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.