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Old 1st September 2010, 01:29 AM   #11
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Bias diodes installed?
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Old 1st September 2010, 03:13 AM   #12
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AndrewT: During the time its on, I felt around, and didn't detect any heat. But, I will try again tomorrow.

Damon Hill: No the bias diodes are not installed, Mr. Leach's board test procedures said nothing of that, but should they be???...Also, in the board test instructions, Mr. Leach says to tack solder a jumper in parallel with c12, which if I understand would, bypass the bias circuit???

Thanks, Andy
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Old 1st September 2010, 06:28 AM   #13
nattawa is offline nattawa  Canada
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In the first pic are you using different sensitivity on the two channels of the scope? What is the measured voltage gain? What was the supply voltage? Can you measure the DC voltage across C2, C3, C13, C14 with the scope and see if they are stable?
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Old 1st September 2010, 07:19 AM   #14
Mooly is offline Mooly  United Kingdom
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I see the input is DC coupled... that's fine as long as you understand the implications but for testing I would add a small cap (0.1 to 1uf poly, not electrolytic as any fault could reverse bias it).

Good advice from nattawa... I would also add to check and measure the voltage across R21 and 22 (the constant current sources) and see if these change as the problem appears.

The DC voltages will reveal the problem.
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Old 1st September 2010, 09:17 AM   #15
AndrewT is offline AndrewT  Scotland
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Hi,
the jumper sets the bias voltage to zero volts.
That ensures that the output stage passes zero bias current.
The amp is then worse than a ClassB since there will be a small range of input voltage for which there will be no corresponding output voltage.

Read the whole Leach paper on his design. Read it until you almost know it off by heart.
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Old 2nd September 2010, 02:22 AM   #16
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nattawa: The sensitivity is the same for both channels. One is the input, the other is the output of the amp. Excuse my unknowingness, but how do you calculate the voltage gain?Click the image to open in full size. Also, the supply voltage was 25v. I will measure the voltages across those capacitors and report back.

mooly: Thank you for the tip. Also, I will measure the voltage across those resistors and report back also.

AndrewT: Thank you for clearing that up for me. I have read the whole paper twice nowClick the image to open in full size..

Thanks again to all for the help so far. I would barely get anywhere by myself. This is a great website!

Thanks

Andy
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Old 2nd September 2010, 04:07 AM   #17
nattawa is offline nattawa  Canada
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A Leach amp has a voltage gain of about 21 by design. You should be seeing an output waveform with amplitude about 20 time as high as the input waveform in your test. However from your first picture you seemed to have a gain of only about 10 by comparing the height of the waveforms, so I was suspecting either the scope had different sensitivity settings between the channels, or the amp had something not right. No rush. While you're at measuring the DC voltages you might want to double check all the PNPs and NPNs are where they should be.
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Old 2nd September 2010, 07:41 AM   #18
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What AndrewT said. I've never used that procedure but it's the best thing to do. I could stand to reread Leach's notes, too.
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Old 2nd September 2010, 12:26 PM   #19
pooge is offline pooge  United States
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Quote:
Originally Posted by elektrogeek View Post
Also, the supply voltage was 25v. I will measure the voltages across those capacitors and report back.

Andy
Why is the supply only 25v. You need more than that for the input stage to kick in.
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Old 4th September 2010, 04:13 AM   #20
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Its at 25v because I was afraid that something might go wrong, since the output would go out, but i shall raise the supply voltage. Also, on r21 and r22 when you power it up the dc voltage on r21 is slowly decreasing and the dc voltage on r22 is slowly rising. Then when the "phenomenon" occurs, there is quick drop of voltage on r21 and a quick increase on r22. I noticed that if the two voltages were added together it would equal the supply voltage.

Thanks

Andy
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