Connecting 2 amplifiers to one set of speakers: making an amp/speakers selector switch

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Gallery Recreation Renovations

 

Is it possible to use two amplifiers or receivers with just one set of speakers? This question gets asked a lot. There’s a numbers of reasons for trying to do that. You could have a home theater receiver for movies and a dedicated stereo amp for music (like I do) and not enough space to accommodate two separate sets of speakers for each amp (or a wife, who vetoes this solution). You could have one big, powerful “performance” amp for active or loud listening, and another, smaller amp that draws much less power for background music and nighttime listening, when the family’s asleep and loud music is a no-no (like I do in another room). Or you simply want to quickly compare 2 amps (like we do very often here at AR), A/B test them without having to make long pauses for switching the amps off, disconnecting speaker wires from one amp and connecting them to the other amp.

So, again, is it safe to connect two amps to one set of speakers? If you do that directly, simply run speaker cables from both amps to your speakers – definitely not. If you do that, and then turn on both amps, you will fry at least one of them, possibly both. Even if you think you’ll be extra careful and always remember to switch off one amp before switching on the other, and you actually are that careful, sooner or later someone else will try to listen to some music using your gear – and poof goes your nice amp. This exact thing happened to a friend of mine. He was careful, but his daughter wasn’t.

Is it possible to use a speakers selector switch in reverse for this? Well, not really. There are three problems with the majority of speaker switching boxes. One is that they usually have common ground. You do not want to have common ground between two amps, this could negatively affect their performance (and sound quality) and in some cases also case damage – there are amps that have fully balanced outputs and cannot have common ground between speaker connectors, for example some Class D amps. Another problem is that many speaker switches have protection and/or impedance matching circuits, and those could also affect sound quality, so you do not want that. But most importantly, most (actually almost all) of these switches allow to select more than one set of speakers at the same time – which becomes selecting more than one input amp when you use the switch in reverse. And if you do that, it’s exactly like directly connecting two amps to one set of speakers – if both amps are on, at least one of them will get fried.

But we’re on the right track now: what you need is indeed a switch – but it has to be a specific type of speaker level switch in order to accommodate 2 amps. Here’s what it needs to do:

1. It needs to be a “break before make” type (break connection to one amp before making a connection to the other amp) to prevent both amps from being connected to each other even for a split second – it should make it absolutely impossible to select both amps at the same time.

2. It needs to switch all 4 connections: L+, L-, R+, R- separately (no common ground).

3. Ideally it should not have anything else inside except binding posts, wires and the switch itself (no resistors, no protection circuits, impedance matching etc.).

With a switch that meets all the above conditions, you can safely connect two amplifiers (or two receivers, or an amp and a receiver) to one set of speakers. They can be on at the same time and you can switch between amps without having to turn them off.

There is one exception: for many tube amps it is very unhealthy to be on with no load (no speakers connected), so If you have a tube amp, better get another set of speakers.

The only commercially available switch I know that does all that is Beresford TC-7220. Unfortunately that switch is quite expensive – it costs $120, which is a lot for a box with a few binding posts, passive switches and some wire. Still, it is pretty much the only thing you can buy that will do the trick safely, so if you want a ready-made solution, that’s what we can recommend. But if you don’t want to spend $120 on a switch, you can built one yourself. Which is exactly what we did here at AR.

BUILDING AN AMP/SPEAKER SWITCH

The simplest and cheapest option is to built a switch box using a 4PDT switch. We’ve built two, the total cost is around $10-30, depending how cheap you can get the switch and what type of binding posts and enclosure you use. Here’s how we built one of them. It’s the more expensive one because of banana plug-ready connectors; if you use spring-type connectors, it becomes very cheap.

You will need the following parts:

1. One 4PDT switch or two DPDT switches (we recommend toggle switches)
2. 3 sets of stereo speaker binding posts
3. 1-1.5 m of speaker wire to connect binding posts to the switch (6 short sections)
4. An enclosure
5. Mounting screws and/or glue

Depending on the type of switch and binding posts, you might need solder wire and a soldering iron to connect internal wiring. Also, depending on the enclosure you use, you might need tools to make walls, drill holes etc.

We used an enclosure of a nice-looking but crappy speaker as a housing for our switch.

We removed the driver and the front wall, and the bass tunnel from the back. As a result, we had a nice box without one wall and with a round opening in the opposite wall.

We cut the missing wall from plywood and made holes for binding posts. We used speaker wire to connect the binding posts to the switch.

 

A 4PDT switch has 12 connectors, usually 4 connectors in 3 rows. In this application, the two outer rows are for amps. The row in the middle is for speakers. Like this:

A1 L+ A1 L- A1 R- A1 R+
SP L+ SP L- SP R- SP R+
A2 L+ A2 L- A2 R- A2 R+

A very important thing when connecting wires to the switch and binding posts: make sure that each wire is tightly secured with a screw or well soldered and there are no loose strands that could cause a short. If you solder the wires to connectors it’s best to isolate them with heatshrink after soldering. Test all connections after soldering for conductivity and possible short circuits between connectors. If you short connectors you will fry your amp, so be extra careful.

We installed the switch in the opening left after removing the bass tunnel.

Then we fitted the rear wall with binding posts.

Wires sometimes get loose during assembly, so at this point all connections of the switch should be carefully tested again for short circuits. Make sure you do that before connecting amps.

That’s it all done. This switching box works for either two amps connected to one set of speakers, or two speaker sets connected to one amp. You can select only one position at the same time. If you want a switch that will accommodate two amps and two sets of speakers (exactly like the $120 Beresford does), you will need one more 4PDT switch and one more set of stereo speakers binding posts. In that scenario, the connections from the amp binding posts to the outer rows of the first switch remain the same; the first switch’s connectors marked SP XX in the above table (the middle row) are connected to the corresponding connectors in the second switch, and the outer rows of the second switch are connected to speakers binding posts for two sets of speakers. The first switch switches between amps, the second switch switches between two speaker sets.

The whole process (including making the enclosure) took us about two hours. The result is a functional, safe, and aesthetic switch that allows to safely connect two amps to one set of speakers and switch between them even when the amps are on, just like the $120 Beresford switch, but ours costs only $22 in parts (again including the cost of speakers that we gutted to make the enclosure). The most expensive part was the speaker wire binding posts – the white ones cost $12. We used them only because they were already sitting in our drawer. There are much cheaper alternatives (both spring-type and banana plug ready) and if you use those, the total cost will be closer to $10.

 

12 comments

  1. François

    Hello Rafal,

    I would like to create a selector-switch box for 3 amplifiers to my set of speakers.
    I’m using the speaker outputs A and B on every amplifier.
    The set of speakers includes : 2 columns (high-medium), 2 bookshelf speakers (med-low) and one subwoofer.

    Using the same principle that you’ve described for 2 amplifiers and 1 pair of speakers,
    – do you think this would be safe,
    – would you recommend it,
    – what would be the schema to make it.

    Very best regards.

    François

    1. rafal lisinski Post author

      Hello François,

      Adding switches in the signal path can always degrade the sound a bit. But with switches of this type any change in sound should be minor or even inaudible. My way of switching is safe for both amps and speakers, so it can be used also in your scenario.

      To accommodate 3 amps and 2 pairs of speakers you can daisy-chain 3 switches made according to the same principle as my switch. Or actually 2 switches like mine to switch between the amps and one cheap, ready-made speaker selector, because switching between speakers is a less sensitive issue and you can have both pairs of speakers playing at the same time and, unlike amps, they can have common ground. Unless one of your amps does not tolerate that, in that case it is better to use my (safer) switch.

      So we have:

      Amp A, Amp B and Amp C
      Speakers A and Speakers B
      Switch 1, switch 2, switch 3

      Remember that my switches can work both ways, this means that they can have either 2 inputs and 1 output or 1 input and 2 outputs.

      Connection would go like this:

      1. Switch 1: 2 inputs, 1 output. Amp A and Amp B are connected to the inputs of switch 1. It selects between Amp A and Amp B.
      2. Switch 2: 2 inputs, 1 output. Amp C and the output of switch 1 are connected to the inputs of switch 2. It selects between Amp C and whichever amp has been selected with switch 1 (Amp A or Amp B).
      3. Switch 3: 1 input, 2 outputs (this one can be ready made if all your amps can handle common ground and more than one set of speakers connected at the same time). The output of switch 2 is connected to the input of switch 3. The outputs of switch 3 are connected to Speakers A and Speakers B.

      With the above system, switches 1 and to select amp A, B or C (only one at the time). Switches 1 and 2 MUST be like the one I used in my project. Switch 3 selects speakers A and/or B. This one can allow to select both speakers and have common ground if ALL your amps can handle that. If any of your amps can’t handle that, use the safe switch (like mine).

      That covers amps and stereo speakers. To accommodate subwoofer I would have to know what kind of subwoofer you’re using and how it is connected to each of your amps. If it’s a passive or active “pass through” subwoofer that you connect to the speaker outputs of an amp, and then connect stereo speakers to the subwoofer – you can connect it the same way to one set of outputs of switch 3. If it has a different connection method, I’d have to know the details.

      Hope that helps,

      Rafal Lisinski

  2. Stephen

    Excellent solution!

    I have a slightly different question and want to be certain I have it right.

    2 receivers/amps, 2 pairs of speakers Utilizing Channels A & B.

    The first receiver is Surround-Using Channels A & B, + Klipsch Subwoofer.

    The second receiver, Marantz 2252, Using Channels A & B (same speakers), but no subwoofer.

    Thank you so much for the knowledge you’ve already imparted.

    Stephen

    1. rafal lisinski Post author

      Hi Stephen,

      Glad my article is helpful!

      Now to your specific situation. I understand that the subwoofer is not important because you only use it with the surround receiver and you only need to connect speakers A and B to two receivers. There are two possible scenarios:

      1. You use either speakers A or speakers B (but never both at the same time) with each receiver
      2. You use both speakers A and speakers B at the same time with each receiver

      I understand that your situation is the second scenario.

      Also, one of your amps is a stereo receiver (the same signal goes to speakers A and B) and the other amp is a surround receiver (different signal goes to speakers A, different to speakers B). That is also important. That means that even if you ALWAYS use both speakers A and B, they cannot be connected to the switch in parallel, because only the stereo receiver uses a parallel connection. The surround receivers actually works as two separate amps (front speakers amp and rear speakers amp). I understand that in the case of the surround receiver speakers are connected to front and rear outputs, and not to front A and front B outputs (that would be a very different connection and actually similar to using a regular stereo amp).

      In that case, the best solution is to use two switchers like the one I described. One switch switches speakers A, the other switches speakers B. Unfortunately there are no simple, sturdy and cheap mechanical toggle switches (that I can find) that would allow to switch a total of 8 separate signal lines (and that’s how many you get with 4 speakers), which is why you need a separate switch for each pair of speakers. It’s not a big problem, you just flip 2 switches instead of one. But it does mean two times the work, materials and time needed to build the switcher.

      The two switches and corresponding speaker binding posts can be fitted into one enclosure, obviously it would need to be larger.

      This solution (2 separate switches for speakers A and speakers B) will work perfectly fine in other scenarios, for example with 2 stereo amps that use speakers A and B, or with a stereo amp and a surround receiver where the speakers are connected to front A and front B outputs of the surround receiver (as opposed to its front and rear speaker outputs). But in some cases a simpler solution would suffice.

      Hope that helps. Let me know if you have any more questions and good luck!

      Rafal Lisinski

  3. Eric Martin

    Thanks for this post
    Managed to put a switchbox together this afternoon and It works a treat, no more ‘protect’ mode.
    Total cost here in Aus, including enclosure was around $17.
    Very happy with the result.

    1. rafal lisinski Post author

      Hi Eric,

      Glad my post could help. I’m curious to see your switchbox, send me a photo if you can. My email is in the ‘Contact Us’ tab.
      So do I understand correctly that you were using a different switchbox before and it was sending your amp into protect mode? What sort of switchbox was it?

      Rafal Lisinski

  4. Steven

    I am currently building one of these and had a question. Could this used to use one turntable with two amps subbing RCA jacks? I see no reason why not do you?

    1. rafal lisinski Post author

      Sure, but it’s overkill. You can also use a cheap ($2) AV switch for that purpose. Search for “AV RCA Switch Selector Box”. Common ground between channels is not a problem with turntables, and you can use the 3rd RCA socket to connect and switch the grounding wire.

        1. rafal lisinski Post author

          I disassembled 2 cheap passive AV switchboxes that I happened to have at home and I did not find any resistors or other components inside that would alter the signal, just sockets, a PCB with tracks and a switch. The quality of components if as poor as can be expected, but there are no wires, so it should primarily affect the switch durability, not signal quality. At least it shouldn’t do any more harm than an average quality cable would.

          I used one of these switches for a turntable in my decent (but not hi end) system without any problems or audible signal loss. I used the video socket to connect (and switch) turntable ground. But whether such a switch degrades the signal could vary depending on the quality of the audio system (there could be an audible difference with a hi end system – just like with better and worse quality cables).

          Active switchboxes are a different story, in my experience they do degrade sound and have their own background noise.

          Of course, toggle switches can also be used for this purpose. A 4PDT switch has the right number of connections (L+, R+, common ground, turntable ground). Common ground is not a problem with turntables in most cases. But i still think building a switchbox with a toggle switch is overkill for switching the source-amp line (including turntables), considering that a simple AV switchbox does the job and it’s very cheap. Unless it doesn’t work (or causes signal degradation) in your system.

          BTW, resistors should not degrade signal quality (unless they’re poor quality themselves), they should simply make the signal weaker, quieter. That, of course, is not preferable in the case of signal from a turntable, which is very weak to begin with.

    1. rafal lisinski Post author

      Hi Adam,

      Buying the speaker switchbox you mention might be an option but:

      – that speaker switchbox costs almost exactly the amount we paid for all parts (including the enclosure) of the switchbox in our article, since then we’ve built another switchbox that was 30% cheaper because we’ve found cheaper sockets; of course that depends on the cost of parts in your area but you would still have to buy some parts you will need to make the conversion: at least a 4PDT switch and probably some wires;

      – you would still have a lot of work to do, you would have to replace the two switches that are there now with the switch we recommend (a 4PDT switch), and fitting a 4PDT switch in an opening left after one of those flat cradle switches might be difficult and it might turn out that you can’t fit a 4PDT switch with screws for wires (it’s quite large and might not fit in this enclosure) and have to use a smaller switch that you have to solder wires to – soldering 12 wires is more tedious;

      – you will probably still have to rewire the speaker wire connectors: this switchbox probably has common ground (they usually do) which is OK if you switch between speakers, but should be avoided if you want to switch between amps;

      – you would still end up with a switchbox that has poor quality wire connectors (these spring-type connectors in the back – we do not recommend them, they are a problem if you use a wire that’s too thick or too thin for them and they break more easily than even the cheapest screw-type connectors).

      That’s the potential problems I can think of from the top of my head. In short: yes, converting that speaker switchbox should be doable, but it might not be easier or cheaper than building a switchbox from scratch. Buying parts could be cheaper depending on the prices in your area, conversion of that speaker switchbox might involve a bit less work or not, depending on its internal wiring and how easy or difficult fitting a 4PDT switch would be.

      Hope this helps. Good luck on your project. If you have more questions, drop me an email, and let me know when you’re done, I’ll be curious to see a photo of you switchbox (whether you build it or convert the one you’ve linked).

      Kind regards,

      Rafal Lisinski

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