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Old 7th August 2003, 01:45 AM   #1
name773 is offline name773  United States
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Question Gilmore amp troubleshooting

built a Gilmore headphone amp and it puts ~1.47v dc at output, w/o an input signal. some resistors were hard to find. here's a schematic of how i built it Click the image to open in full size. the matching isn't very exact in the output stage, any help is greatly appreciated, let me know if you want voltage readings or anything else thanks
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Old 7th August 2003, 06:19 AM   #2
name773 is offline name773  United States
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here's the schematic i tried to build it from Click the image to open in full size.
and one more thing , the 4 2sc1815's on the output stage get hot, and their adjacent 25ohm resistors get really hot. everything else is room temp.
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Old 7th August 2003, 11:55 AM   #3
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All the resistors need to be well matched. The 500 ohm
current source resistors need to be extremely well matched.

The output stage should pull 60ma from each power supply
rail, and does run fairly hot.
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Old 7th August 2003, 06:12 PM   #4
bocka is offline bocka  Germany
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Hi!

Getting hot seems O.K. to me unless the temerature of the output transistors does not rise above 80C or so.

This amp is very sensitive to unbalanced tail currents from the dual FETs. Try to measure the voltage at the 500R resisors near the LED's. They should be exactly the same. Different voltages will be amplfied with a factor of 16.

Thermal coupling of the LEDs and the 2SA1015/2SC1815 is required. Mount the LED and the Transistors together as close al possible. Match the LED's and Transistors and use 1% metal film resistors. BTW, is the -I input of your amp tied to ground?
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Old 7th August 2003, 06:18 PM   #5
bocka is offline bocka  Germany
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Did not see the servo loop...

Seems that the servo loop does not work. Is your amp wired correctly?
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Old 7th August 2003, 09:55 PM   #6
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Yes the servo loop is correct.

Hundreds of these have been built.
For sale too at www.headamp.com (not me)

check out all archived threads at
www.headwise.com and
www.head-fi.com

look too for sijosae's miniature versions.
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Old 7th August 2003, 10:21 PM   #7
bocka is offline bocka  Germany
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Hi kevin,

I didn't meant that the circuit with the servo loop will not work ( it does, a simple integrator...) but if you have a output voltage of about 1.47V this might me caused by a not properly working servo loop, maybe incorrect grounded or maybe two different LEDs used or ???. I would check the output voltage of the integrator first, if it's in the range of +/- 10V this is OK. If it's out of this range the servoloop cannot work.
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Old 8th August 2003, 12:23 PM   #8
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If the servo is at either power supply rail, then it has
done all it can, and can compensate no more. So you
have to figure out which side is low and change out
resistors to fix it.
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Old 8th August 2003, 06:52 PM   #9
name773 is offline name773  United States
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you guys won't believe this. the first datasheet i read on the 2sa1015's said flat part facing you left to right ebc. the second one i read (today) said ecb. so i checked and mine were ecb. thank you both so much for all the help. (btw, i followed your advice, matched the 500r's, diodes, and bypassed opamp supply rails and got 1.32v, transistors in wrong!) thanks again
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