AKG Electret-familen:
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AKG K340
The K340 was AKGs first foray into the territory of electrostatic headphones. It was introduced in 1978/79, and was followed by the K4 in 1982, and the K145 in 1984/85. Both of the latter are supra-aural (on-ear), while K340 is circum-aural (over-ear). The production of K340 ceased in 1993/94.
The K340 is actually a hybrid of electrostatic and dynamic drivers. It builds upon the K240 Sextett design (likely the same dynamic driver, with 5 passive radiators instead of 6), with added treble response from an electret driver. The (full range) dynamic driver contributes up till about 16 kHz, and the electret driver contributes from about 4 kHZ and up, acting as (super)tweeter to add a bit of electrostatic magic (shine/sparkle/detail) to the two upper octaves. It's worth noting that there is no crossover, both drivers will be active at the same time within their respective frequency ranges.
The electret driver needs a stepup transformer*, and in each earpiece the sixth slot (otherwise used for passive radiator) is occupied by the PCB with stepup transformer and resistors. Since it only has to handle frequencies above 4 kHz, it can be this tiny. As well, being electret rather than electrostatic means it can driven directly from the headphone jack of an amp or receiver, though some say the K340 is quite inefficient and needs a dedicated headphone amp to really sing.
It's said that K340 during its lifespan underwent 5 different revisions, though on sound alone only two are generally acknowledged/talked about, the bass-lite and the bass-heavy versions.
There are a couple of known mods to the K340:
A. To custom-tune the headphone by changing the damping (cotton in the K340), and thus the Q, of the diaphragms.
B. To bypass the stepup transformer PCB for the dynamic driver (desoldering).
* Electrostat vs. electret headphones: Both types are electrostatic-- there's no difference in operating principle, just materials used. One uses electret material to supply the high fixed operating voltage, the other gets it from an external power supply. Electret headphones do need stepup transformers, but often these are built into the earpieces (as with the K340, K4 and K145), or an inline pod (as with the Philips electrets), or a fat 1/4" plug (some of the Toshibas). These are the models that plug directly into a headphone jack. Then there are models with the transformers in a separate box (Sony and Stax) that connect to speaker terminals or special high-voltage direct-drive amps (Stax).
http://www.audiofanatyk.pl/recenzja-slu ... k340-es-d/
https://www.headphile.com/page9.html
AKG K4
Introduced in 1982 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 super lightweight K4 (65 g in contrast to 385 g for K340) was probably an attempt to cater for the rising portable market of the time. The hybrid electret/dynamic design of K340 was carried over and adapted to the smaller form factor of K4. The electret is smaller and more efficient, and the construction is far simpler. They used a dynamic driver that is very similar to the K141-style driver used in the K240 and K340 series, but at 400 ohms rather than the 600 ohms in the K141 and K240 or 640 ohms in the K340.
Though small and ligtweight the K4 is not exactly an obvious portable first choice. At 400 Ω / 92 dB they need plenty of gain and and plenty voltage (they don't need a lot of current), not the ideal partner for the walkmans/discmans of the time. Failing sales probably explain the short life span (1982-85), and because of that the rarity of the K4 today.
The headband on K4 is spring loaded, providing a comfortable yet adequately firm pressure on ears and head. There are two versions, one with a single comfort pad, and on with a riveted weave band. If the headband is too tight, a hair dryer can be used to heat up and soften and then stretching it a bit. The foam pads will have deteriorated, and replacement pads are no longer available. The pads can either be rebuild, or replaced with alternative pads (see link below).
https://www.head-fi.org/threads/akg-k4- ... cs.541899/
http://www.hifi-classic.net/review/akg-k4-462.html
AKG K145
Introduced in 1984/85 the K145 was another go at the supra-aural market segment. It accompanied the cheaper dynamic K135, also supra-aural. It is larger than K4, but lighter than K340. At 200 Ω / 89 dB it was still probably to hard to drive for portable players back then, but production continued until 1992. The rarity of K145 today suggest small sales numbers at the time.
Like its predecessors the K145 is a hybrid design, and in all of these headphones what you have is the step-up transformer for the electret in parallel with the voice coil of the dynamic driver (there is no crossover in the usual sense). The electret material is said to vary between production runs, and depending on the electret film used (no specific details are known) you might have a series resistor on the transformer primary, or secondary, or both, or neither. K4 and K145 share drivers, and they are are soldered to each other directly rather than with jumper wires like you see in the K340
There are two versions, outwardly they differ quite a bit, internally they are said to be identical..
- K145: The regular version has a plastic headband with a nylon strap, with red lettering on the band and red accents on the strap. The driver is exposed (apart from the foam pads). Cable and plug are similar to K240-series.
- K145S: The /S version has a wire headband with a vinyl strap (similar to the standard AKG headband), all in black, with white lettering on the strap. A plastic grid protects the driver (similar to the Sextett). Cable is softer and slimmer than on regular version, and plug is similar to K501.
https://www.head-fi.org/threads/akg-k14 ... od.275453/
https://www.head-fi.org/threads/akg-k14 ... ry.363967/
http://audiokarma.org/forums/index.php? ... 5s.409436/
http://tech.hexun.com/2010-12-19/126271502.html
Sound comparisons
"If we had not measured them, we might have concluded that the larger K340 was somewhat dull and lacking in high-frequency response. But a more careful comparison suggests that the superior smoothness of the K4 at high frequencies, possibly aided by its supra-aural design, was responsible for its delightfully open, airy quality. Unquestionably, it did not have the "bottom end" of the K340, but it did have a sweetness at the other end of the spectrum that we found most attractive. In addition, it seems probable that the diminished output of the K340 at upper middle frequencies (where it was 5 to 8 dB under the level from the K4), combined with its stronger bass, had the effect of making it sound dull and distant by comparison to 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 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."
"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."
"For those of you who are still waiting to hear them they have a unique characteristic. The treble is ultra quick. The attack on the treble is noticably quicker than on other 'phones which I found very pleasing and that persuaded me to purchase them. The bass is a little laid back and weak compared to some phones but they always put a smile on my face."
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