2011 © Troels Gravesen
suggest reading the greencone article before reading
this, as the use of the resonanz boxes is a key
component in trying out these vintage SEAS
drivers. The 21TV-G drivers may do well in TQWT cabs too.
the article on "world's worst dome
the SEAS 21TV-G was in the back of my mind,
having an overall smooth response and a sound
that supersedes the German greencones, thus this
article on a Northern equivalent. Let's call them
some "massage" the 21TV-G
drivers display a nice and smooth midrange with
an upper limit of 5 kHz. Above 5 kHz the roll-off
is smooth with a single breakup at 8 kHz. If only
all modern drivers would do the same.
The vintage SEAS 21TV-G is a low-ohmic, high
sensitivity driver, fitted with a lightweight
paper cone driven by a 1" voice coil. The
surround is a corrugated extension of the paper
cone and coated with some sticky goo. This
"goo", coating material, is probably
the most consistent coating ever made as its
elasticity appears unchanged over decades. I have
a suspicion it's a xylene based material and a
no-no due to workers' safety. The downside,
besides possible toxicity, is that it also
The 21 TV-G is slightly larger than the greencone
and due to the 1" voice coil power handling
is vastly improved, also helped by an
acoustically transparent dust cap, not forming a
pneumatic brake as seen on the greencones. Price
to pay for high sensitivity - a healthy 96 dB -
is low impedance reaching 3.5 ohms. But should I
keep a speaker for my 8 watts 300B SET amps, this
speaker would be it. The 300B never felt better
and finally shows what an excellent bass response
the 300B valve can deliver.
a tweeter to match the 21TV-G wasn't
easy until Torbjørn of Norway offered me five
units of vintage SEAS 9TV-LG tweeters. These are
all 16 ohms tweeters and we need two to match the
sensitivity of the highly efficient 21TV-G.
Several configurations of these tweeters are
possible: All drivers on a vertical line or
tweeters side by side, either plane with the
front panel or slightly angled to enhance
dispersion although 9TV-LG dispersion is fairly
good considering a 3" cone speaker. Actually
one above and one below may be ideal.
1. This speaker may be for low-wattage SET amp
aficionados dealing with 2-6 watts. That is, if
you're able to find some old loudspeakers fitted
with these 21TV-G drivers. My friend Michael
found these at a local second hand
store and a few may be available from here.
2. Forget about traditional hifi, we're into a
quite different ball game here and getting into
resonant boxes may be quite a journey of endless
finetuning of resonances, something I didn't even
try with my plywood boxes. The long and the short
of it is that it is possible to make decent bass
from these boxes and due to the lightweight
nature, stored energy is low and the trick is to
control the resonating panels by degree of
bending, thickness (mass), materials (plywood or
low density solid wood), bracing, etc. An example
can be seen here: http://www.musicalaffairs.com/. View video.
I'm surprised the resonant nature of the boxes
apparently doesn't compromise midrange
performance. I had expected this.
3. The use of an extended range driver up to 5-8
kHz supplemented by a super tweeter has its
shortcomings. Severe beaming is the result with
congested treble and increased sibilance. I know
this approach is a key feature of e.g. French PHY-PH systems, but no matter
how well drivers are made, this is something we
cannot overcome. When the wavelength starts
approaching the diameter of the driver, we start
having beaming. The single-cap people seems
prepared to live with this, but to my ears an
8" drivers shouldn't be used above 3 kHz,
some would even say 1.5-2 kHz but it may depend
of the tweeter used and slope of the crossover.
4. The final crossover (v3) driven from my 300B
amp has given me fine moments with my recordings
and should I take it even further I would
substitute the two 9TV-LG tweeter with the Audax
TW025A tweeter having sufficient efficiency to
cope with this remarkable SEAS vintage driver.
Taking the point of crossover down to the 3 kHz
region eliminated most of the beaming and made a
much more tolerable speaker handling most musical
genres equally well. Nothing new under the sun
here. This has been confirmed over and over
again, not least during the more thorough
investigation of the Supravox driver in DTQWT cabs.
SEAS 21TV-G driver.
View data below found on SEAS website.
21TV-G measured TS
data. Added mass used.
Five 9TV-LG tweeter
in excellent working condition. I'm surprised by the
consistency in performance of these tweeters. View
measuring data below.
The 9TV-LG tweeter features alnico magnet, paper cone,
heavily doped surround and an aluminium dust cap.
Finally it comes with a diecast chassis. Quite an unusual
thing for a vintage driver like this.
Left: 21TV-G SPL @ 1
meter, 2.8 volts. Both 21TV-Gs shown.
Right: Five 9TV-LG tweeters; SPL @ 0.25 m normalised for
2.8V/1 meter. This driver more looks like a midrange
driver, but high-end extension is excellent.
Left: All five
9TV-LG free air impedance. Right: 9TV-LG dispersion from
0-35 deg. Evenly distributed readings.
SPL @ 1 meter/2.8
volts for 21TV-G (red) and 9TV-LG (blue). As can be seen,
two tweeters are needed to match the 21TV-G.
- exactly the same
as for the greencones.
in resonanz cabs from crossover v.2.
Left: SPL of drivers
in cabinet, two tweeters here in parallel. Right:
Impedance of drivers in cabinet (no crossover).
Left: SPL from
drivers driven from crossover. Right: Impedance of final
Left: Step response.
Right: Distortion @ approx. 90 dB/1 meter. Purple = THD.
CSD, 25 dB scaling.
in resonanz cabs from crossover v.3.
of crossover version 3. Point of crossover is here taken
down to ~3 kHz.
2.8V, 1 meter. Overall system sensitivity is around 96
dB/2.8V. Right: Same with 200-20000 Hz scaling.
Response from drivers driven from crossover plus summed
response. Right: Final system impedance, minimum 3.5 ohms
A friend of mine wanted the blackcones incl. resonanz
boxes for his 2A3 amplifier, so I put together these V3
crossovers, mostly Intertechnik stuff.