Electro Harmonix Bass Microsynth

[puppiesonacid]

this pedal is awesome.

Respectfully,

Tony

[Aentons]

I agree. I'd love to see somebody write up a circuit analysis type article or post with all the different sections it has. I've always wondered if the filter section was based on or derived from another effect or circuit that you could build or buy independently.

[matmosphere]

I think Brain was working on a board for this at some point. Wonder if it'll ever materialize.

I have never used the bass version, but the regular one is pretty fun to mess around with.

[Aentons]

I have a 2016 Bass version. It's almost all smd but it has all the values and maybe some other useful info on the PCB. Id be glad to take some clearer pics if it would help.

Edit: I noticed that R63 and R102 are different values than listed on the board

[jimilee]

A through hole 1/4" resistor board would be huge, you could probably get away with 1/8", but I would myself after doing it and I had to troubleshoot.


Sent from my iPhone using Tapatalk

[Aentons]

Here is the other side of the board.

[madbean]

I think Brain was working on a board for this at some point. Wonder if it'll ever materialize.

I have never used the bass version, but the regular one is pretty fun to mess around with.

It will!

[Aentons]

I think Brain was working on a board for this at some point. Wonder if it'll ever materialize.

I have never used the bass version, but the regular one is pretty fun to mess around with.

It will!

Total Awesomeness!

[Yahoo67]

Journey said it best ....

[matmosphere]

I think Brain was working on a board for this at some point. Wonder if it'll ever materialize.

I have never used the bass version, but the regular one is pretty fun to mess around with.

It will!

[Aentons]

I found a thread on another site where a Mr Strand was kind enough to post his partial analysis on the big box version. I am linking and copying here in case anyone else is interested or may find it useful:
--------------------------------------

https://www.diystompboxes.com/smfforum/index.php?topic=120180.0



Rob Strand
*****

Re: EHX Microsynth schematic doubts
« Reply #10 on: April 25, 2018, 06:57:36 AM »

Here's some incomplete notes I made about that effect back in 2003.  Perhaps before I got the fixes list.

As it stands, it is what it is.  There may be totally wrong things in there. I kind of eye-balled the schematic and made notes. Some places I did some calculations.

If you can make use of it then all is well.  If you get frustrated with bugs or you disagree
with stuff then just bin it!
==========================================================

EH Microsynth Analysis
by R.Strand, V1.0, 9 Sept 2003

Overview
----------
The Microsynth is a form of analogue synthesizer for guitar.  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). These four signals are combined with a mixer, each signal having an individual level control.  The combined signal is fed through a VCF (basically an envelope follower) which is an upward sweeping third-order low-pass filter.  The output of the VCF is then fed to an amplitude control stage which modifies the attack of the output signal.  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 attack circuits for the squarewave and VCF are really have separate function just that some parts of the circuit are common to both.

There are 11 controls on the unit which control various parameters of each of the stages:

-- PreGain
Preamp Gain before all processing ckts

-- Guitar
Level of clean guitar signal before VCF

-- Octave               
Level of octave-up signal

-- Squelch             
Octave-up squelch

-- Square               
Level of amplitude modulated squarewave

-- Trigger               
Trigger level for note attack (VCF +Squarewave)

-- Sub-Octave       
Level of sub-octave signal

-- Start Frequency   
Start Frequency for VCF

-- Stop Frequency   
Stop Frequency for VCF

-- Sweep Rate     
Sweep rate for VCF

-- Resonance       
Resonance (Q) of 2nd order part of VCF

Note: There are two versions of the microsynth, a guitar version and a bass version. The only difference appears to be the range of the VCF sweep.

The rest of this document gives a detailed analysis of the EH Microsynth.

Power Supplies
----------
The unit is powered from -10V and +8V rails, the two raill are derived from a single input supply.
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.

Input Stage Preamp
----------
The input stage preamp preceeds all other circuity.
The gain is adustable via the PREGAIN control.

Input impedance: 68k,  this will load the guitar pickup.
Variable gain:   x1 to x22

Guitar Signal
----------
The guitar signal is derived from the output of the GUITAR control which is a simple pot connected to the input stage preamp.  The guitar signal feeds the pre-VCF mixer.

700Hz 3rd-Order Low-pass Filter
----------
The 700Hz 3rd-order low pass filter preceeds all of the signal processing elements except the clean "Guitar" signal.  The filter's purpose is to extact the fundamentals of the guitar signal.

Parameters:
1st order:  750Hz
2nd order:  681Hz, Q=1.48

Full-wave Rectifier
----------
The full-wave rectifier is generates the octave up signal and is used as a full-wave rectifier for the attack detection circuit.

The output of the full-wave rectifier is negative.
The input of the full-wave rectifier is high-pass filtered at 56.4Hz (10K//15K and 470nF).
The gain is 27k/15k = 1.8.
The output is filtered by a 3.27kHz low-pass filter.
The SQUELCH control  provides a variable DC offset at the output of the rectifier of -65.5mV to + 81.8mV.

Octave-up Signal
----------
The octave-up signal is a (low Q 2nd-order) high-pass filtered version of the output of the full-wave rectifier.
The level of the octave-up signal is set by the OCTAVE control and the output connects to the pre-VCF mixer.

Loading causing the frequency of the high-pass filter to be affected by the OCTAVE control position.

With the OCTAVE control on minimum:
   R2 = 4.489k
   w0 =2.655E+03 (422.6Hz),  Q = 4.589E-01
  (or, re1= 277.6Hz, re2= 643.1Hz)

With the OCTAVE control on maximum: (nb: the VCF mixer looks like about 450ohms)
  R2=2.955k
  w0 = 3.272E+03 (520.8Hz),  Q=  4.397E-01
  (or, re1 =310.3Hz, re2=874.1Hz)

To simplify we use the geometric means of the real parts,
  re1_gm  = 293.5Hz,  re2_gm = 749.8Hz
  ie.  w0 =   2.948e3 (469.1Hz), Q = 0.4496

re1 only varies +/- 6% which is undetectable, re2 varies +/- 15% which is boarderline

Sub-Octave Generator
-----------
...

Attack Generator Level Detectors
----------
Consists of: smoothing filter, low-level squelch ckt, log-amp, and trigger circuits.

The full-wave rectified signal feed a 4th order smoothing filter. The precise 4th order low-pass filter parameters have not been extracted, however, the following 2nd-order cascade approximation agrees within +/- 0.08dB. ( 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.)

2nd Order Low-Q:   w0 = 353.8 (56.3Hz),     Q = 0.4572
2nd Order High-Q:  w0 = 268.0 (42.66Hz),   Q = 1.2618

The output of the filter feeds the low-level squelch ckt and the log amp.

The low-level squelch circuit goes high when the signal level is low, this corresponds to a filter voltage above -12.2mV.

The log-amp is pre-biased with a -455uA current.  Given the output of the full-wave rectifier can never exceed +81.8mV, the positive current can never exceed 81.8uA and hence the log amp is always bias with the same polarity and the diodes never get below 373uA of current.  If the rectifier is at maximum output then the current through the diodes is about 8.5mA.   If the diode is 0.64V @ 1mA then at 373uA Vd=591mV and the output of the log amp is at 2.366V.  At 8.5mA, Vd = 746mV and the output of the log amp is 2.98V.  The output of the log-amp therefore has a peak to peak swing of about 4 * 1.9 * 26mV * ln(8.5/0.373) = 0.62V. It's possible to write an equation for the log amp output as a function of the filter voltage,

Vlog = 2.404 + 0.1976 * ln(1-Vfilter/0.455)

The log-amp then feeds a 1st order high-pass filter with cut-off frequency 73.93Hz.  The high-pass filter essentially differentiates the log-amp signal to produce a positive pulse for start of the attack and a negative pulse for the end of note.

The high-pass filter feeds the TRIGGER control which is just a divider.  Two comparitors detect the start and stop pulses.  The attack detector is normally low and  goes high when the trigger signal is above 11.4mV. The stop detector is normally low and goes high when the trigger goes below -8.45mV.

Squarewave Attack Generator
----------
The square-wave attack generator is basically an envelope follower which uses the DC output from the full-wave rectifier.

The rectified signal is filtered with an approximated first order filter.  The first order filter is a tap off the 4th order smoothing filter, the tapping means the response isn't exactly first order, it's actually more like a 1st order filter plus a shelving high-cut filter and even then that's only an approximation.  At high frequencies say above 100Hz it is largely a first order filter.  The cut-off is about 58.1Hz, with this cut-off the error is a maximum of 0.7dB in error below 100Hz due to the first-order approximation.

The filterered signal feeds an ideal diode peak detector and an attack circuit with a 1.03ms time-constant - both the input and output signal are negative.  The attack interracts with these two constants, however, the attack dominates the decay ckt by a large factor.  Note the idea diode clamps the voltage on the time constant cap, and this prevents the cap voltage going positive by a significant amount.

The decay time constant is complicated because there are two time constants, one from the low-signal squelch ckt and one from the stop detector.  The low-signal squelch has a time-constant of 32ms  and charges towards 6.3V.  The stop detector has a time-constant of 564ms and charges towards 6.3V. If both signals are active (which is unlikely)  the time constant is 30.2ms.

Squarewave Generator
----------
The square-wave generator connects to the output of the 700Hz LPF.

The square-wave generator is an inverting gain of 10 amplifier preceeded by a 1st-order 58.95Hz high-pass filter.   It is likely the opamp at this stage clips.

The squarewave generator then feeds  into a low-Q band-pass filter.the square-wave amplitude modulation, which is fed by the square-wave attack generator.


Pre-VCF Mixer
----------
...

VCF Attack Generator
-----------
...

VCF Sweep
----------
...

VCF
-----------
...

Output Attack Generator
===========================
EDIT: Given we know SQUELCH is a trim pot there's obviously only 10 controls - which you can see on the front panel.


« Last Edit: April 25, 2018, 07:04:55 AM by Rob Strand »


https://www.docdroid.net/u3cx2Xj/electro-harmonix-micro-synthesizer-revised-2017-pdf