I've googled the web for sample microphone preamp designs using opamps, and I've found that most have either discrete gain settings via rotary switches, or a high fixed gain and a potentiometer attenuator at the output. What I'm wondering is: is there a good reason why potentiometers are almost never found in the opamp's feedback voltage divider which sets the amplifier gain?
In my breadboard experiments, I've noticed problems with radio stations and hum getting into the amp when I've used metal-shaft pots, or long antenna-like wires to the pots, but this is with unshielded parts (and crappy parts), so I'm not sure if these problems carry over to final designs. When I stick a panasonic plastic-shaft pot right down on the breadboard, I can turn it without these problems, and when using conductive plastic pots, without any scratchiness either. It leaves me wondering if it's OK to do it, if you've got the right type of pots.
-Adam
In my breadboard experiments, I've noticed problems with radio stations and hum getting into the amp when I've used metal-shaft pots, or long antenna-like wires to the pots, but this is with unshielded parts (and crappy parts), so I'm not sure if these problems carry over to final designs. When I stick a panasonic plastic-shaft pot right down on the breadboard, I can turn it without these problems, and when using conductive plastic pots, without any scratchiness either. It leaves me wondering if it's OK to do it, if you've got the right type of pots.
-Adam
Thank you all, that is what I needed to know. I am not proposing it for a volume knob, I'm building a mic preamp and what I need to do is have some way of setting levels *once* before recording begins. I found several designs on the web that have a fixed high-gain stage followed by an output attenuator, but this seems to me like it would trash the SNR unnecessarily if used for getting only a few dB gain (which is often all I need to bring electret mic levels up to LINE IN).
Having a pot inside an op-amp's feedback loop isnt a great idea. Because the pot will be mounted mostly likely on the faceplate your going to have a longer feedback loop. The feedback loop will be long and have a high resistance, it will act as an antenna. The shortest feedback path is ideal, this is why many ppl use SMD and solder them directly to the op-amp's pins.
airraid said:
Actually, a pot. in a double inverting feedback network is an exceedingly good way of generating a quasi-log. law volume control from an ordinary linear pot. (due to P. Baxandal)...
Two amps. are needed to ensure gain (and therefore THD+N) at the extremeties of the pot's excursion is well controlled.....
Performance is in fact often superior to that available from the best so-called log. pots. in pratice....
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