Review written by @Resolve
Review unit provided by Ampsandsound for evaluation.
I want to start this review by explaining why I don’t typically do full headphone amplifier reviews. It’s not because I’m not bothered to evaluate them or because I think they don’t make a difference - they absolutely do make a difference - but because they don’t make that much of a difference when compared with one another, or at least my experience with a wide range of solid state amplifiers has indicated this. You’ll see this sentiment echoed by a number of other reviewers like Crinacle and Metal571, who acknowledge that there are differences, but beyond finding the right amping for a given headphone, it’s understandable that there may be fewer reasons to care about amplifiers than there are about the headphones being driven.
So my typical rule, as far as a chain for headphones is concerned that I think is safe to follow, is that headphones make the biggest difference, then amps, then DACs (all of this assumes decent files or streaming). Indeed, you’ll see this rule expressed and repeated commonly in many places online, and these days you can get an ‘all-you-need’ setup at modest price tags. And then, when you get to hear the Ampsandsound Kenzie OG single-ended tube amplifier… you realize that this rule may be worth breaking. At the very least, the Kenzie may be a counter-example or an exception to this rule.
The following is my subjective evaluation of the Ampsandsound Kenzie OG, and on that note I should also stress that while I am able to measure downstream effects of the amplifier on headphones, I do not presently have the tools to provide objective data and measurements for amplifiers on their own. So this is all just going to be a description of my experience with a variety of headphones on the Kenzie.
What it is
The Kenzie is a single ended 1626 tube amp with no feedback. While Ampsandsound has a number of different models, including a line of speaker amps, the Kenzie is their primary entrypoint to dedicated headphone amps. It uses custom high quality output transformers, which leads to a more neutral sound that doesn’t change the downstream frequency response of the headphones, provided the right synergy with output impedance is observed. It’s interesting to note that 1626 tubes are WWII era tubes that were used for transmitters in radar installations. Lastly, the Input tube is a 12SL7 or equivalent (the short one in the middle).
The default configuration for the Kenzie includes a low Z 32 ohm out and a high Z 300 ohm out, however I’m told that should a prospective buyer have a specific output impedance requirement for a set of headphones they own, it may be worth asking the designer Justin Weber if he can accommodate that.
What it’s not
The Kenzie is not an output transformerless (OTL) amp, meaning that while you see some large tubes on the front, it’s a different type of design from what you get with something like a Bottlehead Crack or Feliks Audio Elise. It’s also not the most powerful amplifier, meaning that if you have a HiFiMAN Susvara or Audeze LCD-4, you may need to look to one of Justin’s other models. With that said, most planars still work great with the Kenzie, it’s just the super inefficient ones that may need a bit more amplification.
The Kenzie is also not a jack-of-all-trades master of none type of amplifier like you might find commonly with many solid state amplifiers. So, this is not something that’s going to work the same for every headphone you throw at it, and perhaps unsurprisingly this is also not an amplifier meant for IEMs (although I’m sure someone will find one that works off of it). Instead, the Kenzie is a bit more specialized, and is going to give a unique synergy depending on the headphones you use with it.
One thing to note is that both outputs are on the back of the amplifier, which means that when it's facing forward, the headphone cable has to go all the way around it. This is somewhat awkward - but Ampsandsound have incorporated this feedback with the Kenzie Encore and some of their other models. Other models also have the option to function as a preamp, while the Kenzie OG is not meant for this.
It’s important to note that for both low and high Z outputs, the Kenzie is a completely neutral headphone amplifier. And, as I mentioned at the beginning of this review, while I’m unable to measure the amplifier specifically, I am able to measure the downstream effects of the amplifier on various headphones, and I can confidently say that the Kenzie doesn’t have any frequency response related impacts.
There is of course an important caveat to this statement, and it’s that depending on the headphone being used and its relationship with output impedance, you may end up with a bass boost. So for example, using some of the low impedance Focal headphones resulted in a bass boost by 2-4dB below 200hz from the 32 ohm out, and the 300 ohm ZMF Verite showed a 2dB difference in the bass off the high Z out compared to the low Z out - even though off the low Z out it perfectly matched the same frequency response when being driven by the SPL Phonitor X.
Using the Audeze LCD-X for the same comparison, it showed absolutely no change between the two amps, neither did the HiFiMAn Arya - and the Arya also had the identical frequency response when used from the 300 ohm out as well. So, let me again state that any frequency response changes to the headphones being used is more the result of the relationship between that headphone and output impedance than anything to do with the Kenzie being a tube amplifier, and it’s important to keep the question of headphone synergy in view.
Subjective Sound Impressions
I’ve said that the Kenzie is a totally neutral amplifier, as long as the output impedance synergy is observed. You may want to deliberately add some flavor by breaking the usual impedance ‘rules’ for headphone and amplifier combinations, but again, that’s not a consequence of the tube amp. This raises the question, if the Kenzie is a neutral tube amplifier… what’s the point of that? Isn’t a tube amp meant to affect the sound in some way? Yes and no. With the Kenzie, the real selling point when it comes to sound is what I’m going to call its ‘tube character’. So even though there are no measured downstream effects with the right impedance relationships, the Kenzie still sounds massively different from typical solid state amplifiers - and in a really good way.
The Kenzie is all about sweetness, euphony, and richness of tone. While it’s difficult to correlate this experience with anything in particular, once you hear it, that description should be immediately recognizable. There’s an extra sense of presence for everything when using the Kenzie - on every headphone that worked with it. When listening to individual instrument lines with high impedance dynamic driver headphones, it felt like the Kenzie drew attention to the finer little nuances and small gradations of volume for trailing ends of tones, giving an extra sense of microdynamics and detail. This effect was lessened slightly with the planar magnetic headphones I tried, but still present.
The Kenzie also has an effect on the sense of space, staging, and imaging. It’s creates a more ‘involved’ soundstage, that can come across perhaps a bit cluttered - but not in a bad way. Images have more life and body to them, and stand out from one another a bit more distinctly. Imagine that if a headphone produces sound all around you in a circle (this of course all depends on the headphone in question, but just pretend it does for a moment), that circle on the Kenzie is now contoured inwards at various points, creating a more textured and layered presentation for positional information.
Lastly, when it comes to treble presentation, regardless of the headphones I used, when my music tokened upper frequencies for cymbal crashes or anything else hitting the upper ‘air’ frequencies, the Kenzie made it so that everything came across with a somewhat diffused character. And to that end, this would be an excellent pairing for any headphone that has a bit too much sharpness up top. So for the LCD-X, which is a bit intense up above 10khz, it softened the blow a bit by spreading everything out and giving it extra texture and layering.
- ZMF Verite - Dead quiet off the 32 ohm out, but sounded better off the 300 ohm out despite the slight noise floor that this pairing introduced. If you own this headphone, I recommend asking Ampsandsound about alternative high Z output impedances, but otherwise the sound was fantastic with a sweet, euphonic and hyper-detailed quality to it.
- Sennheiser HD650 (HD6XX) - Dead quiet from both outputs. Retained the same tonal richness qualities as the Verite (albeit to a lesser extent). The main benefit to my ear with this headphone when compared with the SPL Phonitor X is that it didn’t suffer from the three blob effect as much. I found the Kenzie spread the images out a bit more, and provided a more involved headstage. Excellent pairing.
- Focal Clear - Only good to use from the 32 ohm out. No noise floor issues, but it did boost the bass too much for my liking because the Clear is only a 55 ohm headphone, and it has an impedance bump in the bass that elevates off higher output impedance sources. I don’t recommend this pairing, however someone looking for more warmth from a Clear may like it.
- Audeze LCD-X - Dead quiet off the 32 ohm out. The LCD-X doesn’t really require much amping, so I was somewhat surprised at how well it did. The slight upper treble zing of the LCD-X was also softened slightly (even though the frequency response stayed the same). It didn’t retain as much of the sweetness and euphony that I found on the Verite, but it still had a more present and lifelike presentation than on any of the solid state amps I normally run it from.
- HiFiMAN Arya - Dead quiet off both outputs. I was initially worried that the Kenzie might not be able to power the Arya sufficiently but was pleasantly surprised that it worked well. The same quality of softening some of the treble intensity that I found with the LCD-X also happened here. Again, when I say ‘softened’ I mean that it textured some of the bite that may otherwise be there, which lent itself to a better sense of detail in the treble. Interestingly, the Kenzie made it so that I enjoyed the Arya for a wider variety of genres. It’s normally one of my go-tos for my classical and jazz recordings, but I found myself really enjoying some more upbeat genres as a result of this pairing.
- Focal Celestee and Stellia - These are both low impedance headphones that are meant to work with portable devices, so the pairing wasn’t that great with the Kenzie even from the 32 ohm out. It would also boost the bass by a few dB, just because that’s how these headphones behave from higher output impedances, and I don’t recommend that for them. I’d stick with higher impedance headphones, or ones that aren’t as affected by output impedance - unless some boost to the bass is what you’re looking for.
Comparison with the ZMF Pendant
Some time ago I was also able to evaluate the ZMF Pendant, which is a collaboration between Ampsandsound and ZMF. There are similarities between the two, but the design is different. Subjectively, from recollection the Pendant was more about the sense of depth and texture, while the Kenzie is more about sweetness and richness of tone. They both perform well with a wide variety of headphones, but I do think that the Pendant’s newer 100 ohm output may be more appropriate for the 300 ohm dynamic driver ZMF headphones.
Pairing the Kenzie with the Verite off the 300 ohm out yielded a fantastic sound, but unfortunately it had a bit of noise floor, and while the Verite does well off of high impedance sources, this is where pairing it with a 100 ohm output may be more appropriate. With that said, I also think I prefer the Kenzie’s tone overall, so maybe this is where prospective buyers could ask Justin about changing the output impedances of the amplifier to accommodate a specific pairing.
One last thing to note about these two amplifiers is that in my opinion they both demonstrate that it’s not possible to achieve ‘tube character’ for this type of tube amplifier just by using EQ or some type of digital signal processing. Why? Well again, there were no measured downstream effects that stood out in any way. So in short, this type of sound simply is unique to tube amps - at least compared with every solid state amplifier I’ve tried.
There are also a few additional things to note when it comes to using the Kenzie. The first is that while you do get a neutral presentation, it is still a tube amp, and so some of the inconveniences of tube amps are present as well. What I mean by that is you still have to deal with interference issues should your setup be near any kind of wi-fi signal or other RFI intrusions. Additionally, depending on the headphones you’re using, the output you’re intending to use, and the environment the amp is in, you may also get some noise floor. Now, I have to stress that compared to all other tube amps I’ve tried, the Kenzie is one of the quietest for noise floor. But with some pairings it did still show up.
My experience with this has led to several ways to remedy the issue, and I think anyone considering a high end tube amp should also consider ways to minimize noise floor. There are a number of ways to do this, but I’ll discuss the ones that worked for me. Just keep in mind that your environment may be different, so it may not work exactly the way it worked for me.
The first solution is of course to be cognizant of system matching and headphone synergy like I mentioned earlier. There will be some pairings where the headphone is simply too sensitive and it’s bound to pick up noise floor no matter what you do. Interestingly, when using the low impedance closed-back Focal headphones (Stellia and Celestee), which are a bit too sensitive, I found an easy solution to noise floor on the 32 ohm out. Using a 20 ohm impedance adapter between the 3.5mm connector and the 6.35mm adapter completely cut out any trace of noise floor. Now the negative consequence of this was that it boosted the bass by another 2dB, and those headphones really don’t need that. So use this method at your own discretion, but it does work.
The second solution is to use a preamp of some kind. Now, you don’t have to use a preamp with the Kenzie, in fact turning the volume up on the Kenzie didn’t significantly increase the noise floor like it does on many other tube amps. But, I found that using the SPL Phonitor X, balanced from the Matrix X Sabre Pro, and then out to the Kenzie made a noticeable improvement when running the ZMF Verite off the 300 ohm out. Still, this is one that I don’t think is entirely necessary because of the first point about being cognizant of system matching, but for certain pairings, the preamp also allowed for more flexibility that had the effect of minimizing the noise floor.
The third solution is to use what’s called a balanced power conditioner. For me, this made the biggest difference. Now, I have to admit that I was a bit skeptical about this stuff, since I’ve never been one to find the benefits of power devices to be all that meaningful for sound quality. But, when it comes to noise floor from a tube amplifier, in a building that has bad wiring (when I turn on my blender to make fruit smoothies in the morning, the lights flicker), or any situation where the power from the street transformer is compromised in some way, a balanced power conditioner can absolutely make a difference for getting rid of that slight background hum. In my case, I used the Equi=Core 600 from Core Power technologies, which was kindly sent up for evaluation by Underwood HiFi to be used with the Kenzie.
The Equi=Core 600 is essentially a massive transformer (and it’s really heavy), the bottom line is that if you have issues with power in your building, a balanced power conditioner like this can clean it up. Now, I don’t want to suggest that you have to get this to make your tube amp work better - the benefit I’m reporting here is specific to my environment, and it’s something that may benefit your system should the environment be similar. Moreover, this isn't meant to do anything to the sound quality of the system, but rather to eliminate noise intrusions - and in my case the difference was obvious.
So my advice here is to investigate this if you're experiencing noise floor issues with your tube amp and you know the system matching should be good (not using super sensitive headphones off a higher Z output for example). It's also important to consider how old your building is, and any other irregularities with your building's power.
The fourth solution has to do with shielding. You’ll want to use shielded RCA cables if you’re dealing with noise floor issues from a tube amp. And before you think I’m talking about expensive magical interconnects that ‘open up the sound’, shielded RCA cables can be inexpensive as well, and once again, I’m merely speaking about ways to help with some of the inconveniences of tube amps that may arise.
Additionally, if you’re experiencing some kind of RFI issues due to wi-fi or other interferences, I’m told that getting tube shielding around the Kenzie’s 12SL7 (the tube in the middle) can help with that. Now, I personally found that I didn’t need to do this, because where I had the Kenzie set up there was no interference. But this is something that is likely to help for anyone dealing with that issue.
Using the Ampsandsound Kenzie for the last month has been an absolute joy. It’s made me think that my normal go-to rule of “put most of your money into headphones rather than amplifiers” has certain exceptions, and that the Kenzie is one of them. Now, while I still think the best bang for the buck systems revolve around solid state gear, running headphones off the Kenzie provided an experience that got much closer to my other common recommendation of just buying better headphones. I can easily see someone investing in this type of system with far less expensive headphones, and it would make sense to do.
In fact, the Kenzie made every headphone I used with it that was well-matched more enjoyable than any solid state system I’ve used in the past - and it did so without altering the downstream frequency response. For this reason, the Ampsandsound Kenzie is one of the easiest products for me to recommend.
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