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RE: older Sound Labs

Posted by Ralph (M) on May 6, 2021 at 11:49:47

In Reply to: Fairly certain that mine predate that modification.... posted by Simon Templar on May 6, 2021 at 11:02:01:

Have two different backpanels. The earliest uses EI core transformers and works great with MA-1s; IIRC they went out in the late 1990s.

The toroid version which replaced the first one can be driven by MA-1s after a simple mod. The problem is that the impedance of the Sound Lab is a moving target with frequency so a resistor is used to help set the crossover point. Modelling was used to design the crossover and it was off by a country mile.

But it turns out that if you just remove that resistor the speaker becomes much easier to drive. In most of the backplates I've seen the resistor is composed of a number of smaller 20 or 25 watt resistors; the composite resistor is a 200watt device which explains where the efficiency of the speaker went. By simply removing the resistor the speaker sounds better in every way and is easily driven by MA-1s. Sound lab says that the resistor prevents saturation of the high frequency transformer but in practice with the MA-1 or MA-2 that doesn't happen.

The Toroid 2 backplate reflects the design of the crossover using the correct impedance curve of the speaker and so has the same easy drivability.

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