C15,Q10 in Neovibe R.G.

[ohmelter]

Hello. I'm looking over the Neovibe schematic and an original univibe schematic. I noticed on the Neo C15 is in series with the base of Q11. However, on the original schematic c15 or whatever it's corresponding number is not connected to R28 or the base of Q11. Instead it's connected to + C16 and R32. What's up?

Anthony

[R.G.]

Hello.

To the best of my recollection, it's an inaccuracy in the original schematic. Manufacturers sometimes did that, intentionally or not.
It's been a long time (about 1994) since I did the original tracings. As I best I can remember, I found that the boards did not match the "original schematic" there. I may have my original notes somewhere.

On a technical note, changing the C15 connection from the base of Q10, where it matches the structure of the other three phase stages, to the junction of C16 and R28 effectively dumps the high pass side of that phase stage input into a low impedance point on Q10, losing any signal that was present. This is particularly true since the cap side is the high impedance side of the equal-load phase splitter used in the univibe. You might as well remove C15 as connect it as in the original schemo. Connecting it to the base replicates the other phase stage inputs and is theoretically correct.

But the next guy in here will say "But theory isn't everything!!!" OK. I went back and simulated the circuit again. With the original schemo, it removes one of the two phaser notches. With it as in my schemo, it adds the second notch back and as many people have written about in building clones over the last decade, sounds like the original.

So if you want the original sound, build it to my schemo. That's what's up.

[ohmelter]

Hey R.G. Thanks again for the info.
I originally had C15 connected the "in-correct way" for about three years. I removed, and attached it to the base of Q10 and the "choral/phase/vibe" quality is noticeably improved. A bit more lush sounding.

Anthony

[R.G.]

Yeah, it confused me when I went through that first. The connection to the base has to be the right one theoretically, and the boards show it is, but a factory schematic showing an incorrect connection is a powerful misdirection.