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In Reply to: RE: DIY amplifier selector suggestions. posted by maxhifi on October 11, 2016 at 14:09:53
Maxhifi - I did the same thing with my system. I used two DPDT toggles from Radio Shack rated at 10 Amps each. Been using this setup for over ten years now without any ill effects. I don't worry abouit the contact resistance because I chose a heavy duty switch.
But don't switch it hot, when signals are being amplified. This may damage the contacts as they may arc.
Also, and very important, DON'T DRIVE YOUR TUBE AMP WITHOUT THE SPEAKERS CONNECTED. I always turn the volume up slowly, and if I don't hear anything, back the volume down to minimum and switch the speakers over.
Pete
Follow Ups:
Here's how I plan to hook it up in order to make sure I don't damage the tube amp. One switch for left and another for right, but only one shown here. The non active amp is presented with a big power reaistor to keep things safe. I know myself and I will cook the amplifier before realizing the speaker selector is in the wrong position!
Beneath the proposed schematic is how I plan to check the contact resistance of the switch. It's a 15A model, but made in China isn't a wonderful pedigree.
Max - Although your circuit looks correct, it's overblown. First off, the Home Theatre amp (assuming it's solid state) doesn't need a dummy load at all. It can run all day without speakers. No load, no current drawn.
Second, using an 8 Ohm dummy load will parallel it with the output of your tube amp. Assuming an 8 Ohm speaker, the amp,. will always see it as 4 Ohms. Half of your power will be dissipated across the 8 Ohm resistor.
You can use a 100 Ohm, 5w resistor as the dummy load. As long as the tube amp. has a reasonably low resistance to reflect back into the amp, it will remain stable. You will lose about 8% of your power but the amp will be protected. 2 pcs. of DPDT switches should do it.
Have a second look at my circuit, the 8 ohm resistor is never connected to the same amplifier which is connected to the speakers, and just like the dpdt circuit there's never more than two contacts in series with the amplifier at once. (I deem necessary to prevent ground loops). I have a slight ulterior motivation for doing it this way - from time to time I like to experiment with speakers, and I want to have it set up so the switches can also eventually be used to switch two amplifiers between two sets of speakers rather than one set of speakers and one set of dummy loads.
I also have a fairly high beam power tube powered amp, so I want to keep a low enough load reaistor so that under no conditions does the output transformer primary see excessive voltage.
Max - I'm sorry, you are correct. I didn't study your drawing enough. BTW, I measured the switch contacts on the 10amp switches I've been using and with over ten years use, none measured more than 10 milliohms. Use a godd, heavy duty switch and you'll be fine. - Pete
"Max - I'm sorry, you are correct. I didn't study your drawing enough. BTW, I measured the switch contacts on the 10amp switches I've been using and with over ten years use, none measured more than 10 milliohms. Use a godd, heavy duty switch and you'll be fine. - Pete"
Hi Pete
That's fantastic, I appreciate the fact that you both measured it and shared the results. 10m ohms is maximum initial contact resistance spec which is given for quality panel mount toggle switches from quality brands.
It's also an insignificant amount of additional impedance versus the 16AWG speaker wire run I am currently using, so I don't anticipate any changes to the signal. At first I wanted to use relays to accomplish this task, to perform the switching automatically when the home theatre receiver is turned on, but finding one with a 10 m ohm contact resistance begins to get difficult and more or less not worth it. Standard "ice cube" type relays are in the 50 - 100 m ohm range. Two in series makes 0.2 ohms, which is way too high to be considered absolutely transparent.
I think the key to maintaining this spec is not usage, but avoiding any switching under load. If these switches were used to actually switch 10A repeatedly, I doubt that I suspect the Chinese switches I ordered are more or less knock offs of the usual Carling or other US made switches, but obviously at a small fraction of the cost. I think your RS switches are likely sourced from a similar Chinese vendor, like most of their recent parts.
Max,
There is a nice looking Carling DPDT switch on the Antique Electronic Supply site, P/N P-H523, $7.65 ea. Doesn't list the current rating though. You'd have to ask. Looks heavy duty enough, made in Mexico if that makes you feel better. I do.
My Radio Shack switches may have been made in Taiwan, which has much better quality manufacturing.
Pete
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