Today I received an e-mail with a rather interesting question; a MKS-70 owner, uses upper and lower voices as two separate instruments and suffers with crosstalk. He asked, would Nebula reduce crosstalk. Here’s my answer:
Coming off each MKS-70 voice-board, there are four wires; left audio / earth and right audio / earth. There’s nothing connecting the two voice-board audio paths at this point.
The two set of connections (eight wires all together) come on to the original jack-board. Here, Roland implemented an ingenious passive switching system which automatically directs appropriate signals depending on which jack sockets are accessed.
The four audio signals are combined passively on the original jack-board, to provide a stereo headphone output and a mono output. This passive mixing network is where there is potential for crosstalk.
For balanced operation, the jack sockets on Nebula are of course, 3-pole and not 2-pole as per the original jack-board. These sockets don't have internal switching like the 2-pole equivalents and so I simply wasn’t able to emulate Roland’s clever automatic output selection system. Using active components and taking advantage of the original level-selector switch cut-out in the rear of the MKS-70 chassis, the outputs of Nebula are switched manually from stereo to individual operation. On top of that, instead of tapping and mixing the signals for the headphone output mix from the individual signals from each voice-board via resistors (R17 - R20 in the diagram above), Nebula uses op-amps. As such, Nebula's active design specifically at the headphone mixing stage, delivers a hidden benefit; crosstalk resulting from a passive switching mechanism and passive combining of signals is eliminated!
I should mention that similar to the original Roland jack-board, the mono mix signal is derived via passive mixing of left and right signals but only after Nebula's headphone amp. Hence, no crosstalk will be introduced to the balanced individual outputs from this stage but there may be some crosstalk between left and right channels in the MKS-70's on-board headphone output.
Nebula uses dual op-amp devices so theoretically, crosstalk won’t actually be zero as even the best dual op-amps will have some leakage between internal channels. With only one stage in each signal path however, crosstalk due to the use of dual op-amps will be quite negligible. You’re probably aware that dual op-amps are used extensively in audio electronics, anyway.
Each of Nebula's four main outputs, are driven by separate balanced line drivers. These devices are single-channel and other than power and earth, there's no other (common) connections between them. Hence, there won’t be any crosstalk introduced at that stage.
As mentioned, signal switching between stereo and individual output modes on Nebula is done manually. The output mode is selected via a switch on the rear of the MKS-70 that previously occupied the output level selector switch. This switch in turn, operates a pair of relays, the isolated copper contacts of which, means that there won’t be any crosstalk here, either.
So to answer the question; would Nebula reduce crosstalk when using upper and lower voices on the MKS-70 as two separate instruments? YES, I think it would.
Service notes for the Roland MKS-70 can be obtained here: https://www.synthxl.com/mks-70/ but PLEASE leave a donation.