stefanher

Is this safe?

8 posts in this topic

My new pre has two power outputs which makes it easier to compare power amps should one wish to do so. What I want to know is whether it is safe to switch speaker cables from one power to the next if the mute button is on on the pre. I read somewhere that this can short the amp/speaker & that's hardly what I want! All my power amps are valve & a 10 minute delay between comparisons waiting for the amp to warm up is hardly conducive to careful comparisons.Both being valave amps a 10 minute delay waiting for the machine to warm up is hardly conducive to careful comparisons.

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Muting the pre will stop you hearing anything, but if the amps are on and you touch the wrong speaker cables together when swapping them around you are still risking blowing up both amps and speakers. Get a speaker switch and use it backwards to compare the amps, much safer.

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When playing with speaker terminals there is an inherent risk that you may short something out. Most hardware has some sort of short circuit protection but it is normal to switch this stuff off when making these changes. While I fully understand your wich to make comparisons while everything is fresh I think the risk is too great, and I have found greater use in longer term listening than short sharp comparisons. :)

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The danger is not just that you'll short something out, but that output from the valve amplifier for whatever reason - like it picking up a mobile phone base station change - would be at a much higher voltage than normal and could damage the insulation of the output transformers.

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This is an interesting problem, as it doesn't have a fixed answer.

Firstly, with Solid State amps, the main risk is of short-circuits as others have said. SS amps are perfectly happy operating with no load, so the best solution here is to do what Smegger68 suggested, and use a 'speaker switch in reverse.

For valve amps, the risk is different. Yes, you can have short-circuits when swapping cables, but valve amps are reasonably tolerant of brief short-circuits. The main risk with a valve amp is that they re not happy operating with no load. The higher voltages generated can damage transformer insulation as JPG suggested, but also these higher voltages can flash-over the output valves and damage the cathodes. Valve amps should never be operated without a load, and consequently a conventional 'speaker switch won't help. You need a pair of changeover switches which will switch in an 8 ohm dummy load when the amplifier isn't driving the 'speakers. These don't need to be anything very fancy, and you can easily make up a suitable pair yourself.

S.

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^ Hadn't considered flashover - very good point.

And yes, switches with dummy loads would be fine - just make sure that the switches are the 'make before break' type, so that you don't leave the output open circuit when switching.

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Personally I don't think it's worth the risk. It's easy enough to blow things switching them on and off, without adding other variables and opportunities for disaster.

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