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I am working on a design which must drive an audio output into 600 ohm headphones (aviation headsets). Normally, I would just use an off-the-shelf class D amplifier but I can only find ICs designed to drive 4-8 ohm loads. This could still work with an audio transformer, but would require a 75:1 winding ratio. The highest ratio I can find is ~55:1.

Can someone recommend a different design approach?

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  • \$\begingroup\$ Mouser lists a bunch of audio amplifiers rated for 600 ohm loads. How much power do you need? \$\endgroup\$ – JRE Dec 8 '18 at 17:51
  • \$\begingroup\$ Wow, I was only searching on Digikey. I normally assume that they have the same stuff as Mouser, but not in this case... \$\endgroup\$ – user8908459 Dec 8 '18 at 17:53
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    \$\begingroup\$ 600:8 ohm impedance ratio goes with square root: you need about 8:1 or 9:1 turns ratio. \$\endgroup\$ – glen_geek Dec 8 '18 at 17:55
  • \$\begingroup\$ @glen_geek Oh yeah, I forgot about that. It's been too long since circuits class. I think this is what I'm looking for \$\endgroup\$ – user8908459 Dec 8 '18 at 18:03
  • \$\begingroup\$ 1 milliWatt may suffice. Thus 1 volt RMS is a goal. \$\endgroup\$ – analogsystemsrf Dec 8 '18 at 18:21
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A transformer to make 600 ohm headphones appear as an 8-ohm load for the amplifier may be a bad idea...

Low-impedance headphones are ubiquitous. Plugging one into your transformer would present a load of very low impedance to the amplifier, which might dangerously overheat.

Most audio power amplifiers are designed for an 8-ohm load - some for 4 ohm, and a few for 2 ohm. Any of these won't complain if they see a load of 600 ohms. It is difficult to say if you'll get enough volume from your 'phones before distortion sets in. Those 600 ohm 'phones might just give you adequate volume when driven directly - no transformer.
It comes down to Ampere-turns. Drive an 8-ohm speaker with the same voltage as 600 ohm headphones. The 8-ohm speaker gets more current through few turns. Headphones get much less current through many more turns. I have 4000 ohm headphones that sound about as loud as 8-ohm headphones when plugged into an amplifier designed for 8 ohms.
So just try any of the amplifiers that are designed for 8 ohm load.

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Linear or Class D Differential Drive with 100mW may hurt your ears. How much power did you think you needed?

Start with specs 1st for Power, Radiated / conducted noise , spectrum , interface , etc. before you start ANY real design then choose make or buy.

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