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Review and Measurements of RME ADI-2 DAC

Rod

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Tried to plug the headphones into both the Xduoo and the RME amp at the same time. Got a short warning. That doesn't work.
 

amirm

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We lost power due to the storm. Running on the generator.

For now, I remembered vaguely that I had seen a reference to making THD measurements without noise. Dug in and what do you know, it has a sophisticated THD meter in the DSP subsystem. You can instruct it to sum any and all harmonics and derive the distortion total from it!

Here I have all the harmonics selected but I can click on them individually:

upload_2018-4-14_16-16-50.png


Anyway, before the power was lost, I captured one set of measurements. I matched the output level while driving the MDR-V6 to 1 milliwatt per Benchmark test. I then compared both THD and THD+N relative to a 50 ohm dummy load and the 73 ohms of the MDR-V6. Volume on RME ADI-2 DAC was set to 0 dBr and I adjusted the digital samples until I achieved 1 mw (about -32 dBFS):

RME ADI-2 DAC THD into Sony MDRV6 headphones.png


We see that THD measurements were pretty necessary as without it, noise dominates and we see the slightest hump with MDR-V6 relative dummy load. We see a 20 dB rise due to Sony load versus dummy resistor.

Here is the same results from Benchmark:

upload_2018-4-14_16-41-56.png


Focusing on the navy line in theirs, the overall distortion is similar to RME ADI-2 at mid to high frequencies. At low frequencies of course, the distortion remains the same and does not rise as it does with RME ADI-2 DAC. What they show happening to the other two amps is what is happening to RME ADI-2 DAC but at about 8 dB lower.
 

amirm

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Before losing power, I performed another experiment. I connected the grado headphones to the RME and then measured THD while playing 50 Hz and alternatively blocking the back of the headphone versus not. Blocking it reduced distortion by 10 dB! So it seems that this is indeed related to back EMF energy from the driver coming into the output of the amplifier. This is what Mathias is saying is happening. What he does not address is why Benchmark is not sensitive to it, and they are.
 

SIY

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What does the impedance curve of those phones look like? I'm just not seeing this distortion rise at all with the APx1701 driving the Superlux phones that I have on hand.
 

Blumlein 88

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What does the impedance curve of those phones look like? I'm just not seeing this distortion rise at all with the APx1701 driving the Superlux phones that I have on hand.
Superlux phones are too low end to show the difference. They are so low end not only will they not resolve distortion while listening, they won't even do it measuring with an AP unit. :D
 

mindbomb

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Before losing power, I performed another experiment. I connected the grado headphones to the RME and then measured THD while playing 50 Hz and alternatively blocking the back of the headphone versus not. Blocking it reduced distortion by 10 dB! So it seems that this is indeed related to back EMF energy from the driver coming into the output of the amplifier. This is what Mathias is saying is happening. What he does not address is why Benchmark is not sensitive to it, and they are.
I think Benchmark might have been trying to say that the output impedance specification might not reflect actual output impedance at low frequencies, and that's why some amps are more sensitive to it.
 
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Dro

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"The main point of the paper is how to make standard op amps stable for capacitive loads, and protect them from oscillating. This, of course, is no issue if you use circuits that have been designed already not to act like this, and are developed specifically for operation with phones."

I read that as Benchmark use the setup as described by TI to protect their amplifier from misbehaving (which eliminates THD), whereas RME do it differently, so you can still measure the THD fed in by the headphones.

Maybe somebody who is good with electrical engineering in these types of applications could follow up in the RME forums? They sure seem quite responsive to inquiries.
 

Rod

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Its going to take some time to get used to the tube not being in there. The RME dac sounds very neutral to me so I was using loudness and bass boost on it. It does sound very good, but the xduoo amp section has more watts. The distortion levels in the xduoo are higher and I can hear that also. I figure I will be switching back to the RME soon enough after it breaks in. Waiting for the massdrop/THXAAA. View attachment 12079
That didn't take long. Already its sounding better. Had to turn the bass back down. OMG.
 

Sythrix

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Before losing power, I performed another experiment. I connected the grado headphones to the RME and then measured THD while playing 50 Hz and alternatively blocking the back of the headphone versus not. Blocking it reduced distortion by 10 dB! So it seems that this is indeed related to back EMF energy from the driver coming into the output of the amplifier. This is what Mathias is saying is happening. What he does not address is why Benchmark is not sensitive to it, and they are.
Is there anything conceivable that could be done post amp to prevent this EMF energy from coming back into the output? Something akin to a one-way street?

It doesn't necessarily bother me at these levels, but it would be interesting to attempt and compare if a practical solution could be made available.
 
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Seems like a dream DAC.

I wonder:
1. If it can be stackable for doing multichannel (2 units or more) using the USB ASIO driver, the ADI-2 Pro manual hints little about this capability ("multi-interface", if I'm not confusing it) and the ADI-2 DAC hint even less. Could then be just about the best Multichannel DAC/opAMP at about $800 less than the closest contender (exaSound e38) if only 5.1ch is needed (or $1800 less for 2.2ch), with much more features and probably a better headphone AMP(s, as you get more than one unit at this price).

...
You could get a rme ufx 2 for 2k and get 8 outputs in 1 box.
 

restorer-john

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Some of the back EMF from headphones or speakers is returned via the NFB loop to the front end of the amplifier. Unless your amplifier has zero impedance and zero feedback for the buffer stage, I can't see how this contribution to THD can ever truly disappear. This effect has been well understood with speakers and amplifiers for many years.

Many amplifier designers in the 1970s researched the effects of speakers and speaker cables and their contribution to THD and the results led to very fast, ultra wideband, monoblocks with very short cables to pursue the lowest THD.

A fast feed-forward/direct distortion cancelling setup could IMO isolate the back EMF characteristics of the load from the amplifier and let it work in isolation?
 
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SIY

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Two questions, then:

1. Why would the back EMF generate harmonics when measured across the transducer? I have not seen this phenomenon when testing (competent) amps with loudspeaker loads. As well, given the moving mass of headphones (low) and the not-spectacularly-high impedance peak at bass resonance, I wouldn't expect that the back EMF would be super high.

2. Why am I not seeing that same effect with a different amplifier having the same source impedance?

I'm going to try a few more variations and post results. I'm still unsure if this is an amp, Grado-specific, or a test setup issue and would really like to pin that down. Needless to say, if there's anyone in range who has an RME and/or a set of Grados that we can put on my test bench, the reward will be knowledge and a bottle of something excellent.
 

SIY

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And a little more fun. I increased the source impedance by putting 2R1 in series with the headphones, then increasing the drive voltage to maintain the same 630mV across the phones. Two second sweep time, just to get the frequency resolution a bit higher. Lo and behold, I did see a greater amount of bass distortion in the electrical waveform, but still far less than Amir was observing. And still, at the same rated source impedance as the RME, the distortion was lower yet.

FWIW, the distortion was unsurprisingly dominated by 2nd and 3rd order.
 

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Rod

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I used these settings from another forum but its not the same headphone that I have. I noticed it sounded smoother right away. But does someone have settings for the HE-400i for E.Q.?

Hifiman HE-560
B 1, G +4.0, F 120, Q 0.7, SHELF
B 2, G +6.0, F 1.90k, Q 1.2, PEAK
B 3, G -7.0, F 4.0k, Q 3.0, PEAK
B 4, G -6.0, F 10.5k, Q 3.0, PEAK
 

amirm

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I used these settings from another forum but its not the same headphone that I have. I noticed it sounded smoother right away. But does someone have settings for the HE-400i for E.Q.?

Hifiman HE-560
B 1, G +4.0, F 120, Q 0.7, SHELF
B 2, G +6.0, F 1.90k, Q 1.2, PEAK
B 3, G -7.0, F 4.0k, Q 3.0, PEAK
B 4, G -6.0, F 10.5k, Q 3.0, PEAK
I don't but remind me in a few days and I will play around and report back. :)
 

Dro

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@amirm have you measured the noise level of the IEM out with no signal?

I am fairly certain I can hear a tiny bit of hiss with my Andromeda when no music is playing. I think that is to be expected even with the very good specs of the IEM out. Over 135 dB SPL at 1 Vrms is loud. I listen at -30 dB and due to replay gain I basically never have music go above -5 dBFS.
 

dc655321

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I am fairly certain I can hear a tiny bit of hiss with my Andromeda when no music is playing. I think that is to be expected even with the very good specs of the IEM out.
The Andromeda are ridiculously sensitive IEMs. When I had mine, I could hear the screen flicker noises from my iphone se when scrolling quickly.
OTOH, the Andros were completely black when I paired them with the SMSL iDEA.

Great earphones... sometimes I miss not having them any more :-(
 
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