Two transducers with completely opposite functions from the same company — Mike Skeet checks out the K240DF headphones and C535EB microphone from AKG.

AKG K240DF Monitor Headphones

Providing good monitoring is as essential as using effective miking equipment and many companies specialise in manufacturing gear for both applications. The firm of AKG is a case in point, and we examined examples from both these areas.

At first sight the K240DFs look like sealed headphones, but are in fact circum-aural in their coupling to the ear. That is to say that they're acoustically open at the back of the diaphragm but less so than some 'open' phones. As you would expect, the amount of outside noise heard when wearing headphones varies according to the degree of enclosure, so these fall somewhere in the middle ground in this respect. I mention this just in case there's still anybody out there who feels a headphone is not worth considering unless it is sealed, highly boxy and coloured as a result! Suffice to say that I have yet to hear a good closed phone.

The AKG K240DF are substantially made and feature the almost traditional strong double wire over the head. Inside this is a spring loaded headband. It's nice to see that you don't have to struggle to adjust them to a comfortable position. The ear surround pads are soft 'leather' faced and move with respect to the transducers proper and the headband, to readily accommodate all our various head shapes.

There seems to be game among manufacturers (or their stylists!) to hide any L or R markings as carefully as possible. These AKGs follow this current vogue. Manufacturers kindly note; the worst place to indicate this is at the side. It's better that it be seen as one prepares to put them on.

The lead has a sensible thickness but is suitably flexible and the anchorage at the phone end is adequately substantial. A plastic body cover 14" stereo jack plug is supplied at the opposite end and the lead is tightly bushed by a rubber sleeve.

Close examination under the ear pads shows how much development work goes on in headphone design. The acoustic loading around the transducer is interesting with the acoustic resistance of this path apparently important: hence the letters 'DF' (Diffused Field).It makes sense to have headphones of around 6000 impedance like these. They can be used directly on amplifier outlets without the chance of damage, either to the phones or to listeners' hearing, yet they can be driven to reasonable levels by typical line level signals. Additionally, should a reduction in level be needed, series resistance in the hundreds of ohms range suffices. For example, a 6000 series resistance per transducer would reduce the level by 6dB or ¼ the power (if from a low impedance source).

Studio Monitor

AKG make the claim that these should be termed 'studio monitors'. Of course, that begs the question 'What is a Studio Monitor?' Having heard some very questionable monitors in studios, I approached the first audition wondering if I would hear the usual hiss, boom and colouration. I was relieved.

To define a monitor in the light of these AKG K240DF might be interesting. In my opinion a true monitor must exhibit sensitivity at LF and HF, absence of colouration, a smooth feel, ability to take sensibly high levels, and a 'natural' sound reproduction when fed from known 'natural' sounding recordings. Judged according to these specifications, this model does fall into the category 'studio monitor'.

In comparison to other phones there's a difference in part of the HF range (depending on the material used to test them). To investigate further I ran a blank cassette without Dolby on a Nakamichi BX300, as a source of wide band noise.

Inevitably, all the headphones reviewed added different kinds of colouration on the HF content, the K240DF exhibiting a little more 'sharpness' than most. Out of curiosity, I performed the same experiment using my own monitors (Quad ESL63 Electrostatics) and compared the results of both. Again differences were evident, but so were the similarities. To sum up, I regarded these tests as rather inconclusive. I felt more comfortable comparing music on the Quads and the AKG phones; they had more in common than might be expected.

These AKG K240DF headphones are truly useful and effective studio monitors, in the sense that judging sound and balance with them will not lead to a poor balance. Any good or poor decisions will be what the listener makes of the message and not the fault of the messenger!

AKG C535EB Microphone

This is an externally phantom powered electret capacitor microphone.

An electret capacitor transducer is one where the diaphragm and backplate are permanently polarised to a high voltage at manufacture. If a battery is employed it is merely for the impedance converter amplifier in the mic body. However, it is perfectly possible to power this part externally by normal 48v phantom powering, (50v DC in parallel down both balanced signal wires with screen return) and this is exactly what has been done in the C535EB.

The phantom powering does allow for powering from 12v DC or 24v DC if the mic amp is equipped with suitable feed resistors as detailed in the user instruction booklet.

Additionally, a sensible headroom facility can be incorporated in the resultant impedance converter amplifier. Significantly, the C535 is quoted as allowing very high SPLs of 132dB and 146dB at its two sensitivity settings.

AKG have a history of building microphones which remain in production for many years. Classic designs like the D202, C414 and C451/CK1 series. Dare I predict that the C535 will have a similar longevity?

Physical

This slim microphone has a solid feel because it is relatively heavy. Coloured matt black, it is of substantial construction with a very small taper to the body. The built in mesh pop shield is very solid. I cannot resist unscrewing such things and in doing so, I revealed a thin foam lining. The cardioid electret capacitor capsule is very effectively shock mounted. It was a true joy to see such detail! The whole thing is balanced by a brass counter-weight in the body with a rubber gasket pivot and a very soft foam rubber damper.

The 4-position attenuator and LF cut switch is recessed in the body like a C451. A small screwdriver or ball point pen suffices to operate it. Supplied is one of the AKGs marvellous flexible tough plastic stand clips. It's all very neat with a fixed sensible friction mechanism at its pivot.

Performance

In use it performs in all basic ways, very like the AKGs C541/DK1 combination, but with decided advantages.

The built-in pop shield is very effective. Of course, mesh plus foam is better than foam alone. Foam slip-ons do not have still air around the capsule as the C535 model does. Basically, one doesn't bring a C451/DK1 anywhere near a vocal output without it popping, even with foam: it wasn't designed for that, but the C535 is. Handling and stand borne noise is commendably low, demonstrating the effectiveness of the cleverly damped pivot capsule.

So the C535EB is definitely a vocal mic, solo or otherwise. In the former case any LF emphasis due to proximity can be taken care of with the sensitivity/LF cut switch. Position one provides the highest sensitivity and a flat frequency response, apart from a 2dB HF bump around 10kHz. Position two retains the full capacitor mic sensitivity but produces a steep cut below 100kHz, and this is useful for rejecting traffic noise or structurally borne vibrations. The third position gives a reduced sensitivity of some 14dB giving an output rather like a moving coil model D202. This would suit a mic pre-amp which cannot take the levels of a capacitor mic flat out. The fourth switch position maintains the reduced sensitivity with the addition of a shallow LF cut from around 500Hz. For close vocal use, the loudness of the voice might necessitate the reduced sensitivity as well as the shallow roll-off for proximity compensation.

As an instrumental mic, it suits all sorts of instruments from guitar to trumpet, and clarinet to flute and is equally at home around a drum kit. Its attributes are capacitor mic clarity with only a little HF lift in the otherwise flat response. The only inconvenience is that one needs two to get some stereo spaciousness going and it wouldn't surprise me if anyone who acquires one would immediately save for another!

Phantom Powering and Noise

I pride myself that my DIY mixer has quiet pre-amps. Comparing the C535EB with a D202 moving coil shows that at full sensitivity there is little to choose between them in terms of noise. Certainly, I've had a lot of battery powered electret mics which were not in that class. Strangely enough, reducing the C535 output to a level comparable with that of the D202, shows up a little more noise from the electret. Therefore it's best used in low level situations at its full output. Overall, the C535EB has a similar noise output to the C451/CK1 mentioned earlier.

The cardioid directivity seems to have a high standard of uniformity across the range and although there was no opportunity to try two as a stereo pair, I can however see that this would be feasible.

I'm going to stick my neck on the line and predict a possible 'standard' mic has been created here. Solid construction, low handling side effects and an effective pop shield coupled with clean capacitor mic performance assures it of a fine future. That's not forgetting the excellent headroom figures and the four position sensitivity/LF cut switch that further widens its application. Thoroughly recommended.

The K240DF Headphones cost £66.70 and the C535EB Microphone, £173.88. Both prices include VAT.

Further details can be obtained from: AKG Acoustics, (Contact Details).