OBL-7, open baffle experiments
Copyright 2007 © Troels Gravesen
Goodmans Axiom 150 mkII + Supravox 215RTF64 + JBL LE26-waveguide
is my first attempt to make an open baffle
speaker and having a range of suitable high-efficiency
drivers, making a pair of suitable baffles seems straight
forward. My role model is the vintage Wharfedale SFB
(sand filled baffle), which can be studied in detail from
The LE26 fitted with a waveguide
makes a suitable option for high-efficiency systems, read LE26
article. Details on the Supravox 215RTF64 can be
found at Supravox website. A classic,
extremely sensitive high-Qt driver. Goodmans Axiom 150
mkII, details can be found here.
We do need a few crossover components. I've tried running this from a single cap to the tweeter and it just doesn't work. The Supravox has a rising response towards higher frequences and makes the upper treble too aggressive. From a single cap (3.3 uF) the LE26+waveguide does well down to 1-1.5 kHz, where it really shouldn't mingle at all. And the Goodmans does well up to 2.5 kHz where it should leave it to the Supravox. So, basically the bass and mid now runs 1st order after adding impedance correcting RC circuits and the tweeter needs a 3rd order high-pass filter at around 6 kHz to mate the Supravox. The Supravox is really handling most of the treble area here. Let's reiterate frequency bands, so we know what we're talking about:
simulation shown above is made from measurements
done on drivers mounted in closed or vented cabs.
Measuring drivers on an open baffle is major trouble as
has been discussed in Acapella files and the Dipole
Study. Read intro page for links.
The big question in this set-up is the Goodmans bass driver. It's a capable driver up to 2-3 kHz but it doesn't sound good playing upper mid/lower treble. The 3.9 mH coil shown here makes a rather shallow roll-off of 6 dB/octave starting around 200-300 Hz, thus still provides quite a contribution to the whole midrange. Problem with this is that it's slightly out of phase with the Supravox. Adding a high-pass filter to the Supravox would mean inverting polarity of mid and tweeter and I didn't want to do this. The bass and mid are both connected with positive polarity and provides a bass membrane area close to a 15" driver.
So, how does these monster panels sound?
Well, I'm ready for surprises
when it comes to speakers. I've been reading a lot about
similar systems using the single-cap approach, and I've
always been more than sceptical and the first single-cap
run confirms my scepticism. At least for these drivers it
Obviously there's no boxy sound
from this panel. The bass comes out clear and punchy and
listening to acoustic bass is most enjoyable. It's also
obvious that the bass is not as deep as had the Goodmans
been placed in an e.g. Onken vented box. So, how about
the mid running so high? Well, I hear treble crisp and
clean, but dispersion - or lack of the same - in upper
mid and lower treble is very noticeable. In that sense it
resembles electrostatic speakers. Move your head a little
and the soundstage changes a lot. There's a hot spot, no
doubt about it. However, due to the dipole status, this
is more tolerable than expected. A dipole tweeter might
be interesting to try, but where do we find a suitable
planar at this level of sensitivity?
Should you by coincident have these or similar drivers at hand, try it out and prepare for some enjoyable moments. Be also prepared to experiment a lot on room placement and possible damping sheets on nearby walls. It may shake your views on conventional hifi speakers, what they can and what they certainly cannot. Be prepared that WAF is ultra low. And it takes will not to play these panels loud - because they can play very loud - so take care of your hearing.
Philips 9710 fullrange on OB7
I've had a request on
setting up my 9710s on the OB7 and give my opinion on
performance. Without too much trouble the 9710s would fit
into the Supravox rebates.
Some good things too. The
bass isn't bad at all despite a modest membrane area
trying to push the room air. It gives you a nice idea
about what is going on in the lower registers. The whole
of the midrange is good too. No problem here. Perspective
is seriously compromised from treble level being way too
high. Treble as such isn't bad at all, just much too much
of it. I wasn't even thinking in terms of tweeters to
supplement the 9710. Going back to my working desk - in
the next room - and
"listening-from-the-other-room" actually was
pretty joyful. I can imagine the 9710 pointing upwards in
the vintage Carlsson construction
wasn't bad at all because you would never meet the 9710
"face to face".
This experiment took me to the next one: The Supravox run fullrange. Supravox 215RTF64 directly connected to my 6AS7 single ended triodes. So back in went the Supras without any equalisation at all.
Supravox 215RTF64 fullrange on OB7
To the right the frequency
response of the 215RTF64 is seen. A rather wide
measuring window is used with modest smoothing.
With a frequency response up to 9
kHz, most treble information is there. It sounds
slightly "vintage" if you get my meaning. Not
like old AM radio, far from, but you miss some airiness
enhancing the spacial properties of the presentation. I
can to some extent understand why
"uncompromising" audiophiles settle here.
"Uncompromising", because this approach is full
of compromises - to my mind. I too like things to be as
uncomplicated as possible. Simple solutions can hold a
lot of beauty. But the price to pay for this single
driver set-up is high.
Inserting the Goodmans again
via the 3.9 mH coil including impedance correcting RC
circuit is a major step up. From resembling an indeed
very good "transistor radio", it now has body
and weight and gain momentum in the entire frequency band
it covers. Listening to my favourite jazz LPs, I still
miss the airiness of the cymbals and instruments with
high frequency information. They become distant and
somewhat closed-in due to lack of the upper octave. Very
noticeable on piano too, by the way.
The final experiment
fullrange experiments I went back to the 3-way
system and as a final option changed the point of
crossover between mid and tweeter to approx. 3.3 kHz.
Impedance still remains good although dipping to 3 ohms
at 8 kHz. My amps didn't mind.
I do not hear any disadvantage of using these drivers with conventional crossovers. For the purist, even running the Supra to 6-7 kHz may be acceptable. For most musical genres this provides significant improved performance compared to the non-equalised fullrange presentation. For the Supra, very few components are needed to render a balanced sound of instruments and vocals. There's no reason these drivers cannot be used in ways where we can start talking "high fidelity" in the true sense of the words. I mean, the musical signal is passed through countless wires and connectors, transistors, valves, resistors, capacitors, etc., before reaching the speaker. Not equalising frequency response from your loudspeakers - if need be - is like drinking an expensive vintage wine from a mug.