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Old 3rd October 2008, 12:51 PM   #31
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Mooly

Maybe there is something so obvious here I just can't see it
Hi,

Maybe I cannot either . Thinking about it I'm disagreeing with myself ...

/sreten.
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Old 3rd October 2008, 01:03 PM   #32
Mooly is online now Mooly  United Kingdom
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Hello Bonsai,
What you describe sounds to me to be the correct way. The speaker currents have to be kept separate. So too, the rail decoupling. In fact anything that can cause a volt drop "across" the input ground and feedback connections. You have to treat each conductor as a potential source of trouble. You have to keep going back to first principles to see what the amplifier output is trying to do in relation to the input signal and feedback signal and whether those very signals are being altered by unwanted currents.
When I look back on my first amps, hum/noise were always the big problem. When I made my latest Mosfet amp, I actually built as I mentioned earlier two small "models" to investigate this.
The layout I came up with is so at variance to D Selfs layout etc that I started to question whether I was "missing the point" somewhere.
You have to trust your instincts I guess
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Old 3rd October 2008, 01:06 PM   #33
Mooly is online now Mooly  United Kingdom
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Hi sreten,
Email me, I have the page next to me with the layout -- see what you think. Am I overlooking something ?
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Old 3rd October 2008, 05:45 PM   #34
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Mooly
Hi sreten,
Email me, I have the page next to me with the layout -- see what you think. Am I overlooking something ?

Hi,

Why I'm dissagreeing with myself is what should the feedback be
referenced to ? the star point or the earth speaker terminal ?

I think it is the speaker terminal ......
I'll have a look again at D.Self books this weekend.

/sreten.
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Old 3rd October 2008, 06:00 PM   #35
AndrewT is online now AndrewT  Scotland
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the NFB is tied into three points.
The Output, the Audio Ground and the Source.
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Old 3rd October 2008, 07:15 PM   #36
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I can't figure this out either. But, if you throw the noise from the speaker return (load ground) down both the + and - inputs on the amplifier like this http://www.diyaudio.com/forums/showt...02#post1623002 then is it a cancellation or is it amplified?

That sketch isn't a recommend. It is a question.
EDIT: Not shown is the potentiometer at the "X" terminals.

And, if that's done in stereo layout, does it matter how much the "other" channel lifts the voltage up and down--because that may only slightly raise or lower the gain (stereo expansion)? Well, I don't know. So that's why they're questions.
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Old 3rd October 2008, 11:52 PM   #37
jcx is online now jcx  United States
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Quote:
Originally posted by Bonsai
...
One other point - keep all current loops small (i.e. compact) as possible - so return traces must be right next to supply traces on your pcb...
OK so far...
Quote:
Keep the + and - supply lines to the amp apart (to avoid amplifer guru's 'spray harmonics' and induction into the other supply line and small signal stages.
but I believe this last part is wrong

Class B ps currents are individualy very nonlinear but when routed (twist or star quad) close together the far field from the sum of the +/- ps currents should be as linear as your load impedance

spacing +/- supply traces apart maximizes the nonlinear field coupling area

as per the 1st statement it also helps to have the ps gnd return near the closely coupled combined ps leads

and of course bridge drive doesn't have significant return current in "gnd" so close coupling the ps leads is the only way to go
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Old 4th October 2008, 07:21 AM   #38
Mooly is online now Mooly  United Kingdom
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Remember this is for running TWO amps off a common PSU.

This is how I see it. We have to have a single point of reference for both channels. In a way this is a star in its own right. Nothing else should connect here.
Whatever output current the amp sources or sinks this can in no way alter the input reference point ( for practical purposes, as even the small current in the feedback return does flow down the blue connection to the star earth).

When all this is drawn out it all looks very straightfoward but as I keep banging on, to me this is so at odds with DSelfs layout.
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Old 4th October 2008, 08:12 AM   #39
AndrewT is online now AndrewT  Scotland
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and Leach tells us to try moving the Blue grounding connection from the PCB to the input RCA earth tag. I prefer this Leach alternative, since it is more universal than any of the others I have tried.
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Old 4th October 2008, 08:41 AM   #40
AndrewT is online now AndrewT  Scotland
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The Blue signal ground is connected to a very low impedance.
Does this make it less susceptible to either form of magnetic/electric interference?

referring back to post 39.
The two input RCAs have to be commoned at the terminal panel to allow a single Signal Ground connection back to the two channel Audio Ground.
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