Bass/Midrange Drivers
 

I am looking to replace a faulty pair of Spendor BC1 Bass Units. Under consideration are the following;
1)Visaton W200S 2)Monacor SP90 3)Vifa C20WS

Anyone out there have any opinion as to which of the above are the most suitable, or any other suggestions, and would any mods. be needed to the crossovers?

Re: Bass/Midrange Drivers
 

Hi,

Less opinion but more mathematics - the cabinet volume will have been calculated based on the characteristics of the original drive unit used.

Armed with the Thiele-Small parameters for the drivers under consideration and the volume of the cabinet you could calculate frequency response curves for these drivers in that cabinet, then decide which, if any, will work. It should be possible to do this with spreadsheet software, though I've never tried it; I used to use a rather old bit of DOS software for designing speaker systems but haven't done this in a long time.

There should be some stuff on the 'net concerning speaker design; you basically need to do pretty much the same things as a speaker designer calculating a cabinet volume to match a specific driver. The difference is that you're starting with a fixed cabinet volume then selecting a driver which matches the cabinet.

Regards, Kat

Re: Bass/Midrange Drivers
 

Hi,
I seem to recall that the mass and area of the driver work like a weighted piston driving the cabinet like a Helmholz resonator. I also seem to remember that the basic trick is to match the natural resonant frequency of the driver to the resonance of the cabinet when coupled to the driver mass and area. The close coupling results in a broadening or even a double-hump splitting of the resonant frequency of the speaker/cabinet combination. Damping material then smooths out the result. So, in the absence of proper mathematics and physics, choose a driver with the same resonant frequency as the Spendor unit and, if possible, the same mass. My guess, from the original BC1 response curves is that the driver had an open air resonant frequency of about 70 Hz, but that is pure speculation.
Nigel

Re: Bass/Midrange Drivers
 

Thanks, Kat and Nigel,
I will bear what both of you said in mind. Have downloaded quite a bit of info about the BC1s so no doubt in there somewhere is the resonant freq.of the original drivers, plus the other data I need. One thing I do know is that the DC resistance of all three speakers I'm considering is ony abt. 6.3 Ohms, whilst that of the originals is 8.3 Ohms

Re: Bass/Midrange Drivers
 

Have you asked Spendor if they can still supply replacement drivers?

Re: Bass/Midrange Drivers
 

3/10/06 6PM Yes, Spendor have already informed me that none are available for this 30+ years old design.

Re: Bass/Midrange Drivers
 

I haven't used my BC1s for some years. I hope they are still OK, sittingin their boxes in the loft. The suspension in the bass units has a tendency to sag. There is a cure but I have not yet had cause to try it. Use them upside down until the suspension has sagged back! I don't know how long this lasts or how often you can get away with it.

Re: Bass/Midrange Drivers
 

4/10/06
I've already inverted them, Jeffrey. One is much improved, whilst the other still distorts & breaks up if driven hard. As you say there's no knowing how long such a 'cure' will last. These are over 30 years old so I guess new drivers are called for!

Re: Bass/Midrange Drivers
 

4/10/06
Steve - thanks for the info. I will make a note of Wizard distributors' address, but the roll surrounds are OK on my BC1 drivers-they aren't foam in any case.

Re: Bass/Midrange Drivers
 

Wizard Distributors, now of Lymm, Cheshire, can supply roll surrounds ....... http://www.wizard-distributors.co.uk/ ;)