[BUILD] Quantum Audio QM8A-as-CAPI-312 Rebuild

[disjectia]

Hey all, long time lurker first time poster.

I'm modifying an old Quantum Audio Labs QM8A broadcast console (8 channel, 4 buss) as a Quantum/CAPI hybrid. The original build is respectable, mic pres w/75101 input transformers and LM318 ICs, pre-fader patch point for outboard, gyrator-based EQ (selectable 10K/3K and 50/100hZ boost/cut). External linear power supply added (+/- 18, bit higher than CAPI but working ok); 4 buss output card successfully rebuilt as 325 style boosters with gar1731 DOAs driving EA2503/2503HN output x-formers (pair of each).

Currently modifying mic pre on one channel card but have run into an issue. Replaced 75101 with EA2622 (pinouts were mirror images but got around that by installing on the trace side of the card, making sure not to short the casing against traces) and modified the circuit based on a simplified version Jeff's VP312 schemo (omitting phantom power and pad switch); op amp is CA0252 in 2520-to-dip8 adapter (as used for buss outputs).

The result passes signal, but it's very quiet and turning the gain pot has no effect. I've double checked all connections for continuity where expected, as far as I can tell everything is as I intended based on my understanding of the circuit, which admittedly is limited.

Attaching modified schematic (I know I'm missing some elements from original design, like the 1000pf caps on the primary side of 2622; planning to add those but don't think it accounts for the issue I'm having?) as well as an image of the actual PCB with modified layout overlay (signal path in orange, ground in green, resistors in red, caps in blue, ignore the switches bottom right).

I've never wired a gain pot before; the negative side of the 330uF cap is sent to lug 1 of the pot, with lugs 2 and 3 tied together/to ground, but turning the pot has no effect on the level. I have however tried monitoring the circuit with no op amp installed and I can hear the hum of the open circuit being attenuated by the pot.

Can anyone see where I'm going wrong, or should the circuit as drawn/shown operate correctly?

 

[Harpo]

The shunt arm (200R , 330uF/16V, 25K rev.log pot) doesn't connect to the op-amps output pin6. It should connect to the op-amps inverting input pin2 instead.

 

[disjectia]

Ah, copy that - yes I see now, I should connect the shunt arm to the other side of the 20k resistor/120pF cap.

Thanks Harpo, I will make that adjustment

 

[disjectia]

That worked great, 4 channels up and running. For the rest, I'm leaving the 75101s but modding the rest of the circuit like the others.

Anybody built a 312 with 75101 on the input? Trying to understand if I need to omit/change any values using that instead of 2622.

In the original Quantum circuit (crudely rendered schematic attached) the 75101 is wired in parallel, so 1:5 turns ratio. I don't see a zobel network on the output, just a 100K resistor; can anyone tell me the function of 820R resistor and 47uF cap between the inverting input and ground? Basically wondering if there's clues in original circuit so I don't need to measure/calculate changes...

 

[Harpo]

can anyone tell me the function of 820R resistor and 47uF cap between the inverting input and ground? Basically wondering if there's clues in original circuit so I don't need to measure/calculate changes...

The 15k feedback in conjunction with the 820R shunt resistors set a fixed voltage gain of 1+15000/820=19.29 or in dB giving a gain of log(19.29)*20=25.7dB.
The DC blocking 47uF cap in conjunction with the 820R set a HPF @.1/(2PI()*820*47e-6)=4.13Hz.
The 150pF cap in conjunction with the 15k feedback resistor set a LPF @ 1/(2PI()*15000*150e-12)=70.7kHz.
Add the 14dB from your ratio 1:5 hooked up input transformer, giving a total fixed gain of 39.7dB on paper.

 

[disjectia]

Amazing, thank you for the detailed breakdown