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Original Message

(too long) history of Zeros and Sound Labs

Posted by Lew on October 25, 2004 at 07:50:37:
Well, let's see: For several years I owned a pair of U2s, driven by my present amps but prior to making several upgrades to them. My U2s originally crossed over between the bass and treble audio transformers in the backplate at about 1100 Hz (not ~550 Hz as is the current practise). Somewhere in there I bought the Zeros, and they dramatically improved the ability of my amps (Atma MA240s, ca 120W/channel; zero NFB applied) to drive the U2s, but the U2s sounded better yet when I upgraded the backplates to cross over at 550Hz (much better midrange, less midrange "suckout"). Somewhere in there I bought a pair of early Zeros from Paul. This further improved things with the U2s; less frequent blowing of fuses on the output tubes of my MA240s, used at about 2X. Then I sold the U2s and bought the M1s (with toroid, etc). I drove the M1s for a while without the Zeros, but they sounded better with. Then I upgraded to the factory "high impedance" crossover, and tried driving without the Zeros, sounded OK, can't remember whether I tried the Zeros with this set-up. Subjectively, I thought the overall efficiency went down with the "highZ" components in place, not up as one might hope and expect. Then I fell under the spell of Roy et al, and dumped the "high Z" components, altered the cross-over further by subbing a 1.5H inductor in series with the bass transformer in lieu of the several options in the factory set-up (where the "0db" bass setting gives you a 5H inductor). This sounded better than ever before but fuses were occasionally blowing, probably because I was loving the sound so much I was playing the music as loud as possible. Then I added back Zeros at 2X and have now moved up to 2.7X, best combo ever. (I also changed the value of the fuses on my 6C33C output tubes to 1.5A from 1.0A with Ralph's permission; now they never blow.) The current Zero is built to handle ~150W. I once queried Paul Speltz about building a version that can handle more power, e.g., 250 or 300W, but he has never responded.