Saturday, July 22, 2017

Tough Tracks

My system can be loafing along, playing classical guitar, and sounding beautiful and pure.

And then, I can be playing "Hot House," by Arturo Sandoval, and it can sound pretty rough.  Abrasive, screechy, harsh.  I don't think it was always this bad, say when using the Krell amplifier.  When using the Aragon I become more aware of these things, possibly because the Aragon is not quite as good, and possibly because I let myself hear problems more.

I played Hot House on Thursday and then Friday night.  Finally I decided to do something about it.

My first idea was to substitute the Emotiva DC-1 dac for the Denon DVD-5000.  After all, the Emotiva is the cleanest DAC I own, by measured THD and distortion spectrum (which looks almost perfect, and I will not say that I disagree with the measurments sounds like it's not there, only a bit too much so generally I felt in a few minutes of critical midrange listening).

But the distortion I've seen generated by the Denon is around 0.003%.  I think it would be hard if not impossible to hear that, even if it were all 7th harmonic, and actually the Denon has mostly 3rd harmonic with a fair amount of 2nd, plus higher harmonics but only at even smaller levels.

The amplifier might be a bigger factor, but the Krell has not been sent in for repair yet.  (I accepted a freight shipping quote on Friday so it goes out next week.)  I have measured the Aragon at 0.02% distortion at moderate level.  It had been over 0.1% until I fixed the bias problem 2 years ago.  Still, this doesn't seem like the big factor I'm looking for.

So, instead, I decided to go after some of the lumps in the high frequency response of the Acoustats using my DEQ.  I have avoided using EQ on the Acoustats except for the crossover itself and my Gundry/Linkwitz/Peterson dip.  And also a tad of resonance control around 115 Hz.  It may very well need more than I've done (though solving resonances by fixing things is better than correcting them with EQ, but I have zero idea how to fix the response bumps now).

Deeply rolling off the treble seems to do no good.  I still hear the harshness in there, no matter how well rolled back.  And then it just gets boring also.

I am thinking that small peaks which result from tiny resonances in the speaker catch on the natural odd harmonics of the brass instrument as the instrument itself is sweeping through the spectrum.  However you imagine it, it does seem like rough looking response could create rough sound.

I experimented first with attenuating a slightly rough spot around 12kHz, just before the speaker begins to roll off somewhat in my off-axis position.  I can see this same spot regardless of angle with the speaker.  I'm using the 1/6 octave display of an app, so I know that "12 kHz" isn't exactly the spot, but pretty close.  Really when tuning a parametric EQ, you should use something even finer that 1/6 octave, in my opinion a hand tuned oscillator is best--then you can totally zero in on the resonant frequency.

Then it seemed also that there was an elevated sticky frequency on the RTA just below 12 kHz, so I made the bandwidth 1/3 octave and moved the center frequency down to 11.8.  That's where it is now.  Before doing much wider than 1/3 octave a good oscillator test is called for.  After measurements and listening I settled on attenuation of -4dB.  I only weaken such peaks, never total cancellation, because overcorrecting is worse than undercorrecting IMO.  But this did seem to eliminate any tendency to either peak or shelf at 12khz (before plunging down above that), only now there's still a bit of bulge at 10kHz left that wasn't by itself visible before, an indication of the tuning of the parametric correction is still a bit off.

I similarly attacked a small peak around 638 Hz.  When I turned the parametric correction from off to "PARAMETRIC" a previous correction of 638 Hz was turned on (though, when such things are partly saved by the DEQ, the attenuation goes back to 0).  Now it's pretty much gone though there's a similar peak also around 500 Hz.

Despite my Gundry/Linkwitz/Peterson dip (centered at 3.8kHz) there is elevation at 6kHz, the worst frequency for there to be elevation at.  So I added a new 1/3 octave correction just at 6kHz, to keep that down, blending better into the rest of the dip also.

I had also noticed a large difference between the output in left and right super tweeters.  The much more wrinkled looking (from a previous high power mistake) left ribbon showed a nearfield peak (which it needs, in order to have any impact at all compared with the giant Acoustats) which was much larger than the right.  I had always assumed that the ugly looking eft ribbon had less output.  But in fact it has (or at least had) much higher output.  AND in this case, I decided to toss the Harrison Labs 6dB attenuators, dialing in 6dB of attenuation to the Stealth DC-1, and then also using the gain controls on the Parasound HCA-1000A amplifier driving the super tweeters, hand adjusting to about a 6dB (possibly inadequate) nearfield peak.  This may have actually been higher than before on the right side, and now I am worried if there isn't some issue in the right channel, but it could also be the crossover in the left is burned out.  Anyway now both supertweeters are adjusted to the same reasonable level.

One damned thing about the DEQ's is they don't have per channel level--just a convoluted "Image" control.  I'm now appreciating the level controls on the Parasound amp.  The DEQ's should also have  level to 0.1db, per channel polarity controls, and delay up to 10 sec (not 300 msec).

This set of changes did seem to improve the sound of Hot House.  I was able to listen to nearly the whole album again (after chains couldn't have done that) at a Tact level of 89 (approximately -3dB) which is quite loud.  Still, I'd say considerably more improvement is needed, and I'm thinking of doing more testing and possibly switching to graphic eq as well.  But I'm thinking a good oscillator test of the 11-12kHz and 6 kHz resonances might be revealing...

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