Hi, i have this circuit and i wonder if i need to put buffer after the gain control even if it goes straight to power amp ? Thanks guys 😉
No. Output impedance of the last stage is effectively given by the opamp in series with 220 ohms. So very low! Your gain control has a minimum gain of 1. Is that what you intended? I.e. you cannot silence the input with that control.
Thank you, this is exactly what I wanted. i will adjust the volume on my sound card and this control is just for gain.
So I guess one question is why you need variable gain there? The more gain you apply at that point the more noise you will feed into the power amp. And putting a pot in the feedback loop of an opamp does raise layout issues. The usual advice is to keep that feedback loop as physically small as you can - lead out wires to the pot are going to compromise that. why not just implement either a unity gain buffer or set it to the lowest fixed gain that you need.
It is not pot but trimmer that will be on pcb and its only for experimenting and if the trimmer will makes some problems i will make it with just fixed values
One suggestion I would offer is to make U1-2 inverting. That would lower its' common mode distortion and permit you to adjust below unity gain. A small Cfb across R17 would also be a good idea.
To compensate for the polarity inversion of U1-2 in the modified configuration simply redefine CN1 and CN2's polarity by making XLR pin 2 (normally + as shown) feed the inverting input of the differential amp.
Suggestions: design for 2V rms output like most stuff to have a standardized output voltage. Don’t use a potentiometer in the feedback loop. No need for a buffer. Replace bipolar caps for single film caps. Decouple power pins of opamps correctly. Make sure that the power amplifier has input caps for risk management. Add muting relay to prevent power on/off plops. Use a normal volume control of choice between output stage and power amp for risk management. Solved.
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This circuit will have an additional 4uV (at least) of extra noise.
Why not just wire straight from soundcard to amp?
Why not just wire straight from soundcard to amp?
Yes. As it is you have only a 6:1 gain range, ie less than 16dB range. The minimum gain is about 0dB, not useful as a volume control.
So that means instead of 10K on R7,8 i will use 20K so i will have 2X gain and that second stage dont use at all? Sorry if im wrong, im still learning 🙁
When that pot goes noisy (and it WILL) the gain will jump to the open loop state
and you will NOT like it. Better to control R10. If the pot goes open (noisy) the
gain goes to 1.
G²
Why do you want gain of 2x. ? Your sound card will probably be outputting 2V RMS in any case. Keep it simple. Unity gain differential input, into a standard volume control into the power amp. No need for second half of 5532. But be careful - if you don't use the second half do make sure you terminate it correctly. See here https://www.ti.com/lit/an/sboa204a/sboa204a.pdf?ts=1657594932862
No you don't. You either leave it unused or take a single opamp.
You really want to put the signal through another opamp why?
Just connect the +input to gnd and connect the -input to the output.
Jan
You really want to put the signal through another opamp why?
Just connect the +input to gnd and connect the -input to the output.
Jan
This is how to deal with unused opamps - I use it a lot as its just easier to have a stock of duals (5532) than mix or match with singles. So using your diagram its connect pin 5 to ground and connect pin 6 to pin 7. Simple!
I have an idea and i dont know if it will works. What if i use the second op amp for RCA input and connect the two output together. Will it works or does i need some other components between these two outputs?