> Is C4's only purpose to block dc from getting on the fader?
There may be up to 0.5V of DC at IC1 output. 709 offset near 5mV, gain runs up near 100, there is no DC-block cap on the gain network.
You gotta block that.
And because it shaves headroom, you may want a touch of gain after it. IC2's job.
I'd leave it alone, even to the point of putting a fader in and leaving it near -10dB. Removing one cap, or one under-worked op-amp, is not going to be any big "improvement". If you really wanna "improve" it, there's lots more to change.
But these are NOT bad modules. It IS 1973 all over again. Whoever designed this did a fine job.
> IC 2 amp ..You could change it out for a more modern unit
A de-compensated 301 is a very good audio amp. (The 741, internally compensated 301, gave chip-amps a bad rap, but the issue was the heavy compensation needed for idiot-proofing, not the basic amp.) A 301 is not the lowest noise amp, but at any practical fader setting the hi-gain 709's hiss will dominate. The 301 is not lowest THD in heavy loads, but here it is loaded gently.
The 318 is another lost hero. Not low noise, but that don't matter the way they use it here. FAST!!! It aint gonna slew-distort even with 13Vpk of pure cymbals.
Be real-real careful around that '709. They are not short-proof, and are now scarce. In-rack you probably can't kill it, but idle poinking at PCB points killed the first 709 I ever saw.
Huh. That's another reason you "need" C4. A dead-short on Q1 Q2 output, with a strong signal, is the next best thing to a dead short on the 709. With C4, a short at "To Fader" (stuff happens) will not allow sustained high current from the 709.
