add transformer outputs to unsymmetrical preamps

[barthman.de]

I have some old preamps which I want to rack. They have input transformers but the outputs of these preamps are unsymmetrical/unbalanced because they was built in a broadcast console which use unsymmetrical signal flow internally . The preamps have full discrete transistor-based design. I want to get the outputs symmetrical/balanced. I think I should go with transformers. IC based balancing is a second option. There are some questions:

What have I to measure to get the right output transformer (I think I need a 1:1 transformer) or which criteria have I to respect?

 

[ruffrecords]

You first need to check the output impedance a drive capability of the unbalanced output. If you can post a schematic, that would help.

Cheers

Ian

 

[barthman.de]

Unbalanced signal is between 12a and 12b. The documentation says: Output impedance @40 Hz <= 15 Ohm; @1KHz 3 Ohm; @15Hz <= 10 Ohm

And here is the schematic:

 

[CJ]

looks like C206 blocks DC current, this will increase your options,

maybe get a cheap 600:600 with taps off of evilbay to play with,

use the 600 on the sec and try different taps on the pri and check response,

you will get a voltage increase if you use anything other than 600:600,

try and find a 600:600 which handles the largest level,

i guess you could use the taps on both sides to keep voltage ratio at 1:1,

like 50:50 ohms, but this might load down whatever the signal is feeding,

DCR will be very low with a 50 ohm tap, also, as you go down in ohms, flux can go up as the core will have less turns around it, so saturation will go up as the Z ohms go down,

so maybe 150:600 might be the ticket, voltage step up would be 1:2 which is not too bad,

 

[PRR]

Gosh, it's a John Linsley Hood.

http://en.wikipedia.org/wiki/John_Linsley_Hood

The supply is only 18V, and for "Pro" output we want over 10V each side (>20V total supply). So I went to see how much current it can pass, see if it can run a 150:600 transformer for step-up.

But an initial look-thru didn't show how the T203 T204 current is set.

JLH. Output stage current is half of R210 current, times hFE of output transistors.

SH127 is... what? A Russian part?? The only numbers I find are 600mW and hFE=28-560. (The "c" on the end may clarify that, but I can't find that info.)

600mW max at 9V gives 66mA maximum current before melting. (Heatsinks may change this.)

Half of 3.6mA times even the lowest hFE of 28 gives 50mA. Which works, if you sort-out *all* SF127 with hFE over 36. Which could be low-yield. So I think something in my analysis is bogus.

Nevertheless.... about 8V peak at 100mA peak is 80 ohms. IF those transistors run HOT, they can probably drive a 150:600 and deliver +23dBm.

We DON'T really need +23dB in modern studios. We don't drive lines across town past power stations. A 600:600 tranny rated +18dBm will do fine.

 

[menigu]

The transistor is a SF127c. Seems to be from the GDR.

Ic: 500mA
Icm: 1Amp
Ptot: 600mW
IB: 250mA
hFE: 56-140

http://www.tu-chemnitz.de/etit/zentral/ddr/sf126.gif

 

[DerEber]

This is some RFZ-Preamp. East Germany.
Sometimes on Ebay old 1:1 transformer of that brand pop up. I suppose they are 600:600 but I never meassured.
They use them for inside the console or going to "outside world".
pretty sure that you can use any 600:600 - +18dbm with success like PRR sugested.
How manny do you need?
look here:
http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=251453123537&fromMakeTrack=true&ssPageName=VIP:watchlink:top:de

 

[barthman.de]

@ DerEber: Nice Link, thank you. I need 6 of them And yes, this are old RFZ-Preamps from East Germany. Seems so, that the seller lives not far from me. I'll ask him.

@ all: Thank you for your tips. These modules work with -20…-24V, need 45mA @ 24V and have +12dB max. output level. At first I have to clean and recap the preamps. Second I need a power supply to test them. This will take a while, I'll keep you informed about the progress  

 

[barthman.de]

OK. Meanwhile I have built the PSU, the preamps are recapped and tested. And I got 6 of this nice original MU-metal-shielded output-transformators:

Now I have to do the metal-work and plan the front plate. I hope I get all in a 3HE 36 cm case.

 

[barthman.de]

Hi folks,

the job is done.

Here some impressions of the project:

Front:

Preamps:

PSU:

Relays for polarity switch "Phase":

 

[south2577]

Hi Sebastian,
I've bought 4 x V741 and 1 x V781..
I'm looking for 6 output transformers for these.
Do you have any leads to where I might find some?
Any help would be fantastic.
Thanks
Ps. Your rack job of yours looks great! How is it all sounding?
Any important tips for racking these?

 

[filipposamore]

OK. Meanwhile I have built the PSU, the preamps are recapped and tested. And I got 6 of this nice original MU-metal-shielded output-transformators:

Now I have to do the metal-work and plan the front plate. I hope I get all in a 3HE 36 cm case.

Hey do you have a wiring diagram for the transformer? or does anyone know the ratio ?

 

[Newmarket]

Any particular reason for opting for transformer solution over ic/solid state solution ,?