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Releationship between tension and current in audio signals
Releationship between tension and current in audio signals
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Old 25th October 2017, 11:49 AM   #21
Joanito is offline Joanito  Portugal
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I would need about 28 A ( negative and positive ) to feed the two boards. Of course, the consuption will never reach that exact value, but let's just say we need 110vdc 20A and -110vdc 20A
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Old 25th October 2017, 12:06 PM   #22
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Joanito View Post
Hi all,
I was wondering... if an amplifier psu tensions for a 100w rms 8 ohm is about, let's say, 30v and -30v then for a 200w it would be 60v
No, and this shows your confusion, all your problems start from this.
Power is proportional to the square of voltage/tension/potential (pick one).
So a 200W amplifier will not need twice the supply voltage of a 100W one , but one 41% higher.

That with ideal amplifiers and supplies; in practice you will have some less than expected because real World parts have losses, so system is not 100% efficient.

Some "losers" :

* transistors have a minimum saturation voltage or "voltage drop" across them, specially when passing maximum current into a speaker.

For a typical power transistor pair (usually Darlington or Quasi Complementary) you have some 4V loss, so you must increase rail voltage by that amount.

* MosFets can have lower saturation voltage across them, as low as 1V or even less, but need a gate drive voltage sometimes as high as 6V to pass maximum current, and that has to come from "somewhere".

* You usually lose 1 to 2V across emitter ballast resistors ... just do the Math: 5A through a 0.33 ohm or 0.22 ohm resistor, in series with load.

Now 2 BIG Losers:
* typical raw power supplies, as used by 99% of power amps, drop about 20% voltage under full load.
A cheap minicomponent/Home Theater one might drop 30%, an Audiophile type one might drop as little as 10%, but all do.

* under load, you will get ripple on your rails, lots of it.
Substract peak ripple voltage from whatīs available.

Thatīs why although you need 28.3 V RMS to get 100W into 8 ohms, which *would* mean 40V Peak so 40V rails, in practice youīll need at least +/- 45V rails and often +/-50V ones.

Scale up/down for other power and impedance combinations but always remember scale depends on Voltage squared, itīs not linear by any means.
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Old 25th October 2017, 12:37 PM   #23
Joanito is offline Joanito  Portugal
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There's even boards on selling ( that we put between the (vdc and the -vdc) and the amplifiers ) with many ( realy many ) capacitors to reduce the problem...
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Old 25th October 2017, 02:55 PM   #24
turk 182 is offline turk 182  Canada
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just randomly adding capacitors to eliminate voltage sag can quickly turn your amp into an oscillator if your not careful!
reminds me of an old saying "if you build an amplifier you'll end up with an oscillator, if you build an oscillator you'll wind up with an amplifier!"
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Old 25th October 2017, 05:25 PM   #25
Mooly is offline Mooly  United Kingdom
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Releationship between tension and current in audio signals
Yep, I know that one as 'amplifiers always oscillate while oscillators often fail to do so'.
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Old 25th October 2017, 07:35 PM   #26
Joanito is offline Joanito  Portugal
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Yeah, but if the capacitors we're all equal, with a voltage above the vdc, connected all in paralell, that doesn't happen? Or not ?
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Old 1st November 2017, 01:26 PM   #27
Pano is online now Pano  United States
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Releationship between tension and current in audio signals
Quote:
Originally Posted by bwaslo View Post
"It's really loud and the volume control is only X% of the way!". It's pretty dang trivial to arrange gains and signal level so you run out off power in the first 1% of rotation if you wanted to!
Yes, that bothers me too. Most systems have far too much gain and don't have great ways to deal with it. I've often wondered if making the volume knob go to full blast at 9 o'clock is some sort of marketing ploy.
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Old 1st November 2017, 02:44 PM   #28
Mooly is offline Mooly  United Kingdom
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Releationship between tension and current in audio signals
Definitely. Its like small cars with speedos calibrated to 160 MPH or whatever. They have got to be faster... right ?
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Old 3rd November 2017, 05:59 PM   #29
Joanito is offline Joanito  Portugal
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I once had an amplifier like that... it sucks... I think this is where the difference in quality between brands also comes in.
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