PassDIY and Leach paper

[lumanauw]

After reading Leach paper, I get impression of the importance of Re in differential stage. It is to advoid TIM (Differential overload), so Re should be about 100-300ohm attached to source/emitor of differential pair.

But looking at all the designs by Mr.Pass, it never uses Re at all. Starting from A40, ONO, to ZenV7.

Does the issue of TIM is not having any relevance to good sounding power amp? All Mr. Pass designs are good sounding, but not having Re at all.

[Christer]

Few amplifiers use as much emitter degeneration as the Leach,
and some use none at all, as you have noticed. While a high
value extends the linear region and improves overload protection,
it also has drawbacks such as lowering the gain, so it is as with
most things, a compromise. The Leach is a quite old design, using
rather slow output devices, for instance. An amplifier with faster
devices and otherwise designed for higher bandwidth may not
need as much overload margin as the Leach, assuming the input
is properly LP filtered.

[Nelson Pass]

Re is not the only place where you can degenerate the signal,
although it has appeared in my designs at various times.

More often than not, my circuits are simple and have so low
an open loop already, that they don't need degeneration to
lower the open loop gain. Like anything else, try it both ways
and see what you like.

[Bricolo]

Maybe 2 stage designs (Pass) don't have as much gain "margin" so they can't use as much degeneration.
But 2 stages amps have certainly less TIM, due to the simplier circuit path

Am I right?

[Nelson Pass]

Originally posted by Bricolo
Maybe 2 stage designs (Pass) don't have as much gain "margin" so they can't use as much degeneration.
But 2 stages amps have certainly less TIM, due to the simplier circuit path Am I right?

That's the idea, anyway. Personally I think the emphasis on
TIM is too high. Real life measured signals in audio material,
vinyl and CD, don't show the kinds of slew rates that would be
postulated to overdrive the input stage (played at reasonable
levels, of course). This stuff has been looked at by myself and
Peter Walker, both published elsewhere, and supposedly for
TIM to be a problem, it requires slew rates to exceed 1/10 the
rating of the amplifier. As a practical matter, this means that
an intelligently designed amp at 100 watts and 15 V/uS shouldn't
have anything to fear.

At the same time, there have been amps rated at 1,000 V/uS
that didn't go anywhere, so even if it is a factor, it clearly isn't
the only one. The same is true of THD. There have been a
number of amps in the past that measured THD in a few parts
per million that were poorly thought of.

[djk]

" The Leach is a quite old design"

And the 300 ohm resistors are a very recent addition.

"Personally I think the emphasis on
TIM is too high. "

I'm old. So old I own some of those black things they call 'LPs'. Some of those are even scratched.

On a run-of-the-mill amplifier the scratches can seem to last forever, a broken-glass hall-of-mirrors effect.

On the Leach the scratch is a short 'tic', and its done. On the run-of-the-mill amplifier the scratches are totally mixed in with the music. On the Leach the scratch reproduces up front in a different plane from the rest of the music. You have to hear this to totally understand.

It may not be TIM that makes the run-of-the-mill amplifier sound so poor.

It may not be low TIM that makes the Leach sound so much better.

I hear the newest Leach boards sound much better. Maybe my 25+ year old ones will fail and give me an excuse to find out.

[lumanauw]

Like anything else, try it both ways

I've made a simple experiment, about placing Re or jumper it. Re is 100ohm. The result, with Re the sound is dull, without Re, the sound is more detailed. That is why I ask this question. To me, without Re is better sound. But reading leach paper it gives me a slight fear of TIM.
Thanks Mr. Pass, for the explenation. What is really the sound of amplifier having TIM?

Is this "Re eliminating" can be applied also in VAS? I see in A40, Mr. Pass do not use Re in the VAS transistor. If I want to eliminate Re in VAS transistor, assuming the Vbe is 0.65V, what is the voltage drop in the differential collector resistor? Should it be more than 0.65V or less than 0.65V?

[Nelson Pass]

For every diff pair, there seems to be a sweet spot, true of the
Aleph P/ BSOZ and all others. If it sounds better, why worry
about TIM?

[GRollins]

One word of caution--if you intend to build a circuit that creates a balanced signal from a single ended input, you will find that putting too much resistance between the Sources/emitters/cathodes will interfere somewhat with the phase splitter aspect of a differential. Otherwise, have at it.

Grey

[jam]

How? Please explain that.

Regards,
Jam