Wharfedale OptionOne Speaker Repair
I was asked by a colleague to take a look at some special speakers that needed repair, I’m glad I did. Here is an example of an amazing rarity of engineering and design that you just don’t see every day. Word has it that only 8 sets of these were ever made. The attempt was to make the best sounding speaker, and perhaps they did. The cost of these would have been astounding, especially back in the 70’s when these were made.
Some notable feats of engineering here:
– Built in amplifiers in each speaker unit, 4 per side!
– Active electronic crossover, no passive crossovers
– Large bass reflex cavity for air for an integrated sub-woofer
– Dipole speaker arrangmeent
– Balanced inputs for low noise signals
– Cast aluminum base
– The amplifier design appeared to be unusual to me:
It’s essentially a Class-A and Class-AB combined… it’s a rare and innovative design called a “Class A Dumper.” It seems to have some real advantages over Class-A and Class-AB giving high efficiency and low distortion. Quad and Wharfedale apparently had a relationship do carry this out since the Class A Dumper is patented technology.
http://www.quad-hifi.co.uk/our-story.php?id=23
This type of amplifier design gets the best of both worlds, clarity of sound and efficiency. Pretty unique!
It’s difficult to see here but here are the guts found the in base, notably the 4 individual amplifier boards under the active crossover. A total of 8 TO-3 transistors.
For anyone interested, here is the scan of the manual.
In the end, the problem was traced to a faulty capacitor on the active crossover board. It had leaked and shorted one side of the power supply causing the “dumper” part of the amplifier to short the other side also. A tiny little electrolytic cap caused all the problems of blowing fuses – typical of vintage electronics.
And yes, they sounded fantastic and lifelike, amazingly wide sound field and full range.