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Do volume controls waste energy? Repurpose old AVR into Amp

post #1 of 16
Thread Starter 
First thread! Amateur (not newbie) here. After searching over 2 pages of search results for volume control with most the threads talking about impedance matching, I made this thread which I've divided into 2 separate sections, answer what you please.

1. Question regarding basic in-wall low-gang style volume controls:
How do they work?

Does the amp get turned to full blast and the volume knob 'limit" the power to the speakers.
If so, does that mean essentially I could be wasting 170 watts out of 200?
Assuming;
-Zone 2, 1 pair of speakers (20-100W)
-Amp with 100w/channel
-volume control knob rated 100w/channel

2. Can a AVR be repurposed into an amp?

I want to save money by repurposing a 600w (100w/channel) Yamaha AVR as a whole house amp instead of buying another piece of equipment. The AVR has multi-channel inputs, I figured I could just set the audio decoding to "straight" or "direct" and act as just an amp from the Main AVR's zone 2 & 3 pre-outs.
Setup would be:
Main AVR zone 2 pre-outs > Amp (2nd AVR) Audio multi-ch inputs > volume knob > speakers
Seems simple right? I'm just concerned about actual implementation of a volume control knob and also if it waste energy.
Thanks guys
post #2 of 16
Quote:
Originally Posted by Cravit8 View Post

First thread! Amateur (not newbie) here. After searching over 2 pages of search results for volume control with most the threads talking about impedance matching, I made this thread which I've divided into 2 separate sections, answer what you please.

1. Question regarding basic in-wall low-gang style volume controls:
How do they work?

Does the amp get turned to full blast and the volume knob 'limit" the power to the speakers.
If so, does that mean essentially I could be wasting 170 watts out of 200?
Assuming;
-Zone 2, 1 pair of speakers (20-100W)
-Amp with 100w/channel
-volume control knob rated 100w/channel

Yes.
Quote:
2. Can a AVR be repurposed into an amp?

I want to save money by repurposing a 600w (100w/channel) Yamaha AVR as a whole house amp instead of buying another piece of equipment. The AVR has multi-channel inputs, I figured I could just set the audio decoding to "straight" or "direct" and act as just an amp from the Main AVR's zone 2 & 3 pre-outs.
Setup would be:
Main AVR zone 2 pre-outs > Amp (2nd AVR) Audio multi-ch inputs > volume knob > speakers
Seems simple right? I'm just concerned about actual implementation of a volume control knob and also if it waste energy.
Thanks guys

Yes.
post #3 of 16
Quote:
If so, does that mean essentially I could be wasting 170 watts out of 200?

No, most inwall volume controls are variacs, not rheostats or potentiometers.
post #4 of 16
Thread Starter 
Quote:
Originally Posted by SAM64 View Post

No, most inwall volume controls are variacs, not rheostats or potentiometers.

Ok, so for my purpose, what does it mean that the in-wall control is a variac?
post #5 of 16
A Variac is a variable transformer, it doesn't dissipate 'unused' power as heat.
post #6 of 16
Thread Starter 
Quote:
Originally Posted by SAM64 View Post

A Variac is a variable transformer, it doesn't dissipate 'unused' power as heat.

If that weren't chinese to me, I could answer it confused.gif

This is what I'm looking at. I think it's as basic as it gets.
http://www.monoprice.com/products/product.asp?c_id=109&cp_id=10903&cs_id=1090301&p_id=8243
post #7 of 16
Thread Starter 
I have never spent the time to fully wrap my head around electricity.
To me it seems the amp would send the full power amount (200watts) and the volume knob would then "diffuse" that electricity so to speak if I wanted to hear the sound at a low volume.
I really just need to know if it waste a bunch of electricity.
post #8 of 16
Quote:
Originally Posted by SAM64 View Post

Quote:
If so, does that mean essentially I could be wasting 170 watts out of 200?

No, most inwall volume controls are variacs, not rheostats or potentiometers.

True, but that doesn't mean that they don't waste power output capability of the power amp they are used with.

Let's say that a power amp is putting out 10 volts, and we have both a variac and a potentiometer connected across it, and they are both set for 50%. output. The output voltage of each is 5 volts.

The potentiometer will present a far lower resistance load to the amp, so it will be wasting quite a bit of power.

Like you say, the variac will act like a variable step-down transformer, and not dissipate very much heat. It iwll have only minimal losses.

But, there's a 6 dB loss in both and you will have to run the output of the power amp at twice the voltage that is delivered to either load. In some sense the power output capability of the power amp is being lost.

The practical benefit of the variacs will be that you can run more speakers in a system based on them, because there is far less power loss through the volume control for every speaker that is driven.

Each variac output draws less current from the amp so you can have more of them before you are loading the power amp with a load that has too low of an impedance.
post #9 of 16
Quote:
True, but that doesn't mean that they don't waste power output capability of the power amp they are used with.

Google 'variac' for an explanation.
Quote:
The potentiometer will present a far lower resistance load to the amp, so it will be wasting quite a bit of power.

that would depend on the value of the pot, and how it is connected.
Quote:
the variac will act like a variable step-down transformer, and not dissipate very much heat.

Already mentioned, disputed, and then agreed with?
post #10 of 16
Quote:
Originally Posted by SAM64 View Post

Quote:
True, but that doesn't mean that they don't waste power output capability of the power amp they are used with.

Google 'variac' for an explanation.

Please don't talk down to me! I've been an EE for over 40 years and I've been using variacs since the middle 1950s. I own several and still use them from time to time.

BTW, Variac is a trade name and applies to autotransformer-based AC power control products made by a certain company. If memory serves the trade name was originated by a company called Standard Transformer Company (Staco) and now owned by ISE inc.

The proper technical term for the device we are talking about is variable autotransformer. Since AC power is not involved, Variac is not the proper term.
Quote:
Quote:
The potentiometer will present a far lower resistance load to the amp, so it will be wasting quite a bit of power.

that would depend on the value of the pot, and how it is connected.

Believe it or not, if you want to use a pot to control the level of a speaker there are design rules for establishing those parameters. I was keeping that in mind.
Quote:
Quote:
the variac will act like a variable step-down transformer, and not dissipate very much heat.

Already mentioned, disputed, and then agreed with?

Just trying to clarify the issues. Sorry that the explanation was over your head and you missed most of its meanings.
Edited by arnyk - 11/13/12 at 12:37pm
post #11 of 16
Thread Starter 
LOL, you guys are killing me! smile.gif
Does the dang volume control knob waste significant electricity or not? (Significant being relative to 100w max output) It appears that's exactly what an autotransformer does.
post #12 of 16
Quote:
Originally Posted by Cravit8 View Post

LOL, you guys are killing me! smile.gif
Does the dang volume control knob waste significant electricity or not? (Significant being relative to 100w max output) It appears that's exactly what an autotransformer does.

No, it's been answered at least twice.
post #13 of 16
Thread Starter 
Quote:
Originally Posted by Cravit8 View Post

LOL, you guys are killing me! smile.gif
Does the dang volume control knob waste significant electricity or not? (Significant being relative to 100w max output) It appears that's exactly what an autotransformer does.

Quote:
Originally Posted by SAM64 View Post

No, it's been answered at least twice.

Thanks Sam64, would you mind explaining (in less technical terms) how it doesn't waste power, or at the very least direct me to a site.
My understanding (as of yesterday) of how sound is produced leads me to think that if my amp is pushing out 100 watts so my speaker can yeild 89db, that if I use a volume knob to then turn that down to 20db, I'd be wasting ~95 watts. Please correct me where I'm wrong on the fundamentals.
post #14 of 16
Amplifiers don't push power to a load, the load will draw current based on its impedance.
See Ohms law.
Increasing the impedance 'seen' by the amplifier will reduce the current drawn by the load, and thus the power.
post #15 of 16
Quote:
Originally Posted by SAM64 View Post

Amplifiers don't push power to a load, the load will draw current based on its impedance.
See Ohms law.
Increasing the impedance 'seen' by the amplifier will reduce the current drawn by the load, and thus the power.
Yes.

And here is the basic difference between the variable-autotransformer volume controls, and the older, variable-resistor based volume controls (L-pads and T-pads) that they replaced:

The variable-autotransformer volume control will vary the impedance of the load, and operate correctly, waisting little power, as long as the amplifier does not mind an increase in impedance.

The variable-resistor based volume control will keep the impedance of the load constant, and will be transparent to the amplifier, while dissipating the excess energy as heat (inside your wall).

My understanding (although I don't know for sure) is that the variable-autotransformer volume controls do not work well with tube amplifiers, and maybe some lower quality SS amplifiers, because those amplifiers might not work well with higher impedance.

Cravit8, I suspect that the AVR you intend to use will be fine with a volume control similar to what you posted above, and it will waste very little power.
post #16 of 16
Quote:
Originally Posted by Cravit8 View Post

LOL, you guys are killing me! smile.gif
Does the dang volume control knob waste significant electricity or not? (Significant being relative to 100w max output) It appears that's exactly what an autotransformer does.

The autotransformer volume control srongly tends to conserve energy.

A variable resistor volume control tends to dissipate (waste) quite a bit of energy.

This is one reason why autotransformer volume controls are favored - they run cooler because they don't waste so much energy.

That's a simplified view of the problem. Reality is more complicated but its usually better to understand the simple answer first.

For example, if you turn the volume all the way up, the energy wasted by either kinds of volume controls tends to be more similar.
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