Hi all,
I'm trying to adjust the DC offset of a Hiraga 30W design amp. I can't seem to get it "steady".
Anyone any ideas on what could be wrong?
Kind regards,
Sjoerd Smits
I'm trying to adjust the DC offset of a Hiraga 30W design amp. I can't seem to get it "steady".
Anyone any ideas on what could be wrong?
Kind regards,
Sjoerd Smits
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
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
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.
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.
Hi Mooly,
I'm thinking maybee a defective transistor.... It goes up to +80mv. Then I trim the pot back to "zero" and it just keeps going up....
I Agree, I need someone with an oscilloscope to check what's wrong....
I'm thinking maybee a defective transistor.... It goes up to +80mv. Then I trim the pot back to "zero" and it just keeps going up....
I Agree, I need someone with an oscilloscope to check what's wrong....
.... can you ,tell us , rail supply voltage and please post a picture of your amplifier 😉 ...... maybe I will see something 🙂
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.
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.
Hi Mooly,
I took the amp apart so it's impossible to measure anything right now :-(
But nothing ran hot.... Not even the heatsinks. They ran "hand warm".....
I took the amp apart so it's impossible to measure anything right now :-(
But nothing ran hot.... Not even the heatsinks. They ran "hand warm".....
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 🙂
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 🙂
And You used the original transistors?
Matching of the transistors are a must here.
Well, the two NPN-transistors within 2%
And then the two PNP-transistors within 2%
Both Hfe and VBE
No need to match pnp against npn, as the configuration balances it all well.
Matching of the transistors are a must here.
Well, the two NPN-transistors within 2%
And then the two PNP-transistors within 2%
Both Hfe and VBE
No need to match pnp against npn, as the configuration balances it all well.
Yes, both sides are the same....
Tandbergeren: How do I match VBE???? Don't have a clue.... The HFE is 100% matched. I checked.... But VBE??????
Tandbergeren: How do I match VBE???? Don't have a clue.... The HFE is 100% matched. I checked.... But VBE??????
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.
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.
Hi Ian,
The transistors are mounted directly on the beefy heatsinks. Ahey are just 2cm off center to the top. Each transistor has a heatsink of 0,38K/W. I would think that is more than enough???
The transistors are mounted directly on the beefy heatsinks. Ahey are just 2cm off center to the top. Each transistor has a heatsink of 0,38K/W. I would think that is more than enough???
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.
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.
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