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Old 19th October 2006, 04:39 PM   #1
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Default leach amp vbe multiplier

I Just assambled a Leach Amp version 4.5. I made the boards from Mr Leachs original layout. It works, but when I try to adjust the bias current, I get no response, the current just stays the same.

Has anyone encountered the same problem, or has anyone an explanation for this?

I searched the forum, and in this thread

Leach amp question

someone had the same problem, but no one responded to his post

I built everything very carefully and i`d really like to finisch the amp, but i can`t figuere out whats wrong. Please help

Thanks,

Joe
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Old 19th October 2006, 05:49 PM   #2
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Removed the short circuit jumper in parallel with C12?

"Double check the diode assembly. If a diode is backward, the output transistors can blow. You can check the diode polarities one at a time with an ohmmeter. The ohmmeter will not have enough test voltage to forward bias all 4 diodes simultaneously. If there is a short circuit between a diode lead and the heat sink metal, the amplifier will not work and the output transistors can be blown."
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Old 19th October 2006, 08:15 PM   #3
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Thanks for your reply Acenovelty

I checked everything and I found nothing wrong. I already installed the output transistors and the amp does plays music, just the driver transistors are getting a little hot (a little too hot I would say), And I have a little less than 100mV DC on the output.
But the thing that I really don`t understand is that absolutely nothing happens when I turn P1... I connected a signal generator to the input and observed the output waveform on a scope, and it stays as it is when i try to adjust the bias.

Here is the circuit diagram
http://users.ece.gatech.edu/~mleach/...aphics/ckt.pdf
and here the board layout
http://users.ece.gatech.edu/~mleach/...cs/layouts.pdf
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Old 19th October 2006, 08:40 PM   #4
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Defective pot?/ 2K ohms, right? Confirm solder/traces from the pot to other components/cold joint etc. If possible measure pot resistance with DVM to confirm value.
At least you have music. The transistors should be only warm to the touch unless driven hard.
"Do the leads to the heat sinks connect to the right points on the circuit boards? I recently saw a student's amplifier where the base leads for the npn and pnp output transistors on one channel were reversed on the circuit board. The bias potentiometer would not adjust the bias current in that channel."

HTH
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Old 19th October 2006, 09:00 PM   #5
Arius is offline Arius  United States
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Hi Joe,

1) Uhh, what are you trying to do by adjusting P1 with a signal generator at the input and a scope at the output? You should set the bias current without any input and by monitoring the voltage across the output emitter resistor(s).

2) 100mV DC you say at the output? Is this without anything on the input? If so, then that's pretty high for an output DC offset. Might need to check your input stage to see if everything's properly balanced or you may have grossly mismatched input transistors.

Do note that the bias current setting affects the temp of your output devices and not the VAS stage drivers. Good luck with your amp.
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Old 19th October 2006, 09:12 PM   #6
mikeks is offline mikeks  United Kingdom
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Default Re:

Quote:
Originally posted by joe mobile
..leach amp vbe multiplier...
It's not a vbe multiplier..
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Old 19th October 2006, 11:38 PM   #7
AndrewT is offline AndrewT  Scotland
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Come on Mike, please explain.
You have my permission to use more than one sentence
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Old 20th October 2006, 12:12 PM   #8
mikeks is offline mikeks  United Kingdom
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The voltage across Q7 is not a function of Q7's Vbe.
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Old 20th October 2006, 12:16 PM   #9
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Check if Q7 is NPN for sure.
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Old 20th October 2006, 01:46 PM   #10
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i build a pair of those a few years ago......and if I remember... you should use a jumper around this transistor to make previous test on the board if the jumper is still in place you will not be able to set bias it will stay to minimum and will operate in class B
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