Compressor (ML-E only-contd.)
input voltage- a higher voltage at this point corresponds to a lower compressor gain. C97, R98, and R99 are used to bias the output down to
approx. 0Vdc. Diodes D105 and D107 limit the transient output to 2.8V. The 4.7K resistor, R100, sets the maximum gain of the compressor
to about 2. The R108 resistor prevents the RC5532 circuit from oscillating. The analog switch (U3) selects between the compressed signal
and the dry signal from U1/7. When a footswitch is used, a 2Vdc level at U3/2 will select the compressed signal while a 0.14Vdc there will
send the dry signal through. The front panel compressor switch (S2) then selects between the dry signal and the output of U3 (pins 6 and 7).
Thus, the compressor can be footswitched in and out only when the front panel switch is in.
Tone Controls
The tone controls are active and provide boost and cut at four preset frequencies. The Hi-Mid and Lo-Mid controls are peaking filters and
provide boost and cut of 16dB at 4kHz and 18dB at 800Hz, respectively. The treble and bass controls are shelving filters. The treble filter
provides a boost and cut of up to 12dB at 10kHz, while the bass gives 15dB or cut at 80Hz.
Footswitch Circuit
The footswitches themselves consist of two (RF2) or four (RFG4) SPST switches each wired across a green LED that has its cathode tied to a
common ground. When attached to the unit through the proper connecting wire, the internal resistors to +15V will light the LED when its
corresponding switch is open. When the switch gets closed, the LED becomes grounded on both sides and no longer lights. The RF2 is meant
for use with the ML-S and controls channel switching and the boost function. It should be connected with a standard ¼” 3-conductor stereo
cable. The RFG4 should be connected to the ML-E through a 5 conductor DIN cable. This looks like a standard MIDI cable but it must have
all five wires inside rather than just three, which is very common. The RFG4 controls channel switching, boost/compressor, reverb and chorus.
In both cases, the front panel select switch must be in for the function to work properly from the footswitch. The same goes for the chorus
feature of the RFG4. Further operational information on the footswitches (with the functions they control) is covered in the individual circuit
descriptions described herein.
Effects Loop (and Stereo Aux. in on ML-E)
The effects loop consists of the Send and Return jacks. The return is set up in such a way that plugging into it will disconnect the signal that
was previously going to it from the preamp. The E-preamp has a mono return so that it may later be split into stereo with the use of the stereo
effects (chorus and reverb). If a stereo return is desired, the Stereo Auxiliary In can be used to mix in a stereo signal without breaking the
straight signal coming through. It mixes in at U15, which is where the chorus and reverb signals also get mixed together into stereo. The S-
preamp has a Stereo-Mono switch with its return to allow for putting stereo effects directly in line.
Headphone Outputs
The level at the output of the headphone jack is controlled by the Output Level knob. U11 and U12 [U8, 7] are LM386 chips and can provide
about ¼ W each into headphones of 8 ohms minimum. The chips run off of ground and –15Vdc so there is a DC voltage of about 7V on pin 5
of each. The LM386s also have a gain of about 20 so the resistor dividers of R182/R179 [R159/R149] and R194/R190 [R151/R150] provide
attenuation of the signal from the output level that makes the headphone outputs clip at about the same time that the power amp reaches full
power into a load.
Chorus (ML-E only)
After the return, the signal goes through a unity gain inverting op amp, U27. R377 and C376 pass the low frequencies straight through to the
summing amplifier U15. The high frequencies are sent, via the filter of C378, R379, and R380, to the chorus input compressor, U14, and the
chorus select analog switch, U16. The compressor and expander around the chorus circuitry provides for low noise operation. The
compressor is built around one half of U14 and works very much like the compressor circuit described above. Pins 11, 12, 14, and 15 should
all have the 1.8Vdc offset. Pin 16 again has a DC voltage that changes with the level of the input signal. The internal op amp is used in this
case so the output is pin 10 and it is biased to 7.3Vdc for operation between ground and +15V. This goes into a 2-pole lowpass filter, with a
cutoff at 3.6kHz, built around U27. The 1N747 diodes limit the signal so that it doesn’t overload the Bucket Brigade Delay chip, U24. The
center position of the trim-pot R356 is generally an acceptable DC bias setting of around 6.3Vdc at U24/3, but if the output of U24 appears to
be distorting early, it can be adjusted to eliminate that problem. The MN3101, U25, takes a single 0-to-15V square wave clock pulse from a
LM355 timer chip, U26, and generates the two, out-of-phase clock pulses used by the BBD for its sampling. These square wave signals are 0
to 15V and have a constantly changing period of 34 to 44 microseconds (with depth at max). The voltage that defines the frequency of the
LF355 comes from an oscillator circuit around U10. R172 (rate) adjusts the frequency of the oscillation ad R178 (depth) adjusts the amplitude
of the triangle wave that becomes the control voltage at U26/5. After the signal has been sampled, the delayed version appears at pins 7 & 8 of
U24. The two signals are summed together through R369 and R365 and the must be low-pass-filtered to remove the sampling frequency
element. It then passes
