Gain of first opamp in API 536 console (line in)?

[JW]

I was experimenting with a channel in my console in order to add a volume control to 'juice up' the first opamp a bit and have the opportunity to pull it down with the fader.

First, this is for a 'line in' scenario. I don't believe API used to control the gain of the first opamp with an external control ? except when in 'mic' mode. In this case (312 mic pre) we have a 20K pot switched in to the negative opamps input.

My question is, why can't I do that in line mode as well? In line mode, the signal is fed through 10K resistors (no transformer) to the +/- opamp ins. Yet, the same spot that controls the opamp gain in mic mode seems to do nothing in 'line' mode. I'm changing the 200 ohm resistor in front of the negative input out for a pot. I've tried up to 25K. Regardless, no gain change.

Here's a link to Boji's redraw of the schematic. Console signal flow, input cards, general discussion on API styled DIY mixer

 

[abbey road d enfer]

My question is, why can't I do that in line mode as well? In line mode, the signal is fed through 10K resistors (no transformer) to the +/- opamp ins. Yet, the same spot that controls the opamp gain in mic mode seems to do nothing in 'line' mode. I'm changing the 200 ohm resistor in front of the negative input out for a pot. I've tried up to 25K. Regardless, no gain change.

That's because in Line mode, the opamp is reconfigured as a differential amp. By adding a resistor there, the overall gain doesn't change (noise may suffer, though). Making gain controllable of a diff amp requires some modifications to the topology. If not carried with good knowledge of how opamps work, the performance can be degraded.

 

[fazer]

What Abby said. The api console was usually over driven in the mix bus on mixdown . You would pull down the master 2 mix fader and push up the individual channel faders. That means the diff line input amp would stay at unity fixed gain and did not require being driven hard. There is a way to add circuitry with another amp that would keep the instrumentation input unchanged. Search around here. I saw a post recently

 

[JW]

Thanks very much Abbey. So I'm off to research 'differential opamps.' obviously not knowing what the topology means. The second stage can definitely be adjusted at the negative input so it must switch back. . .

 

[JohnRoberts]

Differential simply means there is a + and - input, where - input is subtracted from + input. This allows you to reference signals relative to a 0V ground (any error voltage on both cancels out).

JR