Auto Speaker switch project

K

KG9MM

Active Member
I just built a neat little device for my audio system. I actually have 2 systems in the same room. The main system is a 2 channel system Built up from two McIntosh MC-60 mono block power amps connected to Altec Lancing Valencia Voice of the Theater speakers. The second system is a mono system built up from a MC-60 controlled to a McIntosh C-8 preamp compensator. The room cannot accommodate another big speaker so I was running the mono system using a little book shelf KLH speaker. It sounded ok, but I knew it could sound much better with a proper speaker. This is what I did about it:

First I dug around in my junk box and found a mil-spec Hermetically sealed DPDT relay. It is energized by 26.5 VDC. I also found a 13VAC filament transformer. I used a 2 diode voltage doubler power supply design to produce the 26.5 VDC fro 125 VAC mains. The power supply is built using 2 rectifier silicon diodes and 2 2200uf 50VDC capacitors. I built the circuit on perf board and mounted it in a bakelite project box. I also mounted the transformer & 6 banana connectors in the box. The relay is set up so the speaker is normally closed to the 2 channel system. When I turn on the mono system the relay energies and switches the left speaker to the mono system.

I only see 2 problems with this switch. First, if both systems are on, the 2 channel system left channel will not be connected to a speaker. This should not be a problem because I am the only one that uses the mono system and I turn it off when I am finished.

Second, when the mono system is turned on the relay engages in a few milliseconds. When the mono system is turned off the capacitance in the power supply holds the relay for a few seconds until the voltage bleeds off. This isn't a problem because even if I switch off the mono system and walk over and turn on the 2 channel system there is enough time for the relay to drop out before the 2 channel system comes on line.
 
Register to hide this ad
Change the circuit so that when the 2 channel is on and the mono is off or on the speaker is connected to the 2 channel left channel, and only if the 2 channel is off will the mono amp connect to the left speaker. That way even if you forget to turn off the mono amp and somebody wants to use the two channel all they need to do is turn it on and both channels will play.
Are your Valencias the 8Ω or 16Ω versions?

I see another possible problem. If you run any of those amps without a load (speaker) connected you may damage the amp. I'm not a tube guy but I have read that tube amps really do not like having high (open) output impedences.
 
Last edited:
The Valencias are 8 Ohm.

There are no fools in the house that would turn on both systems or forget to turn one off.

The best way to out smart anyone foolish enough to turn both systems at once is to build a circuit for the mains line on each system. If the mono system is on, then the 2 channel system gets no power. If the 2 channel system is on then the mono system gets no power. An optional circuit would be if the 2 channel system is on and some fool tries to turn on the mono system a lightning bolt will shoot out and take them out of the gene pool. The same thing should happen if the mono system is on and someone tries to turn on the 2 channel system. LOL
 
One more thing. If anyone else wants to build one for themselves here are some thoughts:

First the extra effort to build a power supply to supply the relay can be avoided if you find a relay that can be powered by 12 VDC or some other more common voltage. Then use a wall wart power supply to supply the relay.

Second, this can also be done with two 2 channel systems or a surround system sharing speakers with a 2 channel system. Just make sure you have 2 poles and 2 throws for each switched speaker.

It's best to use a hermetically sealed or vacuum relay. Then you will have no problems with dirty contacts.
 
I have started making safety relays for the two HiFi systems that share the left speaker of the stereo system. The way this is planned to function is when both systems are off both will be connected to the mains. When the mono system is on the Stereo system will be disconnected from mains. When the stereo system is on the Mono system will be disconnected from the mains. I have made one relay box and am working on the second one. This relay system is built from 2 Potter Blumfield 1 phase motor starter relays. The activation coil is 12 volt. I am using 2 wall wart 12VDC power supplies to activate the relays. One PS is in each relay box, but the one in the box controls the relay in the other box. This way I can send the control signal using DC wiring rather than Mains AC wiring. I am taking pictures as I make progress. I will post the pics when I am finished.
 
You could add one more relay and contact set that parallels both speakers to the mono system so as to get sound from both, if the mono system will drive the parallel impedance safely.
 
Yup I know. However, it would just make more work and I have plenty of projects already. My listening position is right in line with the chosen speaker so it wouldn't make it any better. If I wanted to do this for real I would add another amp for the other speaker or rob an amp from the stereo to drive the other speaker. That being the case, why even set up a Mono system. Just push the mode switch on the C2200 preamp and you have mono on both speakers. I set up the mono system, because I wanted to put my 3rd MC60 amp to good use. I don't like it sitting around collecting dust.
 
KG9MM said:
I could take a pic of the black box but it would not be to informative. I should have taken pics before if closed it up. If I ever open it up again I will take some pics and post them.
Click to expand...
Here is a pic of the outside of the speaker relay and some pics of the inside too.IMG_3118.jpgIMG_3117.jpgIMG_3116.jpgIMG_3115.jpgIMG_3114.jpg
 
Here are picts of my safety relay system. The first box controls the power to the Stereo. The larger aluminum box controls the power to the Mono system. When the Mono system is on and someone turns on the stereo, the power to the mono system is cut. If the Stereo is on the power to the mono system is automatically cut and cannot be turned on.IMG_3121.jpgIMG_3120.jpgIMG_3119.jpgIMG_3113.jpgIMG_3112.jpgIMG_3110.jpgIMG_3108.jpgIMG_3106.jpg
 
You must log in or register to reply here.
Back
Top Bottom