In a slightly tangential move compared to the valve amplifiers I generally get to work on, I was approach by Phil, The Count, and asked if I could help with his Snell AII loudspeakers.
In the time before marriage I was lucky enough to have a pair of Snell AII speakers for about 10 years, during which I refinished the wood veneer, replaced the grill cloth and worked on the crossovers, mainly replacing the old electrolytic caps with newer Black Gate bipolar caps
Phil had a problem, one of the tweeters had died. He had hunted down the appropriate replacements and supplied a new pair along with the speakers. First step was the assess the current situation......
Main unit with grill removed:
Inside to uncover the crossover for the midrange and tweeter:
Note the Audioquest speaker cable which is noticeably green under the clear cover
Then inside the bass cabinet to uncover the crossover:
Unfortunately, the "chuffing" noise in the low bass was caused by the perished foam surround on the bass unit:
Not a great result overall, but for speakers originally built in 1983 they weren't doing too badly. The worst problems were the (1) dead tweeter, (2) perished foam surround on one bass unit, and (3) copper oxide cables
Having one fully working unit, a bunch of measurements were taken to give an understanding of the baseline situation before any work was begun.
Acoustic measurements were taken of all 5 working drive units, with and without crossovers.
The crossovers were all taken apart carefully, the circuit drawn out and every component individually measured using an LCR meter.
The crossovers were then modelled in LTSpice in order to understand how they worked, and what the slight differences between left and right did to the output of the filters.
Very interesting
First off, replace the dead tweeter, unfortunately this required a switch of face plate:
Then strip down the bass unit to remove the perished foam surround:
Which seemed to work out quite nicely:
The bass crossover was then upgraded: replace the old cable; replace the inductor with a low dcr unit; and film caps to replace the old electrolytics:
The reduced dcr should help re-align the bass to work more nicely with the valve power amps Phil favours, and the film caps help to really improve the clarity of the bass.
Then the big challenge, the midrange and tweeter crossovers:
The old electrolytic caps were replaced with a combination of new bipolar and film caps, the cement clad resistors replaced, and all the old cable replaced.
All went back together, and it was then a long and careful process of integrating the new tweeters into the midrange. The new units were far more sensitive than the old ones, so they needed to be padded down, and it took a lot of measure/listen/tweak/repeat
before the right balance was struck.
Definitely one of the more challenging projects, but very interesting, and hopefully a nice pair of speakers ready for many more years making music.
