Looking for a good tube mic preamp schematic

[kagliostro]

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  )

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

[RicharD]

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.

[kagliostro]

  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

[RicharD]

>you're a machine

I'm a hack who can't leave well enough alone.   

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.

[RicharD]

Rev1.61 is attached with updated DC telemetry. 

[kagliostro]

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

[RicharD]

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. 

[kagliostro]

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

[RicharD]

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.

[kagliostro]

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 

Franco

[kagliostro]

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

 

Kagliostro

[RicharD]

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.   

[kagliostro]

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

[kagliostro]

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 

Franco

[RicharD]

>>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.

[kagliostro]

OK PA211 and PA522 are the PT (in italy TA) models  :sad1:

SORRY

Kagliostro

[RicharD]

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.

[kagliostro]

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

[RicharD]

>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. 

[kagliostro]

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

[RicharD]

Attached is a usable phantom power supply schematic.

Please check for errors.

-Richard

[kagliostro]

Hi Richard

Thanks for the schematic

saturday I will see to procure me a pair of TL783C

Franco

[RicharD]

>>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.

[kagliostro]

Thanks Richard 

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 

maybe the next saturday I'll be more lucky

Kagliostro

[RicharD]

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.

[RicharD]

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.

[kagliostro]

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