[sdiy] Digital audio demo on Youtube

[Neil Johnson]

Hi Brian,

> Are you absolutely certain that the 74HC4051 is not followed by op-amp
> buffers with capacitors on the inputs, making them S&H stages? I don't have
> the Roland R8 schematic available in my collection, but the 4051 is often
> part of a S&H mux. If the R8 does return to zero while the other channels
> are processed, then I'd like to take a look at how they implement that
> return-to-zero circuit.

Yes, it does, because I *have* looked at the R8 schematic.  You can
find it here:

http://www.synfo.nl/pages/servicemanuals.html

Under "Roland", both the original R8 and the later R8 MkII.
The output stage is on page 11 of the R8 service manual, and page 9 of
the MkII version.  There are minor differences in the filter component
values, but the basic structure is exactly the same:

PCM54 DAC -> 74HC4015 -> 2k7 pull-down -> 3k3 input of inverting LPF,
then into a 3-pole Sallen-Key LPF.  Nowhere is there the usual S&H
structure.

Also on page 5 (6 in MkII) is Figure 1 which shows the timing of the
outputs, and clearly shows the output of each channel of the 4051 as a
sequence of short pulses.

Neil
--
http://www.njohnson.co.uk