This was my first build when I came back to SDIY. A three row/16 step sequencer. Completely build on stripboard. Still working after all this years. Only hand sketched schematics. Nothing to publish. Pictures only.
Sequencer Nostalgia
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Author Archives: Harald
6..36dB VCF Lowpass/Highpass
This is the Eurorack version of my NGF 36dB VCF. I have brought out the 6dB, 12dB, 18dB, 24dB, 30dB and 36dB poles. You have two audio inputs for easy mixing sound sources. And CV inputs for linear TM, log TM, envelope, V/Oct tracking and emphasis. The exponential circuit is temperature compensated with KTY81-110. If the 12dB output is patched back to input 2 the filter can serve as a sine oscillator.
Specs and features
- 36dB voltage controlled low pass and high pass filter
- Two inputs for easy mixing
- 6dB, 12dB, 18dB, 24dB 30dB, 36dB output
- HP/LP switch
- Positive and negative ENV control with sign changer
- Temperature compensation with KTY81-110
- CV inputs for linear TM, log TM, envelope, V/Oct tracking and emphasis
- Usable as sine oscillator
- Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
- Power consumption around 60mA negative rail, 65mA positive rail
The documentation and the Gerber files for download can be found in my website.
Straight forward design. Six state variable filter cells are connected together in series, The output of each filter cell is brought out. There are a lot descriptions of those state variable filters out there. I feel no need to add another one.
Utility Mixer I
This is an often used utility module. The mixer comes in handy for mixing CV sources. The mixer is DC coupled, so you can use it for DC and AC mixing. The input impedance is constant 1MOhm. The input signals are amplified by a maximum factor of two. It is possible to offset every input with +/- 5V. The offset is signaled with LED’s for every channel and for the summing output. The outputs are normalized, so you can remove selected channels from the output mix. The summed output has a -6dB switchable attenuator. There is an inverted summed output added as well. The added volume indicator us useful for finding the appropriate signal level.
Specs and features
- Three inputs, three outputs
- Inverted and non- inverted summed output
- 2x amplification
- +/- 5V offset for every channel
- -6dB switch
- Volume indicator
- Normalized outputs
- Runs on +/-12V and +/-15V
- Power consumption below 20mA each rail
The documentation and the Gerber files for download can be found in my website.
Control board: Straight forward design. The mixer is completely DC coupled. So you can use it for CV mixing as well as audio mixing. IC1B,C,D buffers the three inputs and and keeps the input impedance constant. P2, P4, P6 sets the amplification from zero to 2X. P1, P3, P5 sets the offset voltage. The LED indicates the offset and signal level.
Main board: IC3B, IC4B, IC6B adds the offset voltage and the signal. IC1C, IC1D, IC1B, IC1B drive the low current LED. IC2A sums the signals and drives the negative output. IC2B drives the positive output.
ADSR Euro-rack
This is another derivation off the ADSR for my NGF-E project, adapted to 12V Euro-rack format. Because this one is a stand alone module I have removed all additional features from the Next Generation Formant project. Nonetheless it is still based on original Elektor Formant ADSR schematic. I made some error corrections and added my changes to the design. All parts are updated to today (2021/01) available parts. I have made a few changes to fix some shortcomings of the original. A triple range switch was added for finer adjustment of the ADSR CV-output signal. The attack rise time is shorter now as in the original. The gate input is buffered. The fixes a fault in the original when working with analog sequencers. The output voltage is slightly raised to reach really 5V. Due to the design of the original Elektor Formant ADSR the output of the original ADSR keeps a residual voltage of about 0,5V. I have put an compensation in my design to correct this. The driver circuitry for the output indicator LED is changed for better linearity.
Specs and features
- AD/ADSR switch
- Gate input 5V
- CV output ..5V
- CV output indicator
- Range switch: fast, middle, long
- Attack (A) 0,5ms…16s
- Decay (D) 4ms…40s
- Sustain (S) 0..5V
- Release (R) 4ms..40s
- Power consumption below 15mA each rail
The documentation and the Gerber files for download can be found in my website.
This is a close clone of the Elektor Formant ADSR. Here i only describe the changes i have made. The description of the other parts of the circuitry can be found in the original Elektor Formant documentation. The gate signal input resistance is raised from 33kOhm to 1megOhm with the input buffer IC1A. This protects against double triggering with the falling edge of the gate signal when using sequencers. R30 is used to fix the input to a defined potential when no signal is attached to the input. C1 was lowered to 6n8 from 10nF. In combination with C2 and the raised charging voltage through IC1B/R9 this makes for faster attack time. The load capacitor of 10u was replaced with three selectable capacitors of 2,2uF 4,7uF and 10uF. This makes for a finer adjustment of the response times of the ADSR. The voltage divider R19/R21 was adjusted to ensure that the output level of 5V is reached. If this feature is not used R25 should be lowered to 5k1. Construction conditioned the output at IC1D only reaches a minimal voltage of about 0,5V. To compensate for this i added IC2A. If the ADSR is not used the output voltage is now at -0,5V. The current consumption was lowered with using the TL064 and a low current LED.
Pitch to voltage converter
This is the software driven replacement for my all hardware pitch to voltage converter from my Shakuhachi to Synth project. The software driven approach has the advantage of easily adaption for different frequency ranges. In my case it is the range of the Shakuhachi. To change the range just adapt the software. It is completely temperature independent. The needed input is a pulse train derived from your original signal. You can use my Signal to Trigger converter to provide the pulse train. An offset voltage is added to the V/Oct output to fit the needs of your VCO (Synthesizer).
Specs and features
- Software driven pitch to voltage converter
- 12bit resolution
- V/Oct output
- Offset CV Fine and coarse adjustment
- Runs on +/-15V and +/-12V
- Power consumption around 45mA positive rail, 15mA negative rail
The documentation and the Gerber files for download can be found in my website.
The incoming pulse train is feed to the microprocessor. IC1 (301-F) prevents the microprocessor from negative inputs. Zener D2 prevents from overvoltage. The trigger starts an internal timer of the microprocessor in input capture interrupt mode. The ticks are counted and the count is then looked up in a table. The lookup table provides the values for the V/Oct conversion. The read value is the send to the DAC MCP4921 which is follwed by a low pass (IC1A, 301-B)). IC2A (301-F) adds the offset voltage and IC2B (301-F) corrects the phase.
Compressor with optional pedal steering
This is the revised version of my Limiter/Compressor. First built for my Shakuhachi to Synth project to handle the great dynamic range of the Shakuhachi. Here I left out the limiter and added a make up amplifier. The structure used is derived from “Small Signal Audio Design”, second edition by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal. The compression rate and the make up gain is adjusted by hand or/and optionally with foot pedals. The foot pedals are an additional option particularly made for wind players. It works without this option in your setup as well.
Specs and features
- Compression rate and gain adjustable by hand or/and foot pedals
- Audio path not affected when no compression is used
- Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
- Power consumption below 20mA each rail
The documentation and the Gerber files for download can be found in my website.
When the ratio is set to zero and the gain to one the input signal passes through the circuitry unaffected (IC2C, IC2A IC6OTA1, IC6OTA2, IC2D). When the compression rate is turned up a DC voltage is derived from the input signal wit a precision full wave rectifier and some filtering (IC1A, IC1B, IC1C, IC1D). This voltage is used to open the VCA in the side chain (IC3OTA1, IC3OTA2, IC2B). The signal from the side chain is then subtracted from the main signal (R13, IC2A). The now compressed signal is then potentially amplified (IC6OTA1, IC6OTA2)
Foot switch connector
As a Shakuhachi player I need my hands on the flute. So I use me feet to manipulate parameters and switches on the synthesizer. This module was originally build for my Shakuhachi to Synth project to provide the possibility to connect foot switches with the synthesizer and keep the patch intact when they are removed. The signal is not routed through the foot switch. Instead CMOS switches are used, turned on and off with the foot switch. So the signal stays within the synthesizer and the connection to the foot switch carries only DC. Removing the foot switch does not interrupt the signal flow in the synthesizer.
Specs and features
- Four independent switches
- Signal flow stays intact when foot switch removed
- Runs on +/-15V and +/-12V
- Power consumption below 10mA each rail
The documentation and the Gerber files for download can be found in my website.
The switch in the DG202 is hold in on position with a 100k resistor against the positive rail. With a foot switch attached you can pull down the hold voltage when you close the foot switch.
Signal to Trigger Converter
This module was originally build for my Shakuhachi to Synth project to provide the start/stop pulse for the Pitch to voltage converter. But it turned out to be much more useful. When you have the basics for your synthesizer like VCO, VCF, VCA, ADSR, LFO,… and some controllers and you want more, then using your keyboard to steer the synthesizer it is time for some modules to produce trigger signals out of different sources. Here is one of them. A signal to trigger converter. You can feed in a changing signal and every time the signal went through zero a trigger is generated dependent on the direction from where the zero point is crossed. You can add a threshold manually or CV controlled to move the zero point up or down as well. You can feed the signal in through input one ore two. When both inputs are used the signals are added together. When the signal crosses zero from positive to negative a trigger of about 0.1msec is generated at output -Trig. When the signal crosses zero from negative to positive a trigger of about 0.1msec is generated at output +Trig. Output +/-Trig provides both triggers. This output can be used to generate interesting rhythmic patterns when the threshold is set by a slowly moving CV or some DC offset is applied to the signal.
Specs and features
- Two added inputs
- Threshold manually and with CV
- Output for +Trig, -Trig and +/-Trig: 0.1msec
- Runs on +/-15V and +/-12V with minor resistor changes
- Power consumption below 25mA each rail
The documentation and the Gerber files for download can be found in my website.
The incoming signals are summed up. Every time when the summed signal changes polarity (moving through zero) a trigger is generated. Moving from plus to minus generates a trigger at the negative trigger output, moving from minus to plus generates a trigger at the positive trigger output. Trigger length is about 0.1msec.
The uppermost line (Yellow) shows the input signal. The second line (Blue) shows the trigger when the input signal moves to the positive site. The third line (Purple) shows the trigger when the input signal moves to the negative site. On the fourth line (Green) you can see both triggers added. This picture is taken without any threshold.
Passive Multiple
Just another often needed utility module. A passive multiple.
Specs and features
- Seven passive parallel in/outs
- No power connection
The documentation and the Gerber files for download can be found in my website.
Mixer, DC coupled with volume indicator
This is an often needed utility module. The mixer comes in handy for mixing CV sources and audio sources as well. This version is DC coupled, so you can use it for AC and DC mixing. There is an inverted output added. You can reverse the DC-CV mix with it or experiment with feedback loops in the audio domain. The added volume indicator us useful for finding the appropriate signal level. The volume indicator is optional. You can leave it out with no problems for the other functions.
Specs and features
- Five inputs
- Inverted and non- inverted output
- Volume indicator (optional)
- Runs on +/-12V and +/-15V
- Power consumption below 20mA each rail
The documentation and the Gerber files for download can be found in my website.
Control board: Straight forward design. The mixer is completely DC coupled. So you can use it for CV mixing as well as audio mixing. IC1D sums the five inputs and inverts the signal. P3 is used to adjust the overall signal level. IC1B inverts the signal again. IC1C and IC1A buffers the outputs.
Level indicator: The circuitry around IC2 build an precision full wave rectifier. The output is smoothed with C1. The parts around the BC560C build a constant current source for driving the LED’s. The comparator LM393 (IC1A/B) switches the LED on and off according to the input level.
