Audio Banter - View Single Post - Protecting speakers against switch-on pops and DC

**Paul G.** · September 11th 10, 01:28 AM posted to rec.audio.tubes,uk.rec.audio

On Fri, 10 Sep 2010 14:00:49 GMT, (GregS)
wrote:

In article , Andre Jute wrote:
Paul G asks:
"[W]hat is the best way to protect a
speaker from destruction if the amp is sufficiently powerful, and you
have no control over volume? It happens.... the kids have fun with
your stereo, the grounds come loose on the inputs (huge buzz), the
system oscillates, etc."

I don't know if you want to design and build your own stuff from
scratch, Paul, or merely want to buy a plug-in protector. If you can
solder, here's a halfway house in a couple items to protect your
loudspeakers from switch-on pops and from DC.

Velleman K4700 Loudspeaker protection kit, stereo with switch-on delay

Velleman K4701 Loudspeaker DC-protection, mono

The reason I've identified them is that I have a QUAD 405 MkII which
occasionally eats an ESL-63, which soon gets expensive, and it is
tricky rebuilding them because here is no way to solder except over
the fragile panels... So an audiophile's fancy turns to ripping out
the 405's inadequate clamp circuit and replacing it with industrial
strength protection.

The reason I choose Velleman is I've built some of their kits,
including the multimeter I've used for years and a 100W PP tube amp,
and found their stuff first class, head and shoulders above the
competition (well, in most cases there is no competition, because the
other guys make crap); in the end Velleman is worth the premium price.

Andre Jute
I just listen to one channel at a time. You do have to listen twice.
Adequate stereo separation. -- Al Marcy

For a solid state amp, I usually put in a fuse on the speaker lines,
only if I feel its necessary. The fuse is not a sure thing, and you have to
use the right value. It can save speakers some of the time.
The Velleman devices have nothing to do with over volume, unless the
overvolume cause a shift in average DC offset.

greg

I looked over the schematic of the Velleman unit, although it has
no values shown for the components, it senses speaker voltage, thru a
2nd order lowpass filter, rectifies, filters, then thru to a
comparator and a relay circuit with a another lowpass filter that
gives a 6 sec. or so time delay. Judging from the electrolytics in the
1st 2nd order filter, it will only respond to very low frequencies and
DC.
The unit should protect the speakers from an amp that goes haywire
or blows the output transistors and puts a significant DC voltage to
the speakers.

It would not prevent your speakers from being damaged by too much
volume, and would be of no use with any amp that uses an output
transformer (no DC makes it to the secondary).

I think it is not effective to use a fuse or breaker in series with
the speaker. Those devices require energy to operate, and then must
divert some of the output power to get sufficiently hot. That puts a
resistance in series with the speakers, which may affect their
performance. Magnetic trip breakers would put a series inductance in
series.... that would mess up the treble response. Many of these
devices aren't linear, and could cause distortion. When the device
"trips" it may not switch cleanly (arcs or chatters). The sudden
open-circuit while at high volumes could cause a voltage transient
that could damage the amp if the output circuitry isn't properly
designed.
Even if you get something to work well, which speaker of the
2-or-3-way system are you protecting? They all have different thermal
behaviour, so you'd really need to monitor the voice coil temperature.
Even after that, you might worry about mechanical damage from the
excessive travel of the cone. That would depend a lot on the frequency
and speaker resonance. Looks like a very difficult problem.
I had thought if your speakers had a power rating way in excess of
the amp you'd be OK. But.... I've heard a story where some guy takes a
crappy little amp, runs it flat out (square waves or worse) and
destroys speakers that have a reasonable power rating. I'm not sure if
its an audio myth, but it was supposed to have ocurred when a
knowledgable customer challenged a salesman at a hifi store, claiming
the expensive powerful speaker could be damaged by the little
amplifier.
What all this means is you must carefully manage the volume
yourself, and be very careful with situations that could damage the
speaker, especially a fragile or expensive one. I don't think there is
a cheap simple technical solution.

Paul G.