Attenuating Volume with Op-Amps

[m00dawg]

This is not a MidiBox specific question, but more of a general question that I will apply to my NES audio amp board. I am wanting to add a passive mixer to mix the 2 NES channels plus the cartridge in channel into a summed output. I believe I have that good to go, but the problem now is that the cartridge input requires different resistor values depending on which cartridge is being used.

The original mods mixed the cartridge audio directly into the NES audio pipeline. My design bypasses that (to reduce noise) and uses an op-amp. I think that is where my problem is. Originally I was trying to use resistors before the op-amp to attenuate the volume, but changing values there didn't seem to help much. However, I would like a solution before the op-amp so that my outputs are all consistent (as in my passive mixer output has proper volume for all the channels as well as the discrete output of the cartridge audio). The reason for this is that, by itself, the cartridge input audio is very loud compared to the original audio coming off the NES and I want to be able to balance the volume of the cartridge audio with the NES audio.

So I guess without getting too involved - what is the best way to attenuate volume before an op-amp so I can balance all the input sources to the op-amp and get a consistent output from all of them? Do I just need to increase my resistance by a factor or is there something else going on?

Schematic is posted for the current design, with my desired trimpot and passive mixer, for the curious. The previous revision can also be found here if you want to build your own.

Edited March 12, 2012 by m00dawg

[Nasrudin]

i am no expert in audio circuit design but the easiest way would be reducing the signal gain by using a voltage divider.

For example, you could replace the 100k resistor (from those highpassfilters) at the op amp inputs with a (logarithmic?) potentiometer.

connect pin 1 to the 1uF Cap, pin 2 to the pos. input of the op amp, and 3 to GND. you should now be able to set and reduce the input gain of each source individually to zero.

EDIT: Dont you worry about signal coupling when merging the three sources via those 2.2k resistors?

Edited March 12, 2012 by Nasrudin

[m00dawg]

Ah indeed. That's basically how I have it setup now, except it's before the cap, although in my previous attempt I still had the 100k resistors in place. That makes me think I will need a trimpot with a pretty big range (like 100k-1M?). And yeah I think it should be logarithmic as well.

I don't really need ones on the other inputs since I want the volume of the cartridge audio to be matched to the volume of the NES, although space permitting I guess it can't hurt to have trimpots for all 3.

Hmm...well sounds like I was mostly there. I'll see about making the changes and seeing what comes of it. Thanks!

[technobreath]

Remember that resistors and pots also add noise to the circuit - higher value equals more noise. Not sure how much this means for your application though... Audio circuits is great fun, simple to do whatever u want. Me myself and I are atm designing a line mixer. Seems the biggest part of the job is to actually decide what components to use, as the design by itself is extremely non complicated ;-)

[m00dawg]

Ah hmm that's a good point. Noise isn't as much of a problem for the cartridge audio - it's already pretty noisy from the PowerPak (much less so when using Famicom games though it would seem). Although I hadn't thought of noise otherwise. I was going to add trimpots to all the channels just cause. But if the smallest value is 100k, I'm no worse off then when I had resistors right?

[Nasrudin]

AH now i understand. you want to reduce volume of source 3 in order to get a well balanced signal from mixout (Pin 1 J3).

In this case increasing the value of R14 should do the thing.

PS: i dont think that thermal noise will play a noticable role in here since amplification of the op amp is set to 10 (only).

[m00dawg]

Actually, it would be for both the passive mix and the discrete out. The reason is that the output is so loud that it often distorts if you don't further attenuate it with an analog mixer. The original mod for mixing in the cartridge audio used the original NES audio pipeline so using a resistor before the pipeline worked pretty well. In my design, the same valued resistor isn't helping as much. I figure a trimpot over a decent range would do the trick, though.

That way it works for both the PowerPak and normal cartridges that have their own audio (they both require different resistance values, to taste).

I think I can simulate the same effect on my current design by putting a trimpot in between the cartridge audio and the NES input pin so I may try that just to see what comes of it and go from there and hope. That will at least help me figure out and good resistance if I want to forgo the trim and use a different valued resistor (if I start running low on space as the board is designed to fit inside the original NES case, under the mainboard, without having to do anything like cut posts and things).