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Old October 24th 04, 11:42 PM
Adrian Brentnall
 
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HI again

On Sun, 24 Oct 2004 15:25:07 -0700, The Eternal Squire
wrote:

- not enough decoupling on the 9v rail of the audio power amp (a
couple of hundred uF's as close as possible to the chip)


You mean short the power leads of the 386 with 220uf? Also, the
X100 amplifier is an AMD instrumentation amp, should I short those
with a cap as well?


Certainly you'd do well to get some capacitance across the 386.
The AMD amp is less critical - but you might like to put something a
bit smaller there as well. Can you put the circuits of the two modules
up on the web somewhere ?



- speaker leads running too close to the inputs to the x100 amp


The chain is in a single line, inputs on the left coming from
attenuated audio source, and about a foot to the right start the
leads to the speaker, thoseleads are only a few inches.


Sounds OK


- try running the x100 board at less than 9v (anything from some
resistance and a decent electrolytic in the 9v line up to some
'proper' voltage regulation)


I had read an article today on solving motorboating in tube audio
chains, and the B+ in each of the chains have different voltages. The
final stage had +220 V, the stage just before that +200 V, the stage
before that had +180 V, and resistors were used as the voltage drops.
I do not understand how that solved motorboating.

I mean, what I would do is place an 8V regular between the battery
and my X100 stage, and run the final stage off the 9V battery itself.
But can you tell me why this should work?


I'm guessing (!) that your problems could be caused this way....

The power amp will pull a reasonably heavy current from the 9v battery
on audio peaks. This will result in the 9v 'dipping'. If the x100 amp
stage isn't decoupled itself then it may see the drop in the 9v rail
as a signal, which it will try to amplify. If it succeeds in doing
this then the power amp will pull a reasonably heavy current ... and
so it goes on. This could be the cuase of your oscillation..?
A big decoupling cap at the power amp will help to keep the 9v rail
constant, and mght cure the problem.


- some kind of an earth loop ? - if the board are connected by coax
try lifting the braid at one end of the cable


You mean leave the coax floating at one end? Wouldn't that disable
common-mode signal rejection?


All depends how the other wiring runs. Try it & see ?

Regards

Adrian
Suffolk UK