Amp Problem - 'Breathing Speakers'

[harry]

My guess (semi-uneducated)

See if there are any protection circuits built in. Maybe its some kind of
external servo loop designed to null DC offset, or protect in some
other way. This would be outside the normal audio path, and probably
would be limited to sub-audio in speed...

Or maybe I'm full of SH!T .....

H^) harry

Rob wrote:

> Sounds like somewhere perhaps there is a floating ground somewhere, or maybe
> a sine wave riding on a slowly rising and falling reference. AFAIK, most
> amps are AC coupled in several places, so the only thing I could think is
> the amp that is directly driving the speaker has a very bad problem with a
> floating ground or level. I would take alook at the output to the speaker
> with a scope first, then trace back.
>
> ----- Original Message -----
> From: "J. Larry Hendry" <jlarryh at iquest.net>
> To: <synth-diy at node12b53.a2000.nl>
> Sent: Monday, January 15, 2001 7:01 PM
> Subject: Re: Amp Problem - 'Breathing Speakers'
>
> > Very interesting. I have never seen such behavior.  I would think that the
> > DC current (hard to call 2Hz AC) required to move a speaker in that
> fashion
> > might be thermally damaging to the speaker though.
> > Larry Hendry
> >
> > ----- Original Message -----
> > From: Vic Vector <maddock at execpc.com>
> > To: <synth-diy at node12b53.a2000.nl>
> > Sent: Monday, January 15, 2001 3:12 PM
> > Subject: Amp Problem - 'Breathing Speakers'
> >
> >
> > Here's a peculiar problem with an old Hafler amplifier that my friend
> > mentioned to me.. perhaps someone has seen this type of symptom before.
> > I can't find any similar experiences on usenet, etc.
> >
> > There's a fault somewhere in this amp that causes the speakers to
> 'breathe'
> > at about 2-3 cycles per minute. The voice coil of the speaker (any
> speaker,
> > the Hafler is just a head unit) will slowly and steadily drift from fully
> > impelled to fully expelled at a very regular rate (2-3cpm).
> >
> > It's as if something is injecting a sine signal into the power stage of
> the
> > amplifier and passing straight current to the speaker coils. This occurs
> > in both l+r, l only (bridged mono) and right outputs, with no input to the
> > amp.
> >
> > I thought it was peculiar and people might find it an intriguing problem.
> >
> > The capacitors used in the power stage of the amp are cylindrical _film_
> > caps, and they both tested OK.
> >
> > -maddock
> >
> >
> > --
> >
> >
> >
> >