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Old 27th March 2010, 06:44 AM   #1
GeeVee is offline GeeVee  Australia
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Default Pass Aleph P 1.7 help

Hi all
I have just comleted my 1.7 boards, and have a voltage imbalance issue that I cannot locate.

Both boards are doing exactly the same thing, within a volt or so.

Basically, one half is runing at approximately 25v, and the other half at 36V.

Since I made the boards myself, and both boards have an almost identical problem, I thought OK, it is a track error layout. However after hours and hours of investigation I cannot find any track error, or incorrect part placement.

I know the chances of both boards having the came comonent failure is very rare.

With respect to amplification, there is not much there at all. 450mV in, about 600mV out. Sine wave is very clean though.

Tried the same with a square wave at 1khz, very rounded leading edge.

Anyway, atatched is a sketch showing the voltages measured around the fets etc.

Any help will be greatly appreciated.

Regards

George.
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Old 27th March 2010, 01:52 PM   #2
juma is offline juma  Germany
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Check resistors R17/R18 and R35/R36.
They set the current through MOSFETs.
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Old 27th March 2010, 10:39 PM   #3
GeeVee is offline GeeVee  Australia
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Quote:
Originally Posted by juma View Post
Check resistors R17/R18 and R35/R36.
They set the current through MOSFETs.
Thank you very much Juma

I will check.

Regards

George
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Old 29th March 2010, 11:55 AM   #4
GeeVee is offline GeeVee  Australia
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Default success!!!

Hi Juma

Just a quick note to say that I beleive that I have found the problem.

I had R5 and R6 swapped around. (The 221R and the 221K)

Unfortunately when I made my boards I labelled them incorrectly on the board.

Anyway, the Drain voltages are all within 0.3V (approx 30.5V)

Drain voltages on T1 AND T8 are 11.25V, which by my calcs appears to be about right.

Gain is now excellent, looking at it on the CRO.

Thank you for taking the time to respond
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Old 29th March 2010, 12:09 PM   #5
juma is offline juma  Germany
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I'm glad you found the error
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