Hiraga 30Watt DC offset

42001

Disabled Account
2012-04-23 12:55 pm
Schedule

Hi,

Here's the schematic.... I'm thinking maybee I used wrong parts.....

I used MKP instead of Silver/Mica condensators.....

Thanks for any help!

Kind regards, Sjoerd Smits
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
How "not steady" ? How much is it varying and how quickly.

Have you tried with the input shorted. The cap type shouldn't make any difference. I'm wondering if its oscillating or unstable in some way. A scope check would show any problems.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Difficult to diagnose without actually working on it. I'm not so sure about a faulty transistor tbh, That would tend to cause a huge offset. Are any of the transistors running very hot ?

Is it worth slapping a zobel network on the output just to see if it alters anything. Something like a 0.1uf and 10 ohm in series to ground.
 

42001

Disabled Account
2012-04-23 12:55 pm
Picture

Hi,

Voltage +-33VDC on the amps. Nice and steady.
 

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42001

Disabled Account
2012-04-23 12:55 pm
My thougts: I didnt match any of the trannies..... so that could be the problem??

Now I got good trannies(Q2 & 4 HFE matched and Q1 & 3 HFE matched) I also ordered nice Kiwame resistors and Cornell Dubilier Silver Mica capacitors. And a pair of very, very expensive Bourns 500R Potentio meters.... So I will build the amps all over again :)
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Hmm....you have the output transistors fitted to the chassis which is then cooled by the heatsink.....:(
It would work for OK for lower average dissipation designs like many small class AB amps, but this 30W class A
design pushes the transistors very hard. I would at least remove enough steel to allow the transistors to fit
properly, direct to the heatsink and get better cooling efficiency.

Mounting them near the top of the heatsink is not good use of the sink either.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Yes, I realised a few moments ago, that the steel frame could have been just the small brackets. The pic only seemed to show a steel chassis. As you confirm, it is not and those sides are the heatsinks. Still, I have seen guys do it often enough. Sorry to confuse your pic.

Convection heats the top of the sink rather than getting rid of the heat, so it is better to mount the transistors lower. You can easily check this with a thermometer/pyrometer, where the bottom is relatively cool compared the top. It should be as close as possible to an even temperature.