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How to use (and switch) the same loudspeakers with two amplifiers
How to use (and switch) the same loudspeakers with two amplifiers
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Old 15th October 2005, 08:11 PM   #1
spendorspain is offline spendorspain  Spain
diyAudio Member
Join Date: Apr 2005
Location: Spain
Default How to use (and switch) the same loudspeakers with two amplifiers

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.

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Old 16th October 2005, 12:10 AM   #2
PRR is offline PRR  United States
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Location: Maine USA
You worry too much. Do whatever seems easiest.

> I don't want to waste the precious tube lifetime playing Hollywood blockbusters

The heaters should last many decades. If the plates are not working past the rating, the tubes should last a decade or more even at many hours per day. I sometimes find 1950s amps with original tubes working fine (old caps and resistors give more trouble than tubes). Around 1975 I acquired a 1950 audio console that had been in 24/7 duty for most of 20 years, 78 of the 80 tubes were original, and only one of those was sick when I got hold of it. This included the mil-spec versions of 6V6 working at maximum ratings (as long-line drivers). I was mildly astonished when I picked up a 1938 PA amp in a thrift ship, which had clearly led a long and very hard life, with original 6L6G that "worked" and made full power but had rather a lot of grid leakage. For in-production tubes like EL34, I would not worry about "hours".

> I'm afraid that the separation between channels will decrease if both share the same ...... signal path.

If you merged it to a 3-wire connection (which is a bad idea for other reasons) your separation would be measurably less due to common return resistance. I do run some 3-wire stereo runs, and though separation is "only" 30dB, it is audibly excellent. It all winds up in the same room, after all.

With a 4-wire connection the only effect is capacitance. If the source resistance is zero, separation is high to infinite frequency. The tube amp output will be non-zero. Say 2 ohms (damping factor of 4). Capacitance in switch or connector is probably 10pFd, but the cables into the switch/plug may run close together. 100pFd is reasonable for a meter of bundled cable, but say 1,000pFd (both channels' cables taped tight together for 10 meters). Separation will be near zero at 80mHz, 60dB at 80KHz, 72dB at 20KHz, 84dB at 5KHz (where you can actually hear directions).

If the two signals were a loud-shouted intercom and a fine classical music recording, 84dB might be barely audible if a loud-shout happened in a quiet program passage. For two sides of the "same" program, 84dB is much more than enough.

I have the same reaction to the contact-resistance numbers. In series with >6Ω voice-coil resistance, 0.010Ω or 0.005Ω is "nothing".

Don't be afraid to plug the Neutrik all day long. They are intended for commercial sound systems including portable rigs that get set up and taken down every night, under great time pressure and sometimes in awkward situations (hanging from the rafters). They are pretty tough. They are probably going to last longer than the amp, speakers, or even your home. The 5000-cycle rating may reflect testing difficulty (you sit there and try 5000 plug/unplug cycles) rather than ultimate life. They will sure last longer than the $2 switches I once used in a network application, though a good $10 switch will last "forever".

I would not worry about hot-plugging/switching. House-light switches carry 5A starting surges and do so for hundreds of operations, even the $0.39 ones give years of service. I agree that in your situation, there is no "need" for hot-switching; but if you do occasionally hot-switch by accident I would not worry.

So it comes down to which is easier? The switch is easier for the user, though it does want to be mounted in something solid. Plugs can be left laying around, and if you wire a few extra then other amps or speakers can be quickly patched-in for auditions. A switch does force you to bring all 12 wires into a small space and solder, the plugs have just 4 wires so you have easier soldering access to each terminal.
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Old 16th October 2005, 04:31 PM   #3
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
will you remember to NEVER switch the valve amp ON when the speakers are disconnected?
A switch over relay would be better here to connect in a dummy load. Upto two times the speaker impedance would do.
Two relays in parallel but with antiphase poles, see Borbely for explanation, might be better for each connection.

The surround seems to be using your stereo speakers and some others. Would surround sound better if you had five matching speakers? Then leave the stereo unused when on movie duty.
regards Andrew T.
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Old 16th October 2005, 05:38 PM   #4
dnsey is offline dnsey  United Kingdom
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Join Date: Mar 2004
Location: Shropshire, England
will you remember to NEVER switch the valve amp ON when the speakers are disconnected?
Or even actuated by the valve amp's heater supply, thus achieving 'automatic' switchover when stereo is required. In this case, there would, of course, be no need for a dummy load.
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Old 21st October 2005, 04:17 PM   #5
spendorspain is offline spendorspain  Spain
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Location: Spain
Default Relay switching system


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 contacts) 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,
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File Type: jpg relayswitchingsystem.jpg (48.5 KB, 161 views)
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