Looking for a good tube mic preamp schematic

[kagliostro]

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

[bmack]

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. 

[jjasilli]

google Altec 1566

[HotBluePlates]

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.

[jjasilli]

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

[bmack]

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.

[RicharD]

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.   

[kagliostro]

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

[tubeswell]

Check out the JT-115K-E cascode mic pre from here:

http://www.jensen-transformers.com/apps_sc.html

[kagliostro]

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

[kagliostro]

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

[RicharD]

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
 

[kagliostro]

What can I say

MANY THANKS RICHARD 

Kagliostro

[RicharD]

>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?

[kagliostro]

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

[RicharD]

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.

[kagliostro]

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

[kagliostro]

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

---------

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 ?

[RicharD]

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.

[HotBluePlates]

I found a UA 610-A schematic 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...

[kagliostro]

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

[kagliostro]

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

[bibi]

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. 

[TubeGeek]

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.

[kagliostro]

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

[tubeswell]

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.

[kagliostro]

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

[RicharD]

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

[RicharD]

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.

[DummyLoad]

V1b is a standard grounded grid.

huh???

[kagliostro]

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

[RicharD]

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?

[RicharD]

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.

[kagliostro]

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  (that wasn't a primary necessity)

about High Pass

two steps someting like 75hz/100hz or 80hz/150hz those will be fine

THANKS 

Kagliostro

[RicharD]

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
 

[RicharD]

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.   

-Richard

[RicharD]

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.

[DummyLoad]

phantom power supply should be a stand alone - PS. not part of the mains.

http://sound.westhost.com/project96.htm

[RicharD]

>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.

[kagliostro]

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

--------

about input (150/50k) and output (600/600) transformers have you councils about particular brand/models ?

--------

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

[RicharD]

>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.

[RicharD]

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.

[kagliostro]

Hi Richard  

>> 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  

and that with a lower HV PT the filtering will be of a less quality?

---------

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 ?

---------

About Phantom Power do you think it will be fine to regulate it ?

>> The only question now is to regulate or not.

---------

>> 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)

---------

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 ?

---------

Thanks for the Edcor models indications

---------

MANY THANKS for all this effort

hope the interest is also for other members of the forum

your hard work deserves

Kagliostro

[RicharD]

>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.

[DummyLoad]

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.

[kagliostro]

Hi  

filtering and PT voltages = OK

inches = OK !  

Turret Board = they are my preferred  

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 =   very happy about that


THANKS Richard

Kagliostro

[RicharD]

>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

[kagliostro]

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

[kagliostro]

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 ???  

about Phantom PS I find this link

http://www.elecfree.com/electronic/48v-phantom-power-supply-with-tl783c/

------


MANY Thanks  

Kagliostro

[DummyLoad]

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