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bulgin 3rd June 2007 12:38 PM

Father, I abused my Leak S/20
Hi All

I have 2 setups in a smallish room where I do measurements on transducers. There are 2 sets of speakers (the second pair often doing duty as computer speakers) and for them I use dedicated runs of speaker wire. The same speaker pair is also used with my Leak S/20 amp and another set of speaker cable is used for that. The two sets of cable runs are different types and different bananas are used for each pair of cables to avoid getting them mixed up.

There is also a Quad 405 MK1 in this room with its separate speaker cable pair, going to a second pair of standmount speakers and this setup stays like this permanently.

Both amps (the Leak and the Quad) share the same preamp, using rca adaptor plugs. Obviously only one amp is switched on at any given time.

Well, you know how it is...I had some spare time last night and took out my little Leak Point One Stereo pre-amp last night. It has been refurbished and revalved some years ago but hardly used. At the time of refurbishment, I also made up a 6-wire interconnect and for some stupid reason or other, I made it much too short for convenient use.

So, craving a bit of nostalgia for an all-tube kick, I carefully remade another 6-wire interconnect which goes between the Point One and the S/20.

It got too late last night and I tried it this morning....and nooo sound.

After a lot of checking, I decided to call it a day, convinced that the S/20 had somehow developed a problem.

A few minutes ago, I decided to have another look and found nothing wrong when I tried the amp with my usual s/s pre. Still no sound, until I looked at the speaker cables used for the Leak.

The speakers were wired for my computer setup...very stupid - no speakers, no sound.

Can anyone tell me how much damage can occur when an amp is switched-on with no speaker load? It was only for about less than 30 seconds.

bulgin :smash:

Klimon 3rd June 2007 01:20 PM

Most tube-amps aren't damaged at all by this. If there is any danger, it's that the output transformers would get destroyed, which seems an issue with e.g. Decware zen-amps. None of the tube-amps I've built or owned had a problem when operated without speakers.

What makes most tube-amps immune and some prone to damage is something I'ld also like to know.


EC8010 3rd June 2007 02:14 PM

None at all.

bulgin 3rd June 2007 02:25 PM

Father, I abused my...
Thanks Guys

I'm listening to it now:D All is well. :D



Tom Bavis 3rd June 2007 03:56 PM

Driving an amp into clipping with no load will create some HUGE voltage spikes that can arc over the output transformer. Without overloading it, there isn't much danger.

A few amps will oscillate without a load, but not many - for instance one console amp I had (a console would always have speakers connected). But oscillation won't necessarily damage anything.

ray_moth 3rd June 2007 09:28 PM

You may have the resistors in the NFB loop to thank for that.

Johan Potgieter 3rd June 2007 10:04 PM


Originally posted by Klimon

What makes most tube-amps immune and some prone to damage is something I'ld also like to know.

To stick my neck out: Insufficient design, Simon. (But it could also be that the "immune" amplifiers are so over-compensated that there is little NFB left at high frequencies.)

But nobody used Zobel-networks back then; I do not know why not. Or one can do what Walker did in Quad: Make the feedback resistors low enough to act as a high resistance load but sufficient to prevent a complete open circuit - this is of value especially with pentode output stages. E.g. if the loudspeaker impedance is 8 ohms, you will only loose a few % of maximum output by terminating with say 470 - 560 ohms, and for convenience that could be the NFB network. It also cuts down on the effect of stray capacitances. Many amplifiers came with a 1K resistor on the output for the then 15 ohm loads.

EC8010 4th June 2007 05:15 PM


Originally posted by Tom Bavis
Driving an amp into clipping with no load will create some HUGE voltage spikes that can arc over the output transformer.
But that's two simultaneous fault scenarios, input overload and no output load. If the loudspeaker were to be connected, then you'd be looking for new tweeters.


Without overloading it, there isn't much danger.

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