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Old 24th September 2009, 07:51 PM   #1
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Default Rail Voltage vs Current tradeoff

I could not find much on rail voltage vs. current tradeoff.

My Yamaha RX-V3900 has a very high rail voltage of 70 volts.

My first question is, why so high? It can't produce that much power to one channel before shutting down I hope.

Does a high voltage help in multi-channel AVRs so that when current load from all channels starts pulling down the rail voltage?

What if they lowered the rail voltage? It seems like Harmon Kardon might do this thus giving them low rated power, with more current handling ability.

I am just curious how these tradeoffs work
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Old 24th September 2009, 09:01 PM   #2
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Originally Posted by MichaelJHuman View Post
My Yamaha RX-V3900 has a very high rail voltage of 70 volts.

My first question is, why so high?
The rail voltage is determined by the speaker impedance you intend to work with the amplifier.
It also depends on transients in the audio source.

Music has a wide dynamic range so needs a higher voltage to stop the transients being clipped.

I started off with a high power 45 volt rail amplifier and found the transients got clipped no where near the volume i wanted from the amplifier.
I now tend to work with at least 60 volts to get good dynamic range and high power.
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Old 24th September 2009, 09:24 PM   #3
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Would 60 get you about 200 watts of power for transients if your voltage was not sagging?

60*.707 is 42? 42 * 42 / 8 is about 200? (I realize this is a total simplification of actual impedance and actual audio signals)

Or do you need headroom in case voltage sags a bit, and your peak output is a bit less than 200 watts?

I was trying to figure out some time back why multi-channel AVR channel power dropped so much under load. My conclusion was that heavy loading on the transformer was causing rail voltage to sag. Some receivers have half the power or less than rated power with all channels driven.

I was not 100% sure why rail voltage was sagging that much though. The only answers I got were either transformer IR increasing quite a bit under load, or that the transformer would saturate and be unable to deliver any more power.

Last edited by MichaelJHuman; 24th September 2009 at 09:31 PM.
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Old 24th September 2009, 09:30 PM   #4
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Quote:
Originally Posted by MichaelJHuman View Post
Would 60 get you about 200 watts of power for transients if your voltage was not sagging?

60*.707 is 42? 42 * 42 / 8 is about 200? (I realize this is a total simplification of actual impedance and actual audio signals)

Or do you need headroom in case voltage sags a bit, and your peak output is a bit less than 200 watts?
I allow the power output of the amp plus 50% for the transformer rating and this works well. The output is 60% of the power used and the heatsinks lose another 40%. So for 200WRMS amp I would use a 300VA transformer.

If you watch music on a scope you will see very large transients.
Your voltage at maximum power should allow for these to stop clipping of the transients.
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Old 24th September 2009, 09:40 PM   #5
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I wrote a WAV file analyzer program so I can clearly see peaks over average level. I think I got between 10 dB and 15 dB peaks on a few songs I ripped.

I am sure depending on source material (various type of music, movies and how they are mixed/mastered) this can go higher.

I edited my post above to ask about multi-channel amplifiers and why their power drops so much as more channels are driven. I don't know if you saw that.

I am pretty sure it has to do with transformer IR increasing but I am not sure what the major reason for that is. I know high current draw can increase resistance.

But I would think core size is a big factor? As load increases, is there an interaction between the secondary and primary which causes IR to increase? I have read a number of articles on transformers, and I don't understand everything I read, and did not see a clear explanation of voltage drop under heavy secondary load.
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Old 24th September 2009, 09:47 PM   #6
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Originally Posted by MichaelJHuman View Post
I wrote a WAV file analyzer program so I can clearly see peaks over average level. I think I got between 10 dB and 15 dB peaks on a few songs I ripped.
.
A lot depends on the type of music, a lot of modern pop music is compressed so doesnt have the same transients.
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Old 24th September 2009, 09:51 PM   #7
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Originally Posted by nigelwright7557 View Post
A lot depends on the type of music, a lot of modern pop music is compressed so doesnt have the same transients.
Yes. I am well aware of the loudness wars, and high compression of music.
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