What receptacle for 240V 20A in the States?

You don't need a dryer cord. Your local hardware store has plugs for 220 volts, used for some large window air conditioners and shop tools. The plug looks just like a 3-prong 110 volt plug except the power prongs are turned 90 degrees.

Wagzel:

Your link leads to some great quality Pass and Seymore outlets but only one model (the 6-20 R) is designed to prevent it being used with a common 120 Volt household plug.

Murphy's law: IF someone can plug into an outlet..... they will.

Sure, the Hoover will vacuum better than ever for a couple of seconds, but the smoke and fire is not worth it.

Eldartford's suggestion is better.

The plug he's referring to cannot be mistaken for a 120 Volt nor will the 120 volt plugs fit the slot.

I'm assuming that you're in US/Canada, in which case there would be two hots at 120V each. These are out of phase and are cumulative to 240V, but they can NOT be combined together without a load. That would be a short.

It can be cheated by putting the two hots on a three-prong outlet, abandonning the neutral, and using the ground as normal. This would be 120+/120- or balanced 240V (or close to balanced). Electricians might not be willing to do "bend" the code and your insurance could be void if there were a problem. I usually suggest using GFCI protection for DIY balanced AC.

Europe has 240+/0- unbalanced AC. Technically 220V, but that's splitting hairs.

Just looking through the Hubbell catalog for Nema6-20r hospital grade duplex recepticals and brown is HBL8400 Ivory is HBL84001.These are rated at 20 amps 250 volt.The matching plugs areHBL8576E in white ergo grip.HBL8464V which is the valise style body in white.The 20 amp rated recepticals and plugs differ from the15 amp versions by having one vertical and 1 horizontal blade.Nema 5-20p or5-15p rated 125 volt style plugs can not be inserted in this style of receptical.

Not to through a fly in the ointment. But it is possible you may effect the sonics of the Amp. For better, or worse??

From the mains of the U.S. 240V you will essentially be feeding the Amp from a balanced power system. Also the voltage may be closer to 250V + . You may want to have that checked first. Also check what the maximum input AC voltage rating is for the Amp.

As for the NEMA 6-20R receptacle that would be a good choice. If you already have an extra 120V dedicated circuit by your audio system all you will need is a 2 pole 20 amp breaker installed at the electrical panel and the two insulated conductors of the dedicated branch circuit terminated on the 2pole breaker.