Vintage Spendor BC1
       
Copyright 2005 Troels Gravesen

CROSSOVER      CABINET      MEASUREMENTS     HF1300 TWEETER     BC1 WOOFER       

The Spendor BC1 is basically a three-way system with points of crossover around 3 kHz and 13 kHz. "BC" stands for Bextrene cone Celestion tweeter, according to Ken Grey, former Spendor employee.
The midbass is a small 8" driver fitted with a bextrene cone and an inverted rubber surround, in fact quite similar to the KEF B200, SP1039. From 3kHz the Celestion HF1300 tweeter is taking over and runs up to its intrinsic upper limit around 13 kHz from where it rolls off steeply as can be seen from the graphs below. To help in the top octave, a Coles 4001 super tweeter is used; a small 3/4" tweeter fitted with a plastic dome. The 4001 super tweeter has a 23 ohms DC resistance, quite unusual. Generally the DC resistance of all drivers is high, making this speaker an ideal partner for valve amplifiers.

Overall I find the BC1 a very well executed loudspeaker construction. The use of a thin-walled plywood cabinet has its pros and cons and is not solely responsible for the BC1's somewhat un-precise bass response. The bass driver's Qt is too high for a ported construction and even a larger closed box doesn't leave things ideal. This driver simply needs more magnet strength. On the good side the midbass delivers an excellent midrange and the transition to the Celestion HG1300 tweeter is indeed very well done.
I find the voicing too bright for my taste, but this may easily be adjusted by adding a little attenuation to the HF1300 tweeter.

A large amount of information on this speaker is available on the web.
To get an idea of performance I suggest you read the review by Bob Neill of the current Spendor SP1/2 compared to this old classic:
http://www.enjoythemusic.com/magazine/equipment/0302/spendor12.htm

Read review of BC1 in "Hi-Fi News & Record Review Annual '73". 500 KB, Thanks to Darryl, for the scan.

Also read Hi-Fi News, November 2005, page 66-69. Ken Kessler on the BC1.

Also check out this website: http://spendor.de/warum.html


BC1 cabinet
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Cabinet data:
Dimensions: 300 x 300 x 637 mm (HxWxD). Internal volume approx: 38.4 litre.
Front panel:12.3 mm Baltic birch.
Rear panel: 12.3 mm Baltic birch.
Side panels: 10.5 mm Baltic birch including veneer.
Bitumen pads on internal panels: 10-11 mm lightweight bitumen impregnated masonite (wood fiber).
Damping: polyester foam on all internal panels except front panel, 30 mm thick and relatively light weight.
Front grille frame is made from 6 mm plywood and frame fillets are 40 mm wide. On the outside is added 7 mm polyester foam below the fabric.
Vent: 75 x 125 mm vent is added a sheet of polyester foam: 15 mm thick, thus the vent appear to be 45 (diameter) x 125 mm long
Port tuning: 28 Hz (see impedance measurement below)

The first time you open the BC1 cabinet and find out the cabinet is made from 10 mm Baltic birch, you start wondering what is going on here. The panels being this thin, you'd expect some very heavy bitumen pads to be glued to the panels inside to move the panel vibration modes down in frequency. Not here. The bitumen pads are approx. 11 mm thick and made from some quite lightweight wood fiber material, which in Denmark would be referred to as "soft masonite".


BC1 crossover
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BC1 crossover schematics. Third order for the midbass. Third order high-pass for the HF1300; no low-pass filter for the HF1300.
This tweeter rolls off steeply from above 13 kHz. HF1300 attenuation is achieved from a tapped inductor.
Third order for the super tweeter. If anyone has other BC1 schematics I'm interested.


Crossover layout.


BC1 crossover board.

Measurements
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Frequency response of individual drivers driven from crossover. The midbass in an example to follow. HF1300 tweeter has a dip at 3.5 kHz - or is it a bump at 2.5-3 kHz? Read below.
The Coles 4001 super tweeter does have a lot to say in the upper treble region despite a 3rd order crossover around 13 kHz.

Frequency response of individual drivers measured at 1 metre distance. Red = midbass, blue = HF1300 tweeter, green = Coles 4001 supertweeter. Yellow = combined response with crossover.

The midbass displays an overall smooth response with no serious cone break-ups.
HF1300 tweeter has a deep dip around 3.5 kHz, which is caused by the cabinet lips protruding some 15 mm in front of the front panel. See graphs below on HF1300 response from flush mounting.
Coles 4001 (green) seems to have a much too low response to be able to level with the other drivers and I'm not sure why there is a discrepancy from the responses with and without crossover attached. I'll have to repeat these measurements to see what's going on. The 4001 has a 23 ohms DC resistance, which counts for its apparent very low sensitivity.

The combined response of drivers with crossover connected is remarkably flat. This speaker is more than 20 years old and still has a plus/minus 1 dB frequency response in the entire midrange and up 3 kHz. Excellent! The HF1300 is basically flat with a slightly raised level up to 13 kHz. Had the 4001 supertweeter not been placed near to a 15 mm cabinet edge fillet, I believe the response here would have looked better.


Frequency response of individual speakers. This is an example to follow, a truly matched pair of speakers.
The difference in response throughout the midrange and lower treble is as low as can be.


Frequency response plus/minus front grille. Red = incl. front grille.
I cannot help admiring how remarkably flat the whole midrange, lower and upper treble is from this speaker.


Impedance of system as-is, i.e. port with foam lining. This picture differs a bit above 3 kHz
compared to the HiFi-News profile seen below. There may have been changes to the tweeter
crossovers along the way. With a minimum impedance of 9 ohms and modest phase angles,
the BC1 is indeed a valve friendly speaker - that is, if we're not talking 5 watt SET -
or even 10-20 watts SET. 35 wpc PP amp seems more like it.


Impedance +/- foam in vent. Green = no foam.


HiFi-News impedance measurement for comparison.


Nearfield response of bass driver. Red: as-is, blue = no foam in vent. Green = closed box.


Nearfield response of bass driver with foam in vent (red) and no foam in vent (blue).
Purple = nearfield response of port with no foam in vent, green = with foam in vent.
The port response is remarkably free of any peaks higher up in the midrange. Quite unusual.


CSD, cumulative spectral decay. The midrange is an example to follow in terms of lack of resonances.
The "shoulder" at 2.5-3 kHz is derived from the HF1300 and something is going on around 13 kHz
too where the HF1300 crosses over to the Coles supertweeter.

Spendor BC1 interior. Very rare to see such a degree or order and overall quality design. There may be minor improvements from up-grading crossover components.
Everything in this design appears well considered. There are no resistors in series with any of the drivers. Tweeter attenuation is achieved from a tapped inductor. However, the tapped inductor only has one outlet, thus, you are stuck with the chosen degree of attenuation and the overall balance of this speaker is fairly bright - to my ears.


BC1 tweeters
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Coles 4001 supertweeter

Coles 4001 supertweeter. I still have to measure this fellow flush mounted on a panel without 15 mm cabinet edge fillets in the neighbourhood. The wires from the voice coil are extremely thin and I did not dare unsoldering the wires from the soldering tags on the rear to remove the diaphragm and add a thin felt pad on the polepiece. I wonder why this driver has a 16 ohms rating having a DC resistance of 23 ohms.

Celestion HF1300 tweeter


I've never seen a tweeter like this one before. It has a rather high resonance frequency:


Impedance of drivers in cabinet. Red = BC1 midbass, blue = HF1300, green = Coles 4001.
The Coles has an impedance around 23 ohms!
Generally all drivers have a high impedance profile; good for valve amps.


HF1300 plus/minus "turbo" grille. It's quite remarkable that the grille is able to focus the energy the way it does.
Red = minus grille. Blue = plus grille.



Above the HF1300 disassembled. The diaphragm is some resin impregnated fabric material with a very rigid suspension.
The suspension is very wide and a similar feature is seen on modern tweeters by Vifa and SEAS.

I recently came across a description of this tweeter in Martin Colloms' excellent book "High Performance Loudspeakers", page 219:
"An unusual and highly successful example of a hard dome HF unit is undoubtedly the HF1300 family, manufactured by Celestion (UK). Variations of this design has been produced since the mid 1950s, and have been used in a number of systems. Almost all of the details of its construction have proven to be critical, these range from the particular grade of cured phenolic impregnated fabric used for the diaphragm, to the spacing of phase correction plate. The centre dome is conical, about 19 mm in diameter, and has a shallow, rather broad surround actually larger in area than the dome itself. The unit as a whole has an overall diameter of 38 mm. The surround is in fact the main radiating element; the piston operation holds to be beyond 15 kHz and the diaphragm is particularly free of hysteresis effects. This, in conjunction with its relatively uniform axial frequency response, accounts for its unusually favourable subjective qualities.
The diaphragm's intrinsic pressure characteristic shows a peak at fundamental resonance (1.7 kHz), which is damped by restitively controlled air vents to the cavity within the closed magnet. The centre pole has a conical profile to closely follow the contour of the underside of the dome, this placing the first rear standing wave mode at a very high frequency. The output of the naked diaphragm falls above 7 kHz or so, and to correct this a perforated front plate is fitted, formed so as to follow a similar contour to that of the diaphragm. This results is a damped resonant cavity loading the diaphragm, and also provides a delay path between the dome and the surround radiation. The output is uniformly maintained on axis to 14 kHz. Good examples may demonstrate a +/- 2 dB characteristic from 2 kHz to 14 kHz, with a range 4-12 kHz held within +/- 1 dB."

Great! Finally a thorough explanation on how the HF1300 works. The purpose of the phase plate is well demonstrated in the FR graphs above.

More info on the HF1300 from J. Bright in Australia.
"Love the site especially the vintage and British Spendor stuff. I have a pair of HF1300 and I have some additional information. As I understand it they were originally the 'compression' driver for a horn loaded treble unit. This was to be used with a very early METAL coned unit (8"?). It was around the mid 1950's and it was produced by GEC and the designer associated with was a person called "Brittain", I think, but I'm not absolutely 100% clear about this, but I have a pretty good memory. I used to own a copy of Gilbert Briggs book "Loudspeakers" (5th edition 1958) and there is clearly an early HF1300 in a photo of a collection of tweeters and I am pretty sure it is identified as "A GEC pressure unit". I guy in England called Royal told me that the tooling for the HF1300 was sold by Celestion to Coles who also now make the 4001 after presumably buying the rights for them from STC. If you are into speaker trivia the following might interest you also. I believe the 4001 super tweeter began life as the diaphragm in a STC microphone! You'll find both the tweeters on the Coles Acoustics website but they have developed a new metal mounting plate for the HF1300. But I'm sure it is the same mechanism behind the front frame. I also have some data and a paper on the 4" Jordan-Watts modules (I own a pair). I'll send you a copy if you are interested."
Thanks a lot for the info from Mr. Bright.


Flush mounting of HF1300 by a simple piece of cardboard.
This improves the SPL response significantly. See below.


Red = "as is", blue = flush mounted driver.
Sometimes you wonder about the design criteria behind a certain set-up.
The dip at 3.3 kHz would be visible also with the measuring equipment used at the time of construction.


BC1 midbass
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BC1 midbass driver. A small 8" driver fitted with a bextrene cone applied a decent amount of coating to both sides.
This driver has a ceramic magnet where older types would be fitted with alnico.

The coating applied to the midbass drivers has been repaired on both drivers. I think the only long-term problem with the BC1 speaker is the midbass coating starting to peel off due to use, sunshine or whatever.
However, the coating on the rear of the diaphragm is in perfect condition and on the front it has been repaired with some slightly sticky stuff that may actually be better suited for the front, being more flexible than the original coating. Whether a new coating may produce the same result as the old coating is very unlikely, but in this case it's only some 2-8 mm along the rubber surround and from the SPL readings taken on the bass driver alone, there does not seem to be anything abnormal going on.

I'm pretty sure it's impossible to get the formula for the coating used on the drivers. So, if you are looking for a BC1 speaker, require some very good and sharp photos of the driver membranes near to the rubber surround. On the web I've seen the suggestion of the BC1 midbass driver being replaced by a Vifa 8" paper cone driver, but I would never do this. This is likely to totally change the sonic character of the BC1 speaker. Not even a polypropylene driver may come near this driver. Not that it necessarily would be worse, but only different from how the BC1 was intended to sound.

TS-data:


Left: TS-data for one of the BC1 midbass drivers.
Right: Measured data seems well in accordance with data generated from crosscalc in LspCAD.

Box Simulation


Well, this driver is not particularly well suited for a vented cabinet.
From pure math a BB4 cab of 105 litres is suggested. Won't work.


Not even a closed cabinet is easy:
0.707 Butterworth = 111 litres
0.8 Equal ripple response = 66 litres
The 38 litres cabinet with the vent closed will provide an approx. 0.95 Chebychev tuning.