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Old 26th August 2002, 09:39 PM   #21
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Default LM3875, LM3886, LM12

I think that these "op-amps on steroids" (with apologies to whomever coined that term) can take a lot more than +/- 35VDC.
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Old 27th August 2002, 02:57 AM   #22
alvaius is offline alvaius  Canada
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Here is another thought on regulated power supplies on power amps.

1) If you look at many implementations of biasing on a class-A amp, it is regulated. Typically, the reference current will be generated off a regulated supply. A current multiplier will then be used to generate the main bias current for the output stage. The output current of this will be somewhat independant of the supply rails.....hence a current regulated output.

2) If you regulate the output voltage of a power amplifier, you have essentially created a feedback loop that uses the output of the audio signal as an input. As the voltage of the output drops, which will be a function of the audio output (or input if you want to think it that way), the current into the pass transistor increases, yada, yada, yada......

In either case, the output of the regulator can be modelled as a function of the audio signal and yes, it can be a source of distortion. The question is, is this source of distortion, when worked upon by your amplifier circuit, worse or better than having 60Hz or 120Hz from an un-regulated supply?

Alvaius
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Old 27th August 2002, 05:27 AM   #23
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Default Re: Forgot one

Quote:
Originally posted by HarryHaller
The schematic is from:

“A Power Supply Regulator For the Adcom GFA-555,” Kit Ryan, The Audio Amateur4/89

"Slap on a couple of opamps and you would really have a winner..."

If you can find on op amp with +/- 35 volts rails!
OPA445, +/-45 V rails.

Jan Didden
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Old 27th August 2002, 09:15 AM   #24
ALW is offline ALW  United Kingdom
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Default What causes the temperature to rise with the rectifiers and transformer?

Simply, I2R losses in those devices.

It's exacerbated by the reduction in conduction angle of the rectifier diodes as a result of using lots of capacitance and very little current drain in preamp applications.

I strongly suggest you try the same PSU with only 1 15,000 cap and then see how it sounds - I suspect you'll find it sounds much better, rythmically.

Andy.
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Old 27th August 2002, 11:30 AM   #25
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Default Bridge rectifier life expectancy when run hot?

Quote:
I strongly suggest you try the same PSU with only 1 15,000 cap and then see how it sounds - I suspect you'll find it sounds much better, rythmically.
Thanks for your 2 cents
But the PSU is built for the mono power amp..
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Old 27th August 2002, 02:01 PM   #26
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Default Regulated supplies

Hey Chris,

Not so fast... ALW is right, the same reasoning goes for power amps, although the numbers obviously will be different from the pre-amp case. Those insane reservoir caps of 50.000 or 100.000 or more uF give rise to 50 Amps or more sharp current peaks, and very high levels of harmonics to the power line freq in your amp. I'd rather have some 60 and 120 Hz benign ripple, which the amp can handle perfectly, thank you, instead of a spectrum into the 10s of Khz that is much harder on the amps PSRR.

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Old 27th August 2002, 03:08 PM   #27
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ALW, Jan

Thanks for the advise. I'll try remove 2 of those 15,000uf caps from each supply rail and listen for the difference. If it turns out good there will then be 8 of these capacitors left over for other projects or more amps for bi-amping later I hope. That's good news for me for those caps cost approx. $45 Canadian each. (16 pounds sterling each plus tax and shipping and custom clearing broker fees)

Will it also reduce the heat generated by the bridge rectifiers you think by reducing the capacitance in the rails?

Now then how about those marketing speech trying to sell the upgrades of their amps to increase the caps even as high as 120,000uf? Are they just trying to sell more parts or there is some good reason behind the upgrade? So what would be the nominal capacitance for smooth caps in generic PSU.. 10,000uf?

Chris
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Old 27th August 2002, 03:31 PM   #28
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Quote:
Originally posted by chris ma
ALW, Jan

Thanks for the advise. I'll try remove 2 of those 15,000uf caps from each supply rail and listen for the difference. If it turns out good there will then be 8 of these capacitors left over for other projects or more amps for bi-amping later I hope. That's good news for me for those caps cost approx. $45 Canadian each. (16 pounds sterling each plus tax and shipping and custom clearing broker fees)

Will it also reduce the heat generated by the bridge rectifiers you think by reducing the capacitance in the rails?

Now then how about those marketing speech trying to sell the upgrades of their amps to increase the caps even as high as 120,000uf? Are they just trying to sell more parts or there is some good reason behind the upgrade? So what would be the nominal capacitance for smooth caps in generic PSU.. 10,000uf?

Chris
As for the reasoning behind those upgrade cap banks, you should really ask the guys who sell them.

As for generic cap, I normally use 1000 or 2000 uF per ampere load current, worst case. In my experience, more than 22.000 uF seldom gives improvements.

Jan Didden
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Old 27th August 2002, 03:37 PM   #29
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I've also heard that too much caps may slow the amp, but never did any comparisons. Would be interested to hear results.
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Old 27th August 2002, 03:48 PM   #30
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Quote:
Originally posted by Peter Daniel
I've also heard that too much caps may slow the amp, but never did any comparisons. Would be interested to hear results.
With the gain clones it has been found that the amps don't sound as good with too much C. 1,000uF seems to work best.

dave
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