Preamp Tesla EZL 444

[fjgaston]

Hi,
I got an old preamp, it's all discrete with input and output transformers.
It's supposed to be a mic preamp but the input transformer seems to be a line input transformer :
5AN670 71 1:1 vstupní linkový 1,55V, kryt vejce
Google translate says : "1: 1 input line 1,55V, cover eggs"
I didn't find many documentations, only the schematics for other parts of the desk it comes from. All the other modules I found were powered in -24V, so I guess this one too.
Here is the schematic I draw :

My guess is 4a and 4b are the inputs and 9b and 9a are the output pins.
12b, 12a and 2b are connected to -24v and 1 is connected to ground.
I think 6b should be connected to 6a (directly ? with a resistor ? with a capa ?)
I don't know if 10a, 10b, 9a and 9b sould be connected anywhere.
There seems to be 5 connections on the output transformer's primary and 2 on the secondary.
The primary connection on the board are 5, 7, 20, 22, 24 and the secondary are 1 and 26.
The output tranformer is : 5AN673 85 2+2+1:1 výstupní linkový 1,55V, pult EPP102, jednotka EPE401, EJV441, jádro M12
Google translate says : "2 + 2 + 1: 1 output line 1,55V, counter EPP102 unit EPE401, EJV441, the core of M12"

Here is the only document I found on internet about this preamp :

And a few photos :

 

[PRR]

> Here is the schematic I draw :

Interesting.

If it were alive enough that you could trace the voltages and draw them top to bottom, you would probably get something like the Attached.

Now it looks like a complementary emitter-follower (instead of a no-tail pair).

And the input stages look quite ordinary.

You find this basic plan in thousands of small power-amps. The first two stages give good gain, teh 3rd 2-transistor stage gives strong output.

Your symbol for T2 looks like a JFET. I am 90% sure it is a PNP BJT.

> the input transformer seems to be a line input transformer

Same thing, almost. Mike iron is smaller but should have heavy hum-shielding, and costs more. If they didn't care (or someone has modified) then a line-in "works".
However the guessed current in T1 suggests an optimum source of 2K to 10K. A microphone through 1:1 is nearer 200r.

If this is in a German tradition of HIGH level microphones, and there is no mains-power in the console, then un-shielded 1:1 input iron may be just right.

 

[fjgaston]

Hi,
Thanks for the informations.
I also thought T2 was a BJT as it was marked B, E and K (Base - Emitter - Kollector) but on the transistor I can read HANT 6635 and the only 6635 reference I found on internet was a JFET, that's why I drew a JFET symbol but you should be right.
Thank you very much for the drawing, I'll make some measurements as soon as I have a decent power supply.

 

[fjgaston]

I tested it with three 9V batteries average Voltage was close to 20v, curent consumption 30mA and the gain is 56dB.
Next step is to build a proper Power supply and box for connectors

 

[Dark1]

Hi,
the "EZL" modules were line amplifiers for console inputs/outputs, like bridging, line driver etc. (the "L" in the "EZL" means "line", literally). TESLA modules from this era (late 1960s Czechoslovakia) are usually a strange mix of germanium and silicon transistors, sometimes also early ICs. If you see a black can transistor anywhere, there is a good chance it is a germanium one.

Can not help you with the 444 version, but the earlier 440 and 441 versions had (unsymmetrical) inputs on a4, b4 (shield on a3 or a1, depending on module, check it is not tied to b1/b6),  and outputs (symmetrical, transformer balanced), on a9, b9, with shield/housing on a8 (again, should not be tied to the +24V/common, which is on b1/b6). Power supply was indicated -24V on ab12, and +24V (common) on b1, b6. The a6 pin seems to be unsymmetrical output, in some versions, bypassing the output transformer.

These are just some notes I have on the earlier modules in my notebook, no more information available at the moment, sorry.

As for restoration. Check the dark brown capacitor(s), these are paper caps that had hygroscopic bodies, often cracking and changing values or failing, depending on storing conditions. Checking the electrolytics (aluminum cans with pink plastic wrap) is also a must, these were not the best parts either.

Hope this helps!

 

[fjgaston]

Yes it helps,
I'm now building a rack and power supply, I don't know if there is a way to make it behave more like a mic pre (change the input impedance ?)
If you have informations on the EKL 021 (compressor unit), I'm also interested

 

[Dark1]

I would use it as it is, and should you need more gain, use something else. 

 

[Dark1]

As for EKL 021, I have not worked with one yet. All I have is the same barely readable schematics you have, sorry...

 

[fjgaston]

Just racked it up !!

It's just a wooden box and it's not finished yet but it's enough to try it and I quite like it. I just tried it with my SM7b and it's better than my mixing board's preamp (Mitec EX-31) and much better than my firepod's preamps.
The SM7b is not a very sensitive mic but it worked well. Now I need to add a phantom power P48 and P12 (for my schoeps CMT34) and I'll also rack my compressor when I find how to plug it.

By the way ... does anybody know what is this connector and where I could buy one :

 

[fjgaston]

Hi,
I just drew a new schematics in the way PRR sugested :

If it's a complementary emitter follower I guess the 1000uF capacitor to the ground doesn't make sense ???

I have a compressor with unbalanced input and output I'd like to use it as an insert with the preamp, I don't really know how to do it, I'll measure the voltages.

 

[Dnukas]

Hi, maybe this help.

Lukas

 

[abbey road d enfer]

Now it looks like a complementary emitter-follower (instead of a no-tail pair).

With a trick! Load shared between emitters and collectors eases the work for the driver transistor.

 

[ask]

How did it go, were your assumptions correct regarding the pinout? What about shielding and grounding? I'm curious, also because I'm thinking about getting a pair for myself..

Hi,
I got an old preamp, it's all discrete with input and output transformers.

My guess is 4a and 4b are the inputs and 9b and 9a are the output pins.
12b, 12a and 2b are connected to -24v and 1 is connected to ground.
I think 6b should be connected to 6a (directly ? with a resistor ? with a capa ?)
I don't know if 10a, 10b, 9a and 9b sould be connected anywhere.
There seems to be 5 connections on the output transformer's primary and 2 on the secondary.
The primary connection on the board are 5, 7, 20, 22, 24 and the secondary are 1 and 26.

 

[fjgaston]

Hi,
Sorry for the time it took me to answer. I didn't see the last posts until now.
so my drawings had some mistakes. I now have the full schematics as the one linked by Dnukas.
So for the pinouts :
Pin a4 : input +
Pin b4 : input -
Pin ab3 : input ground
pin a6 and b6 should be linked (you can add a 1M pot to attenuate if your input signal is too high)
pin a9 : output +
pin b9 : output -
pin b2, a12 and b12 : -24v dc supply
pin b1 : ground ? (noted +24v on the schematics)
pin a1 : chassis ground
pin a10 & b10: I don't know (maybe an asymetrical output)

 

[DerEber]

As for the connector they seem to be the same as in east German 700er consoles.

It might  be this one. You can search for the Din number at your supplier.

https://www.buerklin.com/de/federleiste/p/56f117

 

[DerEber]

DIN 41 618

 

[fjgaston]

Hi, I'm coming back to this post again.
Now that we have the complete shematics, do you think there is an easy way to have a variable gain with this preamp ?

 

[analogguru]

Hi, I'm coming back to this post again.
Now that we have the complete shematics, do you think there is an easy way to have a variable gain with this preamp ?

It is already built in: R15, (accessible from the front panel).

 

[fjgaston]

It is already built in: R15

it's the pot in the front but it only reduce the signal by a few dB, is there a way to make it more efficient ?

 

[analogguru]

it's the pot in the front but it only reduce the signal by a few dB, is there a way to make it more efficient ?

Remove R14 and use a higher value for the trimmer R15 e.g 1k, 5k, or 10k according to your taste..