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Old 8th December 2008, 01:49 PM   #961
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As above, the 500 VA 40V transformer will certainly be adequate for single channel Leach operation unless you are planning to use it in a nightclub at full power continuously into low impedance loads.

It should also have adequate margin for your Krellclone monoblocks although some would suggest slightly larger. Again low impedance loads and high continuous power point towards a heavier transformer.
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Old 8th December 2008, 03:14 PM   #962
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thank you for the help

Do you think that 37.5 VAC will be enough for both amps

I will not use loads lower than 4 Ohm and don't need more than 160 W per channel at 8 Ohm
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Old 8th December 2008, 03:29 PM   #963
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37.5 at 115/230 input gives you 39V on the usual 120/240, but even at rated output you only lose ~3.5 volts on your rails. You'll have a little less peak power, but otherwise you'll be fine.
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Old 8th December 2008, 03:43 PM   #964
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AndrewT
.......the 3pair PCB, gave 411W into 4r0 when mains was ~240Vac.
This would require a transformer between 400VA and 800VA for a single channel.....

Quote:
Originally posted by samoloko
.....but If you have to do It with one transformer per channel for the same stereo amp what rating of transformer you will chose
what Is the formula
can't you read?
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regards Andrew T.
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Old 8th December 2008, 03:57 PM   #965
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Andrew, hopefully the extended discussion helped a few lurkers. Sizing a transformer isn't as exact a science as choosing the resistors in a CCS.
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Old 8th December 2008, 05:32 PM   #966
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Default Xformer discussion

It was helpful to me.. thanks Bob!

Best wishes,
Keith
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Old 8th December 2008, 08:15 PM   #967
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Default Missing feedback capacitor for v5.6.10 Leach pcb??

Hi All,

I was comparing the schematics of the various Leach amps we have been discussing, and I noticed that a pair of capacitors in the feedback loop from one pair of the output transistors back to the driver stage seems to be missing. In the latest schematic on Prof. Leach's web site, these are designated C19, C20 and are specified as 0.01 uF, 50 V film caps. These caps correspond to C301 and C302 on the new proposed combo board "Leach power stage." But on the older board, version 5.6.10 from Jens, I would expect to see a similar cap across R41 and R62, and I don't find it. Am I missing something, or isn't it needed in that version??

Best wishes,
Keith
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Old 9th December 2008, 08:35 AM   #968
Ginum is offline Ginum  Lithuania
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Default Re: Missing feedback capacitor for v5.6.10 Leach pcb??

Quote:
Originally posted by kpmadden
Hi All,

I was comparing the schematics of the various Leach amps we have been discussing, and I noticed that a pair of capacitors in the feedback loop from one pair of the output transistors back to the driver stage seems to be missing. In the latest schematic on Prof. Leach's web site, these are designated C19, C20 and are specified as 0.01 uF, 50 V film caps. These caps correspond to C301 and C302 on the new proposed combo board "Leach power stage." But on the older board, version 5.6.10 from Jens, I would expect to see a similar cap across R41 and R62, and I don't find it. Am I missing something, or isn't it needed in that version??

Best wishes,
Keith
Hi,
Prof. Leach about "Low TIM Amplifier" wrote:
"...A current limiting circuit can be inherently unstable and cause oscillations under limit conditions. This can be understood by a simple heuristic argument. Once an overload occurs, the limit circuit is triggered, and the overload condition is eliminated. This cause the limit circuit to deactivate, at which time the overload condition reappears. This is a vicious cycle which can manifest itself as oscillations. Capacitors C15 and C16 help prevent this problem in the VI limiter of Fig.1. In addition, the limiter threshold is set so that it is improbable that the limiting function wil be activated under normal signal and load conditions."
Audio, February 1977
C15, C16 in "Low TIM Amplifier" is same as C19, C20 in original "Leach Amp" or C35, C42 in version 7.4.6 from Jens
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Old 9th December 2008, 11:26 AM   #969
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Congratulations, you found an oversight that nobody else picked up in two years of building this amp.

Since nobody seems to have complained that their amp behaves strangely in protection, it's probably safe to assume that the caps are not strictly necessary. Further into the quoted section Prof. Leach mentioned that bypassing all of the sense resistors caused undesirable behavior.

If you are worried about oscillation in and out of protection (you probably don't need to be for home use), you have two options:

1. Leave protection out completely. Not too unreasonable given how rugged the output stage is usually built and if care is taken to avoid shorting the outputs

2. Place the 10 nF caps across R42 and R62 on the bottom of the board.
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Old 9th December 2008, 11:57 AM   #970
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You may also have noticed that Jens has added all of the output stage coupling caps, which I believe helps stabilize the amp, to each output bjt.
The main question which always puzzled me was raising the rail voltages so far above the Low Tim amps original specs. Is the adjustment for rail voltage at R3, R22 to keep the protection circuit in check or some other function? My current amp seems to be quite stable and unoffensive at very high volume pushing 70+ volts on the rails. Just how high could the rails be pushed in this current design before design changes are implemented?
Seems Dr. Leach covered most of the bases quite well 25 years ago.

Tad
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