Hi - any risks with this setup:

I have a 5:1 setup with an onkyo receiver.
I have added a tube amp and cabled it to the left and right speakers
in addition to the cabling from the receiver
(different sources for each amp).

So, two amps, one pair of left and right speakers, with two sets of speaker cabling, one from receiver and one from tube amp.

I am presuming that as long as both amps are not on at the same time that this is safe.

1 ... is that correct, or am I missing something?
2 ... what happens if both amps end up on?

Advice or experience welcome ...

Hi paulsaints - not a good idea I think.

colinf (or others) may help us out here but when the tube amp is connected, its OPT (output transformer) secondary winding will be connected across the output terminals of the SS receiver. This OPT sec winding will be very low resistance/impedance, almost a 'short circuit' & I don't think the SS receiver (or any amp) will like that at all.

This is regardless if the tube amp is on or off - the OPT sec winding will still be connected.

As far as the tube amp is concerned, I'm not sure what a SS receiver's output will 'look' like to the tube amp.

DarkLantern blog - darklanternforowen.wordpress.com

I agree with Owen, this is not a good idea.

I was faced with a similar situation with home theatre and 2 channel hi fi in the same room and wanting to use my 'good' speakers for both.

One option ( but still not really recommended ) is to have bullet connections on the speaker end of your cables so that you can simply plug in the appropriate system to the speakers when being used. After all you will not be using home theatre and hi fi at the same time.

There are also available switching boxes to switch between 2 sets of speaker cables.

The danger that still remains with this configuration is you may accidently turn on the valve amp with its speaker wires not connected to the speakers. This is a definite no no with valve amps as they do not like an open circuit which can damage the output transformers. My understanding however is the SS amp is OK with open circuit.

I persevered with this configuration on my set up for some time but was always concerned that one day I would forget to connect the correct wires so in the end bought some smaller speakers just for dedicated home theatre use. They work fine for movies and are supplemented with a sub woofer for rumbling effects.

Cheers

Graham

Hi Paulsaints, I agree with Owen and Graham. Not recommended to connect both amps at the same time even if one of them is off while the other is on. It’s best to use a switch on the speaker cable to switch between the two amps.
In the original scenario, if the tube amp is on and the Onkyo switched off, most 5.1 receivers have a relay on the poweramp output which only connects the SS poweramp circuit when the receiver is on. So as long as the receiver is switched off or set to headphones only, the tube amp would work ok into your speakers.
On the other hand, if the receiver is on and the tube amp off, the receiver is now driving the output transformer of the valve amp as well as the speaker, which will put extra load on the receiver, especially at dc and very low frequencies, and also at ultrasonic frequencies where the transformer capacitance and resonance come into play. To fix that you could add a switch in line with the valve amp speaker outputs to disconnect them for only while the Onkyo is on.
Where it gets dangerous though, is if both amps are on at the same time (easy to do), where the negative feedback of one amp fights the negative feedback of the other amp. It will end in smoke or high heat. It will be like watching two moose head butting each other over a female!
Also, if the Onkyo didn’t have a relay on the output, as in if it was a SS poweramp where the amp circuit connects straight to the speaker terminals: the tube amp would attempt to drive the severely non-linear resistance of the switched-off SS amp and not win.

AMR-iFi R&D

Glad I asked - thanks for the advice and the tech behind it. Nothing died overnight

So, something like a Beresford TC-7220 would be the way to go. Any caveats with that approach??

www.homehifi.co.uk/S/tc-7220mk3.htm

Thanks -Paul

Feb 12, 2019 17:48:27 GMT 12 paulsaints said:

Glad I asked - thanks for the advice and the tech behind it. Nothing died overnight

So, something like a Beresford TC-7220 would be the way to go. Any caveats with that approach??

www.homehifi.co.uk/S/tc-7220mk3.htm

Thanks -Paul

Sorry, but NO.

IMO that gizmo will just select an input from one or the other of the two amps.

The one not selected will more than likely be switched to open circuit, and if that is your valve amp the same potential problem still exists.

Boom, goodbye valve amp !!!!

I went through the same exercise and gave up. Buy a second pair of speakers.

When I had a similar set up, I did not run the front L & R spkrs from the receiver, instead running the Pre-outs for front L&R from the receiver to to the separate (integrated) amp. Long time ago - had a Denon 3806 and a Perreaux R200iP - and cant recall what inputs I plugged into on the Perreaux. It could be that the Perreaux had a HT by pass switch, too long ago to recall.

The Beresford switcher would work fine. To stop the valve amp from self-oscillating while it is disconnected from the speakers you could add a resistor of, say, 47ohms 5w across each channel of its output terminals, just so that it has a light load. If the valve amp doesn't have negative feedback you don't need to do that.

AMR-iFi R&D

Thanks colinf - just to check, would the valve amp be "self-oscillating" when the Beresford was switched to the SS amp AND the valve amp is switched off (it has front on off switch, so that's easy to do).

No, it won’t oscillate if it's off and disconnected from the speakers.
Which amp is it? So I can see what sort of circuit it has and if it could do with a slight resistance connected across the speaker terminals while it’s on but not connected to the speakers.

AMR-iFi R&D

Triode TRV m88se mono-blocks

Can I also say how much I appreciate the advice available through this forum - we love the aesthetics, we love the sound, but the technicals are very much part of the "engagement" with hifi, and advice here is superb!

with thanks - Paul

Nice looking amps, built well. It looks like they might use negative feedback, being a push pull KT88 amp. I’d possibly err on the side of caution and put the 47 ohm 5w resistor across each monoblock's 0 and 8 ohm terminals, just so that they don’t have a chance to oscillate without a speaker connected. Of course if you turn them off while the Onkyo is on the speakers you don’t need to add the resistors.
Always happy to help if I can!

AMR-iFi R&D