Moog Moogerfooger MF-105M MIDI MuRF Manual

MIDI MuRF Analog Board 11-487
Theory of Operation,
Note: this circuit description refers to the REV A schematic dated 05/19/09.
SHEET 1
EFFECT_IN is the audio signal taken after the bypass switch on page 6. The input network
C5/58-R5-R6-C4 provides DC isolation, radio frequency filtering, and protection of input
amplifier U3A. The output of U3A is the signal AUDIO, which is the amplified audio input
signal.
Negative feedback around U3A goes through U2-A. U2 is a VCA whose gain is controlled by
the current through Q1, which is the sum of the current through R11 and D17/R121. U3B is a
sample-and-hold buffer with level and offset adjustment. The output of this amp determines the
voltage across Rl1.
U14B and associated components are used to buffer, isolate, and attenuate the audio signal from
the audio bus. It is then fed to the MPU to be used as a 0-2.4V DC reference signal for the
compandor circuit. D3 conducts when the peak voltage at U13A is higher than the
COMPANDOR voltage. The COMPANDOR voltage is held by C30 and a capacitor on the
digital board, where it is scanned by the microprocessor. The purpose of the COMPANDOR
voltage is to determine what the highest peak voltage is at the input to the filters. This enables
the microprocessor to determine the status of the overload LED, and perhaps cut down the gain
of the input amplifier to reduce overload distortion.
UI2B is a sample-and-hold buffer with level shifting and gain. Its output is the Direct VCA
CONTROL signal. It controls the overall gain of the direct (non filtered audio) VCA (Sheet 6).
U14A is a sample-and-hold buffer-level shifter / amplifier. Its output is the Filter VCA control
signal. It controls the overall gain of all filter VCAs (Sheets 2 though 5). Note that the Filter
VCA signal is a diode drop across Q16. The current through Q16 is determined by the voltage
at the output of Ul4A and by R50. This is a current mirror configuration. There is a transistor in
each of the filter VCA circuits that matches the properties of Q16.
U10 is a de-multiplexer that routes one of the microprocessor D-A outputs to the inputs of U3B,
U12B, UI4A, and U14B. The signals Mux_A and Mux_B signals select the output routing,
while the signal Mux_EN inhibits U10 when it is high.