A + B = a trip to Radio Shack?
June 17, 2008 9:54 AM   Subscribe

How can I build my own A/B/A+B speaker selector box?

My lab is two adjoining rooms, with a large doorway (but no door) cut into the wall. We have a crappy bookshelf stereo in one room. It is just audible enough in the other room to be irritating, but not enough to really hear what's going on. I scrounged another pair of 6 ohm speakers from another crappy bookshelf stereo, and I was thinking about how to connect them up so that both sides of the lab can hear Talk of the Nation. Commercial speaker selector switches run $50 - 300, and I'm cheap -- plus I think this might be a project within my limited soldering skills, so if I can make it myself I'd like to.

The stereo is an older one, made by Fisher. The speakers say "MAXIMUM POWER 80W (PEAK), 6 Ω IMPEDANCE"; the stereo says "6 Ω MINIMUM". I'd like to switch the speakers in and out with a break-before-make rotary switch like this one. It's easy to figure out how to run one pair at a time (A or B), but how do I wire it up to drive both pairs safely (A + B)? I'd like to wire the speaker sets in parallel, but I know this will drop the impedance of each circuit to 3 ohms, which is below the amp's rating. Is the solution as simple as putting a 3 ohm power resistor in the circuit, to raise the total resistance back up to what it would be for just one speaker? Is there a better way to do this safely, that will sound OK? Lots and lots of people work in this lab and will be using this switch, so I can guarantee you that it will be switched under load more often than not.

Surprisingly Google has failed me on this issue. I can find lots of schematics online for switching two or more amps between one set of speakers, but nothing for the reverse situation. Please help us hear the sweet sweet tones of Science Friday clearly!
posted by harkin banks to Technology (10 answers total) 2 users marked this as a favorite
You are right that the impedance is the issue in the A+B mode, but a series resistor is not a satisfactory answer. In your proposed circuit, half the power from the power amp will dissipate in the resistor and a quarter in each pair of speakers, resulting in not very much sound.

The only way to do what you want without active components and without wasting so much of your power is with transformers, but those have their own problems; they'd toss your frequency response curve into the toilet. It would sound awful.

The reason Google has failed you on this issue is that there isn't any satisfactory way to do it.
posted by Class Goat at 10:16 AM on June 17, 2008

Response by poster: Ah of course ... didn't think about the power used by the resistor.

So how are the commercial ones made? I accept now that this is probably beyond my skill level, but how would one create something like this? Do you put the speaker signal through one little amp for each set, or what?
posted by harkin banks at 2:11 PM on June 17, 2008

The commercial ones are part of the amplifier, and they use active components.
posted by Class Goat at 2:47 PM on June 17, 2008

So how are the commercial ones made? [...] Do you put the speaker signal through one little amp for each set, or what?

Your basic audio amplifier has two amplifiers; one for the left channel, one for the right channel. A two-stereo-output amplifier would have four of these amplifiers instead of two, I guess.

How complicated is it to construct an audio amplifier? I'm not sure. I have made very simple (non-audio) amplifiers in the past, but judging from the high cost of 'serious' stereo equipment I assume it is complicated internally.
posted by Mike1024 at 3:23 PM on June 17, 2008

The only thing they duplicate is the last stage, and it isn't very difficult at all. 2 power-FETs per speaker plus a handful of resistors.
posted by Class Goat at 4:55 PM on June 17, 2008

Response by poster: OK Class Goat -- exactly how difficult is "[not] very difficult at all"? Any suggestions for resources (online or not) that might help me figure out whether this would be really over my head or not?

... and a hypothetical: if I did go ahead and try to build this, I would want to build two amplifiers per stereo channel that each had an impedance of 6 ohms and increased power by 2x (to maintain output the same regardless of it was in A/B or A+B mode). Correct?
posted by harkin banks at 7:26 PM on June 17, 2008

When you are designing a power amplifier from scratch, it isn't very difficult to include duplicates of the final amplifier stage.

So it's easy for Pioneer and companies like that to include an A+B mode. The overall amplifier design is not easy, if you're trying for quality, but once you've got it, it is easy to clone the final stage and include a redundant pair.

All of which has nothing to do with solving your problem, for which a hack solution is out of reach.
posted by Class Goat at 8:01 PM on June 17, 2008

Does the stereo have a line out or headphone out? You could run that to a cheapo second amplifier that you can turn on and off when needed.

I was going to recommend these HomePlug things that use the power lines to send data, in my case, I picked up an Accurian HomePlug sender and receiver (which is an amplifier, too) on clearance a couple years ago. But it appears that they don't make anything like it anymore! It works beautifully for sending audio from my office to my living room, I'm really surprised they killed this product. Maybe if you kept an eye on Ebay they might pop up.
posted by knave at 8:42 PM on June 17, 2008

FWIW, very few home power amps with A/B/A-B speaker switching have duplicate output stages. What they do have is output stages that will happily handle 4Ω loads, parallel the speakers up in A-B mode, then slap an "8Ω minimum" label on the back (or warn you in the manual that you should only use 8Ω speakers when A-B switching).

Conversely, most of the external speaker switchboxes actually run the speakers in series. Yeah, I know, there's reasons (reduced damping, the effect of mismatched impedances on the crossovers/driver, etc) for not doing that - but, really, the effect is pretty minimal, particularly when the source is only average to start with. The biggest 'problem' you may think is the reduction in power - but remember, 1/2 power is only 3dB down, which is a barely-perceivable reduction in volume.

Switch your speakers in series, and enjoy it.

(Why, yes, I did used to repair stereos in a former life...)
posted by Pinback at 11:38 PM on June 17, 2008

Oh, and most of the external speaker switches don't bother with K (make before break) contacts - they leave the speakers in series, and short out the pair not being used. That is, the "A" button shorts out the B speakers, & vice-versa.

It is possible to get constant-impedance speaker switchboxes, but they generally do it by switching combinations of speakers and series/parallel resistors.
posted by Pinback at 11:47 PM on June 17, 2008

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