[nathanm]

As I understand it, some (or all?) amplifiers don't like being turned on when there aren't speakers connected to it.  The reason I ask is that I want to be able to have all the amps under evaluation to be left on and warmed up without fear of damage whilst switching.

Is it always a bad idea to have speakers disconnected while powered up or is it only if the amp is receiving an input signal?  

Am I right in assuming that there's a resistor you can put across the speaker terminals to act as a 'dummy load'?  If so, what type\value resistor should you use?  I have an extremely rudimentary knowledge of this stuff at best, but I thought I could rig up a dualie banana jack with the appropriate resistor in it and stick that in the jacks before disconnecting the speakers.  Am I cracked or does any of this make sense?

[beat]

I'm with you Nathan,
I understand it to be just as you stated. Amps like to see that load always whether there is an input signal or not. It is my understanding also that you would just use a run of the mill 8ohm resistor (if that is what you normally run) We should wait for a second opinion on that though. I like your idea of just hooking it to a quick banana plug.

[Wayne1]

As long as there is NO input signal, the amps should be okay.

To make absolutely sure, you can get some 8 ohm, 10 watt ceramic resistors to attatch to some double banana plugs. You could also use a shorting RCA on the input to make sure no signal would sneak in.

Plug in the RCA first, then the resistors, THEN power it on.

[doug s.]

afaik, it's only toob amps that need a dummy load, even if there's no signal present.  but, i'd still be nervous connecting speaker wire to a hot amp - one slip & ya could end up w/a fried speaker/amp/etc...

doug s.

[randytsuch]

When I was buring in my amp, I bought some 8 ohm, non inductive, 20 watt (or so) resistors from RS.  They were a few bucks each, cheapest I have seen for this type of resistor.  You want a non inductive resistor for a dummy load.

I don't think a SS amp would care if there is a load there or not, maybe a tube amp cares.

Oh, and I would turn everything off while I was connecting the dummy load, then turn it back on.

Randy

[_scotty_]

nathanm, The tube amps' output tubes' plate is loaded by the output transformer. It may have a 100 to 1 turns ratio,an 8ohm speaker would look like an 800ohm load to the plate.  When no load is connected to the transformer the plate can see a VERY high impedance condition and be essentially unloaded, at this point it can go unstable with bad results.
A solid state analogue amp doesn't care if there is a load or not. In both cases shorting plugs should be present if the amps are turned on.  The digital amp is always switching even when no signal is applied.  It should also have shorting plugs on the input and should always have a load connected to its outputs. A no load condition can cause peaking in the low-pass filter which produces strong resonant currents that can overheat the output MOSFETS and/or other parts in the amp. One other thing, the power supply caps in some amps store enough energy that some amps will continue to play for some time after the power is turned off. That same energy is present when it has not been playing and you have just turned it off to swap cables.  You can potentially kill the amp even when it's turned off if you short the leads.  The only protection against this is a relay that disconnects the speakers from the outputs or waiting until the storage caps have discharged before swapping cables.

[nathanm]

Okay thanks for the answers, now let me see if I've got this straight:

Tube amp : Put 8ohm non-inductive resistors across the speaker outputs in lieu of speakers.  (What about amps with 4 and 8 ohm + taps?  Should they all be bridged with a resistor?)

Solid State amp: short the RCA inputs and leave the speaker jacks open

I can mute the output of my preamp so in both cases I could not have signal going to to amp when removing the line in.

[mgalusha]

Tube amp : Put 8ohm non-inductive resistors across the speaker outputs in lieu of speakers.  (What about amps with 4 and 8 ohm + taps?  Should they all be bridged with a resistor?)

Just one of the taps needs a load. An 8R resistor between ground and the 8ohm tap or a 4R resistor between ground and the 4ohm tap.

I can mute the output of my preamp so in both cases I could not have signal going to to amp when removing the line in.

Be careful with this. If you are disconnecting a device from the preamp's line in, no problem. But if you are disconnecting from the amp's line in there can be problems. Most RCA plugs make contact with the center pin first. This can cause a nasty thump and hum if the amp is powered up. Not sure which line input you are talking about but since you mentioned having all the amps warming up when switching it bears mentioning.

As previously stated, better to switch off while connecting/disconnecting. If everything is warmed up, 30 seconds of being powered off shouldn't have much affect. At least that is my belief, others will undoubtedly say otherwise.

Mike G.

[WerTicus]

yeah my amp takes about 30 seconds to 'turn off' after you turn it off
with 150w rms speakers too

if i have my 75w speakers on the amp it stays playing for 2 mins or so htehehehe...

made me think the power switch did nothing the first time i turned it off!