Not much has been written about the fascinating electret/dynamic AKG K4, so I thought I’d share some of my findings.
First off, this is quite an engineering feat. This headphone was introduced in 1983, I think, and of course it appeared in the wake of the Sennheiser HD414 and the successful Sony Walkman headphones, introduced with the portable cassette player in 1979. The market for small, portable headphones suddenly exploded. And when these slim, low-weight and good sounding headphones appeared – why use the heavy and clumsy headphones of yesterday? AKG realized that the size and performance combination was important. This was the first AKG mini headphone series and it included four headphones: the retractable 20 ohm K1 (inferiour sound), the 200 ohm dynamic K2, the dynamic “multi-diaphragm” 200 ohm K3, and the top of the line 400 ohm electret/dynamic K4.
The voltage transformer. The plastic dish is silver painted, but I have also seen a chromed version.
The K4 is a beautiful design, planned in every detail, admirably executed. This tiny package includes a dynamic driver, the electret element and voltage transformer, and it’s still small and elegant. The style is somewhat straight and angular, and in my eyes quite appealing.
Review of the K4 in Gramophone, July 1983. Note that this is a silver version.
There were also two types of headbands: one with a small "comfort pad" (upper) and one with a riveted cloth/weave band. Headband size adjustment is by the automatic and ubiquitous AKG elastic band, inside the headband itself.
It is one of three electret/dynamic AKG headphones, the others being the famous full-size K340 (introduced in 1978) and the less well-known K145. These constructions were probably triggered by the fact that AKG developed and produced their first electret microphones at the same time, and someone realized they could use that knowledge/production investment for their headphones as well. I guess they reasoned that elctrostatic/electret drivers produce less distortion and has better transient response than dynamic drivers.
The electret driver on top.
Here you can see the ridges of the dynamic driver underneath.
The K4 is an open headphone, with foam pads and ventilated back. Just like the K340, the dynamic driver “plays through” the electret membrane, but the latter is much smaller on the K4.
So how does it sound? In short, the bass is full and midrange a bit forward, the treble somewhat rolled-off. Since the electret driver is smaller than the one in the K340, I guess it works in a more limited range – downward – than the K340 which according to some sources sets in at 3kHz [Edit: 4 kHz]. I suspect a (lower) treble dip at the crossover point of the dynamic and electret driver of the K4, which together with the “eager” midrange contributes to the perception of a softened treble. But in a sense this headphone is more honest than the bass and treble emphasized portable headphones common today.
However, there are remedies for this mild frequency imbalance, of the mechanic variety. First of all, the headband is too tight. With the small pads, this means too much pressure on the tragus part of the ear, which effectively closes the ear canal. All you need is a hair dryer to heat up and soften the headband, then stretch it a bit and let it cool in the proper position. There is a mile of a difference when the pads rest easy on the ear. Here are before and after shots of the distance between left and right earpiece:
Secondly, you’ll probably need to fix the earpads. The foam AKG used has deteriorated ages ago and replacement pads are not available. I renovated the original ones with an outer ring of dense foam, and a centre piece of acoustically transparent foam.
This produced too much bass in my opinion (even though this was before the headband modification, so they might sound fine now), as did experiments with PX200 type pleather pads that fit nicely and look good but left me with a bass heavy, closed-in sound. Grado pads actually fit and sound very good, but are no winners aesthetically. Solution came with simple PX100 (or Portapro) foam pads. They need a small build-up of washer rings, I cut some out of synthetic chamois leather, glued them together with the pads on a plastic ring and then attached the pad assembly to the headphone with double sided tape. As many wise men have said before on these pages, the distance between ear and driver is crucial for the frequency response: closer to the ear = more bass, farther from the ear = less bass (and perceived treble level is lower/higher). I’ve now found a good balance that I don’t think can be much improved upon.
The K4 has an open, appealing and surprisingly full sound with no major flaws. Resolution is good but not top notch, attack also good. It doesn’t show the cross-over oddities I’ve experienced with the K340 with quite audible crossover problems if you listen to female vocal recordings in mono.
These little 400 ohm buggers are power hungry, even if a Cowon D2 manages to drive them to reasonable levels. But a good amp improves things…
The K4 is the aristocrat of portable headphones, now enjoying its old age as a vintage classic. A position it very well deserves.
Edited by MDR30 - 4/5/11 at 11:38am