Pro Audio Design Forum

mediatechnology wrote: Because it's non-inverting, the CM performance of that stage requires careful op-amp selection.

Although it's OT could you give a little lesson on why this is important and what about the 797 makes it a contender?

In the above drawing it's not clear whether the AD797 is inverting or non-inverting. If inverting there should be another inverting opamp before the pot or some way to flip the polarity so absolute phase is maintained. It would take a 3 pole switch to flip the input to the 1240 along with the variab...

Looking at Wayne's gain trim circuit above the only downside I see is that the mono sum will be between 3dB and 6dB up already. Having 3dB of gain at the input might have headroom issues. It seems that to "fix" this you would give up the precise level matching by having makeup gain set by ...

Paul/Ruairi - If the +/- 3dB trim were on a pot would it be precise enough from a repeatability perspective? If not, one could build it as a stepped switch. It sounds like to me that you are leaning towards switches. Maybe we should make the board accept both types of controls. I think +/- 3dB on a...

Thanks for the input everyone. Another option is, of course, a knob with a pushbutton to switch between "fixed" and variable. With only +/- 3dB range the knob would have fair repeatability (and less chunkiness compared to a switch) based on pointer position. The switch or relay would only...

If it becomes too much of a problem to have variable gain so be it. What about having the decode line receiver 1240 feed a switch that is an attenuator with a maximum output of -6dB and have 6dB of makeup gain from an opamp afterward? The output of the opamp would have a low enough impedance to feed...

mediatechnology wrote: The options may weigh evenly.

Seems that way to me. I have no preference.

I would find the gain trims useful. In a mastering rig there aren't usually spare line amps laying around. +/- 6dB would be plenty for me. +/- 3dB is actually all I could see using but others might find that limiting.

Edit: I'm certainly willing to pay Roger for his time and beta test.

I guess I shouldn't have used the word pan in my post. I was thinking of an opamp loss/gain stage at point F in your drawing with a separate control for M and S. A pot would present too high a source impedance to the decode stage, correct? It could be jumpered out if not needed. Would that degrade p...

I hesitate to throw this in but since it seems to getting pimped out... The obvious missing piece is gain/pan between the M and S. Impedance wise I don't think passive attenuation would work anywhere in the circuit (correct me if I'm wrong) but would be fine with me. Level adjustment would be much m...