AudioCats-modded Stax SR-001 MkII (source:

With this article, I am starting a collection of what I consider the most interesting, educational, and authoritivative readings about audio, being them reviews, phylosophies, or other subjects I might have not covered.

This specific article has been written by Catscratch on Head-fi, and I have received his permission to post it here on SoundBsessive. He describes a fantastic mod that Head-fi’er Audiocats designed for the little portable amp. The SRM-001 portable amplifier, in stock version, has always been a bottleneck for the portable version of the tiny Staxes, and with the modifications, the overall sound is easily superior over the desktop version of the same earspeakers (the SRS-005A).

For the record, I have Catscratch’ modded systems at my house (he has two, slightly differently flavoured amps, and the SR-001 Mk2 earspeakers),  and I have been listening to them for a few months now.

He puts in words the way they sound, at least as well as I could do myself.

To add to the review, this is the link to the original modification thread, for any diy’er who’d like to perform them.

Enjoy the reading.

<<Review: AudioCats-modded Stax SR-001 MkII, vs Stock SR-001 and SRM-313/SR-003


The Stax SR-001 Mk II system is an electrostatic portable system that retails for $299 imported from Japan, and is the most practical way of getting electrostatic sound on the move. It consists of the SR-003 headphones and a proprietary portable amplifier. The headphone cable is terminated with a special plug that fits only the proprietary amplifier (though it is detachable and replaceable). The system will run portably off a pair of AA batteries for about 4-5 hours, and somewhat longer with a pair of quality rechargeable batteries. It can also be used as a transportable stationary system via an AC adapter.

It’s not my intention to describe the stock system in detail; there are many reviews available online that do a thorough job of that. The focus of this review will be to compare the AudioCats-modified system to the stock system, and to compare it to its closest-priced stationary rival, the Stax SR-003 driven by the SRM-313 amplifier.

But, if you’ve never heard the stock SR-001 system, I can quickly summarize its sound. It has a classic electrostatic presentation – very lush and full, but nevertheless very airy and transparent. The tonal balance is quite warm, with an emphasis on midbass and lower midrange. The highs are somewhat rolled-off, as is the deep bass. There is a great deal of detail, but there is also a very relaxed presentation of detail that stems from the softer highs and warmer overall tonal balance. The microdetail is all there, but as a listener you have to focus on it in order to hear it, rather than having it be very upfront, as is the case in brighter-sounding portable competitors, i.e. Etymotic ER-4S. Thus, the system contains all of the detail inherent to the music, but it does not allow that detail to detract from the overall presentation of the music. Therefore, it remains a very musical system, and is a very good value among portable headphones, for it gives you a fairly high-end sound at a very competitive price. I prefer it over every universal-fit canalphone, including triple-driver offerings like the Shure E500, but I think that high-end custom-molded canalphones, like the Westone ES2, are somewhat better.

I have used and enjoyed the system for years, but have always believed that, while the headphones themselves are exceptional for the price, the proprietary amp is sonically subpar and is holding the headphones back. So, when I was approached by AudioCats to do a review of a modified unit that involved substantial upgrades to the proprietary amplifier, I knew I would have a chance to find out if I was right.

Gear used in this review:

Stock SR-001 Mk II
SR-003 headphones
SRM-313 amplifier
Rega Planet 2000 CD player
iAudio X5L mp3 player
Headphile BlackSilver RCA ICs
DiMarzio copper RCA ICs
Kimber copper RCA –> mini cable
Kimber copper mini –> mini cable

Music Used for This Comparison:

Stationary listening on CD:

Igor Stravinsky – Petrouchka, Cleveland Symphony Orcherstra conducted by Pierre Boulez
Radiohead – OK Computer
Shpongle – Tales of the Inexpressible
Raja Ram – Raja Ram’s Stash Bag #2

Portable Listening on MP3/ogg:

Spectral – Avalon
Stef – Solar Genius (mix)
Stef – Solitude (mix)

The Modified Unit:

The biggest visible difference when it comes to this unit is the external battery pack. It is rechargeable and will run the unit for approximately 15 hours on a full charge. Also, the battery pack includes a built-in RCA to mini-jack converter. It is operated by a solid and sturdy switch, and includes LEDs for battery level and charge-up. Internal modifications include replacement of the stock op-amp with the 8599 op-amp, replacement of the output caps with 0.022uf Sonic-1 caps, and replacement of the input caps with 0.22uf Auri by passing 47uf BlackGates NX. There may be other internal modifications, but I’m not an electrical engineer and I didn’t dare to open the unit up, knowing my past history with delicate electronics.

The build quality of the external battery pack is rock solid. It may be a DIY effort, but it does not really give up any points in build quality to most commercial portable amplifies, such as the ones from Meier Audio or Dr. Xin. The Stax amp itself doesn’t show any visible signs of modification, but it does tend to run somewhat hotter than the stock unit. Still, I’ve used this system for over a week, and I don’t think that in the long term heat dissipation will be an issue.

The modifications on this unit cost approximately $300, raising the total price of the unit to $599. At this price range, it starts to compete with custom-molded canalphones on the portable front, or top-tier universal-fit models driven by quality portable amplifiers. On the stationary front, quality options start becoming available. Altogether, the modified unit had to perform very solidly to justify its cost.


It did. Big time.

Listening Impressions:

Modified Unit vs. Stock Unit under stationary conditions.

Upon first listen, several differences were immediately obvious. First, the modified unit had a substantially larger soundstage. Instrument separation was improved and there was considerably more air and space between each instrument. Second, there were notable changes to the tonal balance. While the stock unit tends to have an emphasized lower midrange and a pronounced midbass, the modified unit was much more linear, without the midbass and lower midrange emphasis, and with more present highs than the stock unit. The highs were still slightly rolled-off, which is a characteristic of the headphones themselves and can’t really be bypassed via amplification, but there still was considerably more treble presence and extension than the stock unit. Lastly, resolution was considerably higher on the modified unit, especially in the midrange. While the stock unit’s midrange cannot really be considered congested compared to other systems in the price range, in the absolute sense it can lack clarity. The modified unit was much clearer in the midrange, as well as throughout the whole frequency range.

Listening to “Subterranean Homesick Alien” from Radiohead’s OK Computer on the modified unit showed a considerable improvement in the presentation of the vocals as compared to the stock unit. The stock unit’s lower midrange emphasis can sometimes give vocals a chesty coloration. This coloration wasn’t there on the modified unit, and the tone of the vocals was much more natural. Microdetail retrieval was considerably better, and you could hear the nuances in Thom Yorke’s voice with greater clarity.

Likewise, the treble on the modified unit was improved compared to the stock unit. There was a certain raggedy, grainy texture to the cymbals on the selfsame track when it was played on the stock system. The modified system, on the other hand, had more finesse and a better resolution in the highs, and the cymbals’ natural shimmer and decay came through much more clearly. The treble was more present, but at the same time it was smoother and more delicate. Each drum stroke seemed to have a more unique character, giving you a greater impression of a live performance (or live in the studio as the case may be).

Listening to Stravinsky’s Petrouchka (a large-scale orchestral piece) proved the modified unit’s superiority when it comes to instrument tone: strings, brass, and woodwinds sounded more natural and didn’t have the stock unit’s noticeable lower midrange emphasis. However, that emphasis did give the stock unit somewhat more harmonic richness, whereas the modified unit sounded slightly more analytical.

But, the biggest display of the modified unit’s superiority came from electronic music. Here, its brighter tonal balance, considerably better performance in the highs, wider soundstage and better instrument separation allowed it to show every beat, every sample, and every strange psychedelic effect with substantially better clarity than the stock unit. Listening to Psytrance on the modified unit was a treat, and let no one say that electrostatic headphones can’t replicate electronic music with authority.

In general, it is safe to say that I preferred the modified unit by a very wide margin. There really was a night-and-day difference in nearly every sonic category. More importantly, the biggest improvements were in areas where the stock SR-001 was traditionally weak, and I think the modified unit was voiced quite well to compensate for these weaknesses. But, this was expected, given the price of these modifications.

Modified Unit vs. SR-003/SRM-313.

The stock unit’s lousy amp may have been a pushover, but now the modified unit was up against a much tougher customer. The SRM-313 was Stax’s midrange solid-state amp that retailed for $450-700, and is now replaced by the upgraded SRM-323. Together with the SR-003 headphones, it was the closest stationary competitor that the modified unit had in terms of price. Naturally, given that the modified unit runs off battery power and is substantially more transportable, I didn’t expect it to perform as well as the full-size SRM-313. But, I have to say that I was very surprised by how well the modified unit did in this comparison.

The SRM-313/SR-003 combo had a considerably warmer tonal balance than the modified unit. It seemed to me that AudioCats voiced the modified unit to be as natural as possible in the vocals, with a brighter tonal balance than is the norm for the SR-003 (which usually is a warm-sounding headphone). Therefore, the SRM-313/SR-003 combo had a certain euphony to its sound that the modified SR-001 lacked. Also, the bigger power supply of the SRM-313 made itself known, and its presentation was more dynamic, more punchy, and more effortless. But – and this I really did not expect – the modified unit had more clarity in the highs and noticeably better instrument separation, and of course its tone was quite a bit more natural.

Listening to the same Radiohead track I used earlier in my comparison, I was struck by how much more euphonic the SRM-313 sounded compared to the modified SR-001. It had a certain syrupy lushness to the vocals, a very punchy and authoritative quality to the bass, and a delicate softness to the highs. At the same time, the 313 did not have the chesty coloration to the vocals that the stock SR-001 suffered from. The modified SR-001 did have a somewhat more restrained dynamic range than the 313, and sounded slightly more forced. It also had a brighter tonal balance, but with a more natural quality to the vocals that lacked the SRM-313’s euphony. Furthermore, it showed more clarity in the highs than the SRM-313, and noticeably better instrument separation.

The same trend continued throughout all of my listening comparisons. Stravinsky’s masterpiece came across as lush and romantic on the SRM-313, and more accurate and neutral on the modified SR-001. Dynamic peaks did not hit as hard on the modified SR-001, but the softer interludes in between were rendered with more clarity and a more natural tone.

In the end, I had a hard time picking which system I preferred more. Subjectively, I tend to go for euphony rather than absolute neutrality, but that is merely my preference. It works well for some genres, but not for all. I thought that the SRM-313’s extra euphony suited Rock (Radiohead) and Classical (Stravinsky) genres well, but the modified SR-001’s brighter tonal balance and greater clarity/separation in the highs lent itself well to electronic music, and it was my system of choice for Psytrance.

If I had to pick one on sonic preference alone, I’d pick the SRM-313/SR-003, but on technical merits the two systems were quite comparable in my opinion. Given that the modified SR-001 is similarly priced to the SR-003/SRM-313 system, while retaining its mobility, I believe it warrants the cost of the modifications, and acquits itself sonically quite well, and is the better purchase overall given its superior practicality.

Portable Use:

Here, it was clear from the first note that the SR-001 was not going to be the bottleneck in the system. An mp3 player really is not the sort of source that this system needs. Still, even here, the modified unit’s superiority over the stock unit was very evident. The wider soundstage, better detail, more natural tone to the midrange, and better highs really transformed the modified SR-001 into a top-notch portable system. In the past, I always preferred the Westone ES2 over the SR-001 due to the latter system’s sonic problems. But, these problems are mostly fixed in the modified unit, and on the whole, I have to say that it was the best portable system I have ever heard. I won’t even mention my two reference universal-fit canalphones – the Shure E500 and Westone UM2. I was intending to include them in my comparison, but after just a few moments of listening to them my reaction was “Ugh! Get this crap offa me!” They really weren’t even remotely in the same league, though driven by the Ray Samuels Hornet portable amplifier they were overall similarly priced.

The same applied for the HD600 driven by the Hornet. I think this setup is a pretty average performer for a full-size headphone and a portable amplifier, and a good general indication of what a full-size headphone can do on the move. However, it really couldn’t keep up with the modified SR-001 either. The SR-001’s speed, transparency, and detail were all substantially better, while giving up pretty much nothing in bass, fullness, or tonal accuracy.

Of course, being a 300 ohm Sennheiser, the HD600 cannot be driven to its potential by a portable amplifier, and this setup should not be used as a valid comparison between the HD600 and the SR-003 (or the modified SR-001 system). I think that, driven by a quality amp – especially a balanced one – the HD600 is substantially better than the SR-003, but then again, it’s not a fair comparison since a balanced HD600 setup is almost always much more expensive.

I can certainly think of portable systems that use full-size headphones and transportable amplifiers that will beat this system sonically, but I really can’t think of anything that will sound anything like this within the price range. Of course, my knowledge is limited, and there are a lot of quality headphones with which I have no experience, so you should take this comment with a grain of salt.


I wholeheartedly recommend this mod. I believe that its performance justifies its price, and I really do think it takes the SR-001 system to an entirely new level. The main improvements were in areas where the stock system was severely lacking, and the modified amp was voiced in a very synergistic manner with the headphones. Of course, the modified amp can be voiced whichever way the user prefers just by changing opamps and other internal components, but I think this particular system’s tonal balance was spot-on, and my preference for a more euphonic sound is purely subjective.

In practical terms, the extra bulk added by the battery pack did not really reduce the system’s portability. Rather, the extra battery life added by the external battery pack relieved me from the necessity of carrying spare batteries around. The SR-001 goes through batteries like a movie-goer goes through popcorn, and in the long term, the battery pack will save you considerable amounts of money on battery replacements alone.

The best recommendation I can make for this unit is this: I’ll be getting my SR-001 modded in a similar fashion at the first possible opportunity.>>
Catscratch, Head-fi (2008)