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Old 30th December 2004, 10:31 PM   #11
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<Hi there ..I've got a sneaking suspicion (it's getting rather late) that you are using 10,000uF per rail smoothing cap.....for 500W p ch ?I would rate that value as the min ... I don't know what the o/p stage quies current is for such a beast, but probably around the 200ma ballpark..........the residual hum may orig from that unregulated + supply.... it seems to me that the amp grounds go round in a "circuit" around the tranny in which case you have to live with it or up the smoothing cap values......one old trick to find out if it is the tranny is to blame for the earth loop is to fire the amp up via a variac or variable voltage tranny and listen for the change with a 15% voltage reduction. Any change and the tranny is to blame.

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Old 31st December 2004, 10:40 AM   #12
jesper is offline jesper  Sweden
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Hi
It's actually 4*80W or something like that, so "only" a total of 320W. For that I use a 500VA tranny. And I use one PSU for each dual channel amplifier, so 10.000uF per rail for 160W and 2*50mA of cuesent current, the ripple due to that is only some 250mV.

I wouldn't be surprised if the remainder hum is from the tranny, picked up by the psu and signal cables and maybe even the boards. I'll try your variac test.. Maybe some day I'll get hold of some mymetal to sheild the tranny somewhat.
But for now I think I can live with the ramainding hum and start focusing on my other projects which is waiting, a preamp and a sub for instanse...

The sound is acutally quite good (despite the small hum)..

/Jesper
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Old 31st December 2004, 04:50 PM   #13
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Basic hum problems in dual trans/mosfet amps are nasty......this isn't a particulary nice new years gift to sort out. I started by removing the input wiring and measuring afterwards......methodically.... even the end result was a compromise.
The real headache begins when one has actually improved the hum reduction and another problem re-emerges. For example; in my amp by breaking earth loops (to what I thought was ideal) I had ruined the crosstalk perf between A+B to roughly 15dB down on the ajacent channel. Put it simply, the test involves dummy loading the o/p of A, then applying 10Khz sine wave to A input until o/p near clips; S/C the B input and measure the residual volts coming out of B amp with a scope and compare with A. This is the standard crosstalk test and alot of amps fail this test due to bad wiring errors or wiring that has been moved to the wrong place.....check vice versa. Excessive crosstalk at HFreq is usually wiring placement sensitive and also exactly where the LS leads are. These much be carefully placed and not tie wrapped with any other wiring. The effect on audio quality is a pronounced loss of ambience with wide stereo placement.


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