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Old 7th December 2001, 01:50 PM   #1
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Join Date: Dec 2001
Location: Switzerland
Hi,

I have built and begin to test a Elektor HexFet 60 amp with my own power supply circuit (450VA transformer and 4 x 22'000uF). It's my first DIY power amp. At the first hearing, it's seems to work correctly, enough powerfull for 60W. Expepted a very small buzz the sound is more or less ok but really "restrained", "tense", especially in the bass range wich looks poor. I don't know how to explain that with my mediocre english (sorry). Did anyone already experiment this amp? Any suggest why does this thing sound like that?

Thanks.
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Old 7th December 2001, 04:51 PM   #2
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Alex,
I will double check on the bias for drift. As for the buzz, go thru the grounding and make sure that the input lines does not run close to the high current section of the amp, transformers and the AC line.

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Old 8th December 2001, 01:23 PM   #3
bawang is offline bawang  Malaysia
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Join Date: Oct 2001
Location: Malaysia
Wink Schematics and parts list

Ychnaval, also make sure that your ground is taken from the input capacitors. Also, try to link the grounds of the circuit boards together BEFORE connecting to the ground. My own experience told me that getting rid of hum/buzz is as much science as black magic, keep on trying.

It happened to me once too many times, and on one occassion after endless days of headaches, the hum problem was solved by just moving the preamp from the top of the power amp to another location a bit further.

I bought the newer version of the Elektor magazine with the upgraded 60W amp to 90W using Toshiba IGBTs. As IGBTs are impossible to get here (Malaysia), is it possible for you to send me the 60W amp's schematic and parts list? Thanks.
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Old 8th December 2001, 01:47 PM   #4
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Join Date: May 2001
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Despite my vast experience in designing PCBs and making several amplifiers of various power ratings, the 60watt HEXFET amp published in Elektor was one that I did not meet with success. I used the published PCB design, and all parts to spec including the E96/124 resistor values. However, this amplifier has a tendency to oscillate while in operation and could potentially blow the tweeter. Since the inverting input is not capacitor coupled, it is simply not possible to control the DC drift, despite the DC zero adjust given at the input of the amp and sometimes the DC drifts away into the unsafe region. All in all, a good sounding amplifier but not stable and reliable.

Someone in this forum mentioned that he made this amplifier with higher rated HEXFETs and the sound is really good. It is quite possible the the recommended IRF540/9540 HEXFETs are not quite capable of handling the recommened bias (200mA) and the dissipation thereof.
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Old 10th December 2001, 07:16 AM   #5
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Hi,

Thank you all for your advices. I did some more measurements, checking bias current, DC offset, oscillation. Indeed, exactly as Sam wrote, I can't control the DC drift. In the best case, I get 0.5V DC on the outputs. So, there is a mismatch in the P-channel and N-channel bias currents, but it works. And sometimes it drifts away...

Bawang, I'll send you schematic and part-list soon.
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