RETRO: Beyerdynamic DT 990, 600Ω, classic open-back circumaural headphones (circa 1991).

This week, I thought I would do a "retro" measurement of and listen to a headphone from the days of the Cold War. ;-)

Above, you see the 600Ω impedance Beyerdynamic DT 990 (forefather of the current DT 990 Pro, ~US$150), a pair of headphones lent to me by my buddy linnrd that he has had in his archives of audio equipment for quite awhile. As you can see on the box, these were made in W. Germany and purchased back in 1991, a couple of years after the fall of the Berlin Wall; either in Düsseldorf or nearby Essen I was told. The history of these headphones began back in 1981.

For headphones of this vintage (>30 years old), they are in great shape although in the image above, notice that I had replaced what used to be a thin layer of disintegrating black foam inside the earpiece with some bluish 1.5mm felt which I had used in my Dekoni Blue mod last year. So, not exactly a "stock" unit but pretty close.

The ear pads were also deteriorating after all these years but linnrd had some original, sealed, "new old stock" (from the 1990's) velour pads for replacement:

"New" genuine Beyerdynamic velour pads. After removing the deteriorating foam in front of the headphone driver, we can see the driver covered with some kind of white paper-like damping material. Notice the right/left indicator is some white text on the inner sides of the strap tightening piece. The headphones are otherwise totally symmetrical so easy to confuse orientation.

Ear cups easily replaced, before putting some blue felt over to protect the driver. Difficult to repair if paper were to be punctured!

I spent a couple of evenings listening to this baby before measurements to make sure there were no issues with the sound. Yup, both channels working well. Notice the obviously older design with more angled edges, wires coming out each ear cup (non-detachable), an adjustable comfort strap rather than padded headband itself.

Clearly this is an open headphone design. Do not expect much if any isolation and sound leakage is pretty significant. The plastic material is light and the velour earpads very comfortable for longterm listening for hours. Head clamping force was not strong and one might complain that it's too easy to fall off with head movements (depends on head size). The thin leatherette comfort strap did a good job to reduce pressure or irritation from the plastic headband. The plastic material with its various swivel points creaks with motion but it's not too distracting. Coiled wiring is great for a bit more range (12' extension) if one is moving around in a studio, and is nicely non-microphonic.

Headphone cups are 4" diameter, the whole thing with cabling weighs around 0.75lb (slightly less than 350g, ~250g headphones only). 

Let's put these on the miniDSP EARS measurement rig to get some data (as per my standard procedure):

And here's my "big graph" summary of the main results:

Alright then. Immediately I think we can make some predictions about the sound based on the frequency response (with compensation for a Harman 2018-like curve). These are relatively bass-shy. If we use the cursor centered at 1kHz and 92dB SPL to make comparisons, we see that by 50Hz, frequency response is down -6dB and by 30Hz down -10dB. There's an upper-midrange dip at 3.5kHz around -8dB. And then above 4kHz, on average, there is boosted treble frequencies that accentuate the "presence" and "brilliance" regions of the spectrum.

Taken together, we expect a sound that's more on the "lean" side than "warm".

These headphones have low sensitivity with 1kHz 0.25Vrms output translating to 89dB SPL (or around 100dB/V - similar to a more modern 600Ω model measured at ~100dB/V). Compare this to my "standard" AKG K371 with a result of 106dB/0.25V or around 118dB/V. Coupled with the high 600Ω impedance (we see a peak around 900Ω at 80Hz), these headphones will need a good headphone amplifier with the ability to provide the voltage to get the party going. Power spec suggests up to 100mW, or around 8Vrms peaks. As a single driver design, electrical phase is basically flat.

The waterfall plot shown with 35dB range looks good. Frequencies above 2kHz drops off by 2ms. Below 2kHz, there are no major resonances lasting beyond 9ms down to 100Hz. You can compare this with something like the semi-closed Dekoni Blue (Fostex T50RP Mk 3), or completely closed AKG K371 where low-frequency resonances in the cup are accentuated.

The distortion results are alright but higher than modern headphones. We can compare them for example to the AKG Q701 which is also an open-backed design with also limited low-frequency response. At 95dB SPL @1kHz, whereas the AKG is able to keep THD <0.2% above ~150Hz, these older Beyerdynamic DT 990's have elevated distortion >0.2% until about 250Hz. It's primarily a mix of 2nd > 3rd harmonic. The elevated distortion in the bass suggests that we should be careful not to push the low frequencies too much when EQ'ing.

A look at the time domain performance with some 100Hz square waves (bandlimited to 20kHz, using RME ADI-2 Pro FS as headphone amp):

These are very quick transients on the edges of the square waves compared to most headphones I've examined. The waveform looks a bit rough though - I've wondered if this may correlate with a less "refined", less smooth sound. (One of those things which I think can only be answered well with some form of blind listening.)

The lower frequencies are not reproduced well and we see the droop in the square waveform over time. In contrast, headphones that reproduces much stronger low frequencies like the Drop + HIFIMAN HE-4XX are able to produce the signal with a more "square" morphology although the initial transient isn't as fast.

While I typically also measure headphone leakage and isolation from external noise, let's just forego that here. These open headphones will not isolate you from external noises and they'll sound like little speakers leaking your music to the world for anyone sitting next to you on the bus. ;-)

Subjective sound quality...

Let's talk about how these headphones sound. As discussed above, clearly these are not the headphones for "bassheads". You're simply not going to get a noggin-shaking experience with these. They're much more lean-sounding headphones which exaggerates the upper treble and I think most listeners are going to find the "brightness" excessive without at least a little EQ'ing.

Here's some new music to consider and thoughts while listening through the Beyerdynamic...

The other night, I was listening to Keb' Mo's latest album Good To Be... (2022, DR8). Keb' Mo' is typically known to have high quality recordings with better dynamic range than this! Also, the 24/88kHz hi-res version is basically upsampled audio with relatively high noise level so don't bother with that. 

Despite these technical characteristics, this is an enjoyable, uplifting set of acoustic-pop/blues/country songs. In a world of strong polarities and dark news almost everywhere we look these days, this album is a good one to put on for some light stress relief. The remake of "Lean On Me" is comforting, I like the country-twang of "Good Strong Woman" with Darius Rucker (Hootie & The Blowfish), there's a lot of idealism wrapped up in "Louder" (about generational transition), and "Like Love" is a sweet, light love song.

Perhaps you can gather from my description here, this collection of songs might just be too sweet for some. Sound quality is clean and tight although I didn't find the soundstage particularly wide. There's no "wall of sound" potential to overload or muddy up the vocals or details of the guitars or light percussion. 

The last song "Quiet Moments", a duet with Kristin Chenoweth provided an opportunity to assess sound quality of the headphones and compare with others. Over the Beyerdynamic DT 990 (using my Drop + THX AAA 789 amp), the interweaving vocals (Kristin basically contributed background vocals), all the various acoustic and electric guitars, strings, piano parts were easily discerned but I think there's significantly more that can be done to expand the soundstage.

For those interested in audiophile female vocals, have a listen to Melody Gardot's new one Entre eux deux (2022, DR13). By now, I'm sure you're familiar with chanteuse Gardot's vibrato-tinged vocals which retains its sultry, smooth character in this latest offering. A number of these songs accompanied by the light touch of Philippe Powell tickling the ivories on the piano would make for great demo material at audio shows. With a good DR13 dynamic range, this album will for sure show off the "microdynamic" capabilities in many audio system.

The standout track for me on this is the Portuguese / French duet "Samba em Prelúdio (Un jour sans toi)". A good demo track to contrast the male/female timbral qualities and vocal styles.

Here's an example where the sibilance in Gardot's voice simply demanded that some EQ be applied to tone down the upper treble on these DT 990's (talk about this in a little bit).

While I don't listen to music on LP much these days, I do like some of the special DJ remixes and 12" singles over the years ripped to digital. IMO, 16/48 is more than good enough for LP rips, and gentle surface noise reduction with removal of crackles and pops go a long way to improving sound quality especially when listening over headphones. Some of these, especially from the '80s are really good thanks to excellent dynamic range masterings, and are often difficult to find.

The Lionel Richie "Dancing On The Ceiling" single from 1986 is a great 7-minute remix. At the start of the track we hear Lionel walking and asking "Man, what in the world is happening at the end of the hall...?" without the overdubbed party noises like on the original CD release. It reminds me of the brief intro on Michael Jackson's "Thriller" (1983) as the creaky door opens, wolves howl, etc. before getting into the song.

I mention these not just as a comment about rare, good sounding single mixes, but also because with open headphones like the Beyerdynamic, listening to these snips, one can hear the "spaciousness" people talk about with open-back designs as compared to closed headphones which can appear more constrained with less of the "you are there" surround envelopment. Furthermore, the "crispness" of those transients and razor-sharp details impressively create the illusion of listening to something "real".

When it comes to bass reproduction, electronic music with deep lows like say Massive Attack's Mezzanine (1998, DR8) is still enjoyable but clearly missing the low frequency rumble that I know should be there.

Headphones like these are excellent for binaural recordings as well. I think you'll be impressed by samples in this documentary (can start at 5:00), this live mono-to-binaural performance, and this remix of "Bohemian Rhapsody" (this last one very much depends on your taste for surround mixing ;-).

With a bit of EQ experimentation, the settings below in Roon worked nicely for me:

This added a bit of bass back but not overdoing it given the risk of exacerbating excessive distortion. Reconstituted the upper-mid dip around 3.5kHz - feel free to play with this to make sure not worsening sibilance. And toned down the "brilliance" and "presence" frequencies which were just too "hot".

With the EQ, I found it a more comfortable experience with less harshness from excessive tonal brightness. For example, Melody Gardot's vocal on Entre eux deux was now cured of its annoying excess sibilance. Remember, sibilance is a normal part of speech and singing, the point is not to remove all sibilance as that would be very unnatural - just enough to sound normal if accentuated by the hardware/audio mastering.

Summary

While evaluating new products can be exciting, I think it's also nice digging back in time and trying out the sound of products that have come before. In doing so, as audiophiles, we can develop context around whether sound quality has improved as hardware continues to evolve. That sonic evolution can be easily explored for example when we listen to recordings over the decades - the "glare" of early CDs from well-recorded modern titles which over time has improved thanks for maturity of audio production (until of course one runs into Loudness Wars productions, sadly resulting in retrogression of sound quality).

If you have these older headphones, you might be interested in reading a bit more about them on the HiFi Classic website, including the aim for these headphones to achieve uniform diffuse field response.

While I would certainly not suggest that these are the best headphones I have heard, one might be surprised by the sound quality with these, and how they compare with modern open-backed options (especially once a little EQ is applied).

Regarding these sounding overly bright, as I noted above, these are not completely "stock" headphones in that I did have to remove the deteriorating foam cover over the drivers and replaced them with the felt sheet which may have dampened some of those high frequencies in a good way. That I used an EQ to still further drop the upper treble speaks to what I believe is the main sound quality complaint for me. Too little bass, a bit too much treble, so the balance is accentuated to the "bright" side. As we get older and high frequency hearing acuity gradually is reduced, I wonder if that brightness might actually be less objectionable (who knows, maybe even potentially preferred?). Anyone know if there are studies on age-related sonic preferences and headphones?

There have been plenty of measurements over the years on the current-model 250Ω Beyerdynamic DT 990 Pro (like this, and this). As for these older 600Ω DT 990 non-Pro measured here, I have seen measurements by Marvey on the old Changstar headphone site, and on DIY-Audio-Heaven. As usual, each headphone will have variations, and over time, manufacturers may change parts and even drivers without notice. Nonetheless, I think the tendency towards brightness above 4kHz is quite consistent and may even be characteristic of Beyerdynamic's headphones in general. 

It has been a busy and trying week as we go back to work mixed with some of the news events this week. There are audio-related news items worth talking about which we'll address next week...

Hope you're enjoying the music, dear audiophiles!