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[Ital-Rolando]

I remember, not sure where I heard this, that the knob of a power amplifier (for live sound uses) can be thought just like a water tap, because the level inside the circuitry it always running at full, just like the water pressure in our houses.
I must say that, even I do not have any clues, it makes sense to to me and I’ve accepted it.
Could anyone get more in depth about it?

Thank you

[Steve-White]

Think of it as a gain control and not a volume knob.  It's purpose is to set the input sensitivity of the amplifier to the incoming signal level in the system.  High signal level, low gain is needed.  Low signal level, higher gain is needed to drive the amp to full power.

This is a car audio site, but a decent explanation:  https://www.extremeaudio.org/what-is-a-gain-control/

[Paul G. OBrien]

I remember, not sure where I heard this, that the knob of a power amplifier (for live sound uses) can be thought just like a water tap, because the level inside the circuitry it always running at full, just like the water pressure in our houses.

Nope it doesn't work like that at all, the volume control does NOT limit the amplifier output it just changes the amount of input signal level need for full output.

[John Roberts {JR}]

I remember, not sure where I heard this, that the knob of a power amplifier (for live sound uses) can be thought just like a water tap, because the level inside the circuitry it always running at full, just like the water pressure in our houses.
I must say that, even I do not have any clues, it makes sense to to me and I’ve accepted it.
Could anyone get more in depth about it?

Thank you

That is an awkward analogy I won't try to fix it.

Perhaps the relationship that analogy was trying to explain is that power amps are typically designed to run at high nominal gain, because that is more stable and resists oscillation from random irregular loads.

BUT to accommodate integrating into systems requiring less than full gain, most amplifiers incorporate an input attenuator to pad down the signal input. The combination of this input attenuator followed by high fixed gain, behaves like a variable gain amplifier to the user. 

Don't overthink this, the inner workings are mainly of interest to amplifier designers.

JR       

[Ital-Rolando]

That is an awkward analogy I won't try to fix it.

Perhaps the relationship that analogy was trying to explain is that power amps are typically designed to run at high nominal gain, because that is more stable and resists oscillation from random irregular loads.

BUT to accommodate integrating into systems requiring less than full gain, most amplifiers incorporate an input attenuator to pad down the signal input. The combination of this input attenuator followed by high fixed gain, behaves like a variable gain amplifier to the user. 

Don't overthink this, the inner workings are mainly of interest to amplifier designers.


JR       

Thanks John, Paul and Steve.

I had not doubted this analogy up to now also because in many power amplifiers, (i.e. QSC GX5), the knobs are graduated from minus infinity to 0, therefore confirming that the power of the signal inside the amp is always running at full, and you can just limit its full output with the knobs.
What you wrote John, is the closest description to what I have read and has remained in my mind.
I also agree with what Paul and Steve wrote, I have probably just added and accepted a familiar analogy.

[Luke Geis]

To re-iterate, the knob has nothing to do with the power output of the amplifier. If you turn the knob down to -40 ( or whatever its lowest setting is that is not full off ), you would have to drive the amplifier with that much more signal ( from the mixer ) to achieve the same level from the amplifier. The amplifier is a fixed gain device, if there is X level actually getting to the amplification stage, it will produce X output regardless of the input volume knob setting. All that knob does is act as another volume knob.

[Ital-Rolando]

The amplifier is a fixed gain device, if there is X level actually getting to the amplification stage, it will produce X output regardless of the input volume knob setting. All that knob does is act as another volume knob.

Thanks Luke, that's exactly the definition that clarifies my ideas.

[Ivan Beaver]

To re-iterate, the knob has nothing to do with the power output of the amplifier. If you turn the knob down to -40 ( or whatever its lowest setting is that is not full off ), you would have to drive the amplifier with that much more signal ( from the mixer ) to achieve the same level from the amplifier. The amplifier is a fixed gain device, if there is X level actually getting to the amplification stage, it will produce X output regardless of the input volume knob setting. All that knob does is act as another volume knob.

Not exactly.

The amplifier (actually a voltage amplifier, NOT a power amplifier (they actually LOSE power)-but that is a different argument), has a rated gain (from input to output) of a certain amount of dB.

That is ONLY true when the level control is at maximum (turned all the way up).

Whenever you turn it down, the gain of the amplifier also goes down.  SO it takes a stronger input drive level in order to have the same output voltage (wattage into a particular load).

It is also possible on many "amplifiers" to NOT be able to get the rated output if you turn the level down below a certain point, no matter how hard you drive it.

This is because the power supply voltages on the input stage are limited.  They will often clip the signal when driven hard, even if the actual output is well below rated power.

So you cannot turn it down to -40 and still get full output.  The idea is solid, but the actual implementation will simply not allow it (in most cases).

[Miguel Dahl]

I don't know if this is of any help. But when using a Yammie on the "the usual monitor amps". I can either turn down the aux master outputs, to operate the aux-sends from the channels at a normal  -10 to 0 dB level send for monitors, or just turn down the attenuation knobs on the amps without turning down the aux masters. It will be equally loud, but with less hiss through the wedges due to internal hiss from the amps when they are wide open.

[Chris Grimshaw]

If the input attenuators are adjusting the volume of the hiss, chances are the hiss is further upstream.

Chris

[Brian Jojade]

If the input attenuators are adjusting the volume of the hiss, chances are the hiss is further upstream.

Chris

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}]

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.

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]

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.

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}]

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.

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,,,,   

JR

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