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Old 14th December 2004, 11:10 PM   #1
Vuamp2 is offline Vuamp2  United States
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Default verified Audio improvement suggestion on my Leach Super Amp. improve idea?

All,

I am looking for verified Audio improvement suggestion on my Leach Super Amp.

You can see picture of my am at http://community.webshots.com/user/vuamp2.

The components I used match exactly to the list recommended in the Leach Super Amp site.
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Old 14th December 2004, 11:35 PM   #2
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Hi Vuamp2,
What's to improve? Looks like a beauty. The horizontal fins on the heatsink may be a small problem, but they are so big I doubt it.

Prosit
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Old 15th December 2004, 02:03 AM   #3
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Default armchair engineering suggestions to be sure...

The suggestions on C6 aren’t up-to-date; nonpolar Al electro is the only option at these values, series polar caps have been shown (by Cyrill Bateman) to be much worse for distortion, Cyrill also recommends larger V rating for the nonpolar caps, possibly series 2 nonpolar for best possible with electros

If you’re using these in a system with a subwoofer, you could consider raising the corner freq on the main amps to the point that metallized mylar or even polypropylene caps could fit – a DC servo or even DC coupling (with the addition of some offset trimming adj) are other options


I would consider adding a pair of resistors from the amp output to the diff pair bias Rs to get a bootstrap constant current bias rather than modulating the diff pairs with the common mode voltage and hoping symmetry cleans up the even order distortion

Overall I would say the amp has too little open loop gain, I realize that some of that is attributable to the low TIM design concept – but I think better options exist

While the V gain may be low by design (and largely through the application of local, linearizing, emitter degeneration), I really don’t think the current gain has any excuse for being so low, I would expect the linearity to be severely compromised by the nonlinear load presented to the VAS

Without addressing the low current gain, I doubt improving the input and vas cascode/cascade circuits would help much

Leach Super Amp schematic:

http://users.ece.gatech.edu/~mleach/...mp/circuit.pdf

at the Leach Super Amp page:

http://users.ece.gatech.edu/~mleach/superamp/
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Old 15th December 2004, 06:07 PM   #4
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What?
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Old 15th December 2004, 11:59 PM   #5
Vuamp2 is offline Vuamp2  United States
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Thanks all for the comment so far. Please keep it comming.

Some how the URL in my original post does not work. If you have problem with it try this link
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Old 18th December 2004, 12:38 AM   #6
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Why not two of these for c6 in a perfectionist design?:
http://www.partsexpress.com/pe/pshow...=027-447&DID=7
The voltage rating is much higher than necessary but I don't know of lower voltage types at a reasonable price, I'll keep looking.

Most higher voltage polar caps in this capacitance range self resonate around 20 kHz, and are inductive above. I've not measured the smaller electros specified in the Leach design but I'd take an educated guess in the 100s of kHz range where the amp still has enough bandwidth to cause problems:
http://www.capacitors.com/picking_ca...rs/pick8lg.jpg

The design obviously works but I wonder if this was taken into consideration? This combined with the other flaws of electros is a good reason to consider these poly caps in a perfectionist design.

Thoughts? This is probably overkill?
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Old 18th December 2004, 01:51 PM   #7
PB2 is offline PB2  United States
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I agree with jcx about raising the corner frequency if that is acceptable, and the use of a DC servo or DC coupling as better alternativs. The big poly cap would be for the case where one wants to completely follow Leach's specification.
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Old 19th December 2004, 10:58 AM   #8
Vuamp2 is offline Vuamp2  United States
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Quote:
Originally posted by PB2
I agree with jcx about raising the corner frequency if that is acceptable, and the use of a DC servo or DC coupling as better alternativs. The big poly cap would be for the case where one wants to completely follow Leach's specification.

PB2, have you try this out yet? if not, what would yourexpect the improvement going to be?
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Old 20th December 2004, 09:44 PM   #9
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Hi Vuamp2,

Very nice amplifier by the way!

I've not built or worked on any of the Leach designs but have followed them from the beginning. It's been on my mind to try poly caps in the feedback location in the amplifiers that I'm currently using for several years. I'd probably look for ways to get the required value below 100 uF, probably scale the feedback network and live with a bit more DC offset.

It's interesting that the DC balance of the +/- diff amp input is only valid with AC coupled sources since there is no input blocking cap. I believe that grounding the input will upset this balance and increase the DC offset at the output.

The caps that I refer to are physically large with their much higher working voltage but I don't know of a better solution, other than servos or DC coupling.

We should ask, as in the other thread where I posed the question as to why distortion increases at HF, why does distortion increase at low frequencies even below clipping? The nonlinearities in this feedback cap are one reason.

I would not expect it to make an audible difference with speakers given the high levels of distortion that most speakers produce. Still it seems that there should be a better solution for this cap in designs such as this where there's so much attention to detail.

I've not seen anyone discuss the issue of capacitor self resonance and the HF inductive component for this feedback cap. It approximates a series resonance circuit and the .1 uF in parallel is just a very good cap in parallel with this series resonance circuit, it might actually introduce another parallel resonance but I've not measured it. The design obviously works but I wonder if this may explain part of the mismatch between simulation and real hardware, and the need for tweaking the HF feedback network.

Pete B.
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Old 22nd December 2004, 01:11 AM   #10
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Default Leach Super Amp with MJL1302A/3281A

Okay. I just completed two channels of Leach Super Amp with MJL1302A and MJL3281A output transistors. Preliminary test was good – they put out sound without hiss or distortion.

I am going to compare this amp to my original Leach Super Amp (with T03 output transistors) soon. Will there be any surprises?

I use 4 of the CPU heatsinks per channel. They seem to be a little too small???????

I want to get this basic mod to work before I try any of the suggestions I got so far. Stay tune

You can see pictures at this link
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