Hi,
It's usually done for stability reasons. An amplifier may not be stable over the entire range of gains you wish to run it at.
Increasing the gain (or keeping it higher), can allow for a reduction in compensation feedback required to keep the amplifier stable. (Note, some amplifiers are stable only if used above some minimum gain value, say |g|=5, or |g|=10...)
-Dan
It's usually done for stability reasons. An amplifier may not be stable over the entire range of gains you wish to run it at.
Increasing the gain (or keeping it higher), can allow for a reduction in compensation feedback required to keep the amplifier stable. (Note, some amplifiers are stable only if used above some minimum gain value, say |g|=5, or |g|=10...)
-Dan
Use a pot that has a switch built in, so that when you turn it all the way down it clicks off. For inverting op-amp, put the pot in series with a resistor that sets your gain at about 1 when the pot is all the way off. This way, you turn the pot to the point where it is almost ready to click off, and the amp is at unity gain and still stable (and also very quiet) turn the pot a little more and the volume cuts off. For an non-inverting amp I guess you don't need the extra series resistor.
Even if stable, distortion varies with gain. I suspect the origin of this idea came from the use of the feedback loop foor tone controls. That may make some sense there as the gain is frequency dependant and probably does not change over such a wide range as would be needed for a volume control.
If you implement the idea with an opamp in an inverting topology, then I think negative gain (attenuation) is possible, but you still run up against the two concerns stability over the entire range and distortion increasing with gain.
A pot at the input is so much more simple.
If you implement the idea with an opamp in an inverting topology, then I think negative gain (attenuation) is possible, but you still run up against the two concerns stability over the entire range and distortion increasing with gain.
A pot at the input is so much more simple.
lets say u got a power amp with a voltage gain of 30
to controll the volume most of the time there is a potentiometer at the input that will change the overall gain of the amp to less then 30 .
what i'm sayin is why not connecting a potentiometer insted of a feedback resistor , no matter if its inverting/non inverting amp.
that way the signal to noise ratio will be better i guess
and at lower volume level there will be more feedback so ->less distortion
to controll the volume most of the time there is a potentiometer at the input that will change the overall gain of the amp to less then 30 .
what i'm sayin is why not connecting a potentiometer insted of a feedback resistor , no matter if its inverting/non inverting amp.
that way the signal to noise ratio will be better i guess
and at lower volume level there will be more feedback so ->less distortion
it will work
But, real life problems may wish you had not done it this way later. Depending on the pot, you may inject noise into the circuit off the wiper of the pot. AC hum would be the most common.
What happens if the wiper gets grounded to the chassis? Conductive cleaner or insulator leakage could give you a headache here. DC instability may result if the pot is rotated quickly also.
Build it & see. If it doesn't work well, substitute a fixed resistor and go back to a control on the input. Nothing is lost then.
-Chris
But, real life problems may wish you had not done it this way later. Depending on the pot, you may inject noise into the circuit off the wiper of the pot. AC hum would be the most common.
What happens if the wiper gets grounded to the chassis? Conductive cleaner or insulator leakage could give you a headache here. DC instability may result if the pot is rotated quickly also.
Build it & see. If it doesn't work well, substitute a fixed resistor and go back to a control on the input. Nothing is lost then.
-Chris
To add another possible gotcha to the list, I wonder if mixing in a potentiometer with the gain-setting resistors might shorten its life. Some pots, such as those in the Alps RK27 series, expect to only see AC. In a DC circuit, their sliders can suffer silver ion migration and a shortened life. The Alps application engineer I talked to couldn't get any numbers for an excessive DC bias. I got the impression people are careful not to apply one.
sss said:
Preamps are there to attenuate the signal, NOT to increase it. Even with the preamp gain at 1, and the power amp at the usual 30, your sound level will be much too high for comfortable listening. It will probably blow out your speakers, if the amp doesn't clip beforehand. Imagine a CD player with a 2V output amplyfied 30 times, that 60V in 8 ohms = 450Watts.
One reason the pot is normally at the input is to let you attenuate the signal 30 or 40 dB to prevent our ears from being blown out.
Jan Didden\
Inverted Op Amp Pre
I recently completed a preamp using an inverted op amp circuit with a 50K Nobel stereo pot controlling the gain in the feed back leg. The topology whihch always uses maximum input signal does optimize the signal to noise.
I used ADC 825s and set the maimum gain of the Preamp at 6 db (2X input).
I also added a mono L + R output with volume for sub woofer control.
The unit runs off +/- 12 V using AA batteries.
It is very quite and sounds great.
It is currently diving an intersting system.
Bill Strum
I recently completed a preamp using an inverted op amp circuit with a 50K Nobel stereo pot controlling the gain in the feed back leg. The topology whihch always uses maximum input signal does optimize the signal to noise.
I used ADC 825s and set the maimum gain of the Preamp at 6 db (2X input).
I also added a mono L + R output with volume for sub woofer control.
The unit runs off +/- 12 V using AA batteries.
It is very quite and sounds great.
It is currently diving an intersting system.
Bill Strum
I couldn't get an answer on that. You might get some ideas if you click on the link marked "Caution" under "Related Informations" on this this Alps page.lumanauw said:
I've built a few preamps with a pot in the feedback loop of an inverting opamp. The pot used to become noising after a few months but I realised one day that I hadn't bothered to add a input capacitor. Once I added the input capcitor and used a fet opamp everything has been OK (so far)
I've also seen a nice non inverting circuit in electronics world december 1995 pg 1012. This uses a 10klin pot. The advantange of this circuit is that the opamp gain is very low when the signal is reduced to 0. See attached
Mark
I've also seen a nice non inverting circuit in electronics world december 1995 pg 1012. This uses a 10klin pot. The advantange of this circuit is that the opamp gain is very low when the signal is reduced to 0. See attached
Mark
Attachments
Just a point of interest/curiosity. There is a commercial powrr amp, I think it is AccuPhase, that has switchable gain. It's not a volume control, but rather is supossed to let you match the gain with speaker sensativity. Anyway this shows that there is at least one respectable commercial design that does not consider gain to be completely "untouchable"
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