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Power amplifier volume knob question

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Brian Jojade:

--- Quote from: Chris Grimshaw on December 17, 2021, 01:29:27 PM ---If the input attenuators are adjusting the volume of the hiss, chances are the hiss is further upstream.

Chris

--- End quote ---

Not always. I've used plenty of amplifiers that will have a certain level of hiss with the pots wide open, but quiet right up when you trim down 10db or so.

John Roberts {JR}:

--- Quote from: Brian Jojade on December 17, 2021, 01:34:17 PM ---Not always. I've used plenty of amplifiers that will have a certain level of hiss with the pots wide open, but quiet right up when you trim down 10db or so.

--- End quote ---
If the pot is configured as an input attenuator, and the hiss goes down from turning down that attenuator, that hiss is surely coming in on the input signal.

Amplifiers are designed with a lot of voltage gain to insure ability to drive systems to full scale output power from sundry sources. If you don't trim down the amplifier input, that full amplifier gain gets applied to the mixer and/or preceding path's noise floor.  Good practice for fixed install applications is to use amplifier input trims to manage source and interface noise. For R&R live music applications the general advice is keep amps WFO so you can always get full amplifier output power. If the amplifier input trim is turned down too low it constrains maximum output power available from the amplifier, because the source signal saturates (clips) before the amplifier.

JR

Brian Jojade:

--- Quote from: John Roberts {JR} on December 17, 2021, 02:01:43 PM ---If the pot is configured as an input attenuator, and the hiss goes down from turning down that attenuator, that hiss is surely coming in on the input signal.

--- End quote ---

BUT - No signal attached, some amplifiers will hiss when wide open. Turning the pot down it goes away.  Can't blame the hiss coming from the input signal if it's not there to start with.

John Roberts {JR}:

--- Quote from: Brian Jojade on December 17, 2021, 04:12:28 PM ---BUT - No signal attached, some amplifiers will hiss when wide open. Turning the pot down it goes away.  Can't blame the hiss coming from the input signal if it's not there to start with.

--- End quote ---
How does that same amp behave with input sorted?  At the risk of getting all "circuit design" with you, a typical input attenuator is just a potentiometer with the pot wiper feeding the amp input circuit. If the amp is using a bipolar device in its input LTP (long tail pair differential amp), that input will have a noise current associated with the input.

When the pot is full up, with no input, that amp input device will see roughly 10k ohms to ground. The input noise current will be converted to noise voltage by that current times 10K. When the attenuator wiper is all the way down, the resistance seen by the amp input will be more like 0 ohms. Alternately if the amp input was shorted, or connected to a low impedance source, when attenuator was full up it would see a lower impedance (for less noise). In fact with input shorted you might hear the noise rise until the attenuator reaches -6dB then fall again up to 0 dB. At -6dB the source impedance of the simple 10k attenuator pot will be a maximum of 2.5k or -12dB.

In practice this small amount of input noise will be swamped out by the noise floor of the stage feeding the amp. 

Back when I was dealing with customers I would discourage WFO listening tests,,,,    :o

JR

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