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Old 28th November 2013, 09:05 AM   #91
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Originally Posted by AJT View Post
imho, the test of time is the verifier, will people still build your amps say 20 years from now?

the leach amp designed in the 70's and still being built even today, isn't that something?
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Old 28th November 2013, 09:13 AM   #92
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There's more but for the start is OK.
Let's say enhanced VAS, etc.
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Old 28th November 2013, 09:39 AM   #93
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Do following steps for better SQ:
- if needed use Lundahl input transformer for un/balanced input operation
Now talking about sound quality, I've always been very wary of using transformers in the signal path. I see those are extremely expensive, even on ebay, and to me this type of thing is more what we'd find in tube amps, which to me is not something that rimes with sound quality, although some people swear by it.

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- completely omit T1, T6 input cascode
- replace R5, R20 by j-fet CCS
So you don't like the zeners there. What don't you like about that cascode config?

What would using a jfet there bring? How can that help the sound?

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- replace R1, R23 with zero ohm wire
This offers a little bit of separation between the input stages and the more ripply rails. Why would you do that? How could more noisy rails on the input stages improve the sound? I don't get that one.

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- change FB network values to 2,2 k/100 Ohm/2200 uF
- change R2, R21 to 680 Ohm
- change R9, R10, R14, R15 to 22 Ohm
- change R25, R33 to 22 Ohm
All this would simply undo what makes the leach amp a low tim. You would change the feedbacks, reduce the local feedbacks in several stages, which would increase the gain on each of them, and then change the main feedback loop as well. All of this is going exactly in the opposite direction from what leach was doing to reduce TIM. I'm sure you would likely reduce further the THD, but it would increase the TIM in the process. How can this be better? I don't see it. Can you explain the reasons behind those choices? And have you done any sims to compare all this?

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- overall use of better modern transistors
Well, that's in my plans for further versions, but this one for now is done with the idea of using what I have on hand.

I will eventually move on to the flat packs, although they're inferior for heat dissipation, we'll just use more pairs.

The leach amp was designed with one important thing in mind, "low tim", and that's a much harder distortion on the ear than HD. This goes way back to Matti Otala and Leach was applying what Otala has found out, and this obviously was right on target and it's done well.

What you propose seems to me to be aimed at further fighting the THD and not TIM. I don't think the majority of people are able to distinguish a 0.00.... distortion difference, and not if it's HD. However lowering the TIM has much more of an effect on what the ear can distinguish and it does thus improve sound quality. Have you read what Leach was explaining about how the design was done and why the choices were made that way?

I think using a different type of protection that allows removing the VI limiters' action on the drivers could improve things, because the potential remains there for the protections acting when not necessary. Even well calculated, there is a chance the protections can have an effect on sound.

Plus Obviously removing the output coil would improve things a bit, at least on measurement apparatus, most likely nothing a normal human can detect. Or the output coil could be included inside the feedback loop maybe, which would reduce its influence somewhat.

Beyond that, we may be able to improve the current sources as you suggest, but I'm not sure this is enough to really make any real difference.

I don't think a 0.000xx reduction in THD will matter. No one can hear that. But a 0.1 reduction in TIM is a lot more noticeable.

Please prove I'm wrong!!
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Old 28th November 2013, 09:41 AM   #94
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Let's say enhanced VAS, etc.
Looks interesting, and complicated. How does this work and what does it bring?

How can this be brought in the design properly?
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Old 28th November 2013, 09:55 AM   #95
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Steps to be taken improves SQ, each step practically tested, compared to your's basic sch configuration, confirmed by listening tests many times in recent years, now it's up to you what you will choose.

For instance if you don't know what CCS brings over plain resistor in LTP common node, than I see no reason to explain any further. Just telling you what will elevate this amp to higher SQ level.
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Old 28th November 2013, 10:03 AM   #96
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Now talking about sound quality, I've always been very wary of using transformers in the signal path. I see those are extremely expensive, even on ebay, and to me this type of thing is more what we'd find in tube amps, which to me is not something that rimes with sound quality, although some people swear by it.
Extremely expensive

It is in perfect synergy with your's design price level.

Lundhal line transformers are located in top high-end equipment like Linn Klimax, DCS, Jeff Rowland etc. and you're questioning SQ

OPA134 is a compared to Lundhal.
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Old 28th November 2013, 10:04 AM   #97
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Let's say enhanced VAS, etc.
your enhanced VAS falls into the 3transistor VAS category.
Many experts on this Forum tell us that 3transistor VAS are very difficult to stabilise.
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Old 28th November 2013, 10:13 AM   #98
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aside from using more modern transistors, more metal films, i would not change anything on the Leach topology....

if i want to use CCS, i'd build another amp instead...
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Old 28th November 2013, 10:23 AM   #99
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Leach Lo Tim already has CCS.
The tails of both LTPs are CCS.
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Old 28th November 2013, 10:24 AM   #100
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your enhanced VAS falls into the 3transistor VAS category.
Many experts on this Forum tell us that 3transistor VAS are very difficult to stabilise.
But guys all these features I counted are practically tested, no problems found with them, I would not lead spookydd to some odd ways. Aren't we all strive for better SQ, at least that's my priority. For instance if listening tests clearly shows degradation in SQ by using input cascode than there's no place for it to be there.
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