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Old 18th April 2013, 09:01 PM   #1
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Default What does the volume knob do - stupid newbie

Stupid newbie question, but I bet a lot of people (outside this forum) don't know the answer either. In a typical solid state amplifier (such as a home receiver), what does the volume knob control? Current, voltage, wattage or something else?

Speaker impedance curves are rarely flat - a speaker may have a 10 ohm impedance at 1 frequency, and 4 ohms at another, both within the range of frequencies the speaker plays well. If voltage were constant regardless of frequency and controlled by the knob, wattage would be much higher at 4 ohms than 10 ohms, and I would imagine volume would be much higher too. Similarly (but the opposite) applies to current. If the amp holds wattage steady, and is 220 into 4 ohms, what happens if I play the frequency with 4 ohm resistance, turn the volume all the way up, and then play the frequency with 10 ohms?

I figured someone on here actually builds amps and would know.
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Old 18th April 2013, 09:11 PM   #2
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Read this:

http://www.angelfire.com/electronic/...mp-Volume.html
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Old 18th April 2013, 09:51 PM   #3
jaycee is offline jaycee  United Kingdom
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voltage. it functions as an attenuator on the input signal.
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Old 18th April 2013, 11:07 PM   #4
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The volume control alters the voltage going into the amplifier.
The smaller the voltage the less the output from the amplifier.
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Old 18th April 2013, 11:32 PM   #5
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The classic volume potentiometer is nothing but a variable voltage divider.
Quote:
If voltage were constant regardless of frequency and controlled by the knob, wattage would be much higher at 4 ohms than 10 ohms, and I would imagine volume would be much higher too.
True enough, but in any [hi-fi, normal, realistic,... insert your own favorite adjective] circumstance voltage is definitely not constant regardless of frequency.
But let's assume it is for example's sake. The watts would be 2.5x at 4Ω than at 10Ω. That is a difference of 4dB. Enough to notice, but not a greatly significant increase.
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Old 19th April 2013, 02:07 AM   #6
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Originally Posted by scott17 View Post
I understand what a pot is, and I understand the voltage knob probably is a pot. However, I would think it would be terribly inefficient to run 200 watts (for example) directly through a pot before going to speakers. I understand the knob itself directly controls voltage to something inside the receiver - but does it also control the voltage of the signal that leaves the receiver? Or does it control something else? I could conceivably use a pot to raise and lower an amperage in a device that automatically adjusts the voltage to maintain the current I set (my fish tank lights work this way).

If the knob only regulates voltage, won't the current double at frequencies where the impedance is halved?
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Old 19th April 2013, 02:10 AM   #7
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Originally Posted by sofaspud View Post
The classic volume potentiometer is nothing but a variable voltage divider.

True enough, but in any [hi-fi, normal, realistic,... insert your own favorite adjective] circumstance voltage is definitely not constant regardless of frequency.
But let's assume it is for example's sake. The watts would be 2.5x at 4Ω than at 10Ω. That is a difference of 4dB. Enough to notice, but not a greatly significant increase.
I would disagree. 4dB is greatly significant. I understand that if this is the intent of the driver or speaker designer, it could be compensated for or relied on. But I disagree if you say 4dB is small enough not to care about.
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Old 19th April 2013, 02:15 AM   #8
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Originally Posted by jaycee View Post
voltage. it functions as an attenuator on the input signal.
This could work, and I am not disagreeing with you. This might be the best way to do it. But if I attenuate my (analog) signal, won't I lose some clarity? I would think my speakers would sound better played at 100% volume on a 10 watt amplifier than at 1% volume on a 1000 watt amplifier. My experience is generally the reverse is true - high powered amps sound better even at lower volume. Of course, this could be because higher quality components are used and more time is put into design in a 1000 watt amp.
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Old 19th April 2013, 02:56 AM   #9
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Quote:
Originally Posted by bvbellomo View Post
I understand what a pot is, and I understand the voltage knob probably is a pot. However, I would think it would be terribly inefficient to run 200 watts (for example) directly through a pot before going to speakers. I understand the knob itself directly controls voltage to something inside the receiver - but does it also control the voltage of the signal that leaves the receiver? Or does it control something else
Please re-read the information presented.

The "volume control" is an input signal attenuator. It is not connected to the speaker output signal.

Also, what does a "receiver" have to do with this? An amplifier has a certain amount of set gain based on the design. The input signal level, set by the input attenuator, determines the output level of the amplifier.

By the way, why is it true that better components and more time is spent designing a 1000W amplifier?

Last edited by scott17; 19th April 2013 at 03:02 AM.
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Old 19th April 2013, 08:26 AM   #10
AndrewT is offline AndrewT  Scotland
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Bv,
forget about the speaker impedance variation with frequency for the moment.
Concentrate on understanding the input to the amplifier.
When you have that sorted in your head, then you can decide if you want to look at speaker design.
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