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Old 28th February 2006, 11:18 PM   #1
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Default Leach Amp turn on thump?

I'm curious, I've noticed my Leach Amp causes a fairly large (about +4VDC?) "thump" when I turn it on. It's definately audible. Also, about 10 seconds after I turn it off I get a "suck" where it causes the drivers it's connected to to pull inward and vibrate a little bit. The "suck" isn't as large as the turn on "thump". Any idea what generally causes this sort of problem? Thanks in advance =).
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Old 28th February 2006, 11:59 PM   #2
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Howdy m0tion,
Most likely there are no "bleeder" resistors in your power supply.
Take a read of Dejan V. Veselinovic's treatise on power supplies here:
http://zero-distortion.com/start.htm
under the heading
Designing your own power supply
with some practical examples

to learn about this arcane subject.
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Old 1st March 2006, 12:56 AM   #3
Leolabs is offline Leolabs  Malaysia
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Well,if it is a toroidal transformer,big thump is normal.
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Old 1st March 2006, 01:13 AM   #4
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You're right, I'm not using bleeder resistors on the PS caps. Also, I am using a toroidal transformer. Is this fixable? I'd like to be able to use this amp in an active xover situation and without a crossover to protect the drivers from DC ...
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Old 1st March 2006, 01:20 AM   #5
pooge is offline pooge  United States
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The Leach amp should not have a turn-on thump. From the FAQ on his web site:

Why didn't you use current source tail supplies for the diff amps?

I had intended to use current sources in the beginning. However, when I started laying out the circuit board, I found that they added a great deal of complexity to an already dense part of the board layout. Therefore, I opted for resistive supplies to simplify the layout. I never had any problems with the resistive supplies. The amp seemed to work perfectly with them. An additional bonus is that the matching of the tail currents in the two diff amps is not a function of the matching of transistor parameters.

The turn-on characteristics of the diff amps determine the turn-on characteristics of the amplifier. With resistive tail supplies, the diff amps turn on very gracefully at a rate set by the RC time constants in the tail supply circuits. This leads to a thump free turn on, eliminating the need for output relays to protect the speakers. If transistor current source tail supplies were used, the current source transistors tend to turn on abruptly as the power supply voltages come up, which can produce thumps in the loudspeaker. This problem is even worse if one tail supply transistor turns on before the other.
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Old 1st March 2006, 01:32 AM   #6
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Yes it is fixable. I have a Leach SuperAmp with a 80-0-80 15A torroid that makes not one bit of turn on or off noise, thumping etc.
Mr. Veselinovic explains proper use and the math of how to remedy this situation for you. Give it a read.
With all due respect Mr. Leolabs, "if it is a toroidal transformer,big thump is normal." only if you fail to implement proper "bleeder" resistors across the caps on the power supply, Leach amp or no.
Let's not propogate old wives tales as fact, please.
Mr. pooge is also right on with info from the Leach web site.
Re-read all the web site
http://users.ece.gatech.edu/~mleach/lowtim/
for more detail about "protect the drivers from DC" and other catastrophic failures that simply do not occur with this amp if built following the construction details.

Regards
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