No, you don't. You only need to apply an actual load and measure across that load. The AP has 2 floating inputs so no need to measure the headphone.
Using a headphone to look at distortion -100dB would be silly as it would drown in noise and the headphone would be determining the distortion.
It would be like trying to measure microns using a tape measure.
I thought what the circuit does isn't change the output of the amplifier, it changes the electrical properties of the headphone. Therefore, the distortion level and possibly frequency response of the headphone changes, even though the amplifier measures flat.
You need the headphone because the effect is different with each transducer, depending on the impedance curve/resonance frequency or some such.
This is why I am waiting for the speaker version of this, if one ever comes out, to use with my old Klipsch jobbies.
Here's
Jason's description, maybe I am not understanding this:
“So it sounds different?”
“Through balanced, hell yeah,” Tyler said. “Actually I think the balanced is one of the best things you’ve done.”
I shook my head. “’One of the best things’ is pretty marketing-weaselly,” I told him.
“Yeah yeah you know what I mean. This is really really good! What did you do?”
“I tried mixed-mode feedback, kind of a derpy motion feedback thing. Maybe. I mean, if the headphone drivers even flop around that much. Because it’s dependent on back EMF. I mean, we don’t have access to a separate voice coil for real motion feedback, and it might only matter, like, with a 15” woofer in a ported box, but for that woofer you may even be able to get some better measurements, due to damping of the resonance frequency—” (bold added)
Tyler held up his hands. “Wait a sec. Are you saying this might get you better measurements—”
“Not likely with headphones,” I interrupted. “Maybe not likely at all. If it worked at all, it would work best for floppy things, like ported speakers—”
“But if it worked…and it made the acoustic performance better…”
Tyler just trailed off, and we sat staring at each other for a while, not saying anything.
Because if we could create better performance at the transducer, holy crap that was the holy grail. Everything blown up. Because a -110dB and a -120dB amp into a -50dB transducer both give you -50dB for the system.
But if one amp bumped that -50dB to -55 or -60…
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Simple: we don’t have infinite time. Our early results (done by a third party) showed a potential improvement in THD in the low frequencies when run through our mixed-mode feedback topology. Later results were mixed or null. (bold in original) There’s not enough data for us to claim improvement, but there’s also not enough experimentation to say there’s no ‘there’ there.
So, here’s an idea: anyone who wants to try proving a difference, and who has an acoustic measurement rig for testing headphones, contact us. (bold in original) I’m sure we’ll be happy to get you a Midgard to play with. And yes, we promise to report all data, including nulls.
Which, after all, is what I expect: nulls. Because we’re only measuring a tiny corner of the audio universe—steady-state, typically single-sine or low-amount-of-sines for test signals, etc. Transient response, complex signals…we need to look a lot deeper if we expect to find any correlation with subjective results.
Of course I measured the acoustic response of the headphone through Gras 45C and not the amplifier.
