DRIVE UNITS BY MANUFACTURER:
SPH-135C
DISCONTINUED BY MANUFACTURER - SOLD OUT.
The SPH-135C is a versatile drive unit from the Monacor 135 series featuring a woven carbon fibre cone, vented pole piece and diecast chassis. When mounted in correctly aligned cabinets this long throw unit is capable of reproducing bass that is surprisingly deep and accurate - ported cabinets of around 14 litres can give an in room response extending down into the upper 30s (Hz) which is excellent for a driver of this size.
A fine replacement unit for a wealth of mainstream designs requiring a high quality 5¼ bass driver such as Rogers, Castle, Mission etc. Full details and dimensions can be found on the downloadable datasheet.
| Resonant frequency (fs) | 40 Hz | Nominal impedance (Zn) | 8 Ω |
| Mechanical Q factor (Qms) | 4.48 | DC resistance (Re) | 6 Ω |
| Electrical Q factor (Qes) | 0.43 | RMS power handling | 50W |
| Total Q factor (Qts) | 0.39 | Sensitivity (2.83V/1m) | 88 dB |
| Equivalent volume (Vas) | 17L | Chassis diameter (pincushion profile) | 133 mm |
SPH135C, SPH 135 C
Thanks for the review Graham !
Will & Caroline
Great update to the original Castle drivers which started to crack anround the edges after many years of service.
More bass response and energy when playing rock tracks.
Would note that driver surround is slightly larger than original.
Thanks for the review Ian!
Will & Caroline
Ordered two of these to replace the original Castle IsIs units as over a period of 35 years the polypropylene cones had gone brittle due to UV degradation, a common occurrence, units arrived in quick time and came securely packed, they took 20 minutes to solder in and slipped straight in, they sound better than the original units as they develop quite a bit more bass something the originals were very light on, very good build quality, and most importantly, a very good sound quality I’ve just acquired a a pair of Castle Avons for £60 that need the same upgrade, I will certainly buy these again from Willys very competitively priced with a quick efficient service 10/10
Thanks very much Willys HIFI
Thankyou for your review !
Will & Caroline
Friendly technical support
40+ years in the trade
UK family run business
Excellent customer service
Quick dispatch
The crossover was constructed well, I just need to figure out the best way to attach it to my Kef Concertos as they are bigger than the ones supplied by Kef that are mounted on the front of the baffle board.
Very nice deep bass look amazing with my custom made speaker
Great speedy delivery and very easy to fit. The plastic in my drivers had perished and they had suffered one too many parties! No need to make any adjustments (someone above did suggest that). I just screwed them in and the foam compressed into place. Before and after below (before I changed speaker number 2). Thank you!
Thanks you very much for the kit. I found back my kelidh after nearly 27 years. The filter were really dead and one tweeter damaged. Very happy to decide to repair/ upgrade rather than change. I found back tone and stereo stage like in the old days probably even better.
I didn't know soldering material was so important thanks for providing some.
Quite easy to recap filter and change tweeters.
Also wanted to thank you for this little note hand writen in French just for me. Very nice attention to see.
Next step is the next question do I change first the drivers with the monacor as I ve lost dust caps but they still work properly or do I change my old arcam alpha 8. Is there really something to gain moving to an newer arcam sa10 or Radia a5? Are newer amplifier much better than the old days one?
In addition to my post from 17/11/2025, here is some information on the final status of the project: Saving the Mission 752 loudspeakers.
Once it was certain that it would be possible to revise the loudspeakers (thanks to Willy), the desire to also replace the low-midrange drivers and convert the crossover to a high-quality version quickly arose. I was fortunate enough to be able to purchase two Burmester 949 MKII midrange drivers from Audax to replace the existing low-midrange drivers (52LFHDA – still available and can be sold). They are practically identical in design, but significantly more refined and powerful. Revising the crossover proved somewhat more difficult because the values of the coils were not printed on them. However, they can be determined by measuring the originals. Mundorf resistors, coils and capacitors of the two highest quality grades were installed. These are not exactly cheap, but once the speaker is finished, the difference is clearly audible. The crossover was also modified slightly. A shunt was integrated into the high-frequency branch. Following a tip in a forum, the crossovers were also moved to the outside. They are currently still on the speaker without a housing. The metal housing provides perfect shielding. As the Willys tweeters are very efficient, they were ‘damped’ slightly. The final cost of the conversion was approximately £600. In return, the speakers now perform at a level that far exceeds their original condition. An exciting project that was only made possible by Willys – thanks to Caroline & Will ❣️
