Passing DC to speakers
Posted by: Eric Barry on 04 May 2001
Amps have included Musical Fidelity A1 and A100, Rotel 20 watt receiver, Exposure XX, and of course my Naim stuff.
Building is a 60 unit 1920s construction in a dense residential neighborhood in New York. The outlet was put in special (by a previous tenant) for an air conditioner (though it happens on different outlets too).
Questions--
1)How dangerous is this?
2)Why does it happen?
3)Can I do something to prevent it?
Thanks for any advice.
--Eri
If it is during music, then we are probably talking about low bass frequencies that do generally cause bass units to "bloat out" for about 1/2 a second.
Most of these amps you list will be DC protected on the output, either using a servo or de-coupling capacitors.
To answer your question on whether DC is bad for loudspeakers - the answer is yes, sending a squarewave test tone to a loudspeaker is also bad news. As DC is so bad, amplifiers use the afore-mentioned DC de-coupling methods.
Amplifiers generally use DC biasing in the circuitry, without this de-coupling - the amplifier itself would damage the loudspeaker.
Andrew
Andrew Randle
2B || !2B;
4 ^ = ?;
--Eric
How long does the excursion last for?
Andrew Randle
2B || !2B;
4 ^ = ?;
Most amp's (including Naim) monitor the voltage drop across the output stage emitter resistors (i.e. output stage current) and if this is above a predetermined value for a period of time will clamp the drive to the output stage reducing the output.
This prevents damage to output devices and hopefully 'speakers under overload fault conditions - it won't help if the o/p transistor go short circuit though.
Is there any element of your system (e.g. source component) that has remained common over the years? It sounds as if you have either mains-borne switching spikes producing a thump (similar to the amp being switched on / off) or a source components producing some large low frequency noise.
Does the cone stay in a fully in or out position for an extended period? Is it the same as the thump when switching the amp on and off?
Naim preamps will often cause a hum to be heard through the amp when first switched on as the output to the power amp is open circuit at switch-on, usually only noticed when using the amp internal supply to the pre-amp since both have to be switched on at the same time.
Andy.
P.S. I've just thought of a more likely option, bearing in mind the hum you've mentioned - do you have an transmitters (CB / Amateur / Taxi etc) near you. FM transmitters are often the cause of interference of this type, and you will not hear the modulation (e.g. voice) since FM is not easily demodulated by the amplifier circuitry, but a hum or buzz is common. RF problems of this type would be the most likely candidate I'd suggest.
Andrew L. Weekes
alweekes@audiophile.com
You haven't give much info about the events but it can't be a good thing in general to have intermittent dc thumps and buzzing from your speakers. Whether it will damage speakers or electronics sort of depends on your speakers and electronics and the size and duration of the events. Best to get it sorted I think.
As AW said most amps, particularly mid to low priced, do not offer protection against this. Naim amps have excess current protection but it is intended to protect the power transistors not the speakers and so won't help much in this case.
2)Why does it happen?
From your brief description and the fact that several different amps have shown this symptom my money is on mains faults - excess voltage spikes or voltage drops. Could even be a poor connection somewhere in the DIY spur if not a more general problem in the building.
3)Can I do something to prevent it?
Ask an electrician to check out the wiring.
Move.
Buy a mains filter/conditioner unit (there's a thread about this somewhere)
Buy an amp with full protection (this will protect your speakers but won't solve the problem)
If it is a mains problem then you may well see some problems with your picture when watching TV. Do you?
Good luck.
BAM
As to duration, half a second to five seconds, usually one or two. The woofer gets pulled in and held there.
As far as the outlet, it looks professionally done, and besides I've had the problems in different rooms on different spurs.
--Eri
That certainly sounds like DC to me, and I would trot the power amp off to your dealer.
--Eric
The times would tie in with brief transmissions from taxi's etc.
Do you have any friendly radio amateurs near you who could help pin-point the problem, as it would require step-by-step techniques to eliminate the problem, with likely deleterious effects on sound quality. Your dealer may be able to help?
Andy.
Andrew L. Weekes
alweekes@audiophile.com
--Eric
Does this sound like your problem: -
Initial click / thump (transmission start), followed by
Hum / buzz (transmission), then,
Click / thump (transmission end)?
If so I'd suggest one of two options, talk to Naim for advice and try to seek help from whatever organisation assists with such matters in your country.
There is usually a authority who can investigate such matters to ensure that the transmissions meet their required specifications and offer advice on further avenues. Naim, I'm certain, can offer advice also (through your dealer or direct).
The FCC are the organisation in the US who regulate such matters, they could be first port of call, if the above sequence seems familiar.
Federal Communications Commision
Andy.
Andrew L. Weekes
alweekes@audiophile.com
--Eric