Monitor 4, Amplifier, OTL powered

Read more on the Acoustat Monitor 4 in our home audio section

Geoff Burkhalter2004-02-23 22:21

Andy, I have a question about my OTL powered monitor 4\'s. On the amps there are 2 trimpots on the main board, what do these do? Do they adjust the sensitivity or time delay for auto shutoff? I find that my speakers will auto shutdown during quiet passages if the volume is somewhat low. This is the only that annoys the c*** out of me on an absolutely fantastic set of speakers.


Andy Szabo2004-02-23 22:21

I\'m no expert on the Servocharge OTL amplifier, but I\'ll answer as best I can. I\'ve heard of this pre-mature turn-off problem before. I checked the schematic posted on this site, and discovered the following items.

It appears that the two trim pots on the circuit board are for adjusting clipping symmetry, and therefore should not be touched. Special test equipment is required to look at the signal while making these adjustments.

However, Note #3 of the schematic lists several circuit value changes made over time. The only part changed in the turn-on/off circuit is capacitor C20, which was changed from 15 uF to 47 uF. If I read the schematic correctly, this will increase the time delay before shut-down. This capacitor can be a tantalum or electrolytic capacitor, as it is not in the signal path. Be sure to watch polarity (+/-) when you install the new part.

As near as I can tell, most of the other value changes involved improvements to equalization, and some perhaps for reliability. If you have an ”original” amplifier, it wouldn\'t hurt to look at some of the other changes, while you\'ve ”got the hood up”. However, I can\'t comment on their effects or merits. According to the schematic, Acoustat did offer a kit to change all of these values.

Can anyone else provide further advice on factory-authorized changes?

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