Advertisement
Chorus (ML-E only-contd.)
through a 5-pole filter, around U17, that has a cutoff frequency (like the chorus input filter) of 3.6kHz. Finally, it reaches the output expander
that 'undoes' what the input compressor did and expands the dynamic range back to that of the original input signal. The expander circuit is
built around the other half of U14 and uses an external op amp, U13. It also has the 1.8Vdc level at pins 2,3,5, & 8 and at U13/2,3. The
output, U13/1, should be at 4Vdc. And again, U14/1 has a DC voltage that varies with the incoming signal level. At this point, the high
frequency (above 3.6kHz) components are added back in, using the high pass filter of C231 and R233 and the summing resistor of R232. This
signal then goes to U16/3 where the status of the chorus is selected either by the front panel or the footswitch. When U16/2 is low (chorus is
out) the dry signal going to pin 4 is selected and the chorus is off. If the chorus switch is engaged (with or without the footswitch), the voltage
on U 16/2 becomes 2Vdc and the chorus signal going to pin 3 is selected. The output of U16/6 & 7 is summed at U15.
Reverb (ML-E only)
The reverb circuit of the ML-E is a BBD type similar to the chorus but is has an overall longer delay and multiple tapped outputs. The signal
comes from U27/1, full range, and goes into U5, which is a compressor circuit, similar to that of the chorus. In this case, it is used as a turn on/
off gate and as a dynamic range compressor. This means that it will take smaller signals, boost them through the BBDs and reduce the larger
signals so they won't overdrive the BBDs. As with the chorus compressor, there should be 1.8Vdc at U5/11, 12,14 & 15. The output, U5/10,
should be biased at 4.6Vdc. The rectifier output from U5/1 (clean channel compressor) is used as a gating voltage to turn the compressor on or
off. With no input signal, the voltage at U13/7 is +15Vdc and the voltage at U5/16 is 1.6Vdc, which gives the compressor a very low gain , so
in effect it is off. When the input signal gets to be greater than 25mVrms, the DC voltage on U13/6 becomes greater than the voltage on U13/
5, and U13/7 becomes –15Vdc. This causes D208 to become back-biased and drop out of the circuit. Then the compressor starts acting as it
normally would, with the voltage at pin 16 going to about 1.05Vdc, then increasing as the incoming signal increases. From the compressor,
signal then goes through a 5-pole low-pass filter around U18. This is nearly identical to the output filter of the chorus and has the limiting
zener diodes, D269 & D271. R341 and R342 can adjust the DC level going into the BBDs, U21, &U22, in case one side starts clipping before
the other. The MN3101, U23, generates the sampling clock pulses at about 11kHz which is set by R354, R355, and C353. These clock pulses
the drive the BBDs to sample the filtered signal. The signal gets sampled and delayed at U22, the goes to U21 where it is summed with 6 more
delay taps. Taps 2,4,and 6 combine with one side of the stereo reverb, and 1,3, and 5 combine with the other. The final tap,6 (U21/4) is fed
back to the input of the first low-pass filter (U18) to be used as the regeneration of the reverb so that it gradually fades out. The signals that
were summed together each pass through a separate 5-pole low-pass filter that differs slightly from the input filter. The last stage (U20/5-7 &
U19/5-7) has a notch at approximately the sampling frequency of 11kHz. The outputs then go to the dual reverb pot where the level at which
they will be mixed is adjusted. U9 allows the reverb output signal to be turned off with the footswitch. In its normal operating mode, U9/2 is
at 15Vdc (2Vdc if the footswitch and LED are on), and the signals pass freely. But if the reverb is turned off by the footswitch, then U9/2
becomes 0Vdc and the switch between pins 3 & 7 grounds out pin 4. This results in reverb being turned off.
Summing Amps (ML-E only)
The summing amplifiers built around U15 combine the dry, shorus, reverb, and auxiliary signals going to the power amp. The signal at U15/7
contains a 4:1 mix of chorus to dry signal, when chorus is on, and U15/1 has the opposite mix of 1:4. The reverb mix of the MN3007 output
and the MN3011 taps 2,4, and6 appear on U15/7 in a 1:10 ratio with the dry signal and the other taps 1,3, and 5 are in the mix at 1:10 at U15/1.
This mix is what gives the stereo imaging when the effects are on. The signal from U15/7 ultimately goes to the left power amp output and
balanced out, and the tip of the headphone jack. The signal from U15/1 goes to the right power amp output and balanced out, and the ring of
the headphone jack. Also mixed in at these points are the signals from the Stereo Aux. In with the headphone jack mixing from the above
points.
Balanced Outputs (ML-E only)
The signals from the summing amps are sent to balanced outs so they can be used for recording or going direct without miking in a live
situation. The signal appearing at pin 2 of each of the balanced outputs, J3 & 4, is in phase with the summing amp output. The signal going to
pin 3 passes through an inverting unity gain op amp, U28, so that it is 180 degrees out of phase with pin 2. When the Stereo/Mono switch, S5,
is out, J3 (Balanced Out R) carries the signal from U15/1 and Balanced Out L is the same as U15/7. When S5 is pushed in, however, J3 carries
the in-phase signal from U15/1 and the out-of-phase signal from U15/7. J4 then carries just the opposite signals. This gives a mono signal
when pins 2 & 3 get mixed at the board.
Advertisement
