I have a Bryston 5B, and I’d like to be able to switch between two sets of passive speakers, both driven by the Bryston, rather than buy another power amp for the second set. One speaker set is 4Ω, the other has a large range over frequencies from 3.2-8Ω. I’ve read up about the impedance relationship between driver and amp and so on, but am slightly nervous that a simple DPDT [double pole, double throw] switch box, while being the simplest method, may momentarily connect both sets during switching, dipping the impedance to 2Ω and risking damaging the amp. Any suggestions? To put it another way, as they’re both really hard to drive, would accidentally connecting both at once unload the amplifier and make expensive smoke?
SOS Technical Editor Hugh Robjohns replies: If I recall correctly, the Bryston 5B is an old but very substantial and well-engineered three-channel power amplifier, each channel providing around 120W into 8Ω or 200W into 4Ω, with 4Ω being the minimum accepted load impedance per channel.
The impedance ratings for loudspeakers are based only on notional values as the actual impedance usually changes pretty dramatically across the frequency range. So your second speaker would probably also be categorised as a nominal 4Ω.
That being the case, you’re quite right that you shouldn’t connect both speaker sets to the amp at the same time, since the total load impedance would drop to somewhere around 2Ω — below the minimum acceptable figure. Being a Bryston amp, if subjected to that condition I suspect it would probably go into a protective shut-down mode rather than let the ‘magic smoke’ escape, but it’s obviously not worth the risk.
So what you require is a loudspeaker changeover box to select between two sets of speakers, with a guarantee not to connect both at the same time. Sadly, however, most commercial units allow both sets to be connected at the same time, so I fear a DIY project is probably your only option.
As you only need to switch the positive outputs between the two sets of speakers (their negative connections can remained joined together at the amplifier), you will require a ‘double pole’ (for two channels) switch with two states (‘double throw’) — hence ‘DPDT’. The critical aspect, though, is that the contacts must ‘break before make’ to ensure the two speaker sets are never connected at the same time (as opposed to a ‘make before break’ switch type). In your situation, an ‘ON-ON’ type switch would be ideal, because that way one or other speaker set will always be connected to the amp — there is no position that would leave the amp disconnected, with you wondering where the sound has gone!
Of course, the break-before-make arrangement would leave the amp with no load for a tiny fraction of a second. Unlike valve power amps, solid-state amps aren’t bothered about not having a load, but I wouldn’t recommend flipping the switch while the thing is playing music (and especially not loud music). The sudden removal of the load will leave a a lot of current with nowhere to go, momentarily, and that spike could potentially cause issues. In other words, it’s best to mute the input to the amp so there’s silence at its output before you switch speakers.
As for the switch’s current rating, 200W into 4Ω will potentially generate a current of around 7A, so I’d suggest looking for a switch with an AC current rating of 16A or so, to be on the safe side. After you’ve acquired your preferred style of switch — and before wiring it up — I’d check carefully that one side really does disconnect before the other makes. Just use a multimeter to measure the resistance (conductivity) between the two output terminals while operating the switch. At no point should the meter react!