"Expanding" 18i20 to drive HD600s—transport, amps vs DAC+amps, balanced implementations

[lactoseadept]

Hi guys, first post. As the thread title suggests, I'm exploring the sticky subject of headphone amplification "on top of" an existing audio interface. Hopefully someone can chime in or come up with a potential solution to my self-inflicted situation

To provide context, I had been corresponding with an audio engineer at Focusrite via email who basically confirmed to me that the headphone outputs on the 18i20 audio interface are best suited for headphones that are rated up to 200 ohms (math and explanation available upon request)

This led me to question whether I was experiencing the intended frequency response of my 300 ohm pair of headphones, the Sennheiser HD600. After all, I had been enthusiastically EQ'ing, and became quite certain that the bass roll-off on the HD600s led to a rather lackluster listening experience... but now I'm wondering whether my hardware is limited, beyond just volume.

Enter the opportunity to justify spending on a new piece of technology, my first dedicated headphone amplifier. But what about all the bits and bobs? How would I continue to use the platform I had grown so accustomed to (18i20) but eliminate insufficient voltage as a potential factor in the performance of my headphones?

My first thought, which I cast aside rather hastily, was connecting an amplifier via the line-outs of my audio interface—I have plenty of unused ones, and as long as the headphone outputs of the amplifier were capable of driving 300 ohm headphones optimally, I would be good. Or would I? Flashbacks of trying to repurpose old audio interfaces for pass-through widened my eyes and momentarily haunted me with the resulting distorted audio. Would the signal be perfectly transported? It became increasingly clear that buying an amplifier without knowing this sort of information or the potential result would be irresponsible...

What about digital signals, then? Presumably, a digital means of transport e.g. optical/SPDIF coaxial would theoretically mean less conversion (technically, the exact same amount, just using the new hardware instead of the audio interface) but at least a better signal to the amplifier? At this point, I then started to consider a DAC/amp combo to preserve audio fidelity...

I searched the market and found some affordable amplification units; some balanced, others unbalanced. Many with RCA, which stunned me—aren't they inferior? Why would a main headphone output be 3.5mm? Is 4.4mm really the upcoming standard? I was biased towards balanced 6.3mm, but then did some reading that effectively suggested that if it isn't XLR 4-pin, and the amp isn't "fully balanced", as was the catchphrase, my signal isn't balanced, and so I might be missing on that 0.1%+ gain in fidelity. Not if I'm spending hundreds of dollars, I won't!

I then strove to find a headphone DAC/amp with balanced output, preferably single 4-pin XLR, with the proper transducers, and then realised I am woefully out of my element. Did I even need a DAC? Would the existing DAC of the 18i20 be sufficient, or of higher quality than whatever I get? Should I just use 6.3mm between the audio interface and new amplifier, and have that new amplifier provide the right impedance to drive my headphones?

It is with these questions that I seek guidance, and audio science, from the learned users of this forum... how can I get the highest fidelity headphone experience, with a balanced ("fully"?!) amp implementation, presumably with XLR 4-pin (terminations of which I'd buy for recabling)—but then again, are the HD600s even designed for such a purpose, at this point? What would the optimal amplification setup be?!

Dazed and confused, and thankful for any insight,
AM

P.S. Please find attached the HD600 with aftermarket 6.3mm termination, Focusrite 18i20 back panel, and the closest thing I have to a preamp, a Rega Fono Mini A2D

P.P.S. I almost certainly have missed some thoughts or info, so if anything is (understandably) unclear, I would gladly revert. Thanks in advance!

P.P.P.S. If it isn't clear, I'm trying to find the perfect headphone amp, have as balanced a situation as I can, and use the right connections to preserve audio fidelity, all without hopefully breaking the bank. Oh, and understanding why/what would make a superior choice.

 

[staticV3]

To provide context, I had been corresponding with an audio engineer at Focusrite via email who basically confirmed to me that the headphone outputs on the 18i20 audio interface are best suited for headphones that are rated up to 200 ohms (math and explanation available upon request). This led me to question whether I was experiencing the intended frequency response of my 300 ohm pair of headphones, the Sennheiser HD600.

I'm sorry, but that sounds a bit nonsensical.

The only thing that'll happen if you plug headphones with more than 200Ω into the 18i20 is that the internal headphone amp will output less current, therefore running cooler and possibly with less harmonic distortion.

The frequency response won't change and neither will the output voltage, even if your headphones have 5,000Ω impedance.

but now I'm wondering whether my hardware is limited, beyond just volume.

With high impedance headphones, volume is the only thing you're dealing with. There's nothing else besides that (no "dynamics", "impact", or other audiophile buzzwords).

 

[lactoseadept]

I'm sorry, but that sounds a bit nonsensical.

The only thing that'll happen if you plug headphones with more than 200Ω into the 18i20 is that the internal headphone amp will output less current, therefore running cooler and possibly with less harmonic distortion.

The frequency response won't change and neither will the output voltage, even if your headphones have 5,000Ω impedance.


With high impedance headphones, volume is the only thing you're dealing with. There's nothing else besides that (no "dynamics", "impact", or other audiophile buzzwords).

Thank you for the reply, I was under the impression that signal amplitude is influenced by headphone impedance and would yield variations in electrical response measured in dBr across a given frequency range e.g. non-flat, where "flat" would be ideal, or in other words, closest to the "true" frequency response of the unit. Is this only observable in highly mismatched impedance, or am I missing something? I was under the impression that I needed to account for the increased voltage swing, and that maximum power would be limited—If I'm understanding you correctly, that voltage swing and limited maximum power would only affect volume, and not frequency response?

 

[staticV3]

Thank you for the reply, I was under the impression that signal amplitude is influenced by headphone impedance and would yield variations in electrical response measured in dBr across a given frequency range e.g. non-flat, where "flat" would be ideal, or in other words, closest to the "true" frequency response of the unit. Is this only observable in highly mismatched impedance, or am I missing something?

That's possible if you have an old or poorly designed headphone Amp with high output impedance, and a headphone with both low overall impedance (<50Ω or so) and variable impedance response.
In that case, the headphone turns into a frequency dependent voltage divider basically, and so the effective frequency response of your Amp changes.

If I'm understanding you correctly, that voltage swing and limited maximum power would only affect volume, and not frequency response?

Correct.

 

[spigot]

Most new headphone amps or amp/dacs have an output impedance of less than 1 ohm, they're not going to effect the frequency response of any headphones. You can get a headphone amp for $100 which performs way beyond the capabilities of most headphones, including the HD600's. You could then spend similar on a dac, or for less get a combined dac/amp (see reviews on this site). The limiting factor in sound quality will be the headphones, they perform a more difficult mechanical function. Forget all that balanced stuff, it's overkill, HD600's are really easy to drive (I have a pair).

Five years ago this wouldn't have been the case, lots of variation in the budget sector. Wouldn't be surprised if it's principally Amir (audiosciencereview) that's driven this rush to excellence, what with manufacturers no longer being able to rely on reviewers just talking about soundstage to drive sales.

 

[DVDdoug]

This led me to question whether I was experiencing the intended frequency response of my 300 ohm pair of headphones, the Sennheiser HD600. After all, I had been enthusiastically EQ'ing, and became quite certain that the bass roll-off on the HD600s led to a rather lackluster listening experience... but now I'm wondering whether my hardware is limited, beyond just volume.

EQ is fine if frequency response is the only issue and if you can boost the bass without driving the (internal) headphone amp (or the headphones) into distortion.

High impedance headphones are immune to impedance-related frequency response variations but they tend to be quieter (you get less current and therefore less power) through a higher impedance). A different headphone amp might go louder but it should have the same-flat frequency response.

I searched the market and found some affordable amplification units; some balanced, others unbalanced. Many with RCA, which stunned me—aren't they inferior?

Balanced connections are much more immune to noise pick-up and ground loops. For "short" connections RCA is usually just as good. But if you have a home recording studio you might want to keep everything balanced for compatibility (XLR cables everywhere) and for "insurance" against noise.

Why would a main headphone output be 3.5mm? Is 4.4mm really the upcoming standard? I was biased towards balanced 6.3mm, but then did some reading that effectively suggested that if it isn't XLR 4-pin,

There is no inherent advantage to a balanced headphone output* and most headphones are not compatible. With the same power supply you can get twice the voltage before clipping (4 times the power = 6dB louder) since one signal swings positive while the other swings equally negative. But, the headphone amp designer can simply use a higher voltage power supply.



* You can't pick-up enough noise in the wires to "power" headphones or speakers. Noise is only a problem with signals into an amplifier. Balanced connections are most important with microphones where you have a few millivolts and the signal and noise get amplified 100 or 1000 times.

 

[trl]

I had been corresponding with an audio engineer at Focusrite via email who basically confirmed to me that the headphone outputs on the 18i20 audio interface are best suited for headphones that are rated up to 200 ohms (math and explanation available upon request)

Let me provide here few web links with some good writing about headphones impedance and corresponding power levels:
- http://nwavguy.blogspot.com/2011/09/more-power.html
- http://nwavguy.blogspot.com/2011/02/headphone-impedance-explained.html
- http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html
- https://global.beyerdynamic.com/impedance-for-headphones
- http://www.digizoid.com/headphones-power.html

Based on https://focusrite.com/en/usb-audio-interface/scarlett/scarlett-18i20, max. output level of 18i20 would be 7dBu which means 1.73V RMS (see https://reference-audio-analyzer.pro/en/v-dbv-dbu.php#gsc.tab=0).

HD600 has about 97dB/mW, so based on Digizoid link from above the max. 1.73V RMS from 18i20 translates to about 105 dB SPL (peaks) into your headphones, which might be OK for HD600 with 0dB recorded tracks, but definitely not perfect for tracks having lower peaks. Also, given that 105dB SPL is relative to sine-waves and not to actual music, this means that average SPL of HD600 driven by 18i20 is pretty close to 80-85dB SPL (depending on tracks' compression), which might be fine for regular music listening, but not fine for critical listening or tracks mixing.

I usually consider a good-enough headamp the one that can drive a pair of headphones to at least 115dB SPL (peaks) with a THD+N below -100dB, while in your case the 18i20 can not deliver this properly, hence your question about a compatible headamp for HD600.

Given the high no. of headamps tested here on ASR, you may lookup to https://www.audiosciencereview.com/forum/index.php?pages/Reviews/ and search after the "Headphone Amp" string (choose "Exact" match for the search string) and see which one is able to deliver anything between 4.5V RMS to 8V RMS (or even more if really needed) @300Ohms. You can also sort the list after SINAD, Price etc.; too bad the Warranty and Continent columns are not available.

If the headamp will sit close to your 18i20 I guess you won't need balanced interconnects, so close to 100 USD I would suggest: Magni 3+ or Heresy, Objective 2 or Atom would do as well. Of course, more expensive headamps might suit you better, so do give it a search on ASR threads and let us know what you've decided in the end.

 

[lactoseadept]

Thanks so much for the informative replies, that clears things up wonderfully. Very glad I asked and was able to hear from some clearly knowledgeable folk. Elucidating!

 

[8ty8]

So @lactoseadept did you end up getting an amp for your setup?

If so, what did you end up with?

I have found myself in a similar predicament, and would love to heard your experience since your last post on here, on January 9th.