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CharterOak ProAudio - Microphones, Signal Processors, Headphones

acoustic operating principle
Pressure Gradient
Dual 1.07" 6 micron thick gold
sputtered Mylar
12mV/Pa 0dB=1V/Pa@1kHz
Cardioid, Omni, Figure-8, and intermediate stages selectable from power supply
< 200 Ohms
> 200 Ohms
22dB A weighted IEC651
128dB for 0.5% total harmonic distortion @ 1kHz
Custom supply 115V or 230V operation with IEC type 2 AC Cable
CharterOak SA538 & SA538B
By Paul White

Can mics at this price really compete with the classics? Sound On Sound puts them to the test... US-based Charter Oak have been in the microphone manufacturing business since 2002, making them a relative newcomer to the field, but they seem very serious about what they're doing. Essentially, they source capsules, components and other parts internationally from companies in China, Eastern Europe and Sweden, but do all the assembly and testing is back in Enfield, Connecticut, in the US. The design differs from many superficially similar competitors in that very high-quality electronic components are used, especially the capacitors. Although this makes little difference to the paper specification, the subjective sound is improved and it will also have a positive effect on reliability.

OVERVIEW The SA538 and 538B are both multi-pattern tube mics based around a pair of pressure-gradient, 1.07-inch capsules with six-micron, goldsputtered diaphragms (these are clearly different in each model, because the SA538 is edge-terminated while the SA538B is centre-terminated). This is a popular size and specification for Chinese capsules. If they are made elsewhere, I apologise for jumping to the wrong conclusion! Both mics look similar and have the same type of external power supply (PSU), which includes a nine-position pattern selector switch that goes from cardioid to figure-of-eight, via omni.

The PSU appears to be of Far Eastern design, and is a simple but robust folded-steel brick with IEC mains inlet, power switch and voltage selector switch. An included seven-pin, fabric-sheathed XLR cable connects the mic to the PSU, and from there a conventional balanced three-pin XLR accepts a standard mic cable (also included). Such differences as there are between the two mics manifest themselves in their technical performance — which I'll come to later — rather than in their physical presentation or feature set.

Both the SA538 and SA538B look like serious studio tools. They weigh around two pounds each, which means that you need a solid mic stand to keep them stable. An all-metal shockmount is included that seems very similar to the ones I've seen with certain Chinese microphones, but that doesn't detract from the fact that it is both robust and practical. This design incorporates a threaded, locking ring that locates onto the base of the mic so, once fitted to the shockmount, it is perfectly secure whether upright or inverted. Both mics, with their power supplies and all accessories, also come packed in aluminium camera cases fitted with combination locks.

CONSTRUCTION Construction-wise, the microphones are conventional. But they are no less impressive for that, with a heavy machined basket support frame, a dual-layer mesh grille and a slide-on body cover finished in a vintage satin black reminiscent of some early European mics. An embossed silver Charter Oak logo marks the front of the mic, while a heavy, machined ring at the base of the mic holds the cover in The ECC83 dual triode tube, used in both place. Removal of the cover reveals neat construction, with plaited, PTFE-insulated cables connecting the capsule and main circuit board. The tube in both cases is a selected ECC83 dual triode, fitted to a ceramic base arranged so that the tube lies horizontally across the circuit board. None of the other components is visible, as they're all on the underside of the board. The board is shielded by an extension of the transformer housing, which in turn is joined to the basket assembly via four metal rods. There are no pad or roll-off switches on either model.

FREQUENCY RESPONSE Individual frequency response plots are included, and these seem to be of the more honest 'warts and all' variety, rather than having heavily smoothed, and hence meaningless, curves. The response of the SA538 extends from 30Hz to 20kHz (-3dB points) and the cardioid curve is characterised by a flat mid-range, augmented by a fairly high-up presence hump in the 10kHz region. Off-axis, the midrange dips as expected, producing a very happy smile curve! In figure-of-eight mode, there's a dip at around 6kHz but otherwise the response is nominally flat, while the omni mode shows barely a hint of presence peak. In both cardioid and omni mode, the response curve gets a bit bumpy below 300Hz or so, but that isn't unusual. With a self noise of 22dB, A-weighted, this isn't a particularly quiet mic, even for a tube model but, by the same token, the level of background noise isn't high enough to be an issue when close-miking vocals or instruments. For comparison, it is roughly comparable with the noise spec of a good vintage tube mic.

The slightly more costly SA538B has a marginally better noise spec, at 20dB A-weighted, and its lower frequency limit is 5Hz lower at 25Hz, although the maximum SPL is 125dB, rather than the 128dB of the SA538. Both mics have a 12mV/Pa sensitivity at 1kHz and a nominal 200(omega) output impedance. Comparing frequency response plots shows that the SA538B has a little more height in the presence peak than the SA538, but otherwise the two microphones are broadly similar.

THE SOUND OF OAK Before testing, I plugged in the mics and let them warm up for an hour, as recommended by the manufacturers. Though predominantly vocal mics, both models are equally at home on instruments such as acoustic guitar and percussion, with the SA538B having more presence. As I've found with pretty much all large-diaphragm mics, you have to work harder to find the best sweet spot for acoustic guitar than you do with small-diaphragm models, as these tend to be more forgiving for offaxis sounds, but you can get good results. In cardioid mode, the proximity effect is obviously present, but this isn't too pronounced under normal operating conditions and I got very acceptable results from a wide range of preamps, ranging from a cheap-as-chips Behringer desktop mixer to an SPL Gold Channel. Both models are on the bright side of neutral, though the SA538 has less of a presence peak, which makes it sound a little warmer and less edgy than the SA538B. I'm curious as to why Charter Oak found it necessary to create two such similar models, when a model somewhere in between the two could have been used with just a hint of EQ to cover the same territory. For my own vocals, I much preferred the SA538, as it gave more warmth and smoothed out the high end to some degree.

The character of both these mics definitely helps singers who need help with their upper-mid presence and projection (more so in the case of the SA538B), but who want to retain their low-end warmth without hearing that hyped-up, spongy low end that some modern tube mics dish out as a substitute for real warmth. Add a hint of compression and you can get a very refined, classic vocal sound from either of these mics without using much in the way of EQ or other processing. As expected, the omni mode isn't quite as open and natural sounding as from a small-diameter, single-diaphragm pressure capsule but it is still very usable and a nice option if you don't have a wide selection of mics in your locker. Similarly, the figure-of-eight pattern is valuable because of its excellent 90-degree rejection, which can really help separate sounds that are in close proximity. I used a number of tube and solid state mics for comparison, most admittedly a little less costly than the Charter Oak models, and in all cases the tube fixed horizontally across the circuit board. Charter Oak SA538 came across as both solid and present, cutting through a mix rather more assertively than most of the competition but without sounding edgy. The same is true of the SA538B, but I felt it had a less desirable balance of presence and warmth for my own applications, and could easily end up sounding too bright.

IN CONCLUSION These are not the cheapest mics of their type around but, judged on their sound rather than their technical spec or the origin of the parts, they are probably worth the extra cost, as their sound compares favourably with high-end/classic mics costing a lot more, and they somehow help a vocal sit comfortably within a mix without getting buried or being too loud. In this regard, the use of higher quality electronic components certainly pays dividends. Just like the classics they are being pitched against, the noise figures are nothing special, but unless you're recording quieter sounds at a distance that shouldn't be a worry. Most of the time these models are likely to be used as close-up vocal mics, and in that role they are perfectly happy and even seem less prone to popping than the other models I tried for comparison — though you really should use a pop shield whenever recording close vocals.

There's a huge amount of competition in the low to mid-priced tube-mic market at the moment and you should also check out the other models in your price range, especially if the mic is mainly for one singer, as picking a mic with a character to complement a particular voice is something that can't be done by specifications alone. Other mics might be quieter, or capable of adding more character and leaving you with some change into the bargain, but these mics give you that little bit of extra class, and if you're looking for seriously good results in this price range you should definitely consider them.

ALTERNATIVES There are a number of competitors in this category, and the differences are subjective. If you are considering the Charter Oak you might also want to look at mics like Rode's K2 or Classic, the Neumann TLM103, M-Audio's Sputnik, Sontronics' Helios, the Groove Tubes GT67, or the MXL V77S.


by: Garrett Haines

CharterOak Acoustic Devices have been quietly making quality tube mics for the past 3 years. Their first model is the SA538, a squat-looking condenser reminiscent of a short bodied U47. The outer shell is finished in a satin black and appointed with a silver silhouette of an oak tree. The mic is striking in person, and even after viewing high-resolution promotional photos nothing compares to holding this model in person. Speaking of holding it, be careful, it's almost 2 pounds. The heft alone provides a hint of what a quality mic this is.

Internally, the SA538 is a dual diaphragm vacuum tube condenser microphone that uses dual 1.07" gold sputtered diaphragms. The electronics are of U.S. , Slovak Republic , Sweden , and Chinese origin, but the mic is hand assembled and tested at CharterOak's headquarters in Enfield , CT. It is capable of nine pick up patterns ranging from omni to cardioid. The mic is delivered with first-class accessories, including a locking flight case, better than average power supply, and one of the best shock mounts I've ever used. The accompanying cables are coated in a soft webbing. The jacketing helps to avoid tangles, allowing the CharterOak cables to gently slide through potential spaghetti. Frequency response is reported to be 30Hz to 20kHz.

Sometimes you buy a mic for specific purpose, like an SM57 for the snare, or an EV RE-20 for broadcast. So, I set out to see what special purpose the SA538 might have. To make a long story short, I was having trouble finding where this mic could be a proverbial silver bullet. And that's when it hit me: the SA538 is a mic that covers a lot of ground. So, I tested my hypothesis. I took a rock singer I know who sounds thin and harsh on our Korby C-12, but awesome on our Neumann U47 FET. Turns out the SA538 hung in there against the Neumann. Very usable. I took another singer who is the reverse, and usually only sounds good with the C-12. Voila! The CharterOak worked fine. But before I give the impression that this is just a utility mic, let me finish. We took country singer Ian Thomas, who sounds strong regardless of mic choice and had him sing into the SA538 and a BLUE Kiwi at the same time. In blind A/B tests, I could not pick the Kiwi vs. the SA538. In fact, I was wrong 50% of the time.

Another benefit of the SA538 is it gets along with many different mic preamps. From an API to John Hardy to a Mackie VLZ, the CharterOak gave a solid, detailed performance. Studios with smaller budgets should have no reservation using existing mic pres with the SA538.

If you're a small studio that wants to buy a nice vocal tube mic, but can't afford both a C-12-ish mic and a U47-ish mic, you simply have to audition the SA538. While it's not a direct replacement for either - it's a solid performer. Likewise, for location or mobile gigs, the SA538 would be a perfect centerpiece for your mic pack. No need to take a bright and a dark mic because the SA538 can manage nearly any vocalist type. It's sturdy construction (the tube is protected by a rubber guard) make it suitable for the riggors of the road.

I only had two concerns with the mic. First, there is no ring mount for positioning the SA538 in tight applications. You're stuck with the (great - but bulky) shock mount. Second, it seemed to take extra time to find a sweet spot when using it on instruments. In particular, engineers will need to be extra patient when placing it on acoustic guitars.

I believe CharterOak is charging too little for this mic given it's build quality and sonic performance. My recommendation is to seek out a demo before more people catch on to this workhorse mic.

For more information: (List $1499, street ~ $1199 web) -gh



By: Randy Pool

The CharterOak Acoustic Devices SA538 tube microphone makes a good first impression before it¹s even plugged in. The case and build quality are quite impressive, demonstrated by the sheer weight of the product. It has the look and feel of the some of the old classic European tube mics. With a stylish matte black colored body (machined brass) with an embossed silver CharterOak emblem, you won¹t get confused on which side is the front of the mic.

The capsule is a dual 1.07" 6 micron thick gold sputtered variety, behind an open type protective grill reminiscent of classic designs. The shock mount is also of solid contruction with two large elastic bands in a familiar Germanic style. The power supply, however, looks like the standard deal coming from the pacific rim countries these days, while the mic cable has a tight-weaved, fabric-covered outer shell that really adds a nice touch.

In listening tests it was obvious this mic has a good amount of top end‹the kind of high end that feels great on singers who need a little help in that department. With a male vocal, it was compared to some old favorite mics through a UA6176 channel strip. The SA538 had a little more high mids and highs, with a some-what subdued lower mid range. It also had less proximity effect than some mics, allowing a singer to work in close to the mic without it becoming muddy so quickly. There is this sweet spot in the high mids that, for certain vocalists, with the right compression, was found to really make the vocal sit great in the mix, without being overwhelming. There seems to be something unique about this mic¹s ability to handle good amounts of compression and still keep the clarity of the singer right where you want it.

On a female vocal, we achieved similar results, the mic sounding natural and clear. The mic¹s EQ curve really helps those singers that need a little extra high end clarity. I¹ve gotten really nice results with fairly bright mics paired with relatively dark mic pres like a vintage Neve, and the SA538 was suited to that combination.

We also experimented with acoustic guitar recording with the CharterOak, trying several guitars of top-notch session player Mark Bladwin. While the top end seemed to help the darker guitars, we quickly realized the result was generally too bright for this to be this mic¹s best application.

As always, you¹ll want to match microphone and preamp to the voice or instrument that you¹re recording. With its ample high end and detail, the CharterOak SA538 can add a valuable option to your mic collection.


Reviews. CharterOak SA538, SA538B, E700 & S600

“CharterOak microphones reviews in PROSOUND April ’07 Edition”

CharterOak condenser microphones are manufactured in the factory located in Connecticut, U.S.A. Only the parts with well selected and passing the very severe
quality criteria are assembled in the rigid body which is being machining from the brass. And, furthermore, after 7 days aging for the tubes and testing in the studio
environment for more than 30 minutes, the microphones will be delivered to the
customers. In addition to this, such accessories as the flight case, the power supply, theshock mount, the cables, etc. will be checked severely. By taking care of this
fundamental points in order to manufacture the products, the credibility will be
increased and the products will attract customers’ attention and the firm position of the products will be established, which is a short cut to be successful for the products.

The products which I would now like to introduce you this time are 4 types of the
condenser microphones manufactured by CHARTEROAK ACOUSTIC DEVICES in
U.S.A. They are SA538, SA538B, S600 and E700.

First of all, from the external appearance of the microphones, the black shiny finish is afeeling like having its very expensive products and the microphones have a feeling of having its enough weight. Furthermore, except E700 which is solid state type, the exclusive power supply will come with the microphones. About SA538 and SA538B, as they equip the switch with 9 steps of which the directivity can be selected from non-directivity to cardioid.

Let’s try to test them!

First, let’s start from SA538. This is a dual diaphragm tube type condenser
microphone(SA538B is the same.).

Checking it with vocal, it sounds like thinking of a vintage microphone and the
bandwidth ranging from mid-low sound to low sound is solid. Nevertheless, it is not
like a dull sound, but, it has apparently a feeling of existence against the sound source in the back. Listening to only the vocal track carefully, it is understood well that the room noise was not almost felt. Then, after recording while selecting the directivity newly, with transferring gradually from non-directivity to cardioid(middle value), the noise is becoming decreased and it is a feeling that I went to the country side where has clean air, being away from the city. In doing like that, the high technique is necessary in order to decrease only the noise, without spoiling a feeling of air. I feel that CHARTEROAK ACOUSTIC DEVICES has very high technology research level. SA538B is being the same, but, as this is a side termination type, the high frequency sound is much extending with a feeling of hybrid.

Depending upon the vocalist is if it is a male or a female, it is fat or thin, utterance is strong or not, what is the performance style of the back, etc., the choice will be changed. In the studio, I would like to have both. In either way, the microphones can realize sound image very well for which the vocalist is in the very front position of the stage and they are superb microphones, and they are also easy for mixing.

E700 is the low cost version for the above 2 microphones. This is a gold depositioned mylar diaphragm and pure class A solid state type microphone. The microphone equips the switch which can be selected from 3 types of the directivity(cardioid/omni/figure 8) and the attenuator of -10dB/-20dB(sensitivity switch).Though this microphone is in the low priced range, the sound is fairly good and a feeling of clear air with less noise can be realized well.

S600 is a front address type and the tube type, but, this microphone is different in appearance from the above microphones. The top portion of the microphone is aimed at the recording object and I think it is for the purpose of recording the musical instruments. I recorded the Conga which is my favorite musical instrument. Although it is a same feeling as sounding with a feeling of existence, this microphone is likely flat in its frequency response. In its good aspect, it has a sound with non-peculiarity and wide dynamic range.

For myself, in case of recording in stereo against the recording object(2 microphones), especially to drams and percussion(also piano), not standing the microphone inside from outside, reversely, I always request to stand it outside from inside. This means that I consider it phase, but, with using this microphone, when I tried to make both settings and to compare them, I realized that there are big difference in a feeling of existence and a clearness of sound. In some sense, I am impressed with this microphone that it is honest.

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