Model D 'RA's

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dr_k_schneider
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Joined: Sat Aug 30, 2003 1:05 pm

Model D 'RA's

Post by dr_k_schneider » Mon Sep 01, 2003 1:12 pm

i am looking at purchasing a early model D ( serial no 604 .. they think its an 'RA'. i hear the OSCs are slightly warmer.. the question is are they noticeably more unrelyable tuning-wise than the later revisions? is it worth going for this heat sink business? your collective experience would be greatly appreciated ..

cheers

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MC
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Post by MC » Mon Sep 01, 2003 10:54 pm

There were only a couple hundred RAs; if that one is in the 600s then it isn't an RA, more likely a Moog-Musonics.

Whether it is unreliable tuning-wise depends on the VCO card installed in it. There were three revisions of VCO cards. This one is early enough it might have the original one with all discrete component (zero ICs) but is the worse for tuning. By serial #1300 the second VCO card was fitted as standard, used 3046s and was better tuning wise. By serial #10175 there was a third VCO card with ua726s and this was the most stable.

New VCO cards were available for direct replacement so it is possible for an early RA to have a late VCO board. Heat sinks will not help tuning stability in a Minimoog, there are a lot of circuit corrections that will improve it. I'm an EE and I found a lot of corrections, and mine (RA #53) stays in tune really good.

Soundwise, the VCO cards all sound the same. I heard all three, all in early RA Minimoogs. What is different is the VCF board. The first few hundred Minimoogs had matched transistors pairs in all five pairs in the ladder filter; to reduce the labor in hand matching, they later only matched the top and bottom pairs.

Every single RA Minimoog I heard with the 100% matched pair ladder filter sounded the same - even with three different VCO cards. But when I heard the later VCFs with only the top and bottom transistor pairs matched in the ladder filter, the difference was immediate. Even those with the same VCO board as mine.

This is also the reason why one Minimoog sounds different from another - because the transistor pairs in the middle of the ladder aren't matched, that affects the pole positions in the frequency domain response of the filter.

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