Poweramp with adjustable gain?

[heinrichs]

i was talking to a freind of mine the other day.. and we came across the subjekt of preamplifiers..

and after sometime we both agreed that the way you normally control the volume, were you reduce the input signal to the poweramp, were the wrong way... seen from a perspective of noise and signal integrity.

why not make a power amplifier, were you can control the gain? that way you wouldnt interfere with the signal.

i am pretty sure lyngdorf/tact does that in some of their amps and meybe B&O in some of theirs icepower.. but not sure about that one....

i know that i only listed Class D amps.... but shouldnt it be posible with a class AB or class A?

just a ideer... i have no experience in designing power amps just wanted to get some opinions on the subjekt..

[juma]

Originally Posted by heinrichs

...why not make a power amplifier, were you can control the gain? that way you wouldnt interfere with the signal.
...

Ooo yes, you will. In most amps your idea means messing with NFB loop and that's far bigger intrusion than simple attenuation of input signal

[fotios]

Putting a signal attenuatting or input sensitivity control in a power amplifier, believe me, it is a PITA! As you know, a potentiometer it is a variable resistor. If it is connected accross the input socket and GND (representing thus the Zin of amplifier which must be from 10KΩ and above) and its wipper directly to the input of amplifier, when turned arround the middle of its track, it is tuned with the input filters of amplifier. The results are ringing, overshoot and amplification of enough dB of the unwanted noises existing inside the amplifier like buzz, hum, power supply ripple etc.
To not presented such effects, the value of pot must be 1,5ΚΩ as much. A such pot can't placed directly accross the input socket because the Zin becomes very low. The only solution, it is a very quiet buffer circuit between the input socket and the input of amplifier. The 1,5ΚΩ pot thus can placed in the output of buffer to control the level of input signal injected in the main amplification circuit.
With other words, we have to include a preamplifier (a buffer is not a kind of preamplifier?) inside the power amplifier case.

Fotios

[heinrichs]

#fotios
i wasnt thinking of using a potiontiometer at the input of the amp.. i want a power amp were i can adjust the gain on the fly. so i wouldnt need the in line potentiometer.

#juma
hmm ofcourse there was a problem to such a obvius ideer..



hmm.... but the ideer should be posible with a class D amplifier, since it only puts out squares before the filtering, so by adjusting the + - power supply, you would change the gain? since you would change the amplitude of the squares.?


but... i could use someone to tell me if this ideer is just stupid... or if there is a point in pursuing the ideer...

[fotios]

Originally Posted by heinrichs

#fotios
i wasnt thinking of using a potiontiometer at the input of the amp.. i want a power amp were i can adjust the gain on the fly. so i wouldnt need the in line potentiometer.

#juma
hmm ofcourse there was a problem to such a obvius ideer..



hmm.... but the ideer should be posible with a class D amplifier, since it only puts out squares before the filtering, so by adjusting the + - power supply, you would change the gain? since you would change the amplitude of the squares.?


but... i could use someone to tell me if this ideer is just stupid... or if there is a point in pursuing the ideer...

Yes, i got what you are thinking. A power amplifier, it is operating like an IC in non inverting mode (Av=1+Rf/Ri). So you can adjust the gain by changing the value of Rf if you put a potentiometer in its place. I have tried this in small power circuits like a microphone preamp with success, but with supply rails of +/-18Vdc and an output of 4Vrms only. In a power amplifier, as pointed very correct by juma this practice it is doubtful to not say very risky, as can cause amazing instability problems because should be applied in the main feedback loop.
But who knows? Everything in our world it is under revision. Why you don't make an experiment in a small amplifier of 2X40W for home use? If you don't try, you will not know. Are you experimenter?

Fotios

[jaycee]

See the preamp in the Musical Fidelity A1 amp as to why this is a bad idea. For a start, the DC across the wiper will make the pot noisy quickly, which makes the preamp noisy and unstable. Imagine that in a power amp and it's a recipe for disaster. Many power amp designs are only stable at certain feedback gains and changing it would make them oscillate.