Just a brief few questions about the Leach's components:
1) Dr Leach specifies 100nF film for
C2,C3 (current source decoupling)
C12 (Vbe multiplier C-E connection)
C17,C18 (protection circuit)
C15,C16,C23,C24 (supply rail decoupling)
and 10nF film for C19,C20 (protection circuit)
Although there is merit in using high dV/dt polypropylene caps for the supply rail decoupling caps, would there be an audible difference in using standard polyester film caps for all of these? I figure the quality C2,C3 and C17,C18 in particular shouldn't have any adverse effects.
2) It's hard to find a quality 390pF cap for C1 in my country (and I'm sure as hell not going to import 2 small caps). If it's changed to 470pF, the bandwidth reduces from 200kHz to 170kHz. Although 10x audible bandwidth is the rule of thumb for 1st order filters, would this make any difference? One could of course decrease R2, but that might change the feedback and therefore influence DC offset. The other alternative is to change C1 to 330pF, increasing the bandwidth to 240kHz.
3) Although 1N5250B 20V zeners are specified, I figure any 20V zener with the same power rating would also suffice.
Any comments?
Pierre Watts
1) Dr Leach specifies 100nF film for
C2,C3 (current source decoupling)
C12 (Vbe multiplier C-E connection)
C17,C18 (protection circuit)
C15,C16,C23,C24 (supply rail decoupling)
and 10nF film for C19,C20 (protection circuit)
Although there is merit in using high dV/dt polypropylene caps for the supply rail decoupling caps, would there be an audible difference in using standard polyester film caps for all of these? I figure the quality C2,C3 and C17,C18 in particular shouldn't have any adverse effects.
2) It's hard to find a quality 390pF cap for C1 in my country (and I'm sure as hell not going to import 2 small caps). If it's changed to 470pF, the bandwidth reduces from 200kHz to 170kHz. Although 10x audible bandwidth is the rule of thumb for 1st order filters, would this make any difference? One could of course decrease R2, but that might change the feedback and therefore influence DC offset. The other alternative is to change C1 to 330pF, increasing the bandwidth to 240kHz.
3) Although 1N5250B 20V zeners are specified, I figure any 20V zener with the same power rating would also suffice.
Any comments?
Pierre Watts
1. Polypropylene is ideal, but polyester works just fine.
It's what I've almost always used.
2. I'd like to use polystyrene or polypropylene here, but I've
always used mica caps with good results. Using different
values probably won't hurt, but I couldn't guarantee
stability. You could also parallel smaller capacitors to
get the specified value.
3. A 40 volt zener would work too, or any combination of
zeners that adds up to 40 volts. If the resulting voltages
are more than 5% off, you'll have to do some matching.
It would be ideal to use the same type part in all cases
and to closely match the voltages as per the instructions.
--Damon
It's what I've almost always used.
2. I'd like to use polystyrene or polypropylene here, but I've
always used mica caps with good results. Using different
values probably won't hurt, but I couldn't guarantee
stability. You could also parallel smaller capacitors to
get the specified value.
3. A 40 volt zener would work too, or any combination of
zeners that adds up to 40 volts. If the resulting voltages
are more than 5% off, you'll have to do some matching.
It would be ideal to use the same type part in all cases
and to closely match the voltages as per the instructions.
--Damon
I believe the diodes are specified for low noise.
C1 value can affect stability of the amp. Don't go smaller. Micas are OK. polystyrene/foil ideal, but COG (Class 1) ceramic is low distortion, according to article called "Capacitor Sound" in Electronics World last year. The article didn't deal with possible issure of microphonics with COG ceramics, though, so I don't know if this is an issue with Class ! ceramics vs. the old style ceramics, which did have this problem. One more thing with the Leach amp: use a series pair of non-polar electrolytics in the feedback, instead of the single non-polar or series polar he recommends. They should each have the value of the series non-polar he recommends, or larger.
C1 value can affect stability of the amp. Don't go smaller. Micas are OK. polystyrene/foil ideal, but COG (Class 1) ceramic is low distortion, according to article called "Capacitor Sound" in Electronics World last year. The article didn't deal with possible issure of microphonics with COG ceramics, though, so I don't know if this is an issue with Class ! ceramics vs. the old style ceramics, which did have this problem. One more thing with the Leach amp: use a series pair of non-polar electrolytics in the feedback, instead of the single non-polar or series polar he recommends. They should each have the value of the series non-polar he recommends, or larger.
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