• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). Come here to have fun, be ready to be teased and not take online life too seriously. We now measure and review equipment for free! Click here for details.

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amirm

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We live south of Seattle in Washington state. So it would be a 4 hour trip down from the Canadian border. I am interested in some of that but you really think you would want to bring all that gear with you on the airplane?
 

tktran303

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Hello Amir,

I'm moving to Vancouver for 12 months...

The smaller items will come, and yes I probably won't need to bring 6 channels of amplification in the move...

just let me know...
 
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I'd still love to see the Mayflower Arc reviews. To my ears they are greatly underrated but I'd love to see if my subjective experience is backed up by the facts!
 

amirm

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I'd still love to see the Mayflower Arc reviews. To my ears they are greatly underrated but I'd love to see if my subjective experience is backed up by the facts!
Are they O2 clones or separate design?
 

Dialectic

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Separate and original design, just the same case. One of the more interesting things about it is its complete lack of op-amps. To my ears is is VERY neutral sounding.
The absence of op-amps is not evidence of good engineering.
 
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Amir, can you share your technique for measuring output impedance of DACs (for instance, the totalDAC is measured at 37 ohms)? Perhaps you have a post that describes all your test set-ups in more detail? Tks.
 

amirm

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Amir, can you share your technique for measuring output impedance of DACs (for instance, the totalDAC is measured at 37 ohms)? Perhaps you have a post that describes all your test set-ups in more detail? Tks.
I don't think I have documented that. Briefly, the output voltage is measured with a load of 100 k ohm of my analyzer (essentially "no load"). I then load up the output with 33 ohm and measure that voltage. The output impedance is then = 33 * open voltage/(closed voltage -1). Basically you are solving a resistor divider formula.

By the way, are you the author of the SignalPath Youtube channel (Shahriar)?
 
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Thanks Amir. I'm not the YouTube signalpath, but that's an excellent resource. I connected with you on LinkedIn not long ago (my email URL is my work). So I assume you run a full-scale signal at 1kHz, measure at 100k load, then remeasure at 33 ohm load and calculate the ratio drop? For instance the TotalDAC's 37 ohm output impedance means that its signal dropped roughly 6dB at 33 ohm load?

Also, I'm not clear if you're measuring the source impedance of the main out or the headphone out. In your commentary, you say "poor source impedance can impact the frequency response of headphones" -- so it seems like you might be measuring the HP output, not the mains (?).
 
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Amir, additional questions for you. On the page "Audibility Thresholds" you use -120dB as a "strict" THD threshold. Why? Are you using psychoacoustic data to back this up?

I ask because there are double blind psychoacoustic tests to determine the threshold of audibility of common THD. In the worst case, I believe the lowest detectable distortion was around 0.1% (-60dB) with most trained listeners unable to detect THD under 1.0% (-40dB).

Agreed that lower THD numbers seem nice, but how did you arrive at the -120dB strict threshold? This will make certain excellent tube and / or transformer designs look inferior (-60 to -100 range), when in fact they may perform with stunning ABX perceptual transparency and realism. In reality, many .000x% THD numbers are arrived at via massive internal loop gain (ick), which often makes audio sound less accurate, less transparent -- even though the THD numbers are ultra-low.

In one section, you note "-60 dB is NwAvGuy's guideline for crosstalk, so half (numerically) of the strict limit of -120 dB." I trust your realize that -60 isn't "half, numerically" of -120dB :) The actual audio level numerical difference between -60dB and -120dB is 1000 to 1, or 3 orders of magnitude.

Also see that you cited NwAvGuy's recommendation for -85dB masking level between signal and noise. Keep in mind, this measurement is pure noise, not THD+N. In virtually all program cases, noise will be ABX masked at -40dB under signal. The exceptions are rare, and generally only with signals like very high freq pure tones against broadband noise, which indeed requires -70 to -80dB masking ratio.
 
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amirm

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Amir, additional questions for you. On the page "Audibility Thresholds" you use -120dB as a "strict" THD threshold. Why? Are you using psychoacoustic data to back this up?
Those graphs are not mine. But yes, there are multiple ways based on psychoacoustics that we can calculate the distortion-free channel we need for full transparency. One is based on ear's dynamic range (about 115 dB). Another is based on threshold of hearing relative to the loudest playback SPL. The latter results in 120+ dB SPL.
 

amirm

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we can calculate the distortion-free channel we need for full transparency...
OK thanks for that. I have no problem with using -120dB as an arbitrary target for THD measurements. That's your call. But be aware that -120dB has no credible basis in ABX quantitative perception -- that is, based on psychoacoustic literature of THD thresholds. For an "audio science" website, you might want to reconsider this 'target" number. Many readers will likely assume that -120dB is required to eliminate all perceived distortions, which isn't good for the cause of audio science. Suggest it is better to teach readers that THD numbers and sonic performance are (beyond a -60dB threshold) generally not correlated.

Here's a real-world example from our analog design bench over the last 25 years. We've tested and listened to nearly every audio-grade op-amp IC on the market. Some of those op-amps deliver stunning THD performance (better than -120dB). But many of these op-amps sound different, and (surprise) many of them with extraordinary THD performance may exhibit less than ideal sonic performance. In many of our products, we have opted for our own discrete amplifiers --- which we find closer to ideal sonic transparency and realism. And, yet, these discrete amplifiers exhibit THD performance, between .001% and .01% (-80dB). Design techniques used to achieve ultra-low THD in today's IC op-amps (strong internal feedback, massive loop gain, etc.) can translate into inferior sonics, in our experience. Something to consider when using an arbitrary -120dB "quality" target. It's simply not the right way to judge audio.

Also, could you please revisit my earlier question (Aug 3)? Is your 33 ohm impedance test performed on headphone output or main output, or both?
 
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