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Old 11th October 2004, 12:51 PM   #1
foxyb is offline foxyb  United Kingdom
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Question One Channel of JLH 80w Mosfet Louder than other

I have build that jlh 80W mosfet amplifier, from the ETI articles in the mid 80s, using Hart provided boards, and upgraded components.

I have replaced most of the transistors in the differential input stage, the voltage amp, but NOT the output stage, as they are relatively expensive SJ50, SK135 mosfets.

One of the channels is about 20% louder and may have a higher (correct?) frequency content than the other. The volume difference can nearly be adjusted out with the aid of a 2k2 trimmer pot. It has been like this since I built it, and I have ignored it till now.

To fix this, do I have to have access to a variable frequency voltage source and scope (like I did when I built it), or is there a more basic series of checks I could do?

If it needs specialist AC sources and scopes, then it will have to go to a local hi-fi repairman, whom I will have to show the circuit diagram and hope for the best.

The circuit is a 3 stage ac coupled amp with a differential input with active loads, a voltage amp with active loads, and mosfet emitter follower output stage. It looks like it has 2 stage feedback, a loop from output, and another from the input to the emitter follower.


Any assistance would be much appreciated, as in spite of the volume difference it does sound very nice.

Regards,
John
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Old 11th October 2004, 05:07 PM   #2
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Maybe this is obvious, but did you check to make sure all resistors are the proper value? Maybe a wrong value slipped in there during assembly.
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Old 12th October 2004, 02:14 AM   #3
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Hi John

Inspect the accuracy of the feedback resistors...

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