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Old 24th December 2002, 06:32 PM   #1
diyAudio Member
Join Date: Aug 2002
Location: Toronto, Canada
Unhappy buzzing sound from mini-a's

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!

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Old 24th December 2002, 06:48 PM   #2
The one and only
Nelson Pass's Avatar
Join Date: Mar 2001
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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Old 24th December 2002, 08:57 PM   #3
diyAudio Member
Join Date: Feb 2001
Location: Columbia, SC
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.

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Old 26th December 2002, 01:46 PM   #4
diyAudio Member
Join Date: Aug 2002
Location: Toronto, Canada

Thank you all for the suggestions.

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Old 27th December 2002, 08:18 PM   #5
Thomas is offline Thomas  Denmark
diyAudio Member
Thomas's Avatar
Join Date: May 2001
Location: Right here
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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