why a cap and resistor at output of leach? - diyAudio
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Old 12th August 2004, 11:41 AM   #1
Wagener is offline Wagener  South Africa
diyAudio Member
Join Date: Mar 2004
Location: Paarl
Default why a cap and resistor at output of leach?

I am still battling with my leach amplifier. I am now only working on the one channel. The one which gives out dc from time to time.

First off. In the leach amp plans there is a 10ohm resistor and a 100nF cap connected in series which is to be connected at the output.

I do not have these two components at my output. What is the purpose of them?

I noticed this: When my amplifier idles (4-ohm load connected, input connected but without any signal, ie silence) the output transistors get extremely hot. This also causes the amplifier to give out DC. I found this out last night when I left for about 20minutes leaving my amplifier on with no music. When I got back my subs where making the typical dc noise and I noticed that the output transistors (and heatsink) were very hot. I could not touch it. I switched it off. connected Dc v meter and found that it once again gave out random dc.

I suspect that it is this overheating (while idling) that destroyed my other channel's output transistor.

Does it have anything to do with the resistor and cap that I don't have?

When I completed the amp I tested it on a oscilloscope I did not find any oscillation. I did however at that time still had the resistor and cap on the output. Removed it later because I believed it was just some filter.
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Old 12th August 2004, 11:48 AM   #2
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Join Date: May 2002
Location: Switzerland
This thingie is called a Zobel and is actually there to improve stability, i.e. avoiding oscillation !!!!


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Old 13th August 2004, 07:08 PM   #3
Wagener is offline Wagener  South Africa
diyAudio Member
Join Date: Mar 2004
Location: Paarl
I missed the part in the leach amp plans which clearly states that R50 and C25 (which I removed from the output) prevents parasitic oscillations which causes the output transistors to become very hot.

That was the problem with my amplifier all the time.

I feel like an idiot!!

Well I am relieved that I found the problem

Thanks for all the help and suggestions from you guys!
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