Does having two amplifiers hooked into the same pair of speakers cause any problems?

[The Paulinator]

Many times in the past I have had two diffferent amplifiiers hooked into the same pair of speakers so I could switch between them to see if I could hear any differences, and recently someone has absolutely insisted that I never did that because it would cause one of them to blow. I don't even think that would happen if they were both on at the same time, but I could be wrong... any input?

[sreten]

I suggest verifying this with both on at the same time could be expensive.

With similar rated amplifiers as long as one is turned off then there is no real
danger, though cable loading and possible zobel correction will be unrealistic.
As well as reverse voltage diode practise differences in the amplifiers.

Its a practise of little merit for direct comparisons.

sreten.

[Cal Weldon]

The possibility of send high level input back into the speaker outputs of an amplifier doesn't sound very productive. I agree this should be avoided.

Cal

[jneutron]

For a simple class b output, that would be rather dangerous.

With the supplies off, it is possible to reverse bias the emitter-base junctions, and forward the collector-base junction.

That would mean that somewhere in the 5 to 15 volt region, you would be able to put the e-b junctions into reverse avalanch breakdown, with very little to prevent destroying the output transistors..

Do not do it for any solid state amps..

John

[sreten]

Originally posted by jneutron
With the supplies off, it is possible to reverse bias the emitter-base junctions, and forward the collector-base junction.

That would mean that somewhere in the 5 to 15 volt region, you would be able to put the e-b junctions into reverse avalanch breakdown, with very little to prevent destroying the output transistors..
John

Can't see this happening myself, sreten.

[jneutron]

Originally posted by sreten


Can't see this happening myself, sreten.

Hmmm...

Pop up a schematic of a simple B output stage, emitter out..

Short across the supply capacitors, to represent the off condition.

Put 20 volts positive on the output line.

You are taking the NPN emitter above the supply voltage present on the collector.

C-B junction will be forward biased, E-B junction reversed.

Typical E-B reverse breakdowns are 5 to 15 volts..

If good current is drawn at 15 volts, the base dissipation will fry the unit.

Cheers, John

[sreten]

Hi John,

don't really want to argue about this as we both agree its not a good idea.

Seems to me quite complicated if you include the drivers (which supply base
current so we are talking 10V to 30V for both junctions) and the driver base
resistance will limit current, which is usually ridiculousy high.

sreten.

[jneutron]

Originally posted by sreten
Hi John,

don't really want to argue about this as we both agree its not a good idea.

Seems to me quite complicated if you include the drivers (which supply base
current so we are talking 10V to 30V for both junctions) and the driver base
resistance will limit current, which is usually ridiculousy high.

sreten.

We appear to disagree on a point...we are not arguing. We are having a nice discussion...

I have been talking about when an amplifier is off..you have obviously been talking about the amplifier being on...a funny discontinuity of understanding...

On, I agree with you completely..

Off is what I have been talking about...where the output devices are subjected to reversed voltages..

Cheers, John

[sreten]

No I'm talking off.

The output pairs bases are not connected to the voltage rail,
they are typically driven by the emitters of the driver pair.

This is for the most common emitter follower pair, CFP is different....

sreten.

[Svante]

At the risk of making a fool of myself:
isn't the back direction of the typical B-E "diode" somewhat like a poor zener, at about 5 volts? Given this and the diode between B-E, there seems to be a great risk of damage if the supply is grounded and say 20 volts is applied to the output of the amplifier.
For sure, I'd recommend him to have a (non-shorting) switch or relay to flip between the amplifiers.