ZMFheadphones Aegis Amplifier


Headphoneus Supremus
Pros: -- Impressive resolution & clarity
-- Big/wide/tall soundstage
-- Very powerful
-- Allows granular customization of output impedance
-- Gorgeous looks
Cons: -- Maybe make the front panel VU meters ¼" larger in diameter…?
-- (sound of crickets)
I've heard big transformer-coupled tube headphone amps at shows and a few times in my system, including amps by Feliks, DNA, Woo, Cayin, Auris and others. When the ambient noise was low I often heard things I associate only with large, powerful tube amps: a deep soundstage; a big, expressive midrange; hyper-real spatial placement of notes and voices, each seeming to sit on its own little puffy cloud in 3D space; and sometimes a slight roll off in the upper treble and a somewhat less deep and dynamic bass. I assumed this is what all big tube amps do.

Then I heard the ZMF Aegis, which upended all assumptions and exceeded all expectations. It sounds different and is different. It's also gorgeous to look at.

My sincere thanks to ZMF for the loaner Aegis; and to Zach and Keenan for their very helpful answers to my pestering
questions about this new amp.
My apologies to readers for the length of this review. There's so much ground to cover with the new Aegis.

The Aegis Backstory
L0rdGwyn (Keenan McKnight) is well known here for his innovative DIY tube amps, the OTL Airmid and transformer-coupled Aegis. A lengthy Head-Fi thread is devoted to them. He also supports these amps with very helpful, informative instructions and cogent answers to user questions. ZMF Headphones' owner/chief designer, Zach Mehrbach, is also well known here for designing a succession of beautiful (and beautiful sounding) artisanal headphones starting in 2014. Keenan and ZMF are collaborating on the commercial version of the Aegis. The ZMF Aegis is designed by Keenan, manufactured by Cayin, with design aesthetics and marketing by ZMF.

Amp Design & Tubes
The design of the Aegis is quite unlike that of other tube headphone amps. Keenan believes the Aegis is the first commercial tube amp to use this design. As Keenan told me,
"It's a transformer coupled amp, but not like all the other ones out there, it's an unusual circuit. The output tubes are wired as a cathode follower, much like how tubes are wired in an OTL, but it is transformer coupled instead. This isn't the case in a conventional transformer coupled amp. Cathode followers have no voltage gain, so to make a transformer coupled cathode follower work, the input stage has to be very high gain, the reason 6SL7 is used. Another unusual bit is the 6SL7 input stage is loaded by a choke, not something you'll see much in commercial amps!
So why go to all the trouble for the transformer coupled cathode follower? 1) lower distortion and output impedance and 2) going cathode follower removes the cathode bypass capacitor out of the output stage that is present in almost all transformer coupled amplifiers. The more capacitors you can get out of the signal path, the better, which really improves dynamics and clarity in my experience."​

The other point I would make about design is how much user-customization of output impedance the Aegis offers. There are 3 user-selectable output types (XLR; ¼"; and 4.4mm; switch on front panel). The 4.4mm is significantly attenuated for IEMs, so I'll focus on XLR and ¼". The Aegis is single ended; its input is an RCA L/R pair. The XLR is a pseudo-balanced output with the output transformer acting as a phase splitter; it's offered for user convenience. The XLR output puts out somewhat higher power than the 1/4"; it also has a higher output impedance at each setting of the impedance switch. Thus users can co-vary the output type with the 3 impedance switch settings to precisely dial in the output impedance that reaches the headphone. This matters because output impedance subtly affects the sound of the amp

The Aegis uses 5 tubes. On my loaner the stock signal tubes are 6SL7; power tubes are EL34, both JJ; and the 5AR4 rectifier is Sovtek. There are many possibilities for tube-rolling with this amplifier:

Input tubes: 6SL7 and all of its variants​
Rectifer: 5V and up to 3A, includes 5U4, 5R4, 274B, 5V4 / GZ32, 5AR4 / GZ34, 5Y3 (best to use lower power output tubes), others​
Power tubes: Almost any power pentode will work: EL33, EL34, EL37, EL38 (w/an adapter), KT66, KT77, KT88, KT90, 6550, 6V6, 6L6, EL84 (w/ adapter), others​

Conservatively rated, the amp's power output is 2 watts per channel. That is BIG tube power, plenty for anything out there.

Physical Stuff
The Aegis is a full-size tube amplifier. It measures 11½ x 14 inches and is heavy. It's also gorgeous to look at (apologies for the crappy flash pictures):

4 - AEGIS (side-by-side).jpg

My loaner has verdigris-painted transformer covers, a teak faceplate, wooden volume pot & on/off button (a 1st in my experience) and a painted metal bar with enclosed ZMF logo running along the bottom of the faceplate. This mix of wood and earth tones is very attractive. The Aegis stands out. This design gets small details right: the figured circular brass rings encircling the base of each tube contrast nicely with the dark grey case, highlighting each tube from below. The Aegis puts on a discrete, attractive light show, with soft yellow light emanating from each of the two VU meters; the on/off button subtly outlined in orange light; and the volume pot circled by 16 small, orange back lights. And of course there's the soft orange glow of the 5 tubes. IMO this Aegis is knock-out pretty.

2 - AEGIS Top View (Flash).jpg

The Sound
After a day's burn-in I began listening via the ZMF Ori to check how much power the amp had for this relatively power-hungry planar (hint: a lot), then switched to my best planar and dynamic headphones, the Caldera open and Verite open. From minute-1, the sound characteristic that jumped out at me was clarity. I've never heard this much resolution from a tube amp. Even 50 year old studio tracks that were never conceived as "audiophile" recordings sound bigger, better, and more expressive on the Aegis. Every musical note rings clear as a bell. Part of this clarity is the truly black background; the Aegis is quiet as the proverbial grave.

Years of audio experience conditioned me to distrust apparently high resolution. Too often it comes at the cost of brightness, edgy transients, and spikes in the upper mids or treble. But the Aegis sounds nothing like that. This amp captures the soul of music. Its considerable resolution is natural, organic and dynamic. The amp's overall tone is neutral, well balanced and level, top to bottom. For me, the "voicing" of the Aegis is ideal—and this is with stock tubes.

I quickly became accustomed to Aegis-level resolution. I heard new things in tracks I've heard countless times: keyboard parts I could barely make out before; precise articulation of strings and percussion; ambience of the recording space. Latin Jazz tracks with multiple percussionists sound utterly clear; every drum strike is distinct. The vibraphone, so challenging to record, is beautifully and realistically rendered here. The Aegis' resolution extends to the accurate timbre and body of instruments and voices. With this amp, resolution enhances tone.

It's easy to get used to high resolution that is so musical. Tracks from different recordings sound noticeably different. With the Aegis I easily hear into any track and understand the effects the mixing engineer was going for. It's also easy to hear the slightest reverb or delay in microphone processing, the relative "wetness" or "dryness" of the recording acoustic, and so on. I tried a number of music genres with the Aegis, everything from ethereal classical to electronic pop with strong beats and bass pulse. The Aegis nailed every genre. This is not an amp that's good with certain things but not others. If there's a music genre that's "eh" on the Aegis, I couldn't find it.

Soundstage: I've never heard soundstage this wide and tall. At times it felt like a sonic bubble surrounding my face and head. Some sounds were actually behind my head. All 3 headphones I heard on the Aegis sounded even larger & more spatially expressive than usual. This amp's dramatic soundstage was heightened by the clarity of each note, precisely anchored in sonic space. The Aegis' soundstage could be huge, intimate, or somewhere in the middle; it reflects the music source. Tube amps often have strong spatial and soundstage qualities; the Aegis simply has more. When I paired the Aegis with the Caldera open, a headphone that has ZMF's ADS system and soundstages heroically, it was like sitting midcourt at Wimbledon.
  • The Aegis' soundstaging can be startling. In a Scary Pockets live-in-the-studio YouTube video, the song's chorus features a loud, heavily distorted bass guitar. Those bass notes appeared suddenly beneath my chin and across the bottom of the entire soundstage. It felt like one those lead radiation aprons used for X-rays was draped around my shoulders.
Tone: Frequency response of the Aegis is easy to discuss because bass, midrange and treble are so equitably reproduced. I can hear no bloat or significant frequency elevations/depressions. Every instrument and voice sounds just right. Timbre is accurate and life-like. The bass deserves special mention. The Aegis has bass for days (including sub-bass), deep, textured and dynamic in ways tubes often are not. Treble is equally impressive. If there's a roll-off in the upper treble, I don't hear it.
  • I'm reminded of 2 measurements (rise time [small signals] & slew rate[(large signals, measuring an amp's response time) commonly used for solid state amps but not tubes.
  • I could be wrong (please weigh in, Keenan), but I'd guess the Aegis has a fast rise time and/or slew rate, maybe also a better than usual S/N ratio. This amp goes from 0 to 100 in no time.
At times I heard a slight forwardness of certain voices or instruments in the upper midrange of the Aegis. This was not present in all tracks, just some. I suspect what I'm really hearing is the mixing engineer's decision to highlight the lead vocal or instrumental. Either way, the effect is small. It's the kind of thing tube rolling could readily change.

Impedance Matching: When first listening to this amp I used the XLR output (all my cables are balanced). After Keenan explained how the XLR and ¼" outputs differ, I tried the ¼" output. Then the sound really snapped into focus:
  • The ¼" output sounds more controlled and somewhat smoother than XLR, at least to me. I did most of my listening on this output
  • I appreciated the ¼" output's somewhat lower gain. The Aegis is so powerful that the extra gain with the XLR output is not necessary, at least in my 2 systems
  • The ¼" output has lower output impedance at all 3 settings of the impedance switch vs XLR. For me, "Low" worked perfectly with the two planars, and either "Low" or "Med" worked well with the VO. I didn't care for the "High" setting's higher gain, probably because both my systems have single-ended DACs (MHDT Labs Orchid; Metrum Onyx) that output well over the 2.0v standard
To my ears the lower impedance settings produced a somewhat smoother, calmer, more controlled sound. Perhaps I traded off a bit of dynamic punch for this (not sure). If so, the tradeoff was worth it (this amp has dynamics & punch to spare). The Aegis gives users unusually granular customization of sound via combinations of output type & impedance settings. This is both elegant & practical.

Aegis impedance TABLE (100%).jpg

The stock tubes sound very good. But of course, tube rolling beckoned. I replaced the stock EL34s with matched pair of NOS Siemens EL34s. After 5-6 hours burn-in I did some listening. The Siemens sounded subtly different, a bit sharper on transients and slightly tipped up; drums & percussion, already crystal clear, became borderline spectacular. But on less than ideal recordings, these tubes let me know it. Still, vocals stand out and the treble is sky high. Bass is very clear and controlled. This is pretty much what I expect from Siemens tubes—relatively small but audible differences.

Keenan believes the unique circuit design of the Aegis predominates in determining the amp's sonics. He also says using different types of power tubes in place of stock (ie, KT88s or any other compatible tubes) may produce more audible results. I suspect he's right ... but that doesn't stop me from wishing I had a pair of late '50s Mullard Blackburn EL34s to try in the Aegis.

Aegis vs Icon Audio HP8
It seemed unfair to pit a compact 3-tube amp against a 5-tube, full-frame amp, but the contest was far from a blow-out. Both amps sound rather amazing via the VO and Caldera Open. Sonic differences are small but meaningful:
  • The black background of the Aegis continues to impress, but the HP8 comes close. Neither amp telegraphs "tube sound" -- there's no grunge or sonic dirt with either. Both are very quiet even with no music playing and volume pots at medium loud (impressive)
  • Soundstage on the Aegis is somewhat wider and taller than on the HP8
  • But soundstage on the HP8 is somewhat deeper
  • Bass on the Aegis is articulate and textured. It's the best bass I ever heard from a tube amp, maybe the best bass of any amp
  • But the HP8's bass was a bit more impactful and a touch deeper, albeit not as textured as the Aegis. This is probably due to the NOS RCA 6NS7s I use on the HP8. When I switched to them, the HP8's bass became noticeably richer
  • The Aegis is fast and neutral; its articulation of multi-track music & ability to untangle complex passages is a next-level, though the HP8 is also above average in this respect
  • The HP8 imbues music with a certain beauty of tone, slight warmth, and palpable "thereness" that is quite alluring. IMO this is the tradeoff for the Aegis' greater clarity and articulation
  • Stock tubes of the Aegis sound very good—tube rolling is not a necessity--while the HP8 can benefit from the best NOS tubes
I would love to revisit this comparison after systematic tube-rolling on the Aegis (I can dream).

The Aegis is the best tube amp I've ever heard—maybe the best amp, period. This is big-league tube amp sound worth seeking out.
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@Ridewave, the ZMF Aegis definitely lights up both planars and high impedance headphones. And yes, that is cool.
I'd be really curious to see how the Aegis compares to the Cayin Tube amps. I love my HA-3A but the look of the Aeigis is stunning.
Thanks for the informative writeup, the gears are turning here to see how I might figure out a way to get my hands on one! :wink:

I'd appreciate knowing what else you have in your chain, (i.e. DAC and solid-state amps), as we share some similar headphones. It would give me some reference points. :beerchug:

Thanks for replying on the other forum, it's too bad you're having issues there! :angry: