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Old 14th March 2005, 03:30 PM   #1
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Default Puzzler!! Using 3 amps w/ 1 pair speakers

Have searched these forums and others looking for guidance on using a pair of speakers with three amps. Hope to complete Brian's Rev C 3875 chipamp this month, and also have tube amp and solid state amp.

Would like choice to enjoy any one of the three amps, selecting with a switch as opposed to rewiring the speakers each time.

Why is this such an uncommon thing (there are very few related posts)?

My thought is to use simple rotary switch circuit, no relays. Mouser has an Electroswitch C5 series 4 pole 3 position rotary switch for $6.73, rated 1A@28VDC, 1A@125VAC.

Would use 2 poles in parallel for left, 2 in parallel for right to achieve 2A@125VAC, or 250 watts per channel. Max amp output is wll under 100 watts per channel, so I think this should be fine, although the contacts are brass with silver plate as opposed to the preferred gold. No LEDs to indicate selected amp, just knob pointer.

I understand I will need to be careful to turn down or off all amps before rotating the switch, and be careful to not allow and amp to try to drive speakers when none are swithed in.

I would really appreciate feedback from experienced members of the forum to see if I am on track. Sometimes, I am learning, in the world of audio, things are not always what they seem...

Ransom Peek
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Old 14th March 2005, 04:03 PM   #2
AndrewT is online now AndrewT  Scotland
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100w amp uses 40v peak to drive 5a into 8 ohms. If your speakers have a low ohm somewhere in the frequency range you could be asking for short term currents approaching 10amps.
Nothing wrong with silver contacts, very low electrical resistance but poor environmental resistance. Use a proprietary non abrasive cleaner fairly regularly to keep corrosion at bay.
Ensure the switch has break before make contacts to avoid shorting amp outputs together.
After you have found which amps produce different sound try experimenting with bi or triple-amping your speakers using one make for the treble and another for the mid etc.
regards Andrew T.
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Old 14th March 2005, 04:39 PM   #3
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Default Follow up to AndrewT's input

OK, so we know that a rotary switch to handle the power is not feasible - there are some high power rotaries but they can cost $100+! I'll use a rotary switch to drive relays then.

So, for three amps, do I really need to switch 2 channels x 2 leads per channel = 4 leads per amp, so the two speaker pairs (4 leads) nead to switch to three sets of four? Right?

Is there a way to ensure break before make with a rotary switch controlling relays circuit, and is it not an issue if you were to always turn off unused amps before switching?

Could you kindly explain what you mean by bi amp/ triple amping...
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Old 14th March 2005, 05:27 PM   #4
dnsey is offline dnsey  United Kingdom
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A search of surplus outlets should reveal high current rotary switches at sensible prices, as they are widely used in industry. Most are modular designs, so you can configure them to suit your requirements, and many have fast-break actuation, which would be ideal.
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Old 14th March 2005, 06:03 PM   #5
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A couple of questions come to mind.

Are you using a single preamp? If so, you would need to switch the signal coming from the preamp as well as the speaker outs. You wouldn't want to be driving an amp with no speaker hooked up, especially the tube amp. This means you are going to need a dual rotary switch capable of switching 4 signals, (2 preamp and 2 speaker)

I think you're right about turning the amps down before switching but what if you or someone else forgets? Could be hard on your speakers if switched while music is playing.

I've toyed with the idea of using a tube amp for the highs and solid state for the lows in a biamp system. Mabe some day.

Blessings, Terry
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Old 14th March 2005, 08:18 PM   #6
Giaime is offline Giaime  Italy
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Originally posted by still4given
especially the tube amp.
NEVER run a tube amp unloaded. This can cause major damage to the transformers. And avoid switching the outputs of tube amps: most of the time when the rotary switch become dirty or the relays become malfunctional, you have a dead amp. Trust me.
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Old 14th March 2005, 08:25 PM   #7
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Default No alternative???

If switch and relay contacts failure is too much of a concern (dmage to tube amp) then the best idea is to buy additional set of speakers for the other amps, or, perhaps, use a patch cord/ patch panel - just plug in each system as chosen....

No reasonable alternative?
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Old 14th March 2005, 08:29 PM   #8
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Have a relay with a 8r resistor on the 'normally closed side' on all your switches. an amp would never accidentally run unloaded.
Crazy Yankee.
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Old 14th March 2005, 08:49 PM   #9
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Default Safe switch for amp output

mpmarino - that is such a simple fix! I love it! But, it means the more complicated relay solution, and assumes, of course the relay doesnt hang up in no man's land with neither contact made.
Would the resistor wattage need to be at least the wattagge of the max amp output? So, for the LM3875 at least 56 watts? And I would guess you cant use wirewound resistors because of inductance issues...

Is there a similar fix if using a power rotary switch, no relays?
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Old 15th March 2005, 08:38 AM   #10
Enzo is offline Enzo  United States
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The tube amp definitely needs to be loaded, but solid state amps do not. They produce an output voltage, but it is the load that draws the current from it. AN unloaded SS amp just sits there cool.

I think the reason you don't see this sort of thing is that there is little market for it. People like us like to tinker, but for the most part, people want to listen to the music rather than play with what the different amps sound like. In the same way as I might use my word processor program to write letters, but I would not be interested in the ability to run three different word processors at once with the ability to switch the same document back and forth between them. On the other hand, stereo equipment sales showrooms do have such switching gear, because there the differences are what they are listening for.
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