Advertisement
Figure 10
As an example of how far this sort of thing can be carried, figurell uses two 4711 mixers to allow
any one (or combinations) of 6 voices to be knob selected.
By bringing up the level on inputs 1 or. 2 of mixer #1, either the triangle or square wave output of
the oscillator may be selected.
Panning the selected input channel to either the left or right routes
the signal through either the low pass or band pass filters.
At mixer #2, bringing up the level on
inputs 1 and 2 can select either the triangle or square wave before they have gone through any
processing.
The level controls of channels 3 and 4 of mixer #2 should be used to select the output
of the modifier path being used.
The modifiers at the output of mixel` #1 don't have to bc filters.
They could be balanced modulators,
Figure 11
VCA's or other processing elements.
Similarly, the signal sources can be from separate oscillators
or noise sources and up to four can be input to the first mixer and selected as required.
Finally, the
channels of the mixers don't have to be either on or off and many pleasing results can be obtained by
varying the percentage of each input sigrial that goes through each modifier group.
SOME THINGS TO WATCH OUT FOR
There is very little that you can do to the 4711 Mixer to damage it but there are some configurations
of the module that will produce unpredictable results.
Be careful of feed-back loops external to the mixer.
Some interesting sounds can be created by picking
up a signal from near the output of a patch and mixing it back in near the input but this can lead to
oscillation of the mixer.
For example; figure 12
shows a mixer patch that will definitely oscillate.
Here the right channel output drives one of the inputs which is panned fully left while the left output
drives an input panned fully right.
This turns the 4711 into a very slow astable multivibrator with
15 volt square waves appearing at both outputs.
=---11 -=
Figure 12
Within a single synthesizer system a common ground should be est,ablished through the power
supplies and because of the low output impedances of the processing elements short signal runs
(less than 2 ft. ) can be made without using shielded wire.
But if the signal source is external to
the synthesizer (and the synthesizer's ground system) co-:ixial shielded cable must be used for
the input connection.
Similarly, shielded co-ax. must be used for the final connection between
synthesizer and amplifier.
DESIGN ANALYSIS
At the heart of the 4711 mixer are two integrated circuit operational amplifiers, Icl and IC2.
The
configuration of these amplifiers is as simple summing circuits as shown in figure 13.
Because of feed-
back the negative input of the amplifier is at a vil.tual gI.ound and since there is no voltage variation at
this point there can be no interaction between the two inputs ei and e2.
Gain in this configuration is inversely porpol`tional to the amount of output sigml that is fed back to
input. Master gain controls R43 and R44 (figure 16) increase the gain of the amplifiers by attenuating
the amount of signal that returns to the input as feedback.
®o - el + e2
'3
Advertisement
