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Old 28th January 2009, 07:06 PM   #1131
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Can't tell from the photos...

Anybody using TO220 type heatsinks on the driver transistors in this amp project? If so how big should they be?

-Joe
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Old 28th January 2009, 07:14 PM   #1132
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If you don't want to use the big holes Jens provided to attach the drivers to the main sinks, you can use something like Mouser P/N 532-504102B00.

Early versions of the board mounted the drivers and heat sinks on top of the board. There was a potential issue with interference between the sinks and the snap in caps. This version solves that issue by allowing the devices to mount to the main heat sink.
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Old 28th January 2009, 07:49 PM   #1133
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Quote:
...you can use something like Mouser P/N 532-504102B00.
I looked up the Mouser part, and the dimensions are listed as 19.812 mm L x 17.78 mm W x 21.59 mm H

The reason ask is because I already have some TO220 'sinks measuring 18mm x 10mm x 20mm.

Would these be large enough for the drivers in a 120-150W extended Leach amp?

-Joe
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Old 28th January 2009, 08:02 PM   #1134
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That sounds like the same basic part. The difference could be tolerances in part and your measurements.

I've had my Leach amps running for almost 10 years at 60V rails with similar sinks with fins only about 12 mm high. Yours should be fine. Be careful that you don't short the sink to ground or the snap in cap case.
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Old 30th January 2009, 06:48 PM   #1135
clm811 is offline clm811  United States
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Default Power supply question

Guys,

Granted, Dr. Leach has recommended 12,000uf for the main PS rails in his writings, but I'm guessing that a few here have experimented with this(more or less)...

In a class AB amp to be used with a (sub)woofer system, what do you find to be the minimum amount of PS filter capacitance that will provide good bass power and control (assuming the user has already optimized the woofer design and placement)?

TIA,

-Chas
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Old 30th January 2009, 09:27 PM   #1136
AndrewT is online now AndrewT  Scotland
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Hi,
the PSU smoothing capacitance is linked to the passband of the amplifier.
PSU RC >=sqrt(2) * NFB RC
NFB RC >=sqrt(2) * Input high pass RC.

If the input RC =80mS (1u5 & 51k), then NFB RC >=113mS. 1u5F & 53k
If the NFB RC =113mS (150uF & 750r), then the PSU RC >=160mS.

For 8ohm speakers C must be >=+-20mF/ch.
For 4ohm speakers C must be >=+-40mF/ch.

For very high power amplifiers, a second rule becomes applicable.
>=2mF/Apk of output current, to reduce cap ripple to an (arguably) acceptable level.
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Old 31st January 2009, 01:23 AM   #1137
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Chas,
The depth of your money stack also comes into play here. Good quality 75 plus volt capacitors are not cheap. I chose to use 75000 uf per channel and the results are good. Using a film cap to parallel the electros has favorable results. If the music continues to play for about a minute or more after you turn off your amp you can bet there is adequate reserves for most musical transients even as sub woofer duty.
Tad
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Old 31st January 2009, 01:28 PM   #1138
rob3262 is offline rob3262  United States
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Tad,
the results are good, or outstanding? I had planned on 22,000 per 70V rail.

I just can't quite figure out AndrewT's formula and how it relates to the rail capacitance (I know, go back and read the stuff.. ) I popped for the 1000VA tranny to help mitigate my other poor design choices
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Old 31st January 2009, 02:25 PM   #1139
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by rob3262
I just can't quite figure out AndrewT's formula and how it relates to the rail capacitance
it's not mine. I simply adopted it when I discovered it worked well on all my amps.

Pick your LF roll off frequency (F-3db).
Select the input filter components to achieve that frequency.
Then use the two formulae to determine first the NFB DC blocking cap value and secondly determine the smoothing capacitance to suit your speaker impedance.
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Old 31st January 2009, 03:55 PM   #1140
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Rob,
There is a lot of exceptional information that Andrew provides to us that I do not understand. But if not for his and other devoted members input I would be farther in the dark than I am. When I first started reading posts here I did not know the difference between a mosfet and bjt transistor.

Slowing absorbing little breadcrumbs along the way may one day lead to construction of a truly monumental amplifier or some other using item. I only wish I had started 45 years ago

I think we have a truly great pcb layout here, thanks to Jens and others, from which we can construct and amp the quality of which most of us could not afford at retail. I have read nothing but praise for Dr. Leach's protege.

Thanks Tad
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