RE: Thanks.

OTL Asylum

OTL, Output Transformerless Amplifier User Group.

RE: Thanks.

Posted by Lew (A) on December 25, 2013 at 07:45:32

In Reply to: RE: Thanks. posted by AJ on December 25, 2013 at 01:52:12:

If you read on the SLOG website, you may find where I and a few others have made a major mod to the "backplate" of the Sound Labs that to my ears is revolutionary. The idea was not mine; it was brought to me by email from a guy in Australia. We removed the hi-pass crossover and the toroidal treble transformer from the backplate and replaced the RC network and the toroid with a full-range 1:90 EI type ESL step-up that is made in Australia. Then we run the AU transformer essentially in parallel with the Sound Labs bass transformer. The only remaining passive element in the original crossover is a 1.5mH inductor, in front of the bass transformer. This and its own limitations sharply roll off the bass transformer above about 2kHz. The net effect is a huge upgrade in efficiency and transparency and elimination of a lower midrange suck-out (because the impedance used to dip to as low as 2 ohms somewhere between 500Hz and 1.5kHz, due to the R in the old RC network). Further Z of the speaker from 50Hz to about 5kHz never goes below about 20 ohms! It's at 8 ohms at 5kHz and rolls off gently from there. This makes the SLs easy to drive with a tube amp. I am quite sure that 50W from a good OTL or other tube amp is all you need to play LOUD.

Sound Lab countered this idea by a re-design of their toroidal treble transformer, such that now it goes down to lower frequencies and crosses over below the midrange, but they still use cursed resistors and more inductors. Nevertheless, their mods also improve the speaker, or so I am told. I am not tempted to adopt the new SL toroid, however, not at all.

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Follow Ups Full Thread

Topic - Roger Modjeski OTL - drummer 16:55:01 12/13/13 (20)

RE: Roger Modjeski OTL - tyu 05:16:09 12/14/13 (19)

RE: Roger Modjeski OTL - airheadair 18:19:56 12/14/13 (18)

Hi, Tyu - Lew 07:44:01 12/18/13 (16)

RE: Hi, Tyu - tyu 05:51:51 12/19/13 (0)

RE: Hi, Tyu - airheadair 22:36:40 12/18/13 (14)

Modjeski and Beveridge - Lew 12:23:49 12/19/13 (13)

RE: Modjeski and Beveridge - tyu 08:36:50 12/20/13 (12)

RE: Modjeski and Beveridge - Lew 07:22:15 12/23/13 (11)

RE: Modjeski and Beveridge - 6bq5 08:45:21 12/23/13 (10)

Dynamic range, Model 2 vs 2SW - Lew 11:25:28 12/23/13 (9)

Interesting.... - AJ 14:51:07 12/23/13 (7)

ESL panels in the Model 3 - Lew 07:07:24 12/24/13 (6)

RE: ESL panels in the Model 3 - AJ 09:41:43 12/24/13 (5)

Wow and thanks... - Lew 11:46:25 12/24/13 (4)

Thanks. - AJ 13:45:40 12/24/13 (3)

RE: Thanks. - airheadair 21:33:41 12/24/13 (2)

RE: Thanks. - AJ 01:52:12 12/25/13 (1)

RE: Thanks. - Lew 07:45:32 12/25/13 (0)

RE: Dynamic range, Model 2 vs 2SW - tyu 13:28:59 12/23/13 (0)

RE: Roger Modjeski OTL - tyu 03:31:22 12/15/13 (0)

Follow Ups

RE: Thanks. - Lew 07:45:32 12/25/13 (0)

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