Back in 2011, John told me that he was working on a new portable amp with a slim form factor. I was so impressed with the JDSLabs Cmoy’s sound quality that I told John that I would be very happy if I can get the Cmoy’s sound quality unchanged but in a slimmer package and with USB recharging. I think the sound quality of the Cmoy is high enough for most people (me included), and the only thing missing from it are general convenience features such as USB recharging and a slimmer form factor. John assured me that the C421 was going to be better than the Cmoy in every aspect. Honestly I had my finger crossed then, as I’ve witnessed the Cmoy beat many amps costing more than it.

Prior to the C421, there were already several slim portable amps in the market. The TTVJ Slim and the ALO Rx (currently at Mark II) were among the first slim portable amps, and though they are both great sounding amps, I wanted something lower priced that I can recommend to a wider group of people. The Headstage Arrow was a pretty good amp with great features and price (when it first came out), but since then have seen a price increase to the current $299.00 price tag, and I’ve always thought that it fell short on the soundstage performance. Aside from these models, I know that Leckerton’s UHA-4 amp has been getting good reviews but I’ve yet to listen to it. You can also get amps like the RSA Shadow and the Pico Slim that are both slim and tiny, but their limited power output and relatively premium price means that they are quite niche. Perhaps the most phenomenal entry to the slim amp category is Ibasso’s D-Zero, released last year with an aggressive pricing of $109, complete with a built in USB DAC. It was a solid entry from Ibasso and I do recommend it once in a while, but the sound wasn’t quite as nice as I want it to be.

I’ve evaluated a lot of portable amps on the market and I think the sweet spot is on a slim form-factor amp, priced within the $100-$200 range (of course the cheaper, the better), have a relatively nice sound (even on par with the Cmoy would’ve been enough), and comes with all the convenient features such as USB recharging and gain settings. The C421 seems to hit all the checkpoints, except for the sound part, which at that time I’ve yet to listen to. And to be honest, I wasn’t that enthusiastic about it, as I only see it as another amp builder’s attempt to move up from a basic Cmoy assembly to something more advanced, but hardly ground breaking.

Initial Impressions on the C421

I don’t remember when I first received the C421 amp prototype, but I was definitely not too excited about the sound. It was the AD8620 version, and the only thing I can attribute to it is that it has a pretty good, punchy bass. Soundstage was quite flat, high and mids are quite dry. I gave it to some friends for them to audition, and they were also quite lukewarm about it. At that point I thought that the C421 would be just a slim amp with no other special qualities and I was quite disappointed. I wrote John an email explaining my impressions, and to my surprise he replied by sending me another C421. (I don’t intend to bash the AD8620 version, rather I just want to tell the story as it happens. I also am aware that JDSLabs recommends the AD8620 version so it’s up to you which version to get)

The second C421 was different. It had the OPA2227 op-amp, the same op-amp that John installed on his JDSLabs Cmoy. The treble/midrange/balance was among the best I’ve heard on any amps. The soundstage were extremely good, possessing all the right width and depth and with good ambiance. Midrange is among the sweetest and clearest I’ve heard from a portable. I immediately fell in love with this version of the C421, and I knew then that the Cmoy is not going to get much use from that moment on.

I noticed that the bass was a little loose and not as punchy as I’d like it to be (bass boost OFF), but overall this is among the best sounding amps I’ve listened to. It wasn’t as powerful, as dynamic or as impactful as the TTVJ Slim or the ALO Rx, but to my ears the C421 has the best tonality among all the slim portable amps I’ve listened to. In fact, I find the tonality to be so good that I very rarely use the bass boost feature as that adds bass quantity significantly, and to my ears the bass quantity is just right on the flat setting (I just want it to be a little more punchy).

(from now on any references, both mine and Lieven’s to the C421 will be based on the OPA2227 version).
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