Wednesday, June 1, 2011

Oscillator Drift Iteration

Ye Olde Oscillatore Drifte question...

The Buchla 258B, with the 2N3802 dual transistor in a single package and the uA726 heated transistor pair (again in the same package/substrate) provide a very decent amount of pitch stability without going into the land of totally uninteresting. I wanted to see what component variation would cause/effect.

This especially centers around the removal of any dual transistors in a single package. Using modern transistors, matching isn't terribly hard for the 1% and better range, but not having them in a single package/substrate provides for less overall accuracy. Having them physically contacting each other with heat grease can improve things but in some designs such as the Moog 901a/b, these transistors are not placed close enough to each other to allow for physical contact.

Individual transistors with no physical contact per pair seems very important.

-The 726 and 3802 I have replaced with selected BC550C and 560C and of course, find myself in maximum driftland. Adding a TEMPCO noticeably reduces this but does not eliminate it. Might be nice to have a single module set up this way. Didn't even bother to heat grease them together to see if slightly less drift could occur.

-Tried a TEMPCO plus uA726 plus 2N3802 and it powers up and STAYS there. "No, too much!" -The Who, "Magic Bus", "Live at Leeds"

-I note that the J3RK/Verbos 258 design using the THAT340 is stable enough without a TEMPCO. I hope to have a future iteration of the 258B design using a THAT340 to replace both the uA726 and 2N3802. I would have pads and components added so that you can use the vintage parts instead, if desired.

The J3RK design uses the second half of the 340 IIRC to replace the 2N3565s. A tech tells me that the THAT has a quarter or so of what is needed to correctly operate in place of a 3565 pair...could be interesting to compare how 3565 and THAT versions sound/look on a DAW.

-There is indeed a certain solid "feel" to the uA726; it grabs the pitch nicely but not absolutely.