Inserting an Eq into my ADM pre amp. Where??

Hi.

I am racking up a pair of ADM Pre´s with 2 ADM1540 eq´s.
Since the pres are balanced in and out and the eq´s are not i would like to insert the Eq somewhere inside the pre.
I know that there are pins for that on this pre originally, but mine has been modded for a shorter audio path.
Now they go to imput transformer, to the op-amp with gain control, to the aux pot thats used as an outut gain control, and finally out through the output transformer.

So, where do i put the eq´s??
Just before the output transformer, or before the aux pot, or somewhere else??

Any input appriciated!

Regards,
Dennis

If you have the schematic, post it up. Not that I have the answer, but it's not very easy to help without seeing the circuit. It's like asking a mechanic how to fix your car without lifting the hood (or bonnet for other parts of the world). :wink:

Hi.

I do not have the exact schematic for my module, but its almost the same as this (or exactly the same, i am not sure).
ADM 780 schematic
But my module have been modded for a shorter audio path, ell logic controld have been disconnected, and it uses the AUX pot as a master output fader.

Here is the schematic and layout of the ADM 202 op-amp:
adm 202 layout
adm 202 schematic

Does this help??
Will try to see exactly where the audio is taken out to the AUX pot.

Regards,
Dennis

Can't tell from your schematic where your aux pots are wired. However, the only obvious point to insert EQs is after C2, but the levels here will be wrong. They will be about -16dBu + 20log(output transformer turns ratio), assuming +4dBu nominal at the output. It will also depend on what is on the other end of lines P and N.

Ok,
Had a long hard look at my module.

Its modded so the output of the op-amp goes directly to the input of the AUX pot, this is just before C2 on the schematic.
Then the output of the AUX fader goes directly to the in of the output transformer.

All IC´s have been removed, and there is alot of trace cutting to make this work i guess.

Does this make any sense??
Would it be possible to insert the eq somewhere, or will the levels be totally off?

Regards,
Dennis

It wouldn't be my choice to drive the output transformer from the wiper of a pot.

If you want to splice your EQ in after C2, I would consider resurrecting A3 and its output transistors to amplify the EQ output and drive the transformer.

Hi.

As i said, the signal is taken BEFORE C2 to the AUX pot.
I think C2 and A3 are both removed.

May i ask why its not good to drive the output transformer with the AUX pot??
Is there something bad that can happen??
It does sound good, and i cant hear any problems at least.

Dennis

Since it sounds like someone has already severly modified this unit, I will offer this solution:

1. Restore the amplifier A3 and the unity gain buffer following it.

2. Move the feedback connection from the output of the buffer to the output of the amplifier A3. Leave the feeds to all other circuitry connected to the buffer output.

3. Insert the eq between the output of the amplifier A3 pin 6 and the bases of Q1 and Q2.

Its not good to drive a transformer from a source of variable impedance, which is what a potentiometer would look like to the transformer.