Vintage SEAS Kit 503
Copyright 2006 Troels Gravesen
ORIGINAL
CROSS-
OVER
MODDED
CROSS-
OVER
MEASURE-
MENTS
AFTER-
MATH

SEAS 33F-WK + SEAS 13F-MB + (ScanSpeak) D3804


Download:
SEAS Kit Brochure, 810 KB pdf file
SEAS Kit 503, 525 KB pdf file.

Now this is some dirty old fellow, in particular the 13F-MB midrange. I've never seen so much dust stuck to any midrange coating. Some gentle alcohol treatment didn't help much. However, performance doesn't appear to be compromised from this "additional" dust-coating. The bass drivers are in mint condition and so are the ScanSpeak D3804 tweeters due to the grille in front of the large dome.
The chipboard cabs have no bracing but due to the almost 100% glass-wool filling, the cabs appear less resonant than we might expect.

One of my friends bought these old SEAS speakers for ~100 US$ and I'm having them for some days to hear how they perform and to see if an upgrade is worthwhile.
The original SEAS Kit 503 had a SEAS 1" 86H dome tweeter, here replaced by the ScanSpeak D3804, the ancestor of the current D3806/8200 used in my SP38 construction.
Unfortunately the former owner of these speakers had connected the tweeter with the wrong polarity and may have "enjoyed" a less than ideal sound for a couple decades. See measurements below.

To summarise:
"As is" these speakers can be fine-tuned to a performance that leaves most modern speakers behind. Giving the tweeter the right polarity plus some further attenuation, the frequency response is reasonably flat and I'm almost stunned by the performance of these speakers: Dynamic, transparent, etc.
With a new crossover these qualities are further enhanced and the only thing missing is a little bit of airiness due to the early roll-off of the D3804 tweeter.
A new modified crossover allow the incorporation of the SEAS 27TDC tweeter and to make a long story short: Is it worthwhile? Indeed so. Should you have a pair of these hidden in the garage or attic, don't ever throw them away. A new cabinet and a new crossover may be all that's needed.
My own 33FWK drivers will surely end up in some 60-80 litres cabinets and I'll use the new SEAS MCA15 for mid.
This is "an-easy-to-listen-to" speaker.


The original crossover:

The crossover is simplicity itself and I guess the original tweeter, the 86H, could have performed better compared to this D3804 replacement. This is what often happens if we uncritically swap tweeters: Polarity may get wrong and the frequency response of the new driver may not fit at all with the other drivers. The 2R2 (later changed to 4R7) is not in the original crossover. Here inserted to level the treble response. Only the midrange driver has to be connected with inverted polarity.
The bass driver is taking care of quite some frequency range and the apparent point of crossover between bass and mid is around 700-800 Hz, well in accordance with the SEAS specs.


Measurements:

Red = as I received the speakers. Tweeter had negative polarity and a huge dip around 2.5 kHz was the result. The treble level in the 5-10 kHz range is some 6-8 dB above average level. Not a pleasant sound I can tell.
Changing tweeter polarity made the blue graph and adding a 2R2-4R7 resistor in series with the tweeter made an even smoother response as can be seen later.

Individual response of drivers without crossover attached. The bass has a significant bump at 1700 Hz, which is a little strange as I have a pair of 33F-WKA drivers that hasn't. See below.
The mid response has been seen worse and I'm not sure this is up to the original performance. The fabric outer suspension is rather rigid, but maybe it was supposed to be so when new.
The metal grille probably compromises the D3804 performance and the roll-off above 10 kHz is in accordance with the current D3806, though the latter reaches higher.

The 33F-WK bass drivers in these cabinets are not identical to the two 33F-WKA drivers I have. The "33F" was made in a number of versions: 33F-WB, 33F-WK, 33F-WKA and 33F-ZBX and some of these with "DD", dynamic damping:.The "DD" refers to "two high conducting rings placed at each end of the motor coil. These act as eddy current brakes, firmly halting additional cone excursion as the rings enter the magnetic field". Thanks to Martin Colloms/High Performance Loudspeakers, page 79, 6th edition.
Looking at the data files on the
SEAS website, the drivers in these cabs are close to the 33F-WB although the membrane mass appear to be somewhat higher. My own bass drivers are close to the 33F-WKA, having a higher BL - and most importantly, a 4-layer voice coil causing a steep increase in impedance towards higher frequencies.
Most likely the bump at 1700 Hz is caused by a thinner cone compared to my WKA.
None of the drivers in the SEAS files indicate a fabric dust cap and I think we have to conclude these drivers were made in quite a number of versions, not all recorded in the brochures available. Here the TS data as measured by the added volume method (47 litre closed cab used).

:

Above the individual response of drivers from the crossover. Apparent points of crossover are 700-800 Hz for bass and mid and 4-6 kHz for mid and tweeter. The latter not easy to determine, not even by reversing polarity of tweeter.


Modified crossover for
33F-WK + 13F-MB + D3804


New crossover from LspCAD simulation and some manual fine-tuning. Generally this will provide a more balanced presentation and now the tweeter and mid doesn't share the 5-10 kHz region.

Measured response from new crossover. This is actually well in accordance with the LspCAD prediction. The mid and tweeter now handle what they're good at respectively and despite the lack of level above 10 kHz, this sounds very good indeed. Most likely the 86H tweeter may still be in place should you own a pair of these speakers and should you decide to give the 503 kit a second chance, read below.

More 3804 images:
(- just had to)



New tweeter, SEAS 27TDC


Components in blue are different to the D3804 version.

Point of crossover is maintained at 4 kHz. Don't pay too much attention to the overall balance presented here. Merging frequency response files can be done in numerous ways and the 250 Hz chosen here is not correct in terms of displaying the sensitivity of the system in the 100-500 Hz region.


Aftermath

Routing front panels is one of my routines in the workshop and while doing so today - or rather in between - due to the noise - I listen to music, and this afternoon I had brought two CDs: Nils Lofgren/Acoustic Live and Mark Knofler/Sailing to Philadelphia. I wanted to check out male voices on these old speakers. After some listening I increased the attenuation of the tweeter by replacing the 2R2 with 4R7 and this took away some of the (still) treble forwardness from the D3804 dome (the original crossover still in place). It's a bit tricky with the D3804 as you would like it to do a little above 10 kHz, which causes the 5-10 kHz region to be ~2 dB too high. But better level the 5-10 kHz region and forget about the top octave.
It's not often you sit in front of a pair of speakers and have this spine chilling feeling when the music starts. Play track 5 from Nils Lofgren, play it loud - real loud - and it's like sitting front row, looking up on the guitar. It's all there.
Forget about silver cables, golden binding posts, Mundorf Supreme and all that crap! Get an old 12-13" (good) paper cone and let it run up to 5-600 Hz. Give it 50-100 litres of volume or as much as your wife allows. Get a decent 5" paper mid and don't pay more than 50 $ for the tweeter! Get some electrolytic caps and - most importantly - make a good crossover and you're there.