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Old 25th December 2001, 11:18 PM   #11
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Yeah i know Grey, thanks for the input.
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Old 28th December 2001, 04:23 PM   #12
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Thank you all,

Ive ordered 4 2mH airchokes of 1.4mm wire (0,38 Ohms). Since I use 50 paralleled 1000 microfarad capacitors after every choke the impedance at 100 Hz will be even lower than Nelsons estimated 0.13 Ohms.

The only problem left is the shielding of the chokes. What would be the easiest way to do this?

william
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Old 28th December 2001, 05:07 PM   #13
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easiest way? try to keep them away from the front end of the the amp or hide them behind those big caps..... saves you the time and trouble
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Old 28th December 2001, 07:09 PM   #14
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Mr Pass,

I'm sorry, I cann't agree with you. Attenuation of 20 dB can be insufficient to avoid audible hum in the vicinity of a good loudspeaker : 20 dB is only 1/10 in voltage, and, depending of the supply rejection of the amplifier, significative residual signal at 120 (or 100) Hz can be measured at the output...

Regards, P.Lacombe.
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Old 28th December 2001, 07:36 PM   #15
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Why bother with PI filters on power supplies for solid state amplifiers? A well designed amp will have a good power supply rejection ratio.

If you really must, the Radiotron Designer's Handbook has full details. A CD version is (was?) available from what was Audio Amateur.

regards, Keith
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Old 29th December 2001, 06:41 AM   #16
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Yes Keith,

But ... the solid state amps such as the Zen have really poor PSRR so a clean supply is critical.
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Old 29th December 2001, 09:33 AM   #17
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Quote:
Originally posted by AudioFreak
Yes Keith,

But ... the solid state amps such as the Zen have really poor PSRR so a clean supply is critical.
Accepted. Does this mean that the Pass amps are poorly designed?

The design setup for a PI filter between a PSU and an amplifier is NOT intuitive. The filter is just that... a filter and a low pass one to boot. This means that it'll have all the electrical characteristics of a filter and, if not properly handled, may be worse for the amplifier than a capacitive power supply.

Again, see the Radiotron Designer's Handbook for all the gory details.

regards all, and have a great musical new year.
Keith
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Old 29th December 2001, 04:37 PM   #18
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For those who don't remember a certain thread from last summer, Keith likes to argue more than anything else in the world.
As for intuitive...PI filters seem quite reasonable to me; make perfect sense. Low pass filter? Of course it is. But so's a simple cap. The cap just has a lower slope: 6 dB vs 18 dB. Caps work fine. Inductors work fine. Caps and inductors work fine together. For that matter, if inductors bother you, you can use a resistor between the caps in a PI filter, albeit with some loss of power.
I think that you'll find that many circuit elements have poor PSRR. Does that mean that every stage in the circuit has to be a differential? Of course not. Don't be silly (or should I say argumentative?). Even portions of circuits with good CMRR benefit from a quiet power supply, even if the residual is, say, 80 dB down. Why have unnecessary racket?
(The Radiotron Designer's Handbook aka Radio Designer's Handbook is the third book down on my shelf even as we speak.)
Please, Keith, if you're going to start arguing purely for the sake of arguing again, at least refrain from foolish statements such as the PI filter being a low pass filter, whilst ignoring the fact that a simple cap is also a low pass filter. It reflects poorly on your comprehension. You did enough damage to your credibility last summer without chipping further at it now.

Grey
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Old 29th December 2001, 05:59 PM   #19
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Hi,
Whether the PI-filter is a lowpass filter or not does not bother me, but most power amplifiers have <B>un</B>regulated supplies.
The PI-filter did improve the <B><U>SOUND</U></B> of my GAS Ampzilla amplifier and that is interesting.
I also experimented with larger values for the inductor up to 22 mH but got no further improvement.
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Old 29th December 2001, 06:05 PM   #20
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Power supply can be critical, even with a good *apparent* PSRR. Extensive experiments have demonstrated that strange behaviours can occur in some circumstances, in class AB amplifiers, specially BJT, whose the idle current is (necessarly) low.

Hum at output can be inaudible, but sound quality can be poor when the listenig level is high : increase of power consumption causes more hum, which is inaudible because of the loud music, but the amplifier act in the same way than a balanced mixer or ring modulator, causing intermodulation products at +/- 120 (100) Hz of each music tone. This can be clearly seen with a spectrum analyser, or a good sound-card in a PC compatible, and the suitable software.

This is why I always use a very good filter design in PS filters. Iron-core inductors don't causes any distortion, even saturated, because audio signal don't flow across them, don't need shielding, and are less expensive than coreless ones.

Regards, P.Lacombe.
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