vintage kenwood amp help

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#21
Hi Duke,
I agree to a point.

Hi pistollero,
Your transistor checker will not match transistors unless it holds the transistors at the same exact temperature - which it doesn't. The jig I designed puts the two transistors in thermal contact in a long tailed pair arrangement and isolates them from the air. You measure between the two collectors for a null. You could use a million dollar semiconductor grading system and unless you can maintain that exact temperature - you cannot get proper matches.

-Chris
 
#22
How does a Schottky diode be more efficient than a 1N400x at mains frequencies?
Or is it just a fad? Anybody try higher rated diodes just to compare?

Schottky are better at much higher frequencies, and the supply is getting smoothed out by filter circuits. But at 50 or 60 Hz?
 
#25
NareshBrd said:
How does a Schottky diode be more efficient than a 1N400x at mains frequencies?
Or is it just a fad? Anybody try higher rated diodes just to compare?

Schottky are better at much higher frequencies, and the supply is getting smoothed out by filter circuits. But at 50 or 60 Hz?
Click to expand...

Schottky diodes are more efficient at mains frequency because they have a lower voltage drop. But they leak - orders of magnitude more than regular ones. So at high voltage the power efficiency is worse. You’re supposed to use them on low voltage supplies where the leakage isn’t an issue but voltage drop is. The use of SiC schottkys is a fad, at least where audio is concerned. They are for high frequency use only, and the forward voltage drop is even higher than for *any* silicon rectifier - even the fast ones.

The way to get rid of diode commutation noise, which is actually technically a high frequency parametric oscillation, is with a snubber. Whether or not it happens is more dependent on the transformer and loading conditions than what kind of diodes you use. Fast diodes just push the frequency up, where it might be harder to eliminate. It tends to get radiated and go all over the place. I never see it in a solid state power amp. I have seen it in tube power amps and low power supplies for digital, preamps, and the like - where a small high reactance transformer, heavily loaded, is used.
 
#26
very clear, that's educational, thank you.

its misused here in this amp, I agree, for this is a common vintage amp.

a title here in 2017,

Bridge Rectifier vs. Schottky Diodes for F4

in that title , use whatever makes you happy....

so I am happy.

thanks
 
#27
Good points wg_ski, got Schottky mixed up with the super fast rectifiers.

Fully agree with everything else. The best way to reduce HF noise is to avoid creating it to begin with. Then use snubbers if you have a problem.
 
#30
Lossy capacitor, usually. Same thing we use for supply decoupling caps in RF power amplifiers. The Cap With The Built In Resistor. I always add a resistor, to act as a FUSE if the cap fails. On a 600V supply (Or even 100V) you sort of want that.

They also might not be calculating it correctly, just using convention which tended to work for the last 20 years.
 
#31
They are normally disc capacitors, not the ones with built in resistors. I do believe you are right about them just sticking in what worked before.

Yes, you absolutely want the resistor in case the capacitor fails. That does happen.
 
#32
A cap with a high dielectric absorption has “a built in resistor”. Most of the ferroelectrics they use in class 2 ceramics are pretty lossy. That’s why “they worked before”. Put in a high Q (or audiophile) cap and it rings worse than it ever did.
 
#33
I didn't think DA would be lossy enough. Whatever, that's what they stuck in there. I thought the ones across the transformer were for line noise. The ones across the rectifiers didn't make sense, so I ignored them.
 
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