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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 30th Nov 2008, 12:41 am   #1
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Default Quad-powered sub-woofer

I have successfully restored to daily use a Quad II valve audio amplifier and preamp, bought two years ago for under £20, together with a Tannoy Monitor Gold 12" speaker which I inherited from a friend's late father. Normally, this setup is used to play my collection of 78s, but I have also created a sub-woofer function to use when playing my CD collection of cinema organ music.

I constructed a simple 2-stage passive low-pass filter, with a cut-off frequency of 10Hz and a 6dB/octave roll-off. This is fed directly from one speaker channel of a relatively elderly Sony hi-fi system into one input of the Quad QCII mono preamp, and enables me to boost the really deep bass. The Tannoy speaker is mounted in a relatively basic sealed corner enclosure tucked away at floor level in one corner of my workshop.

At normal listening levels, the sub-woofer output is less than 1 watt, and the location of the bass speaker is not noticeable due to the non-directional characteristics of such low frequencies. The results are dramatic! Wurlitzer organ music sounds so realistic. Tonight I made a few rough measurements using a signal generator, and I am getting smooth bass response down to below 30Hz, no doubt due to the frequency response of the Quad II (20Hz-20kHz + or - 1dB) and the very compliant suspension of the Tannoy speaker.
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