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AKG Solidtube

Valve Capacitor Microphone By Paul White
Published January 1998

Paul White tests AKG's new budget studio valve mic to see whether it's really possible to build a quality tube capacitor mic for under £1000.

Over the past two or three years, there's been a trend for microphone companies to release large‑diaphragm capacitor models priced to appeal to the private studio sector of the market, and though I can't be sure, I think the influx of cheap Russian and former East German mics must have played some part in influencing their market strategies. Capacitor mics are complicated and expensive things to build compared with dynamic models, but one of the ways to keep the cost down is to build single‑pattern mics, usually cardioids, rather than multi‑pattern models that require dual diaphragms and complex switching arrangements.

AKG SolidtubeAKG were one of the first companies to embrace this new challenge with their C3000, which continues to be a top seller today. The C3000 employs a single‑sided capsule, one inch in diameter, which is roughly based on one side of a 414 capsule. This capsule includes a microscopically thin, gold‑sputtered diaphragm and delivers the crisp top end associated with the 414, though it doesn't claim to sound exactly the same as a 414.

With the budget capacitor mic cracked, the next stage was to find a way to satisfy the growing demand for valve microphones, but valve electronics are much more expensive to build than transistor or FET circuitry, and need separate power supplies — you can't run a valve mic from phantom power. Nevertheless, AKG saw companies such as Audio Technica, Rode and Groove Tubes bring more affordable valve mics to the marketplace, so now they're fighting back — and very aggressively, it seems.

Solid Tube?

Why the name SolidTube? Well, AKG's new budget baby combines solid‑state power‑supply circuitry with an all‑valve signal path to provide the warmth and tonal colour of valves at a lower cost. The design starts with a capsule based on that used in the C3000, hence my description of that model earlier, but the housing is much larger, both to provide more air space around the capsule and to accommodate the ECC83 valve. Apparently the ECC83 dual‑triode valve was chosen because it is still in production, unlike some of the valves used in true vintage models, and it is run at a full HT voltage rather than in the 'starved' configuration used in some low‑voltage designs. Surprisingly, for a low‑cost mic, the output stage employs a transformer rather than the more usual electronic balancing, and no doubt this also contributes to the sound.

The SolidTube is a fixed cardioid‑pattern model and, as you can see from the photograph, it's tubular in construction, though in real life it looks somehow larger than you'd expect. As with virtually all studio mics, the capsule is mounted for side‑fire operation (you sing into the side, not into the end!), and a red perspex window lets you see the valve glow — those long winter evenings must simply fly by! Overall, the standard of construction is just what you'd expect from AKG — solid and nicely engineered. For direct stand‑mounting there's a threaded insert in the base of the mic next to the XLR output connector, and a recessed 20dB pad switch is fitted about halfway down the mic body. Because tube mics have special requirements, the output XLR cable has six pins and connects to a separate power supply box.

I think it would be both fair and accurate to say that this is the nicest vocal mic I've tried in the sub‑£1000 price bracket.

The PSU box is mains powered, and has a conventional balanced XLR output, but in addition to simply providing power, it also includes a switchable 12dB/octave 100Hz low‑cut filter, plus a ground‑lift switch. All necessary cables are provided with the mic, but you also get a bright yellow pop shield and a tough elasticated shockmount thrown in — which is a nice surprise when you think that many of the more expensive mics have these accessories as chargeable options. On top of that, AKG pack the whole kit in a foam‑lined camera case to keep your mic safe when not in use.

Paper Performance

Though good tube mics can exude a kind of sonic magic, their Achilles heel is noise. Tubes run hot, and in electronic circuits, more heat means more noise. Fortunately, by using a hybrid design incorporating solid‑stage amplification in the more vulnerable areas, AKG have managed to produce an unweighted equivalent input noise of 30dB, or 20dB A‑weighted. In practical terms, this gives a 74dB A‑weighted signal‑to‑noise ratio (ref 1 Pa), and although there are quieter mics around, this is a good figure for a valve mic, and in typical close‑miking studio situations background hiss should not be a significant issue.

The frequency response of the mic is nominally flat from 20Hz to 20kHz, though there is a deliberate presence peak at around 10kHz, with a smaller peak at around 4kHz. This is predominantly a vocal mic and the presence peaks have been tailored to that application, but the SolidTube is also eminently suitable for acoustic instruments, percussion, and even electric guitars. Though quite sensitive, with a quoted 20mV/Pa sensitivity, this mic can handle SPLs of up to 130dB without the pad, or 145dB with the pad switched in, so sticking it in front of a guitar stack isn't likely to frighten it one little bit!

Studio Test

As expected, when used in most normal studio situations the SolidTube is more than adequately quiet, and its sensitivity is comparable to other capacitor mics in my locker. Tonally, the mic is very well behaved and has the smooth yet detailed character associated with good valve microphones, making the sound seem almost slightly compressed. There's plenty of high‑end detail, but in a less sibilant way than with most of my solid‑state capacitor mics, and the proximity effect can be used to create really warm, intimate vocal sounds without making the result too muddy.

I was surprised to note that in direct comparison with microphones that I know are good studio performers, the SolidTube sounded less honky or nasal, even though these were characteristics I'd never have associated with the other mics without this direct comparison. It's almost as though solid‑state mics leave the voice sounding somehow exposed, while the SolidTube produces an integrated, very comfortable tonality, making the voice seem bigger and helping it sit better with the backing track. If there's a downside to this, it's that people whose voices lack edge or projection might find they don't cut through a mix as well as they do with a more conventional mic, but for the majority of voices I think this mic will just make them sound more as their owners would like them to sound.


I think it would be both fair and accurate to say that this is the nicest vocal mic I've tried in the sub‑£1000 price bracket — and there are several competitors well on the wrong side of £1000 that don't sound as sweet. As with all nice mics, it's difficult to put your finger on exactly what makes them sound so good — it's almost as though they project the whole of the voice rather than just taking one element of it and emphasising it. How much influence the valve has in all this I wouldn't like to say, but the sound certainly has that slightly compressed quality normally associated with valve mics, and the delivery of high‑frequency detail and transients is very smooth, without dulling the sound in any way.

Other considerations include the impressively solid construction (which should be good for client confidence), the excellent shockmount, and the inclusive camera case. At under £800 full list price, including VAT, the SolidTube can hardly fail to be a winner. This is one mic you simply have to try — the SolidTube is no pipe dream.


  • Exceptional value.
  • Good standard of engineering.
  • Flattering, classy sound.
  • Accessories included.


  • They want it back!


Though around twice the price of a budget solid‑state capacitor mic, this model has a wonderfully sweet sound that makes most voices sound larger than life. It also includes most of the accessories you usually have to buy as options.


£799 including VAT.