resonance at 500 Hz using headphones - maybe need a different DAC?

Have you checked the possibility that it's the headphones?

+1 @koestner First, this doesn’t even sound like a DAC problem, and second since it happens to some degree across several DACs it’s most likely not a DAC issue and more likely an issue with the headphones.  Have you even tried different ‘phones?  What are you using for an amp?

It only happens with the Gustard. Did you think the other equipment I mentioned like the Aurender or Zen Stream were DACs? Those were servers/streamers driving the USB input of the Gustard.

I've tried two different headphones, two different headphone amps, and when listening closely to speakers there seems to be some of the peakiness, too.

I did another experiment which helps to convince me that it's the Gustard. I set up my modified iFi iDSD Nano, which is using a very hq linear power supply, and I put my best power cord on the supply, using the Aurender as USB source. The sound had a lot of the qualities of the Gustard, in terms of transient speed, dynamics (micro and macro) etc. But almost no resonance. I then played piano notes through the DAC, and almost no resonance. 

What the Gustard is doing technically... frequency response, etc. ... I don't know, but somehow it makes this resonance worse.

Note that all my equipment has modifications by a technician in the N.J. Audio Society. He adds bypass caps, damping, and ERS fabric.

Yeah, well, try something he hasn't touched.

I was thinking, DACs do have some distortion, and I listened to a 500hz tone, thinking maybe I could recognize something. Especially in my ’sharpest’ DAC. What do you mean by ’resonance’? A hum at 500 hz? Echo? Distortion? Or that the fundamental tone at 500 hz has a lot of overtones or exaggeration? Could it be that the Gustard is actually doing its job - showing more of the resonance in the recording?

By resonance I mean a sharp peak in the frequency response curve. If I play increasing tones on the piano they sound normal until I get around C above middle C. Suddenly the tones will get much louder and sound distorted for a few notes around middle C. They it will calm down as I keep going up. There's another, less prominent, resonance around an octave above, that is, two octaves above middle C.

The resonances are more obvious with some USB sources than others. They are much more obvious on an Aurender N100, and less obvious on a Windows 11 NUC. Generally the more transient power I'm getting from source/DAC combined, the worse the resonance.

Maybe the DAC is broken. I don't know any other way to explain it. 

Thank you, interesting, I hope you solve the problem!

I am quite familiar with the technician you used. Unfortunately, he is retired, otherwise, I would suggest that you have him investigate. I owned the same Beringer DAC that he modified. It was an excellent DAC, and I only replaced it because I wanted to go beyond 96/24. I don’t do much listening with cans, but I have never heard any frequency peaks from this DAC, or, for that matter, my turntable, which he also modified. Perhaps there is an impedance curve mismatch between your headphones and the DAC? Also, it could be some issue with the USB receiver. Not sure how you can address this with the fellow being retired. You could still contact him. He might find your issue puzzling enough to take a look at it. If you need contact info, private message me.

There is a resonance starting right around 500 Hz on the LCD-2's - see https://www.stereophile.com/content/spectacularly-yummy-audeze-lcd-2-and-lcd-3-measurements

Could be a bad solder joint that's causing some distortion and making the problem worse. Might be a good idea to have your modder listen to your setup and check over his work.

Have you tried this test on a real piano? Possibly you are experiencing a temporary sensitivity in that range. This happened to me and it soon went away.

A tone from an open string bass guitar, or guitar, tends to sound louder than a closed string further up the neck. Some DACs seem to give this fundamental sound more preeminence than others. But maybe not relevant for choir music ca 500 hz.

To address some of your comments:

Re: LCD-2: I don't think it's the headphone because it only happens on the Gustard. It doesn't happen on the modified iFi iDSD or the Beringer. Also it happens with another headphone, the AKG K712. And furthermore it's a very high-Q resonance. Very sharp, spans only about 3 half steps.

Re open strings: it is true that the 500 Hz resonance is triggered by notes with a lot of fundamental, because it's when the fundamental is at 500 hz (i.e. C above middle C) that it's loudest. It's much more noticeable on close-miked instruments. So for example an album by Maria Scheider, Walking By Moonlight, has a lot of close-miked instruments like piano and saxophone that move up and down scales and keep hitting the resonance. Whereas a chamber music album by Sheffield lab that has more of a perspective miking doesn't trigger the resonance much.

Bondmanp: so obviously you know were talking about Igor Kuznetsoff. He is still doing things from time to time and I'm in touch. I'm having him do one of his Dynamo power cords right now. I'll ask him if he can look at the Gustard. I was thinking I might solve this resonance by tweaking something, but at this point it's obvious I need the original expert to look at it.

I have found that some DACs or systems emphasize the fundamental tones, more than others. As a test case I use Joe Cocker: Seven days, on his Sheffield steel album. This is a great track, with Sly and Robbie on drums and bass (if you want to define "tight", go here). You can really hear what bassist Robbie is doing, changing from the fundamental to the two upper chords in the three chord pattern (melody; Bob Dylan). When he goes up to a higher chord, the volume of the bass goes down. It is like Robbie takes a step back. Now the interesting point: this seems to vary, between my DACs. Two steps back, on some, just half a step on others. But I will need to test more. Just an impression.