A bad idea to.....

...connect two amps to the same set of speakers without a switch box ?
Given that only one amp would be on at any given time, is it such a bad thing to do ?

Just curious as to any potential negative effects .

 

if it went wrong, yoiu may find the magic smoke escapeing from one or both of the amps i suspect

 

Measure the impedance across the speaker terminals with the amp off. If there is no circuit on either amp then it is fine. If there is a circuit then I think there'll be a problem. It's been a while since I did any electronics, so if I'm talking bunkem then anyone feel free to step in, but as I see it you are putting two loads in parallel. When you put loads in parallel you get:

Z1 = impedance of speakers
Z2 = impedance of amp two

The load amp one will be seeing is:

Z1*Z2/(Z1+Z2)

So you are dropping the load and drawing more current from amp one.

You are also drawing current into amp two, how much will depend on the impendance across the speaker terminals as compared to the impedance of the speakers. Remember that less impedance is more current. This is almost certainly not going to do the amp any good.

All in all, I'd say best left alone. Worst case you could blow both amps.

 

Now that I think about it it would be rather amusing :MILD: !

Curiosity-Cat-etc.

 

And just how do we do that? Impedance varies with frequency. The DC resistance can easily be measured but this is no guide whatsoever to impedance.

 

if you really don't want a switching box then you could simply swap the speaker cables over when changing amps. If you use banana plugs it's not going to take longer than a few seconds.

 

But make sure both amps are OFF when you change the cables over.

 

Absolutely !!!!

Else it'll be :chainsaw: BZZZZZZZZZZZZ Central !

 

Fair point, although presumeably an amps output impedence would have to be pretty constant with frequency just like a speaker's?

I'm pretty sure that connecting two amps is a bad idea BTW. Unless the amps do break circuit when they switch off (which I don't think is the case) then it would be a problem as described above.

 

The amplifier manufacturer will have the details on input impedances for each amplifier.

 

It's the output impedences which are relevant here though .

 

Don't do it.

Output Impedance is (intentionally) very low, otherwise speaker control would be interesting (Damping Factor anyone?). Think of them as current sources.

I'd put money on the fact that at least one amplifier would not survive the experience if you tried it.

Cheers

 

Well, the input impedance of the speaker terminals is what the amplifier 'sees', is it not? Or in this instance, is it one and the same thing?

This is all a long time ago, so I'm a bit hazy.

 

Absolutely. When it comes to an amplifier it's input inpedence is that 'seen' by a source component (e.g. a CD player) i.e. it is the impedence measured across a pair of the amplifiers inputs. Similarly an amplifiers output impedence is that measured across a pair of its speaker output terminals. With the combination of two amps to one pair of speakers discussed, it is this output impedence of the second amp that the one trying to drive everything will actually see. Just a question of terminology that's all; of no real relevance at all. Sorry I mentioned it.

 

yes don't do it.

I did it once and blew up a few transistors in the other amps output. fortunately, my brother fixed it for me, as I couldn't.

basically, you will be putting loads of current into the other amp, you 'on' amp will be going into a near short circuit, and your off amp will be cooked.

A typical amplfier output impedance might be 0.2 ohms, maybe a good bit less. It is usually equivalent to the bigish high power resistor in the emitter circuit of the output. So have a look under the lid if there are any of those white ceramic resistors, and that is your output impedance.

This would be in parallel with say 8 ohms for the speakers, giving well, less than 0.2 ohms load. (8x0.2 / 8.2).
Speakers are around, say 4 ohms minimum, at other frequencies, the figure usually rises, often to 30 or more ohms, so the most current is at the lowest figure, you can see how much lower the 0.2 ohms is than say 4 ohms. When amps baulk at 2 ohms loads, 0.2 is FAR lower than that.

an amplifiers input resistance, nothing to do with the output is usually 10k-47k, 100k for valves.

You could try a switch box.

 

I was just trying to find a quick solution as I'm still waiting on TACT getting back to me about an A/D input card for my power amp (needed to hook up the Home Cinema) but it's taking them aaaaaaaaaaaages .
I don't want a switch box I'll never use again and just wanted a simple solution for the g/f which didn't involve switching cables.

Nice to know somebody else was curious enough to try it .

Cheers,

Voo

 

Can't yOu just dO the usual thing Of sending the frOnt Outs frOm the av amp tO the stereO amp? NOt sure Of yOur set up with the TACT, but can't yOu dO this? Or is that what yOu'rse waiting on the AD card for?

 

That last post gave me a headache.

 

nO need tO wait fOr the A/D card nOw as the g/f has said she has nO prOblems with me MOving the main system tO my Office/spare rOOm .

All I need nOw are sOme decent standMOunters .

 

Not advisable to read MO speak when operating heavy machinery/women otherwise :chunder: !