Help choosing transformer for opamp output stage

Hi everybody

I am currently working on a pcb for the Neumann PEV-c equalizer.

Most of the pcb is ready, but couldn't find information regarding the output transformer - probably a custom made Haufe. So I am looking for a suitable transformer off the shelf.

My idea was to use Lundahl LL1517 or LL1524, mainly because they are available on the used market for less than a brand new transformer.

https://www.lundahltransformers.com/wp-content/uploads/datasheets/1524.pdf

https://www.lundahltransformers.com/wp-content/uploads/datasheets/1517.pdf


Lundahl datasheets say that the transformers have to be used in a mixed feedback configuration, to provide a negative impedance to the transformer.

From what I can understand looking at the PEV-c schematic, this doesn't seem to be the case.

This circuit is made of two OA10 opamps in push pull configuration. The OA10 datasheet states 250 ohm output impedance (f=100Hz, open loop)

Lundahl datasheets say the optimum source impedance is minus 14 ohm, minus 18 ohm.

I expect the transformer needs to be properly chosen with respect to the impedance of the output stage, but on the Lundahl datasheet displays the turns ratio only, and a negative source impedance is recommended.

How can I figure out, first of all the exact impedance of the output stage, and then the correct transformer able to perform well with such source impedance ?

Also, I guess the transformer should best be connected 1:2 to maximise output level, but that would means a higher impedance is reflected at the primary ? So this has to be taken into account as well ?

Really hope someone can shed light on this subject.
Thanks in advance for any reply.

This is the schematic of the Neumann EQ circuit

Datasheet of the OA10 opamp

beatnik said:

This circuit is made of two OA10 opamps in push pull configuration.

Click to expand...

It's symetrical configuration often used when single power supply is used.

The OA10 datasheet states 250 ohm output impedance (f=100Hz, open loop)

Click to expand...

And it is lower when the loop is closed .
I believe that two OA10 are just fine to drive LL1524.

Also, I guess the transformer should best be connected 1:2 to maximise output level, but that would means a higher impedance is reflected at the primary ?

Click to expand...

I would use 1:1 ratio here.

I’m not 100% sure if this applies but there is a good chance it may. It’s an article about negative impedance and how to use it with transformers. I never knew there was such a thing. This guy’s a kind of a genius. Maybe you’ll get something out of it. It’s a little over my head.

http://sound.whsites.net/articles/negative-z.htm

beatnik said:

Lundahl datasheets say that the transformers have to be used in a mixed feedback configuration, to provide a negative impedance to the transformer.

Click to expand...

The datasheets don't state that the have to be used with NIC / mixed feedback but it is recommended for optimum results.

eg:
"The LL1517 has sufficient low copper resistance to meet broadcast specifications in a conventional drive
configuration, but is (as all output transformers) ideally used with mixed feedback drive circuits. (See separate paper
for mixed feedback design principles)."