leach amp vbe multiplier

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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

http://www.diyaudio.com/forums/showthread.php?s=&threadid=2925&highlight=

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
 
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."
 
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/lowtim/graphics/ckt.pdf
and here the board layout
http://users.ece.gatech.edu/~mleach/lowtim/graphics/layouts.pdf
 
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
 
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.
 
Much better:

:cool:
 

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hello everybody

To Arius: I did not try to adjust the bias by that, but if I turn P1 the waveform should change I think, thats why I mentioned that.

To Acenovelty: The pot is ok too and it is 2K, and the leads to the transistors are in the right places too.

to Darkfenriz: I checked it, it is mpsa 06, so it`s npn.
and to Audiofan: the jumper is removed.

AndrewT:
I don`t know that much about electronics and it`s my first amp, so I can`t explain that much, only what I measure and what I see that happens... But if you need to know something perticular to make an assumption what might be wrong I`ll try to explain it.

I`ll go on trying, if nothing helps i`ll just build a new one, good for practice...

Thanks to all for helping.
I`ll be away for the Weekend, so nice weekend to everybody

joe
 
I'd be checking for a change in the voltage drop across the Re for the voltage amp or a change in the voltage across the (disputed) Vbe multiplier. Also, is the drop enough to bias on the outputs or not. There was no mention as to whether the bias was low or high (measured across the output emitter resistors) and not changing, or none existent.


A quick run through of the various stage currents might point to what's working or not.

Regards, Mike.
 
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