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Comtech 400-- How to Boost Internal Slew Rate?

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I know that modding these amplifiers is often frowned upon, however I was wondering if anyone has had any success in modifying the Comtech 00/10 line in order to boost the slew rate. These amplifiers have their slew rates limited to around 17 v/us in order to protect against RF and ultrasonic interference, however in a home environment this type of interference is unlikely. Many other amps from the same time period (the PSA-2 for example) have much higher slew rates (around 40 v/us).

This leads me to believe that with a couple modifications that it would be relatively easy to "unleash" higher slew rates on these amps and reduce Intermodular Distortion. Comparing the PSA-2 to the Comtech 410, the PSA-2 offers a TMD of about .01% while the Comtech offers a TMD of around .05%! I am convinced that the slew rate limitations are one of the major differences between the PSA-2 and the Comtech line of amplifiers. I could be wrong, however many of the older Macrotechs also seem to have their slew rates limited for the same reason.

I tried replacing the opamps in my Comtech with much faster opamps (20 v/us-- new LME49740 vs 7 v/us-- old MC33079), however there seem to be other parts of the circuitry that are incompatible with the faster opamps. When I try to run the faster opamps, I notice a medium-pitched hum coming from my 2 speakers that wasn't present with the old MC opamps.

If not one else has any idea or recommendations for modifications to make, I will eventually get around to purchasing the proper analytical equipment (a scope and frequency generator) in order to narrow down the other slew rate limitation(s) in the Comtech circuitry.

Comments appreciated!


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