Hi Bob,
Finally, I got it !.
It is related with "the RC network connected between the output pin 5 and
ground, required to keep the amplifier stable at high frequencies at
different loads", as you mention on your previous post.
I checked the RC network and I even installed a new one: same behavior.
If I removed this RC network completely, the oscillation now started with
lower volumes !.
Using the technique "try and error", the problem was solved with a capacitor
of 68 nF in parallel with this RC network, that is, just from pin 5 to
ground. No more high frequency oscillation with different loads !.
By the way, the DC voltage at pin 5 was half the supply: 3 V. I even
installed a short leads 10 uF tantalum across the supply of U4, without
positive results.
I don't know the reason of this behaviour, maybe, as you say on your post,
is due to the high gain of this stage (46 dB). Who knows... .
Many thanks Bob and have a good day.
73,
Paco
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