The log-amp is pre-biased with a -455uA current. The low-level squelch circuit goes high when the signal level is low, this corresponds to a filter voltage above -12.2mV. The output of the filter feeds the low-level squelch ckt and the log amp. ( It was found by splitting the circuit into a 3rd order and 1st order circuit extracting the parameters from those then plugging the parameters into two cascade 2nd order in spice an tweaking frequency of the 2nd real stage.)Ģnd Order Low-Q: w0 = 353.8 (56.3Hz), Q = 0.4572Ģnd Order High-Q: w0 = 268.0 (42.66Hz), Q = 1.2618 The precise 4th order low-pass filter parameters have not been extracted, however, the following 2nd-order cascade approximation agrees within +/- 0.08dB. The full-wave rectified signal feed a 4th order smoothing filter. Re1 only varies +/- 6% which is undetectable, re2 varies +/- 15% which is boarderlineĬonsists of: smoothing filter, low-level squelch ckt, log-amp, and trigger circuits. To simplify we use the geometric means of the real parts, With the OCTAVE control on maximum: (nb: the VCF mixer looks like about 450ohms) Loading causing the frequency of the high-pass filter to be affected by the OCTAVE control position. The level of the octave-up signal is set by the OCTAVE control and the output connects to the pre-VCF mixer. The octave-up signal is a (low Q 2nd-order) high-pass filtered version of the output of the full-wave rectifier. The SQUELCH control provides a variable DC offset at the output of the rectifier of -65.5mV to + 81.8mV. The output is filtered by a 3.27kHz low-pass filter. The input of the full-wave rectifier is high-pass filtered at 56.4Hz (10K//15K and 470nF). The output of the full-wave rectifier is negative. The full-wave rectifier is generates the octave up signal and is used as a full-wave rectifier for the attack detection circuit. The filter's purpose is to extact the fundamentals of the guitar signal. The 700Hz 3rd-order low pass filter preceeds all of the signal processing elements except the clean "Guitar" signal. The guitar signal feeds the pre-VCF mixer. The guitar signal is derived from the output of the GUITAR control which is a simple pot connected to the input stage preamp. Input impedance: 68k, this will load the guitar pickup. The gain is adustable via the PREGAIN control. The input stage preamp preceeds all other circuity. The different + and - rails more than compensates for the differences in output swing on each rail of the opamps, as a result the opamp clipping slightly assymetrical. The unit is powered from -10V and +8V rails, the two raill are derived from a single input supply. The rest of this document gives a detailed analysis of the EH Microsynth. The only difference appears to be the range of the VCF sweep. Note: There are two versions of the microsynth, a guitar version and a bass version. Trigger level for note attack (VCF +Squarewave)
There are 11 controls on the unit which control various parameters of each of the stages: The attack circuits for the squarewave and VCF are really have separate function just that some parts of the circuit are common to both. There is also an attack generating circuit, which has a trigger threshold, and performs three functions: control attack of the generated square wave, control attack of the final output, control start of the VCF sweep. The output of the VCF is then fed to an amplitude control stage which modifies the attack of the output signal.
The combined signal is fed through a VCF (basically an envelope follower) which is an upward sweeping third-order low-pass filter. These four signals are combined with a mixer, each signal having an individual level control. It consist of four signal generators whose frequency is derived from the guitar input signal: Guitar, Octave-up, Sub-Octave and Square-wave (at the input frequency). The Microsynth is a form of analogue synthesizer for guitar. If you get frustrated with bugs or you disagree If you can make use of it then all is well. I kind of eye-balled the schematic and made notes. There may be totally wrong things in there. Perhaps before I got the fixes list.Īs it stands, it is what it is. Here's some incomplete notes I made about that effect back in 2003.
I am linking and copying here in case anyone else is interested or may find it useful: I found a thread on another site where a Mr Strand was kind enough to post his partial analysis on the big box version.
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