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Old 27th May 2004, 12:36 PM   #11
wopo is offline wopo  Austria
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Hi!

I built up the Leach-Amp and I can say NICE SOUND. Even the bass. The mids and the highs were reproduced very clear. The bass comes very clear and not bumpy. The reason for that might be a big power supply. I used a 500VA transformer with 2x66.000F reservoir capacitors. You can visit my homepage, were I present some pictures and the layout. Unfortunaltely it in German, but I plan to translate it into english. You will find it under DIY.




All the best

Wopo
http://homepage.univie.ac.at/Wolfgang.Postl/
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Old 27th May 2004, 05:33 PM   #12
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Send a message via ICQ to Damon Hill
I upgraded my old Leach amp from a modified 2.something
to the 4.4 and found it much improved; possible
oscillation on the old boards was suspected anyway.
I found the imaging much improved, a faint hum had
disappeared due to the newer grounding scheme, less
residual noise. I was a very happy camper.

I tossed in a few premium parts, matched resistors
practically down to the quantum level, and rewired
with some teflon wiring. Used MJ21193/94 outputs
as they're more rugged and a bit more linear.

One modification: a 1 uF film cap between the driver
emitters, as per Self and Sloan. Supposed to help
turn off the outputs and improve high frequency
distortion. I set bias for a minimum of crossover
distortion, there's a definite null.

I don't have any pictures of my amplifier; all I can say
it's mostly power supply...
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Old 27th May 2004, 05:46 PM   #13
pooge is offline pooge  United States
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Quote:
Originally posted by Damon Hill
One modification: a 1 uF film cap between the driver
emitters, as per Self and Sloan. Supposed to help
turn off the outputs and improve high frequency
distortion. I set bias for a minimum of crossover
distortion, there's a definite null.

Don't think this is an issue with the T-circuit output. That's supposed to help turnoff. I don't think Self ever mentioned the triple Darlington, at least that I could find. He mostly discussed regular two-transistor Darlington. Not as familiar with Sloan.

I'm curious about Leach's removal of the zeners in the input cascodes, and later reintroduction. I never heard anything about that history.

I had an input ground loop problem at first. I couldn't really do the fix he suggests, because I have my input jacks on opposite sides of the back panel, rather than close together. I didn't want to ground one jack through the other at that distance. I put in some Jensen input transformers, and all hum dropped out completely. In fact, the amp is almost silent, even with 107db sensitive speakers, until I put my ear up to the tweeter.
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Old 28th May 2004, 03:34 AM   #14
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Quote:
Originally posted by pooge
Don't think this is an issue with the T-circuit output. That's supposed to help turnoff. I don't think Self ever mentioned the triple Darlington, at least that I could find. He mostly discussed regular two-transistor Darlington. Not as familiar with Sloan.
If you listen you will see that the capacitor here improves the sound quality. It doesn't matter that it's a triple emitter follower. When it comes to making something sound better, thinking is good but listening is better.
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Old 28th May 2004, 03:45 AM   #15
AJT is offline AJT  Philippines
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i never had any hum problems in any of my amps, i used the star grounding system....

my power traffos have their secondary windings bifilliar wound to exactly locate the center tap....this practice was mentioned by jim bonjiorno in hi GAS Ampzilla article published in AUDIO magazine sometime in 1975.....
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the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
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Old 28th May 2004, 04:08 AM   #16
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Quote:
Originally posted by pooge

.... I don't think Self ever mentioned the triple Darlington, at least that I could find. He mostly discussed regular two-transistor Darlington. Not as familiar with Sloan.
Self just says triples are troublesome and prone to oscillation, so he would not consider using them. Says much the same about MOSFET outputs -- a bit of a head in the sand attitude, or head somewhere else perhaps. Slone follows the same line, but Slone actually does some SPICE modelling to show the potential instability, so I respect that more. Self just handwaves and calls them impractical.

Yet we have ancient designs like the JBL T-circuit from 1967 and the Leach that work quite well to this day. With care, triples work just fine.

While I greatly respect the methodology and objective findings Self has presented regarding the front-end design, his output stages are simply too conservative given the outstanding devices we have available today.

Quote:
Originally posted by pooge

I'm curious about Leach's removal of the zeners in the input cascodes, and later reintroduction. I never heard anything about that history.

....
It should make a slight improvement in PSRR, but I suspect a resistive divider with small cap filter would work almost as well.

Overall, the Leach amp is a superb, well-balanced compromise and much ahead of its time (1976). This amp is about as bulletproof a design as you can find, has been beat up by thousands of students for over almost three decades. No gimmicks, just solid engineering design and great sound.
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Old 28th May 2004, 01:46 PM   #17
pooge is offline pooge  United States
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Quote:
Originally posted by slowhands

Self just says triples are troublesome and prone to oscillation, so he would not consider using them. Says much the same about MOSFET outputs -- a bit of a head in the sand attitude, or head somewhere else perhaps. Slone follows the same line, but Slone actually does some SPICE modelling to show the potential instability, so I respect that more. Self just handwaves and calls them impractical.

As I understand it, "Self's" Complementary Feedback Pair output is also prone to oscillation. If I recall, his published "blameless" amp used EF output.


Quote:
Originally posted by slowhands

It should make a slight improvement in PSRR, but I suspect a resistive divider with small cap filter would work almost as well.
I was just curious why he removed them in the first place.


Quote:
Originally posted by slowhands


Overall, the Leach amp is a superb, well-balanced compromise and much ahead of its time (1976). This amp is about as bulletproof a design as you can find, has been beat up by thousands of students for over almost three decades. No gimmicks, just solid engineering design and great sound.
It seems like he's eliminated oscillation problems with this amp, despite the darlington's.
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Old 28th May 2004, 01:49 PM   #18
pooge is offline pooge  United States
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Quote:
Originally posted by Damon Hill
One modification: a 1 uF film cap between the driver
emitters, as per Self and Sloan. Supposed to help
turn off the outputs and improve high frequency
distortion.

Could you point me to references for this topics.
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Old 28th May 2004, 02:34 PM   #19
pooge is offline pooge  United States
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Quote:
Originally posted by Damon Hill

One modification: a 1 uF film cap between the driver
emitters, as per Self and Sloan. Supposed to help
turn off the outputs and improve high frequency
distortion. I set bias for a minimum of crossover
distortion, there's a definite null.
Quote:
Originally posted by Pooge

Don't think this is an issue with the T-circuit output. That's supposed to help turnoff. I don't think Self ever mentioned the triple Darlington, at least that I could find. He mostly discussed regular two-transistor Darlington. Not as familiar with Sloan.
I guess I made the mistake of speaking before re-reading. Leach says that turnoff of his output transistors is aided by running the drivers in class A instead of class AB. This has nothing to do with the stage being a triple.
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Old 28th May 2004, 03:33 PM   #20
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Wink What version of Self are you referring to?

Quote:
Self just says triples are troublesome and prone to oscillation, so he would not consider using them. Says much the same about MOSFET outputs -- a bit of a head in the sand attitude, or head somewhere else perhaps. Slone follows the same line, but Slone actually does some SPICE modelling to show the potential instability, so I respect that more. Self just handwaves and calls them impractical.

You might want to refer to the third edition of Self's book, as well as some of his more recent publications; in it his prime candidate for a load invariant amp, and the circuit with which he studies the extended beta devices is a straight EF3 T circuit, with the class A configuration for the drivers.

There are potential issues with stabilizing the T circuit because of the gain and bandwith of the transistors- the parastic capacitive load can make the circuit look like a Colpitts oscillator at very high frequencies. The oscillation, though, is local to the transistor, though it propagates through the rest of the circuit. But it's not a local loop to stabilize, unlike his dalliance/romance with the CFP, which is substantially harder to stabilize over a wide range of loads without an output inductor.

It's possible to make a CFP circuit that is stable with capacitive output loads with no output inductor, but only in my experience by using low GM devices (MOSFETs) as a class A driver, and having very low turn-off impedace for the upper devices (5 ohm driving the distributed gate load). The output stage was pretty brute force, having 8 each P and N channel four chip Magnatek iso modules.

The net output stage transfer function isn't as whimpy this way as a straight MOSFET follower; I'm sure CH would argue it doesn't sound as good, but the client was looking for bottom end control, as well as a smooth, clear mid and highs, and he was quite happy with the results.

Regards,

Jon
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