MOTM

Op-amps for mixers...

OK I specify the TL072 in my multimix. Its OK for audio, and less than
perfect for CV. But still usable in most if not all situations. I would
be glad to hear of any problems that people have had regarding the use
of this unit. But you can use the OP275, great for audio and IMHO good
enough for pitch CV use. The other one to think about is the dual OP-27,
(OP272??)

But I would hasten to add why we really need to be 'hugely' accurate at
DC for an external mixer. This only really applies for VCO pitch. The
MOTM VCO has a 1V/oct input which are used for the pitch CV already.
Presumably we need to mix other CV sources to our VCO through the two of
the other potted inputs. The mixer performance that we need to control
is therefore offset drift. I don't think absolute offset is that
important unless you are feeding the mixer into a VCA then the VCO's
pitch control. In most cases this could be nulled out anyhow. So for
offset drift this will only be apparent if the op-amp does drift badly
with temperature, and the VCO's input pot is set to its maximum
senstivity. It is best then to use your mixer pots over their fullest
ranges and set the VCO input to the lowest setting to get the sound you
require.

Another issue for mixers, particulary at DC is the gain. This is worse
to set up accurately in a reversible attenuator system. In other words
accurate calibration of +1 and -1 settings.

Regards,

Tony Allgood  Penrith, Cumbria, UK

Modular synth circuits, TB303 clone and Filter Rack
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan