Transformers for API 2520

[Kshitij]

Heyy everyone!
So I have been in the process of designing a set of pre amps for my studio which will eventually reside as the pres for the channel strips on a custom analog console I am building. I love the sound of the 515s, 313s and 1608s for which I believe the soul is the 2520 discrete op amp. I have already built a 2520 based on a schematic but I am absolutely stuck on what the specs of the transformers are.
I believe the ones in the APIs are 2622 for the input and 2503 for the output. I have learnt the the 2622 is of the ratio between 1:7 and 1:10 and in the primary has two sections for 150 ohm and 600 ohm, and the output should be 3:1 step down. But a spec sheet I found shows that the output actually is a step up with 75 Ohm in the primary and a variable/selectable 150/200/300 ohm in the secondary. Would like to know if anyone can help me because I am by all means clueless. Thank you very much.

 

[shabtek]

output is generally step-up 1:2 or 1:3. and/or isolated feeds to bus/line/insert out etc.

 

[CJ]

The output is wound with one piece of four strand wire which is usually twisted or braided like Litz wire.
This means that all the winds have the same amount of turns in relation to each other, so one wind to any of the other winds is 1:1

So you can pick 1,2 or 3 windings for your primary or secondary.

Usually the opamp is loaded with one winding leaving the secondary with three winds.

You can use 1 wind for a vu meter and use the remaining 2 winds as a series or parallel output, this would mean you would have either a 1:1 xfmr or a 1:2 (series connection)

Or you could skip the vu meter and use 3 sec winds for a 1:3 xfmr(series all 3)

You have to watch the phase to get things right but the wires are colored so no problem there if you have a diagram.
Standard wiring is one pri, 2 sec in series and 1 vu out. So 1:2.

You could reverse this process to load the opamp with 1 2 or three winds in series and thus gave a step down, I haver never tried this , it might change distortion or bass response.

You can wind your own xfmr if no cheap ones available.

 

[Kshitij]

There is a schematic I found on the web. The only one I could find which showed the schematic for the variable gain stage. I do not understand how the transformers are wired in this. I was under the impression that the output transformer is supposed to have a single primary of 75 ohm and a three part secondary of 150 300 and 600 ohm to provide for different impedance outputs. If there is the need of only a line level output, can we use a 75 ohm to 600 ohm output transformer? There is also a lot of vagueness around the core materials of the transformers. Also, can someone explain the variable gain stage, wouldn’t it be easier to have a negative feedback with the input on the positive and a variable resistor on the feedback?

 

[Kshitij]

output is generally step-up 1:2 or 1:3. and/or isolated feeds to bus/line/insert out etc.

So it’s like one part of the primary is 1:2 for a certain impedance and then 1:3 for the line feed of 600 ohm? So like a single part primary and the three part secondary of 150, 200 and 600 ohm?

 

[shabtek]

Z (impedance) is transformed by the turns ratio squared...opamp is low low output... we are not usually concerned with matching a 600Ω load, 1:3 will be close enough though. 1:1 or 1:2 will also work...for a stand alone preamp you can try any and all available options and see if you can tell/prefer.
in a console design having options concerning operating level/bus headroom etc. is beneficial

 

[Kshitij]

There is a schematic I found on the web. The only one I could find which showed the schematic for the variable gain stage. I do not understand how the transformers are wired in this. I was under the impression that the output transformer is supposed to have a single primary of 75 ohm and a three part secondary of 150 300 and 600 ohm to provide for different impedance outputs. If there is the need of only a line level output, can we use a 75 ohm to 600 ohm output transformer? There is also Also, can someone explain the variable gain stage, wouldn’t it be easier to have a negative feedback with the input on the positive and a variable resistor on the feedback?

Z (impedance) is transformed by the turns ratio squared...opamp is low low output... we are not usually concerned with matching a 600Ω load, 1:3 will be close enough though. 1:1 or 1:2 will also work...for a stand alone preamp you can try any and all available options and see if you can tell/prefer.
in a console design having options concerning operating level/bus headroom etc. is beneficial

Ohh ok. Thanks a ton! Will try the options