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Another consideration may be noise. If you make the resister values 10 times higher, the Johnson noise will be 10dB higher. This only matters if you want very low noise obviously, and then you need to use a low-noise opamp otherwise the opamp noise will be higher than the resistor noise anyway.
Feedback, yes, but the idea is to use negative feedback to reduce the ambient noise inside the headphone cup. As with any other negative feedback system, you have to pay attention to the loop gain and phase to avoid instability and oscillation.
That can work too but, IMHO, the equalisation is...
1) There's no reason to use an arduino or any kind of microprocessor.
2) Before starting the circuit design, you need to know how noise-cancelling headphones work. e.g. that the microphone goes inside the headphone, in the cavity between the loudspeaker and your ear. Also the frequency response...
IMO, it depends if you're talking about power gain (ratio of output power to input power) or voltage gain (ratio of output voltage to input voltage..
For power gain: dB = 10log(Po/Pi) = 10log{(Vo^2/Rload)/(Vi^2/Rin)} and the Rs can be different as you say.
For voltage gain: dB = 20log(Vo/Vi)...
The TL071 is not very modern, but it is good for audio (and general purpose), is cheap, and is easily available (even in through-hole packaging).
TL072 is the dual version, TL074 is the quad version.
Datasheet is here.
As FvM mentioned, it's a really good idea to split the gain into two stages...
@barry:
No, that will give the average, not the sum. (time for a cup of coffee? :grin:)
@Rafael:
If the two signal generators are completely isolated, you can just connect the outputs in series.
If not (e.g. if they have a common earth), then you can use a transformer to isolate one of them...
Why?
You mentioned elsewhere in this thread that the project has already been scrapped and that the people who had been working on it were sacked. You also mentioned that you left that company as well.
I don't understand why you want to go back to a company you used to work for and explain to...
@ gonadgranny
It's hard to guess what's going on without knowing what sort of scales they are. Presumably they're some sort of electronic scale, other wise you wouldn't be worried about electronic interference. If they are mains powered, the problem may be caused by low/eratic/dirty mains...
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