| Dennis Hui |
Hi everybody,
I've spent the last few days building two channels of
the mini-a. I built a single power supply with 70000uF
on each rail. Each channel sounded fine when hooked
up to the power supply but when I tried hooking both
up to the same supply, I got buzzing sounds (high frequency)
on certain music material. I tried powering this with
two different transformers. With a cheap radio shack
12.6-0-12.6 centre tap transformer, the buzzing occured
at higher volume levels. I then tried a 300VA 2x12V toroid
and it was much worse. The buzzing was noticeable even
at very low volume. :( (I don't think this is ground loop
since it's not a hum, but rather a crakling type buzz on
the tweeter.)
Any idea what the problem might be?
Another thing: The AC hum at the output seems to be worse
with the toroid than with the cheapie transformer.
Thanks in advanced for any suggestions.
Merry Christmas!
Dennis |
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| Nelson Pass |
When you run into this sort of stuff, you need to isolate
the problem down to the following categories:
oscillation: high requency buzzing, none of the other
approaches helps: play with frequency compensation,
look at output with a scope.
ground loop: If the problem doesn't go away when you
disconnect the source and short the inputs, it's not a ground
loop, probably.
transformer pickup: Move the transformer / bridge / cap
assembly away physically, and the problem foes away.
poor ripple rejection: CLC or CRC filter in the supply
fixes this. |
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| GRollins |
If it's only happening when you hook up both channels, you might try isolating the left and right channel rails from each other with a resistor or inductor.
Cap (first one after rectifier)<two resistors or inductors, each leading to a separate cap for the two channels.
Grey |
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| Dennis Hui |
Gentlemen,
Thank you all for the suggestions.
Cheers,
Dennis |
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| Thomas |
Hi Dennis
I had this problem too. I solved it by installing a 4.7mH choke in a CLC config. The choke is a ferite core / 1.4mm wire diameter from a cross over. My mini-A is biased at 1.6A and has 66000uF pr. rail pr. channel.
The complete supply for both channels looks like this:
66000uF -> 4.7mH -> 66000uF (pr. rail.)
The amp is now 'dead silent'. The only penalty is a voltage drop of about 0.6 V across the choke. |
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