API 325 make-up gain in A&H Saber

Hey everyone,

I've installed rebuilt 325 cards to replace the makeup gain opamp in a Saber console. Two of the output secondaries are used in series and wired directly to the Mains XLRs as a balanced signal. The 325's gain is set accordingly by fixed resistors. The third output secondary feeds the monitor/Ctrl room section of the board through a T-pad attenuator to match the gain of the replaced op-amp.

The modification vastly improved the overall sound of the board, especially by added clarity and focus to the low end. So here's the problem... On both Main output channels I'm seeing an AC voltage (test tone) on pin 2 exactly 3x that of pin 3. So it seems like a simple problem in regards to the transformer wiring. Maybe I need to ground the junction (CT) of the secondaries?? Someone please tell me what's going on.

Thanks.
- Andre

You don't necessarily need to do anything.

You're not accounting for balanced load impedances. -Your meter has such a preposterously high input impedance (assuming that it's a DMM) that ANY slight leakage impedance to ground on one leg will affect a difference in reading.

In fact, you shouldn't really be able to read ANY reliabel voltage between EITHER leg and ground, becuase they are utterly unconnected at the preamp... It's a completely open-circuit.

So don't fixate, and don't think that a center tap to ground is automatically a good idea. -Stop for a moment and consider that a galvanically-isolated signal is VASTLY better in terms of ground-loop avoidance. -Connect the center tap, and you throw away the galvanic isolation, for no good reason other than making the numbers look pretty on the DMM.

Keith

very cool, I've been thinking about doing this for a while, what DOA did you use?

Keith,

Consider my worries a thing of the past. Thanks for the advice.


AJ,

The DOA is a 2520 variant from Scott Liebers and Eddie Ciletti of www.tangible-technology.com. They offer two versions- a lower noise modern version and a vintage. I bought the vintage version and am pleased with the sound.

- Andre

Cool, Andre.

I need come by and give my ole console a listen !!!