Relationship between Supply (V) & Output (Watts) - diyAudio
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Old 2nd October 2012, 12:46 PM   #1
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Default Relationship between Supply (V) & Output (Watts)

I'm after some advice on how easy/practical it is to change the supply voltage on an 'average' 4-6 transistor amp - like this one on Wikipedia to change (lower - in my case) the output power?

I've been studying some simple Class A and Class AB circuits in preparation for my first DIY amp build. Originally I was looking at 10W but now I'm thinking about starting with something closer to 1W.

If I take a schematic designed to run on e.g. +24V DC and just halve the voltage to +12V DC - would it just halve the output power? Would there be any major component value changes? Are there specific parts of the circuit I should be looking at adjusting?

Thanks in advance.
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Old 2nd October 2012, 01:29 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Generally an amplifier can't swing to the voltage rails. Halving output
swing quarters the the output power for the same load impedance.

So generally halving the supply will cut output by more than a quarter.

rgds, sreten.

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Class A discrete op-amp / headphone amp / low power power amplifier

Designing discrete opamps.
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Last edited by sreten; 2nd October 2012 at 01:32 PM.
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Old 2nd October 2012, 01:35 PM   #3
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A majority of circuits have some "fudge factor" when it comes to supply voltage. Halving the supply tends toward the extreme, though. The amp you linked to doesn't show any values, but it's almost certain that the values of the resistors would need to be changed 24V vs 12V.
That circuit has a differential input with feedback. Q1 and Q2 should be closely matched. You can probably buy these as a pair, or test a bunch of singles to find 2 that are (ideally) identical. Maybe not the best choice of circuit for a first build, but that's your call.
As to your specific question, in broad terms yes, output is halved. But it is rarely that simple because of other factors, like overall efficiency.
Hope that is (correct enough to be) helpful.
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Old 2nd October 2012, 01:36 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Generally an amplifier can't swing to the voltage rails. Halving output
swing quarters the the output power for the same load impedance.

So generally halving the supply will cut output by more than a quarter.

rgds, sreten

Class A single ended discrete op-amp / headphone amp / low power power amplifier

Designing discrete opamps.
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Last edited by sreten; 2nd October 2012 at 01:39 PM.
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Old 2nd October 2012, 03:02 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

Generally assume about 5V is needed to operate a typical Lin type amplifier,
so a 24V supply might swing 19V pk to pk whilst a 12V circuit might swing
7V pk to pk. (Output voltage swing is not always symmetrical).

19V pk to pk is about 4.5W 8 ohms, 7V pk to pk about 750mW 8 ohms.

Low voltage power amplifiers generally use circuit arrangements that allow
them to swing nearer the voltage rails that higher powered amplifiers.

rgds, sreten.

http://www.ti.com/tool/tina-ti is a great free circuit emulator.
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Last edited by sreten; 2nd October 2012 at 03:08 PM.
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Old 2nd October 2012, 04:06 PM   #6
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Thanks - that's helpful. Much appreciated.

Quote:
Originally Posted by sreten View Post
Ugh I'm on a PPC Mac so AFAIK simulation for me is limited to MacSPICE which doesn't interest me that much. Luckily, I'm pretty happy to breadboard stuff - but thanks for the links.
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