I have several pairs of speakers, and the collection keeps growing. I need a way to switch easily between them to avoid having to reach round the back of the amplifier every time I want a change.
I found this QED MB44 for about £8 on ebay and thought nothing of the word Transmatch on the front. I assumed it was just a simple box of switches and connectors.
It turns out the Transmatch units contain transformers. These allow you to switch in several pairs of speakers, up to 5 in this case, and the impedance seen by the amplifier will never drop below 4 ohms.
Four pairs of speakers are connected in parallel and then to the primary winding of a transformer, the secondary winding connects to the amplifier presenting a consistent load of 8 ohms. There are separate transformers for left and right channels. The 5th pair are connected directly to the amplifier for "Best Sound Quality".
As this implies, having transformers in the signal path is only going to degrade the sound quality. Metal Oxide Varistors are provided to protect the transformers from too much amplifier voltage, and these no doubt don't help with sound quality either.
So all this is going to have to go.
Much simpler now. Each switch connects a pair of speakers to the amplifier. I just have to be careful to make sure only one set of speakers is switched in at a time as they are now connected in parallel and the total impedance would drop too low for the amplifier.
I was hoping to leave the transformers in place, unconnected, to give the box some weight but it was too much of a struggle getting the wires back into the box with them in the way. The rubber feet on the bottom of the case do a good job of stopping it sliding round anyway.
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