Re: Groundplane in poweramplifier PCB design

In article <e22mml$rqk$1@xxxxxxxxxxxxxxxxxxxxxxx>,
Wiebe Cazemier <halfgaar@xxxxxxx> wrote:
On Tuesday 18 April 2006 02:27, Ken Smith wrote:

[...]
The latter part goes against what I know about line input connections. For
example, in my current state of design, I have one SPK_OUT on the PCB, and the
return goes to the PSU's 0V. When you connect the input ground to the 0V of
the PSU, instead of close to the input stage on the PCB, it will oscillate. I
have tried this, and indeed it did.

I'm not talking about moving the ground of the input connection. Only the
ground side of the resistor is moved to the speaker's return.

My PCB has a 0V pin, which is used as a starpoint for all the sections which
require 0V. That means there is one for the input stage as well. However, the
input ground is not fed to to this star seperately. Instead, it's connected to
the input stage (near R2) and from there, one connection goes to the 0V PCB
pin. That's how it should be, as far as I know.

I'm renumbering this to match the schematic. I think you'll see the
point.


R4 R5
(in-) -+--/\/\/----+---/\/\/----
! ! !
[?] ! !
! ---!-\ !
GND ! >-----+-- (Out+)
---!+/
!
!
(in+) ---/\/\------+-----/\/\---+-- (Out-)
R1 R2 !
[?]
!
GND

I'm beginning to. But, this setup doesn't hold true if I adhere to what I
mentioned about the input ground connection above. I cannot have the speaker
return and input ground at the same points. It will oscillate, and/or speaker
return current pollutes the input ground.

Again look carefully at what I suggested. Notice that the R2 ground is
the only thing moved. I've also assumed that R5/R4 now equals R2/R1.
Also, I've assumed that the signal source has low impedance. If it
doesn't the values have to change to include it's effects.

[...]
Yeah, OK. But in practice, this is not really necessary. In fact, I have a
first version of this amp running with the servo not in contact with anything.
It doesn't suffer from thermal runaway.

You often have to run the amplifier at about 1/2 power at a low frequency
to get it to run away.


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