Dumb question about PA amps

Subject: RE: Dumb question about PA amps
From: The Fooles Troupe
Date: 30 Nov 04 - 06:57 AM

If it really was a shorted plug being plugged in, then it is possible that it was a mono amp intended for 2 speakers in series, and the shorting plug just allowed one speaker to be omitted - in that case the thing would not work without the second speaker connected - the plug may also have had a loading resistor inside to help things.

Subject: RE: Dumb question about PA amps
From: s&r
Date: 29 Nov 04 - 07:00 PM

I could nit-pick about some detail here , but it's a good straightforward summary

Stu

Subject: RE: Dumb question about PA amps
From: fiddler
Date: 29 Nov 04 - 06:45 PM

It used to b*gg(r the old HH amps to ru n with onloy one speaker - I don't know why it jsut did!

I desn't bother newer amps!

Andy

Subject: RE: Dumb question about PA amps
From: s&r
Date: 29 Nov 04 - 06:18 PM

Impedance

Stu

Subject: RE: Dumb question about PA amps
From: Richard Bridge
Date: 29 Nov 04 - 06:06 PM

Let's keep it simple. Valve amps: do not EXCEED the recommended resistance (no speakers-infinite resistance)
Trannies, do not go below the minimum resistance.

Subject: RE: Dumb question about PA amps
From: Bernard
Date: 29 Nov 04 - 05:22 PM

Just to get technical...

Yes, stereo amps need a load on both channels, though if you can prevent the side with no speaker from being 'driven' - turn the volume off! - it will be okay as the load is only required when a signal is present.

Putting a shorted jack into the effects loop (on a mixer-amplifier) may also cut off drive to that channel, but be sure you know what you are doing!! Don't short the amplifier output!! If it's a slave amp, just don't plug the input lead in on the unused side...

Mono amps are okay with just one speaker, but you must be sure the single speaker can handle the extra load...

Two eight ohm speakers on a 100w amp must be able to handle 100w between them, but one eight ohm speaker on the same amp is likely to be subjected to around 70w because the load is inductive, not resistive. In other words, Ohms Law doesn't strictly apply - there are other considerations.

Ooops! Another can of worms!! ;o)

Subject: RE: Dumb question about PA amps
From: mkebenn
Date: 29 Nov 04 - 08:14 AM

Stu was on point. Never run a stereo amp without both speakers connected. Lack of resistance can damage the amp. Mono amps are safe as there is only one channel, and only one speaker is needed to provide the load. Mike

Subject: RE: Dumb question about PA amps
From: GUEST,Songster Bob
Date: 28 Nov 04 - 09:57 PM

Older tube amps with output transformers need a load on each (if they're stereo), so you'd need a plug with a resistor across it for the second channel. Solid state amps are not liable to such damage, and can have any number of speakers attached. I have one with two output jacks, but you can use just one, even daisy-chaining the speakers if you have two and room for 'em. So the amp can be between the speakers (using both jacks) or at one end of the speaker chain.

Bob

Subject: RE: Dumb question about PA amps
From: Big Al Whittle
Date: 28 Nov 04 - 06:31 PM

Right got the message. If I disappear in in a puff of smoke in the middle of the gig. I will take a most serious view of the matter.
thanks for all contributions
best wishes

al

Subject: RE: Dumb question about PA amps
From: treewind
Date: 28 Nov 04 - 06:28 PM

Some valve (tube) amplifiers used not to like operating with no speaker connected. With any transistor amplifier should be totally harmless to operate it with nothing plugged in the speaker connector.

Anahata

Subject: RE: Dumb question about PA amps
From: Grab
Date: 28 Nov 04 - 05:56 PM

If there's no speaker connected, plugging in a cable does nothing, so it can't affect the amp at all, so that's bogus. *Unless* this was some home-brew amp which had some bizarre switching arrangement or a strange screwed-up output stage that required *some* loading (in which case there would have been a cable with a load resistor, not just an unconnected cable). Bottom line - if it ever was true, it isn't now.

Graham.

Subject: RE: Dumb question about PA amps
From:
Date: 28 Nov 04 - 05:03 PM

It might have been a problem in Tube situations. It's been a LONG time!

Subject: RE: Dumb question about PA amps
From: s&r
Date: 28 Nov 04 - 01:56 PM

If it's a stereo amp, turn down the channel that's not plugged in. If it's a mono amp the jack connectors are likely to be in parallel. No harm will result from using only one speaker.

Stu

Subject: RE: Dumb question about PA amps
From: Hand-Pulled Boy
Date: 28 Nov 04 - 09:41 AM

Put the other speaker in the toilet but make sure it's plugged into the amp. You'll reach a whole new audience in there!

Subject: RE: Dumb question about PA amps
From: mack/misophist
Date: 28 Nov 04 - 09:20 AM

A jack that's not connected to anything wouldn't change the characteristics of the system. ie. It might as well not be there. I guess the best compromise would be to put a large resistor across the plug at the speaker impedence. But my training was decades ago, too.

Subject: RE: Dumb question about PA amps
From: Leadfingers
Date: 28 Nov 04 - 08:49 AM

From the depths of (very old) involvement in electronics I would say the problem would be the loading of the output stage of the amp ! It would depend on what the output on the amp is , but it would only be a
problem at 'full' volume on most amps . The imbalance would not help the sound , and I would rather put both speakers on for safety's sake . If space is a serious problem just put one speaker on top of the other !

Subject: Dumb question about PA amps
From: Big Al Whittle
Date: 28 Nov 04 - 08:30 AM

Recently I saw a folk duo doing a small pub and they only had room for one PA speaker, so they plugged just one speaker in.

Now many years ago I was told that you could 'do some damage to the amp' and that you should always plug a jack into the empty socket in the amp - even if you didn't plug it into a speaker.

is this an old electricians tale.

I need to know cos I'm thinking of doing the same gig and it is very small.