adding vibrato to bank of 921b's

[rarecomponent]

I'm struggling a bit with adding vibrato effect to my 921b oscillators .
I used to take a low sine wave from a dot .com module and then route it thru a 995 attenuator into the frequency input of the 921a but I found that tuning became unstable with this method. With various degrees of vibrato the pitch would alter.
Going straight into the ac input of a 921b was far better regarding tuning but then you have quite a few patch leads to contend with when using three oscillators.

Is there a better stndard method for adding vibrato to a bank of 921b's ?

[analogmonster]

Does your sine wave contain a DC offset? If so this changes the tuning. The sine wave must be symmetrical to zero if used for vibrato.

[Rarecomponent1]

Thanks for this....I imagine this could be causing the tuning anomaly.

So.......I'm using the Dot .Com VCO with a low sine wave output as vibrato signal control ...how would I determine if it has DC content ( volt meter? ) ...and eliminate it or is it 0 symmetrical? (Oscilloscope?)
I didn't realise a low sine wave had complications.....
Also....is feeding it to the frequency input of the 921a the established procedure.?

[analogmonster]

Thanks for this....I imagine this could be causing the tuning anomaly.

So.......I'm using the Dot .Com VCO with a low sine wave output as vibrato signal control ...how would I determine if it has DC content ( volt meter? )

No, I wouldn't rely on a voltmeter because AC measuring is often caibrated to frequencies of electric supply networks (50 or 60 Hz) and the results on other frequencies are often unpredictable. I recommend to have a closer look at the scope.

...and eliminate it or is it 0 symmetrical? (Oscilloscope?)

To eliminate DC offsets of low frequencies I use a DC mixer and another voltage of -(DC offset). A big cap might do the job also, depends on the LFO frequency.

I didn't realise a low sine wave had complications.....
Also....is feeding it to the frequency input of the 921a the established procedure.?

I would do this that way, too. LFO Modulation of the 921Bs might be interesting if you use different LFO frequencies for each. The original 921B has an AC coupled and a DC coupled frequency modulation input. An AC coupled input has an input capacitor which eliminates DC offsets (470nF in an original 921B) but can be used for higher LFO frequencies only for the same reason.
The original Moog 921A's frequency modulation input is DC coupled, so an LFO sinewave with DC Offset causes detuning.

[Rarecomponent1]

Thanks Analog....I understand what's going on now ....it was certainly causing me a problem.