opposed to dual PNP or Dual NPN). This was done because the converter is really a PNP-
NPN-NPN: Q1B and Q1D are both driven off Q1A. Why two exponential current outputs?
Well, remember the diode bridge likes to be balanced, specifically, it likes having the same
amount of current injected at the top as pulled out of the bottom. So, Q1D provides the
current sink at the bottom, whereas Q1B operates a current mirror made from Q5, which
provides a current source at the top. These identical source and sink currents are of course
the “control current” for the filter frequency. The “left over” transistor Q1C is used for the
Q control circuit as described previously.
Highpass Mode
The operation of the filter in highpass mode is essentially the same as for lowpass, save
that the input signal is introduced into the now series-capacitor (SW2 switches C12 from
being grounded to being used for series input) and the impedance element Q10 has the left
side fixed at a DC constant (again by SW2) of half the power supply voltage in order to most
efficiently use the bridge as a parallel (to AC ground) resistor. Series R with parallel (to
ground) C being the simplest lowpass model, it is reasonable to assume Series C with
parallel R is the simple highpass model. And such is the case: the filter now behaves as a
second-order highpass filter save for that region where the input signal is close to the cutoff
frequency, at which point it assumes a bandpass within the bounds of Q.
PRELIMINARY CHECKOUT & TROUBLESHOOTING
The first thing to remember: this filter will not self-oscillate — Blame Yamaha :)
The most common mistakes when building this filter are:
a) Make sure the 3 film caps are in the correct place. C10 and C11 have caps marked
“223” and C12 has one marked “473”.
b) Verify the 2 trim pots are correct. TP2 must have a mark of “P 203” and TP1 must
have the “P 202” mark.
c) Make sure C9 is the 3M3 non-polar cap
To verify operation of the filter, an oscilloscope is most helpful. But I’ll assume you don’t
have one for now (start saving, there are many good ones on eBay for $100). But you will
need a signal source, such as a MOTM VCO (you cannot plug a mic or guitar into the ‘485
directly, you first must amplify the signal).
a) Set both trim pots to the center positions (use a small flat screwdriver).
b) Set the top switch to LP, the bottom switch to F, RES to 0 and FREQ to 0.
c) Apply power, and apply an audio signal to IN 1. A VCO Saw at ~A440 is a good
starting place.
d) Monitor the OUT jack with an audio amp: turn the volume down and do NOT use
headphones!
e) Slowly rotate the FREQ knob towards 10. At first, the volume should be very low
because the VCF (if it is working) will be attenuating most of the signal. As you
rotate the FREQ knob, more and more of the signal should pass through.
SYNTHESIS TECHNOLOGY
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MOTM-485 ASSEMBLY 8/11/05
WWW.SYNTHTECH.COM
Содержание MOTM-485 GX-1 Diode VCF
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