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Old 20th June 2008, 08:55 PM   #141
Dave S is offline Dave S  United Kingdom
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hi Mooly,

Just had a look at your layout in post #19...

Shouldn't the speaker OV return go to the PSU star point not to the PCB? Seems to me that your method will suffer from common impedance coupling to the input caused by the current flowing in the wire from the speaker 0V on the PCB to the star point.
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Old 21st June 2008, 01:22 AM   #142
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hi mooly,

i am a bit confused about somethings. output having some voltage it is about 5.6v and showing without connecting the speaker, showing -v on the positive output line, isn't it funny? when i connect the speaker the output voltage come down to 0.++ . do you think it is a problem?
i think its a problem. but across the R21 & R18 voltage showing 0.0+....its looks ok..

need comments pls.
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Old 21st June 2008, 07:08 AM   #143
Mooly is offline Mooly  United Kingdom
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Hello Michael,
If the voltage at the output is Zero with speaker attached but has a voltage without the speaker it could be a stability problem caused by all those wires. Did you move the 270 ohm resistors onto the legs of the FET's. You will not measure any voltage ACROSS R21 and R18. No D.C. current ever flows into the gates of the FET's.
This is where you really need a 'scope Michael. One other thing, have you got the Zobel network connected to the correct side of L1. Also, when the DC voltage was there did the resistor R25 in the zobel network get warm.
I have your other message as well
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Old 21st June 2008, 07:28 AM   #144
Mooly is offline Mooly  United Kingdom
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Hello Dave S,
Very very good question Dave, did you read post#12 by the way. You are absolutely correct in what you say.
I know this is a real "cop out" but I followed Doug Selfs articles all those years back on his blameless class B amp, and learnt a lot in the process. I even went as far as building one, not on home made PCB's but on the official boards approved and designed by Doug and Gareth Conner I believe. And guess where the speaker return is -- exactly .
What can you do !!. I understand the arguments about "the amp does not care what goes on between it's ground and the PSU ground" etc and that it's "where and how the feedback and input nodes are referred to".
It's what happens as you so correctly say " when those input grounds are tied together " wherever that may be.
This would be a good -- no an excellent topic in it's own right.
It's easy to say how it should be done -- we all KNOW how it should be done Just does not work does it.
That has to be the best question to date !!!
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Old 21st June 2008, 08:14 AM   #145
Dave S is offline Dave S  United Kingdom
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Hi Mooly, just read #12 again, from this I assume you are using one PS for 2 channels.
Nowadays I alsways take the easy way out and have a separate supply for each channel.

I reckon Leach has got the grounding right (although his scheme is for a single PS). This is borne out in my experience because the Leach amp is the best sounding amp I have built and I've built quite a lot, including the JLH80W (the SQ of whcih was helped by mods to the front end PSUs), Aleph3 (should be called Alaugh3) AKSA, NCC200 etc etc.
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Old 21st June 2008, 08:28 AM   #146
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Quote:
Originally posted by Dave S
Aleph3 (should be called Alaugh3)
Why so?
__________________
Sam
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Old 21st June 2008, 10:53 AM   #147
Dave S is offline Dave S  United Kingdom
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Because I reckon Nelson's having a laugh!
The Aleph is certainly not a straignt wire with gain.
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Old 21st June 2008, 11:28 AM   #148
Mooly is offline Mooly  United Kingdom
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Hi Dave S,
Yes it is a shared PSU. It's much easier with one PSU per amp as you say.
One real advantage with the MOSFET design over Bjt outputs is that the driver currents are tiny and so do not add to the problem of circulating ground currents. I actually used a "ground lift" arrangement around the volume control/preamp connections to help isolate and reduce the effect of this problem. The preamp is grounded back to the star earth but the preamp output ground is not connected directly (not zero ohm) to the power amp input grounds.
I find it easier to think about if you analyse it as a DC problem with say the left channel supplying 10 v across 10 ohm positive going, then try and visualise whats going on, putting in say 0.1 ohm wiring resistance. It all gets interesting Particularly when you then join the input grounds of the two channels.
And then we are back where we started, ha ha.
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Old 22nd June 2008, 07:24 AM   #149
KLe is offline KLe  Australia
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Quote:
Originally posted by Lumba Ogir
KLe,
your pondering is reasonable. Many years ago a friend of mine had a British style single ended input amp, despite many impurities and low quality parts the sound was kinda pleasant and I just could not understand why. It took a long while to find out due to extremely sluggish thinking but enlightening spiritual exercises expand our consciousness and enhance our ability to perceive clearly the inner nature of reality around us. There is no doubt in my mind that this amp is as musical as Mooly says, however, the transcendental guru`s penetrating insightfullness cannot pass by unnoticed, stating: everything can get better. Reassuringly, later on in chapter twelve, indicating nearly infinite wisdom, we also find: all problems have solutions.
Now, the bothering problem with the bias current in the drivers still remains, leaving them without a chance when encountering those huge input capacitances. Moreover, something deep inside tells me that output stages should not have voltage gain...
Hi Lumba Ogir
Wow ...

Quote:
Now, the bothering problem with the bias current in the drivers still remains, leaving them without a chance when encountering those huge input capacitances. Moreover, something deep inside tells me that output stages should not have voltage gain
... are you able to provide more detail(s) of your concerns. We are only to happy to discuss. I probably will not understand (maybe) but hopefully Mooly and others will.

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Old 22nd June 2008, 07:33 AM   #150
KLe is offline KLe  Australia
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Quote:
Originally posted by Mooly
Hello Michael,
As Andrew says, just put it in parallel with L1. Anything
As the amp is so stable you could even try shorting out the coil all together. Just keep the 0.22 ohm series resistor R26.
Hi Mooly

Are you saying, because the amp is so stable, that the output Coil and parallel 10R resistor can be left out?

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