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Can a dual throw switch be used to cut off speakers when the headphones are used? Will it hurt the OPT's? What is the best way to splice it into the circuit?

If solid state, yes. If tube, no. It would need to switch-in a dummy load for the amplifier.

Also, you'll want to add a ressitor in series with the headphones whose value depends on the power of the amp and impedance of the phones.

Cheers!

I and others have had endless arguments with a guy who insists of running his high-Z headphones directly on the speaker OPS of a Musical Angel (EL34 or KT88 - don't remember) and he has suffered from some problems. When pointing out that his stubborness will eventually cause his amp to break, he ignores our warnings . But, well that's another story.

Howevere I ran into an interesting design in the Austrian or Australian amp I am building:



The componentsin red had to be added to have the automatic switching working correctly.