The AKG K 250 (history, pictures, and a review of this forgotten dynamic K 340/K 241 hybrid) - Page 7

The K 250 don't seem to have Sextett drivers, though; not according to the service doc, anyway.

Hard to tell what sort of standard equalization, if any, was aimed at with the 250 and 340 based on the graphs I measured...

There's a picture here comparing free field and diffuse field. I suppose you could see some evidence of the free field trace in what Tyll measured for rhythmdevils' pair of K 340.

The K 250 as I've measured it fits neither of those.

Here's the raw (i.e. un-diffuse field compensated) graphs for the K 250, K 340 and DT 990 (vintage) as I measured them:

If you look at the DT 990 graph (middle), then compare it to the diffuse field graph I linked to above, the similarity is pretty striking. (Though could be incidental, as my recording setup isn't pro.)

Adda linked a graph (here) comparing the 240 DF to the 240 M. The graphs are presumably compensated, but I don't know with what curve. My compensated curve for the K 250 isn't actually too far off the DF as it was measured by that person – the DF has a hump around 5 kHz as well. (But again, I've no idea what sort of a recording setup that person had.)

I haven't had a chance to test the K 250 with any other sources yet. And to be honest, I don't really have any. But I'll have a look at the bass department when I get around to my 250/340 thing.

I've never heard a 240 M, but I do own the Philips SBC 3178, which are a Made in Austria type of thing, and most people seem convinced that they're a rebadged K 260. The K 260, in turn, has been described as a better version of the 240 M. I don't know how close of a rebadge the Philips version is, but they don't sound as good as the 250 anyway. Not bad, but a bit honky.

I'll measure the Philipses tonight; gets us a bit more context.

Alright, so the similarities for the K 250 & K 240 DF, based on written confessions:

- lean bass,

- neutral sound (mids?),

- bright treble.

(More on that after the graphs.)

Here's the graph for the Philips SBC 3178 (presumed AKG K 260), averaged from three different ear positions and compensated for HRTF:

(The peak at 50 Hz is noted on my graphs of other headphones as well; most likely ambient interference.)

Again, even though the Philipses do sound bright, I have a feeling the treble area may be pushed up a bit too much in dB in the measurements. The shape of the graph (both the detailed light one and thus the abstracted dark one) otherwise agrees very well with what I hear in a sine sweep. The dip at 3.5 kHz I think is also exaggerated in this graph; I hear it, but not as distinctly as the graph would have it – maybe half of that in amplitude would be about right. I hear the slight wobbliness between 2 and 3 kHz. There's a definite hump in the treble after the 3.5 kHz dip, regardless of whether the graph has it a bit stronger than it should. The 10 and ~12 kHz peaks I hear as well, but I think there's one peak between them that's not on the graph. The drop-off after about 13 kHz seems right.

This is the combined DF/M graph from this post:

The Philipses have the same notable dip at ~3.5 kHz as the DF and the M. It even seems to have that little v-shaped notch in the bass (or lower mids as they are, at 550 Hz in my graph). To me (based on the graphs) it seems like the DF, the M and the Philips are in the same family of phones, and as such I guess it's not so far-fetched to say the Philips is a rebadged K-something (definitely looks like a K 260).

(Curiously, the DF in that graph has a similar post-4 kHz notch as the K 250. It's most likely a coincidence, but still neat.)

Here's the Philips compared to the K 250:

If we take the post-3 kHz dip to be a mark of a certain family of headphones (can we? do we have any other measurements of the 240 M or DF – or the 250? was it lejaz who didn't find a 3 kHz dip on the DF?), then the 250 might not belong in that family. The DF, M and K 260 are all studio phones as far as I know. The K 241, on the other hand (the one the K 250 apparently shares drivers with), were supposedly marketed for home use (or were they?).

It would be very interesting to see the freq response of the 241. Deep Funk has compared the K 241 and the DF by ear and found them similar. Deep Funk, would you mind checking if you can hear a dip on the K 241 around 3 kHz like in the DF/M graph?

So despite the similarities of the K 250 and the 240 DF that I listed at the top of this post, I'm not exactly sure how much they're related. Based on certain features in the graphs, it seems that AKG were maybe after a different type of sound with the 250. Or, maybe, the differences are simply due to the bigger cups on the 250. The graphs (again, my graphs are definitely not absolutely reliable) would indicate that the K 250 actually got very close to being neutral, closer maybe than the DF. Why would AKG abandon the 250 design?

I'd very much like to know what sort of use the 250 was made for, but AKG haven't yet responded to my email. They've either ignored it as meaningless by now, or they're having a tough time figuring out the K 250 as well.

I'd also love to measure the 240 DF, so if someone has a battered DF for veeeeeery cheap.... (no need for pads, headband, name plates etc., as long as the sound is intact).

No problem, Adda, measuring them is pretty interesting.

While the 250 mids seem very neutral, they're also a bit grainy. It's a subtle thing and I didn't notice it at first, only after I'd compared them to some very smooth ortho mids. But I think mids are a strong point for orthos in general.

Early DFs (and Monitors) do share the same baffle as the K 241 – however...

That's the K 250 baffle. Catch the difference?

It's something I'd like an explanation for. The different-colored sound port coverings (black and white) apparently have different densities, so one assumes they have different acoustical properties. On early DFs as well as the Monitors the white rings are aligned neatly on one side while the dark ones are on the other side (as in the picture you showed), but on the 250 the pattern is disturbed.

Also, the pattern on the 250 isn't symmetrical. The left baffle has a different one than the right one, and they don't match up. This isn't just on my pair – you can see it in all the 250 baffle pictures on the internet. How's the sound port placement on the DF – do they align so that when you bring the cups together, the white rings are against each other and same for the dark ones?

I really don't know what AKG was getting at with the sound port differences between the 250 and the DF/Monitor/241, but it's pretty intriguing.

(Just for the heck of it, I'll do a measurement tomorrow where I'll rotate the artificial [blu-tack] ear so that in relation to it the white and black rings align the same way as on the early DFs. Probably won't be any different than what I measured normally, but why not give it a try.)

The Philips baffle looks kinda like the one in the last picture, except the driver hole is covered as well.

I did measurements on the K 250 on tilting the ear:

(Peak at 50 Hz is background interference.)

Each measurement is averaged from three different positions – up, middle and down – and HRTF compensated. (When moving the ear down and up, I did it relative to the tilt of the ear – i.e. for measurements where the ear was tilted, I was actually moving it slightly sideways in relation to the cup itself, but down and up in relation to the ear's tilt, if that makes sense.)

Black line is with the ear in its normal state. Green/blue (whichever way you interpret it) line is with the ear tilted so that the white sound ports are aligned on one side of the ear and the dark ones on the other (+ then tilted slightly as the usual ear is). The sort of powdery-colored line is the measurement, with the ear in its normal state, that I posted and did earlier.

The earlier measurement is slightly different than the one I did today, which is most likely an unfortunate side effect of having the ear made out of blu-tack. Meaning it's impossible to have it in the exact same shape from day to day. The differences aren't big, though, and mainly in the treble, which is where you'd expect to find ear-related differences. (These differences are why I re-measured the K 250 normally as well for this test.)

Between having the ear normally and tilted, I don't see a large difference in the graphs (if any at all), and based on that, I'm not sure if it matters much that the sound ports on the K 250 are rotated differently than on the DF/M/241. Another matter, however, is the asymmetrical baffles, meaning the pattern of sound ports doesn't match. I don't know if the very slight differences together would create some acoustical effect when placed on the ears. I'll do yet another measurement tonight (or tomorrow) where I'll measure both sides. The current measurements are from the right side only.

Here are un-averaged measurements for the three positions (up, middle, down). First with the ear tilted:

Then with the ear normally:

It seems like having the ear non-tilted was more sensitive to how the cup is placed on the ear. I don't know what to make of that. Though looking at the last graph, the ear position 'down' seems to have produced the most notable dip at 4 kHz, which is also closest to how I personally hear it. I'll have a listening test tonight and see how the placement of the cup on my ear changes what I hear.

Here's the channel balance measurement for the K 250 (raw data):

(Peak at 50 Hz is background interference.)

For each channel, I placed the recording setup (without ear) so that the ear canal tube was in the middle of the cup. I took three measurements (varied the ear canal position by one or two mm on the extra two measurements) and averaged them into a single measurement.

The graph contains only the ear canal resonances – I didn't use the artificial ear for this measurement. Thus the graph is also not corrected for HRTF. The problem was that I only have a right ear, so couldn't have reliably tested the left channel with that one.

The graph would most likely not show any effects of the asymmetrical placement of sound ports, if there were any. I have a feeling the different sound port materials are chosen based on some anatomical model of the outer ear, so certain sound port materials would correspond to specific areas of the ear. But I could be wrong.

(More detailed channel balance measurements here.)