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Author Topic: Power amplifier volume knob question  (Read 486 times)

Ital-Rolando

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Power amplifier volume knob question
« on: November 16, 2021, 03:05:09 AM »

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 Iíve accepted it.
Could anyone get more in depth about it?

Thank you
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Steve-White

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Re: Power amplifier volume knob question
« Reply #1 on: November 16, 2021, 06:24:08 AM »

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/
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Paul G. OBrien

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Re: Power amplifier volume knob question
« Reply #2 on: November 16, 2021, 09:43:38 AM »

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.
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John Roberts {JR}

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Re: Power amplifier volume knob question
« Reply #3 on: November 16, 2021, 10:00:31 AM »

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 Iíve 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       
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Ital-Rolando

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Re: Power amplifier volume knob question
« Reply #4 on: November 16, 2021, 04:56:14 PM »

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.
« Last Edit: November 16, 2021, 05:54:32 PM by Ital-Rolando »
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Luke Geis

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Re: Power amplifier volume knob question
« Reply #5 on: November 16, 2021, 10:18:54 PM »

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.
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Ital-Rolando

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Re: Power amplifier volume knob question
« Reply #6 on: November 17, 2021, 05:43:28 PM »

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.
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Ivan Beaver

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Re: Power amplifier volume knob question
« Reply #7 on: November 18, 2021, 12:07:48 PM »

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).

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Re: Power amplifier volume knob question
¬ę Reply #7 on: November 18, 2021, 12:07:48 PM ¬Ľ


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