Owner of local high-end hifi store that I do some service work for, bought
these in with the surrounds rotted away on the bass drivers. Apparently,
these things were 8 grand apiece 12 years ago when new. He managed to get
some new cones / surrounds from the States somewhere for a reasonable price,
and fitted them. One of the amps also needed a new level pot, so he put one
of those in, too. When he repowered them, one worked perfectly, but the one
that he had put the pot in, buzzed so loudly that it nearly destroyed his
new cone work. He couldn't see anything that he had done wrong, so he sent
them both to me. One double hernia later, I had them both up on the bench.
They are signal-sensing for main power turn on, so first I had to work out
which was the relay for this, and bridge it such that the amp could be
powered via a variac. At about 40v input, the amps started working (separate
amps for bass and mid / top drive) but the bass amp started motorboating
violently. I spent a very long time checking caps in the power supply and
bass amp. Interestingly, I found one with very poor ESR on the driver board,
but replacement made no improvement, and then a 470uF that read very good
ESR - in fact so good, that it rang alarm bells, so I whipped it out and put
a good old analogue ohm meter across it. It was short circuit (near enough).
Remember we were talking about just this a couple of days back in the thread
about SMPS cap testing, where someone asked if an ESR meter showed a cap as
good, could it still be bad, and someone else then commented that a straight
ohms test should be carried out "for sanity" I think was the phrase. Well
here we are with a good real life example. Anyway, that turned out to be a
red herring too. I don't know what that cap did as I have no schematics, but
a replacement had no effect on the instability problem.

At this point, having already spent too long on the job, I phoned the store
owner to tell him where we were at with the problem. He then went through
what he had done again, and then casually tossed in that the voice coils on
the bass units that he had re-coned, were unusual in that there were two of
them per driver ...

Now some real alarm bells started to sound in my head. I asked him if he
meant that the drivers had four wires going to each of them, and he said
yes. I hadn't actually seen the backs of the drivers, as they come out from
the fronts of the cabs, and the backs are packed with damping material. I
rang off, and pulled one out, and indeed, there were four terminals, two of
which had a fairly heavy piece of twin connected to them, and the other two,
had a piece of thin screened wire connected to them. Looking at the
soldering, he had definitely had these wires off, so I removed the screened
wire, and reversed it. That cured all the problems, so I have to conclude
that the second 'voice coil' is in fact some kind of feedback winding, and
when the cone went say forward, this winding, being reverse connected, was
telling the amp that it had moved backwards, playing havoc with the
stability.

In many many years of servicing all sorts of hifi and audio amplification
equipment, I don't ever recall having come across a bass driver with a
feedback coil like this. Has anyone else on here, or anyone have any
knowledge of the system, and what the advantages are claimed to be ?

Arfa