the better it is (he has some old, pardon, vintage mic)
Then let's continue with the 1.61 version. It has enough gain to amplify any old ribbon mic. If used with a modern mic, simply turn down the gain.
>a compromise is acceptable because a flat response is a must to have option
I retested the frequency response last night. That dip at 20Hz seemed odd. I must have collected bad data because I could not replicate it. This circuit is flat within -2dB at 20Hz and -1dB at 20kHz. In my opinion, that's quite acceptable. You could probably get the low end up a little more by going with 10uF coupling caps instead of 2.2uF. I'm using 2.2 caps because I have plenty of them on hand in an "audio grade" variety vs. using electrolytic caps for coupling. Please note these tests were run w/o input or output transformers so as to not color the data.
>the phantom power will have a separate transformer
Yes. My thoughts on using a bias tap were quite flawed. Since the B+ is derived from a full wave rectifier, that means the CT is grounded. The bias tap is referenced to the CT which means I could only use a 1/2 wave rectifier which is completely out of the question.
I will update the layout this weekend. I still haven't ordered a regulator for the phantom supply. Sorry I've been busy with real work plus I hate to place an order for a single $2.00 part.
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Hi Richard
don't mess about the time
real work is on the top
all other things comes later
only one thing
my 220v/48v 50VA transformer I think is a "little" overdimensioned
as the 1 and the 8 of may I go to a fair of the electronics (1 may) and to a very big (international) radio amateur flea market (8 may)
if I found a near 48v transformer, in which range is better to have it (how many VA / mA) ?
THANKS AGAIN
Franco
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Attached is a usable phantom power supply schematic.
Please check for errors.
-Richard
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Hi Richard
Thanks for the schematic
saturday I will see to procure me a pair of TL783C
Franco
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>>Please check for errors.
Errors galore. Delete the previous schematic because it does not work. First off, the isolation caps and zener diodes are not necessary with a transformer coupled input. That's solid state stuff not necessary here and unwanted in the signal path. I had a diode and a cap in the adjust circuit that caused problems. That's what I get for hijacking circuits from the internet instead of reading the cut sheet on the part I'm using and doing what they tell me to do.
http://focus.ti.com/lit/ds/slvs036m/slvs036m.pdf
Page 10 (top left corner) spells it out exactly as we need it. Vout = Vref(1+R2/R1) Vref for this device is 1.25 V so....
Vout= 1.25{1+(3000/82)}=47V close enough. Exactly what I'm getting in the lab using 10% parts and a 55VAC transformer.
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Thanks Richard :smiley:
at the moment I didn't got the TL783C (only find LM338K for another use)
but now I know were to find it
the 1 of march I lost the way to get an used 2U rack for cheap :sad:
maybe the next saturday I'll be more lucky
Kagliostro
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There is no law that says phantom power HAS to be 48V, that's simply the recognized standard. I used to have a really crappy mic that used 2 - AAA batteries for phantom. All that being what it is, I'd still shoot for 48V. It looks like the LM338 has a max output of 32V and a max input of 40V. It'd probably work for a phantom supply but 5A is quite excessive. As you said, use it for something else.
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That LM338 would work just fine in place of the LT1084CT-12 with the addition of 2 resistors just like TL783C. Same Vref=1.25V so leave R1 the same (82 ohms 1/2 W) and make R2 = 680 ohm 1 watt. That puts you right at 11.6VDC.
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Hi Richard
Today I was able to find a case candidate for the preamp
it was a computer hub, it is near 450mm 430mm large and 65/70mm 60mm tall and 250mm 180mm deep (not measured but it looks very close to that measures now measured) ad it has also a 65/70mm 60mm fan mounted (tat I think is an useful feature, or the fan can waste the signal inside this kind of preamp ?) and also was very cheap and nice to look
I also get a 0-290v/300v 0.085A transformer with a 6.3v filament winding (not able to find a 330v small transformer)
but unfortunately it is near 1-1.5cm 0.5cm too tall and don't fit the case
so I must go on looking for the PT
(the 48v PT that I have for the phantom power fits exactly in the case)
looking from behind the intake of the 220v is back into the right and near is the fan and the original on/off switch
Kagliostro