Bob Cordell Interview: Power Supplies - Page 3 - diyAudio
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Old 20th November 2006, 09:10 PM   #21
Netlist is offline Netlist  Belgium
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Old 21st November 2006, 12:28 AM   #22
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Another thing that may influence the role of power supply filtering and quality is if the amplifier is of a minimalist design. A number of designers feel that the best sound is obtained with the simplest possible signal path. In some cases this leads to designs that have less power supply rejection than a more complex design with many transistors. Thus, it may turn out that great attention to the power supply will make a bigger difference in these types of designs.

I am not in the minimalist camp, and I will spend a transistor to improve power supply rejection without half a thought, and I have a strong goal to achieve very good PSRR out to fairly high frequencies.

Bob
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Old 21st November 2006, 01:10 AM   #23
gerhard is offline gerhard  Germany
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Quote:
Originally posted by AKSA

These caps have two purposes; one to smooth the power pulses coming off the rectifier (and these are pretty savage), and the other to accommodate speaker earth return current. If both these currents coexist in the same cap, then intermodulation results, so I like to use two caps per rail, separate them with a small resistor (typically 0R22, or even a tiny inductor around half a millihenry) so that the two currents are separated.
For intermodulation to occur, a nonlinearity is necessary. Otherwise, the superposition principle applies.
And if there was a built-in diode or whatever in a cap, 0r22 would not linearize it.

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Old 21st November 2006, 01:25 AM   #24
gerhard is offline gerhard  Germany
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Quote:
Originally posted by Bob Cordell


Another thing. If the VA rating is primarily an indicator of maximum core temperature, then different transformers of the same VA rating could have vastly different useable d.c. rectified current capabilities and regulation in the real world. For example, copper is now very expensive. A transformer manufacturer could probably arrive at the same thermal VA rating by using less copper and more iron, and maybe arranging that iron to dissipate heat better. The resulting transformer would have significantly more winding resistance and would not have as good regulation, even though it had the same VA.
Copper loss dominates at maximum load while iron loss dominates in no-load situations. An optimal transformer is usually designed so that copper and core losses are equal for the intended application. So a transformer for a class A amplifier should have relatively more copper than a transformer for something that is spec'ed for PMPO.

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Old 21st November 2006, 01:33 AM   #25
gerhard is offline gerhard  Germany
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Quote:

me:
> For a typical tube amp with Vplate of several hundred Volts, 1mF should be enough.
>Stored energy goes up with the square of the voltage.

Tube_Dude:
If that is obviously true from a energy point of vue , we must not forget thet the rail capacitor is the path that close the signal to ground.

So this parasitic series impedance must be the minimum that can be practically implemented...

Ideally it must be 0 Ohms at all signal frequencies...
For an amplifier at 300V/100mA, the impedance level is 100 times higher than for an amp with 30V/1A, so, everything else equal, we could afford a much higher power supply source impedance.

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Old 21st November 2006, 06:53 PM   #26
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Quote:
Originally posted by gerhard


For an amplifier at 300V/100mA, the impedance level is 100 times higher than for an amp with 30V/1A, so, everything else equal, we could afford a much higher power supply source impedance.

Gerhard

Good point. I use about 1000 uF at 500V in my KT-88 tube amp, so that would correspond to something on the order of 100,000 uF/rail in my solid state amp, not far off.

Bob
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Old 21st November 2006, 07:03 PM   #27
MikeB is offline MikeB  Germany
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Quote:
Originally posted by Bob Cordell

I am not in the minimalist camp, and I will spend a transistor to improve power supply rejection without half a thought, and I have a strong goal to achieve very good PSRR out to fairly high frequencies.

Bob
Bob, can you tell any number, what level of PSRR is recommendable ? Is -90dB a good number, or should it be even more ? (getting difficult)

Mike
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Old 21st November 2006, 07:13 PM   #28
PMA is offline PMA  Europe
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Mike, firstly, the PSRR is frequency dependent. Speaking about single number without showing frequency plot is speaking about nothing.
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Old 21st November 2006, 07:19 PM   #29
MikeB is offline MikeB  Germany
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Pavel, you're right, my question was way too generic...

Mike
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Old 21st November 2006, 07:21 PM   #30
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Quote:
Originally posted by AKSA
[snip]However, topology must play a part. These caps have two purposes; one to smooth the power pulses coming off the rectifier (and these are pretty savage), and the other to accommodate speaker earth return current. If both these currents coexist in the same cap, then intermodulation results, so I like to use two caps per rail, separate them with a small resistor (typically 0R22, or even a tiny inductor around half a millihenry) so that the two currents are separated.

Cheers,

Hugh

Hugh,

I have a problem with that. The speaker current ultimately comes from the first cap, the one closest to the rectifier. It therefore also needs to return to that. The ripple current may divide in some way between the caps but you can't say that one current goes to one cap and another to the other cap.

Jan Didden
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