MOTM

I second this approach.  I'd rather have a good audio mixer that does DC than a
hugely accurate DC mixer or a very expensive mixer that does both.  The voltage
summers on the inputs of the modules should do the accurate DC mixing. When I'm
mixing control voltages through a mixer, the offset and drift are usually not as
important because I'm, well, mixing up a bunch of control signals and probably don't
care as much about 1V/oct scaling, etc.  Besides, a little drift and non-linearity
can be very creative now and then...

Eric


--- Tony Allgood <oakley@...> wrote:
> 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
> 
> 
> 
> 


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