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Kilowatt 27th December 2001 11:08 PM

Leach Amp
 
Here's my progress

I'm building 2 Leach amps for my home stereo system. Eventually I will probably upgrade to 4. I'm done with the power supply, and I'm nearing completion of one channel. My power supply measures +/-60.5V at no load (40-0-40 1500VA toroid, 11200uF per side). This should give about 120W into 8 ohms and nearly twice that into 4 ohms according to the article about the amp. That ought to be plenty for my subs. Has anyone here built the Leach amp? How does it perform into 4 ohms? I would tend to think that it would try to put out more like 300 or more watts into 4 ohms instead of 240 because of Ohms Law, which would probably be too much for the outputs. Am I right?

I'll probably build some ESP project 03 amps for mid and trebble instead of tube amps because big tube amps are too expensive for me to build.

I am reading G. Randy Slone's High-Power Audio Amplifier Construction Manual and will attempt to build the 550W amp that's in there once I'm done with Leach amps, but before I try for my big amp.

Damon Hill 28th December 2001 12:23 AM

You might be pushing your luck to get 300 watts out of even two
pairs of MJ15003/4s; perhaps try three pairs and slightly higher
values of emitter resistors. You'll need a higher power supply
voltage too and that's where you'll be pushing the margins.

An alternative might be multiple sets of MJ21193/4s; anyone have
other recommendations? How about TO-3 versions of the MJE15030/31? (see next paragraph)

I've got a monster wind tunnel heat sink that's hard-wired for up
to five pairs of output devices and I'm tempted to try out an old
batch of MJ15003/4s to see if I can squeeze 500 watts continuous. I'll have to get a transformer rated for that voltage/power.

I've tested my Leach amp at full power into 3.75 ohms, but never
for more than a few minutes--the heat sinks are too small and
poorly ventilated for that kind of power. I'm rather pleased with
the sound of the ver 4.5 boards and can't wait to get a set of
matched MJ21193/4s installed, to see that improves the sound.

Probably can't go wrong with Sloan's designs, either. Just be sure
you've got lots of heat sink area and air flow.

Kilowatt 28th December 2001 01:19 AM

I'm not out to get much more than 200W from each Leach amp because that's what my speakers are rated. I've got 4 Sony Xplod 12s. They're 200W RMS, 4 ohms. I'll have two of them in series for each of my 2 Leach amps (120W/channel), and when I make my other 2, there will be a Leach amp for each sub (at least 200W/sub). So the 2 x MJ15003/MJ15004 with my supply should be fine, but I might add more outputs in parallel, so I can get more power out reliably, maybe even into 2 ohms.

I don't know well 1500VA will supply 800W worth of Leach amps, and I might want to add more filter caps too.

I wasn't thinking when I made my ver. 4.5 boards, and I ended up with my foil pattern a mirror image of the original artwork, because I just printed it out (foil side), layed it ink-side-down on my board and developed it like that. Because of this, I have had to turn all my transistors around, so the drivers (TO-220 case) do not lay flat on the board as they are supposed to, and I had to bend the base lead on the metal can transistors the other way, but it shouldn't cause any real problems with the function of the amp. At least I hope not.

By the way, Damon Hill, when you tested you're Leach amp into 3.75 ohms, what was the supply voltage, and what was the power output?

I wonder if I could use 2N5684/2N5686 transistors for outputs especially for 2 ohm loads. Those things are beasts! 300W, 50A, 80Vceo. They're expensive, over $5 each, and I have 2 of each in the junk box, so I might as well try to use them for something.

djk 28th December 2001 07:32 AM

I have several factory built Leach amps in addition to home brew. The LSR&D 101 has a 90VCT 1KVA transformer with a pair of 20,000F cans for +/-63VDC and does 300W+300W@4R. A couple of these have been driving 2R for the last 10 years. LSR&D used 3 pair of MJ15011/15012 with one pair of MJE15030/15031 as drivers. The Super Leach from Heathkit, AA1800, used four pair series connected MJ15024/15025 and did 400W+400W@8R with +/-95VDC. The AB International 1100A uses the Leach topology with a tiered power supply and does 1000W+1000W@2R with only three pair of MJ15024/15025 with rails at +/- 0, 48, and 95VDC.

Damon Hill 7th February 2002 12:17 AM

For whatever it's worth, I've just replaced the MJ15003/4s in my
Leach amplifier with MJ21193/4s. Initial tests and listening
suggest, unsurprisingly, no difference in stability or sound
quality.

Both batches of transistors tested very close together for beta
at 1 amp, so matching seems to be a given. They replaced
a motley collection of unmatched devices with four different date codes.

The newer devices have a greater SOA, somewhat better
linearity, and a Ft of 4 Mhz vs. 2 Mhz; I suppose it may be possible
to modify Leach's design to take better advantage of the new
devices, but I'm not sure where to begin.

I recently upgraded my amplifier to the ver. 4.5 circuit and am
very pleased with the sound quality. Imaging and sound field
are much better, and a slightly strained and edgy quality from
the older 2.something circuit seems to have vanished; the sound
is almost effortless now.

Needs a reality check against some known high-quality amplifiers.
I may audition it on a friend's Martin-Logan electrostatics.


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