switching one pair of loudspeakers between two amps

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Hi, diyers

I have two amplifiers, one stereo for music (Copland CTA-501, 4x EL34 valves, 30w) and other multichannel for Home Cinema use (Denon AVR-1802, rated at 5x80w, but perhaps about 5x40w when all channels are driven). I don't use the valve amp for Cinema (I don't want to waste the precious tube lifetime playing Hollywood blockbusters), but I do use my main L&R loudspeaker pair (Spendor SP1/2E) for that, connected to Denon's "front left" and "front right" outputs with centre channel in "phantom mode" and shared by both L&R speakers (a "4.0" setup). I believe this is the best way to integrate the cinema and music systems in the same room, but each time I want to see a movie, I have to manually swap the cables from the binding posts(bananas) of the valve amp to the ones in Denon´s backplate, and viceversa when I want to hear stereo music again. Swapping all four leads is annoying but now it would be difficult too, because a new arrangement of the equipment racks makes the binding posts on the rear less accessible.

My question is what is the best easy way to select between one or other system. The use of a relay and its associated power supply is too complex for my DIY capabilities, so I've searched on high current toggle switches (it seems there is no rotaries for this use) and fast and short-proof loudspeaker connection systems like Neutrik SpeakOn. What are your suggestions?.

I'd like to switch both channels simultaneously (the faster and easiest way), but I'm afraid that the separation between channels will decrease if both share the same 4P2T switch or the same 4-pole SpeakOn plug (for L+, L-, R+ and R-) in signal path. Do you recommend to use two separate L and R switches/plugs (a kind of "dual-mono" arrangement) or it would be overkill?


Regarding the quality/integrity of connection, what do you think it's better? The specs of a high quality high current toggle switch, like the NKK "S-series", are:
25A/125v- 9A/250v rating, phenolic resin dielectric case, silver-plated "silver alloy on copper" and brass contacts, without gold-plating given the high amperage (but perhaps this will be a problem as the switching will be done always "dry", with both amps off and no signal through the switch), a contact resistance of 10 mOhms maximum and a life expectancy of 25000 cycles.
The specs of the SpeakOn SPX 4-pole plug and chassis socket are:
30A/250vac rating, PBTB plastic body, silver-plated brass contacts, 2 mOhms(plug) or 3 mOhms (socket) contact resistance and a life of 5000 mating cycles.

It seems that toggle wins in reliability (and of course in convenience) and SpeakOn wins in lower contact resistance but it is more fragile and the repeated insertions perhaps will worsen perfomance with time. Have you any experience with this kind of switching? (I've searched in DIYaudio site and I've found little info about this). What are your recommendations based in sound quality? Any suggestion will be welcome. Thank you.

Regards,
Jose


PS.: Excuse me if I've posted this message both in amplifier forum and loudspeakers forum (is it allowed to do this?), but the questions I have are applicable to both.
 
it seems there is no rotaries for this use
There are plenty of industrial switch systems which will be suitable if you want a rotary switch. In general, though, it would be better to use relay switching - if you can wire a switch system you can manage a relay.
There's no need to go 'dual mono' - the impedence is too low to allow any significant crosstalk.
Why do you need to use Speakons (or similar)? They're good connectors, but once your switch is set up you won't need to reconfigure the cables. What's wrong with flying leads from the switchbox, with appropriate connectors for the speakers and amps?
 
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I'd also say go with the relays. If you are worried about the powersupply just get a dc plugpack (wallwart) of the correct voltage for the Relay's coil. then it really is only a case of connecting the switch and inputs and outputs to the relevant pins on the relay.

If you are really worried about channel separation just use one relay for each channel :) you wouldn't even have to make a circuit board, you could glue the relays to the bottom of the switchbox case (upside down) and p2p wire it :)

just observe voltage and current ratings for the relay as you would for a switch...

Tony.
 
relay switching system

Hi

Thank you for your suggestions and my apologies for the delay in my answer. Following your recommendations I’ve think twice and I’ve sketched a switching system using two relays that fulfill my needs. However, as I’m a newbie I’m posting a picture of it to let you know. I hope there will be no mistakes, but I’m open to further suggestions.

The switching would be done this way:

A) Position 1 (with the lever of the AC toggle switch down): The music valve amp is powered on, the relay power supply is off, so the coils are unpowered and the relays stay in their “normally closed” (spring-loaded) position connecting the music amp to the loudspeakers. Thus the valve amp is never “on” with no loudspeaker connected. This position is for listening to stereo music and, as the relay coils are totally disconnected, there will be no damaging effects on sound quality from them nor from their Switching-mode PSU.

B) Position 2 (lever in central position): Both music amp and relays are off. This position is used when I want to power off the main system and listening to a separate headphone amp I have.

C) Position 3 (lever in up position): The music amp is “off”, the relay power supply is “on” and it energise the coils of relays with 24volts DC so the contacts made are those of the “normally open” position. The audio signal is routed from the front L and front R binding posts of the Cinema multichannel amp to the L&R loudspeakers. This position is used when I want to see a film. Any magnetic influence of the relay coils or the effect of having a Switching-mode PSU in circuit is less problematic when listening to a movie rather than to music (moreover, as the relays and PSU probably will be fitted into the music amp, they would be far from the cinema amp circuitry connected at the same time).

What do you think about this arrangement? Any comments? I have yet some doubts that I hope you can answer. These are the following:

1) The Amplimo gold relay special for speakers, mentioned elsewhere in this forum, is DPST and not suited for my needs. I’ve found in Mouser a Fujitsu DPDT relay (FTR-F1) with sealed silver alloy contacts and gold plating. It is rated at only 5A (switching) but 7A (carrying). My amps have low power outputs (30- 40 watts) and the relay will be never switched with amps on, so I hope this 7A rating is enough. Am I right?

2) What is the best way to mount the relays? I have no problem with space, even if I put the switching box into the music stereo amp (there’s plenty of room inside)

3) The power supply for relays could be a high quality 24v-0.5A regulated (I think) switching-mode PSU that I already have (Stontronics T2500ST). Is the voltage the only relevant parameter to consider for this PSU (the relays I’ve found have widely different Ohms and power coil ratings)? Are the connections in the picture correct (+24v to one side of both coils (in series) and the minus pole to the other ones)? The 0.5A rating will be enough for both coils?

4) I’ve seen that usually a diode and/or a resistor are connected across the coils terminals, I believe for protecting other circuits from a “pulse” (I think) that the coil generates when unpowered. Do you think this will be necessary in my case? (there is no other circuit involved except the two relays). If so, what types and values do you recommend?

5) The snubber circuit you can see in the AC switch (between AC in and AC out to music amp) was implemented in the original AC switch of the music stereo valve amp, I think to protect a rotary DPST switch with lower ratings. I’ll use a NKK S-333T DPDT (on-off-on) toggle switch rated at 15A. Is the snubber really necessary? If so, what values are needed?

I apologise again, this time for the length of this post, but I wanted to fully explain this rather complex switching system (well, not so complex… I think that, if all above is correct, this is a neat solution to my needs). Thank you to everyone for your help.

Best regards,
Jose
 

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Just another Moderator
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Hi Jose,

i think your setup looks good. Since the switchmode power supply will never be running when you have your Tube amp on, then I'd say there isn't a problem housing it inside the Tube amp's case, provided you are comfortable with adding the extra speaker terminals on the rear to take the speaker in for the other amp :) and re-routing the existing speaker out to the relays isn't a problem.

for mounting the relays you could just glue them to the bottom of the case with something not to brittle (silicon or similar) so that if you want to take them out again it isn't too hard.

on the question of diodes and resistors with the relays, I'm not sure about the diodes, but the resistors may be necessary/advisable for current limiting purposes. if for examle the relay's coil is rated at 24V 20ma then you would use a 1.2K resistor to limit the current to 20ma.

I think on the switch snubber the best approach would be just to duplicate it :) shouldn't be that critical, you could just use the existing parts :) I believe (but could be wrong) that it just helps to stop switch bounce noise...

with regards to current handling, my 100W amp has about 3.5A at full power into an 8 ohm load so I think 5A should be more than enough for 40W :)

Tony.
 
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