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Leach amp treble problem
Leach amp treble problem
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Old 18th February 2009, 06:13 PM   #1
fabix is offline fabix  Germany
diyAudio Member
Join Date: Feb 2009
Default Leach amp treble problem


I'm new to this forum but I think I'm at the right place asking about problems with the Leach Amp V4.5.
At the moment I build my own Leach Amp. One channel is already finished but the trebles are quite muffled. The rest works fine.
First I had no bass response but then I recognized that I forgot the jumper over C6B.
After solving this problem I saw that D16, a zener-diode, is in the wrong way. I changed that but the problem didn't got better.

My speculation is that maybe one transistor in the Input-stage is blown. Could this be possible?
What other reasons could cause this problem?

Thank you,
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Old 18th February 2009, 06:26 PM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
measure the voltage across each of the LTP emitter resistors.
measure the voltage across the Zin resistor.
measure the output offset.
regards Andrew T.
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Old 18th February 2009, 06:35 PM   #3
pooge is offline pooge  United States
diyAudio Member
Join Date: May 2003
Location: Northern Va.
There is no way to answer your question without more information. Perhaps you are not used to the subjective impression of a treble that isn't glaring. We don't know that. Also, we do not know what you've checked yet, other than what you posted.

Have you measured frequency response or any of the other tests Leach recommends?

Have you made sure all the parts used are correct, such as the cap in the input? Diagnosis cannot be performed from afar without solid info.
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Old 18th February 2009, 08:42 PM   #4
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
You might want to check the capacitors C8 and C9 - if they aren't correct it might roll off high frequencies in the feedback loop.

Also check C1 is correct, this forms a low pass filter with R2, if it is incorrect it may be rolling off trebles.

You also need to check Q3, Q4 and Q6 are not damaged by the incorrect installation of the zener.
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Old 18th February 2009, 10:04 PM   #5
fabix is offline fabix  Germany
diyAudio Member
Join Date: Feb 2009
I think the sound hast to be a lot better because there isn't even a bit of brilliance. It's just muffled.
The parts are all correct, I think. Every transistor is right(NPN, PNP) and all the diodes are in the correct position.
With a sine wave on the input it looks fine on all frequencies. There is also a lot of power in the amp. There is no distortion or anything, just muffled.

Is there a typical mistake which can cause such a sound?
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Old 19th February 2009, 01:40 PM   #6
fabix is offline fabix  Germany
diyAudio Member
Join Date: Feb 2009
So, I just solved the problem.

I changed Q3,Q4 and Q6 but nothing got better. Than I soldered out C1, so that there is no lowpass but even then nothing got better.

Finally I changed the loudspeaker and the cable and everything works fine. Such a great sound and so powerful. I like the amp already.
Now, I just have to finish the other channel, anodize the case and search a preamp for full power.

Thanks to all who tried to help me with my quite embarassing problem.
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