I keep stumbling across things in the most unexpected of places .... just as well I don't trust my audio memory!
Some posts back we introduced the idea of an instantaneous switch-over box containing heavy duty relays which under foot switch remote command, will divert the audio signal from A to B. It can be used in several different ways because relays are non-directional and the terms 'input' and 'output' are interchangeable.
In this example from several years ago, I was working on the design of the NRG bookshelf speakers. I'd arrived at two very different crossovers in my simulator, named X1 and X2. You can see from the (poor quality) picture that these crossovers have different components in different places and a different crossover frequency yet according to the simulator model the end result is a virtually identical loudspeaker frequency response on axis. But would they sound the same or would there be a sonic winner? That the simulator cannot tell.
I really don't like situations like this where there may be a technically superior solution, but for lack of care or time or listening fatigue the less good solution is picked. Considering that I do all development listening in stereo (that's unusual I'm told, even the BBC designers listened in mono to one speaker) I'd have to disconnect and reconnect two wires to each tweeter, two wires to each woofer and two wires to each amp output - a total of 12 connections to be made to connect either a pair of X1 or X2 crossovers: in fact, to make a change over there would be 24 wires that had to be swapped. Assuming that I moved really fast, paused the music, rushed over the the speakers and made the twenty four connections at the fast rate of 10 second each, it would be 10 x 24 = 240 seconds minimum before I could be back at the hot seat and playing music again. That's far, far, far too long a time gap to draw a reliable impression about whether X1 or X2 sound best. You may as well flip a coin.
The solution was simple, and took a few hours of construction time. A switch-over box to which the pairs of X1 and X2 input and output were permanently wired as were the tweeters and woofers and amp connections. At the touch of the foot switch, the two tweeters and two woofers were driven from the X1s or in the other position, driven from the two X2s. Change-over time about the blink of an eye.
And the result? Interesting. The simulator was right (of course) that the resulting frequency response was, subjectively, virtually identical. Listening to one pair for ten seconds or so, or until some interesting feature of the music made you wonder how that would sound in the other position, I must have switched between them twenty of thirty times over as many minutes before the very subtle real sonic differences started to show themselves to my ear/brain. Those extremely subtle differences - which amplifier exponents claim are 'night and day' to them could never be positively identified with one or two minute gaps between switch over.
One circuit did sound very slightly better and we went into production confident that the best one had been chosen, and also that if the same test were run today, the outcome would be the same.
The proper evaluation of all and every audio component (or recording) really should be done under instantaneous conditions or the outcome is, at best, questionable. It's satisfying turning in at the end of the day assured that you have done the best job possible and can truly justify why. I do understand that to consumers disconnected from the mysterious design validation process this seems a cruel, cold, unemotional way of comparing A with B. And maybe it is. I doubt that one audio designer in ten uses such a method because the outcome is rarely flattering to his hard work.
When I came into audio manufacturing twenty six years ago, I had no one to instruct me otherwise. It's a perfectly natural and obvious way (for me) to work.
There's not much more to say about the methodology. All that's needed are a handful of relays (Maplin sell suitable one), a hand or foot switch, a battery or PSU to energise the relay coils, a box to mount them in (merely to disguise the audible 'clonk' when the operate) and, most important - and you absolutely do need to consider this - a means of being 100% sure that the contacts have definitely changed over. I considered offering the use of my relay box, but to avoid any suspicion of biasing the result, after inspection of your solder joints and general assembly, I'm perfectly happy to permit you to use your box. Be sure that the relay contacts are of good quality since they must be beyond reproach: gold contacts are readily available.
P.S. I would like to help you as much as you'll allow me to let you win the pair of M40.1s so here is how I'd build your A-B comparator box. If you assemble this neatly it would be entirely adequate to do the job. Links to all parts provided.
Relays - suitable type available from Maplin. Cost £3.99 each. I suggest you buy the 6v ones and wire them in series/parallel to be sure they all switch together. I suggest that you buy four, and use one pair of contacts on each one not for signal, but for a confirmation of change-over (details up to you).
Switch - suggest a foot switch also from Maplin here at £7.99. Warning: this is in a cheap metal can and will make a tinny clunk when operated. You may need to apply damping (car panel bitumen sheets) to the tin box to suppress the mechanical noise upon operation. An alternative is a door-bell push switch. That has the advantage of being non-latching and returning to the rest position (contacts open) and is silent. But it means you have to keep your finger pressed down during the entire time you are listening to one of the two amplifiers.
Or ... you may prefer a flex switch, also from Maplin £2.99 which is virtually silent when gently rocked-over in your hand.
Connectors - either make-up some leads (to speakers and amp) and solder directly to the relay or arrange some sort of speaker connectors, for example fancy ones from Maplin here at £2.49 each. An entirely acceptable alternative would be heavy-duty connector blocks also available from Maplin here. I suggest the 15A (scroll down) at £7.49 for three.
Power supply for relays - either battery power or mains. Battery is easy - two of these 6v brutes in series from Maplin (check store stock - these are unusual) at £8.49 would be ideal. Tip! If possible design the circuit so that the normal switch position puts the relays in the relaxed state, drawing nothing from the battery.
Speaker/amp cable - basic low resistance cable is all you need. Again, Maplin at £1.39/m. Better to buy too much than too little. Maybe black sheath for the left channel to avoid confusion?
Little box to mount relays in - again, Maplin. The biggest one of these would probably do. Tip! The tinny plastic box will make quite a clonk when the relays 'throw'. You may need to mount the relays on some decoupling (sponge?) or maybe cover the box with some cushions to hide the mechanical noise. Or perhaps use slightly longer heavy duty speaker wires and put the box in an adjacent room?
LED's - Once again, your friendly Maplin store. 12v LEDs here which just need a simple hole drilled in the mounting box.
Off-on battery switch - You may want a battery off/on switch (this would do) or just disconnect one side of the battery.
That's all you need.
Assembly time: about 2.5 hours.
Test equipment needed: none.
Tools needed: wire cutters and strippers, medium duty (40W?) soldering iron and clean solder, drill (if case is used), screwdriver for case, pliers to tighten speaker terminals (if used), small screwdriver (if connector block used) - all available from Maplin.
Most important skill: patience and really good quality soldering.
Age suitability: successful assembly should be possible by an interested 14 year old. Parental guidance may be needed for the hot soldering iron.
Provided that you follow the general principle of this simple switcher, you are free to adapt the design as you see fit. The most important precaution is that the relays are new with good quality contacts (like the Maplin ones) - the signal will be passing through those contact points.
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Alan A. Shaw
Designer, owner
Harbeth Audio UK
