syntha vcf historical

Description: This module is a tribute module based on the Steiner-Parker Synth VCF. It is notable for its unique sound, which differs from the Moog ladder filter and possesses significant character. The behavior of the resonance is somewhat atypical; once it begins oscillating, the resonance control must be adjusted down significantly to stop the oscillation, and it can also be frequency dependent. The CV input should be connected to a voltage source such as a keyboard, envelope generator, or sequencer. The output should be connected to a VCA or amplifier. Signals can be fed into the high-pass, band-pass, or low-pass inputs, and unlike the original design, this version allows for simultaneous input into all three. When the same signal is used across all inputs, the result is similar to a phaser effect. If different signals are used, it creates a frequency-based "interpolating scanner," allowing for panning between sound sources, influenced by their respective frequencies. The circuit employs a standard non-inverting amplifier configuration, with the three modes (HL, BP, LP) achieved by injecting the signal into three distinct points of the circuit. Increasing the amplifier gain enhances the filter's Q, which remains nearly constant across the audio spectrum. Diode strings function as voltage-controlled resistors, with differential-amplifier transistors applying bias voltage to the parallel diode string RC networks in opposing phases, effectively canceling the control voltage at the output. The final stage is a simple gain stage, modified from the original design to improve output volume. The LEVEL pads allow for the installation of a level potentiometer, with recommended values of 50k or 100k (or higher, such as 250k) based on the desired gain. A fixed resistor can also be used, or a link if the 47k in series is increased for compensation. The level pot does not reduce the output to zero due to the nature of the non-inverting buffer feedback circuit, but it can adjust gain from a default of 5:1 to up to 15:1 with a 100k pot, effectively varying the output level from standard to three times that level. Historical notes indicate that early PCB runs had some design issues, including incorrect placement of input capacitors and a reversed output capacitor on the PCB. Modifications suggest removing the input capacitor in the high-pass input and not installing the associated 390k pull-down resistor. Variations in levels and CV response may occur depending on the transistors used, and experimentation is encouraged. The 2N2222 and 2N2907 transistors were chosen for their gain characteristics, although other types like the 2N3904 and 2N3906 have been successfully implemented.

The Steiner-Parker Synth VCF tribute module is designed to provide versatile filtering capabilities with a distinctive sound profile. The architecture of the circuit allows for three modes of operation: high-pass (HP), band-pass (BP), and low-pass (LP), which can be utilized simultaneously for creative sound design. The non-inverting amplifier configuration ensures that the output maintains phase integrity, which is crucial for applications in synthesizers where signal coherence is necessary.

In practical applications, the module's CV input facilitates dynamic control of the filter characteristics, making it suitable for integration with various control sources. The ability to connect multiple signals to the filter inputs simultaneously opens up new avenues for sonic exploration, especially when different audio sources are utilized. This feature can produce complex phasing effects or frequency interpolation that enhances the richness of the sound.

The resonance behavior of this VCF is a point of interest; its tendency to oscillate requires careful adjustment, which can lead to creative sound shaping opportunities. The use of diode strings as voltage-controlled resistors allows for precise control over the filter's response to the incoming CV signals, ensuring that the filter can adapt to a wide range of input levels and characteristics.

The final gain stage has been optimized to address volume concerns, allowing users to achieve a broader dynamic range. The LEVEL pads provide flexibility for users to customize the output level according to their specific needs, whether through potentiometers or fixed resistors. The historical notes regarding PCB revisions provide valuable insights for builders, ensuring that they are aware of potential pitfalls and modifications that may enhance performance.

Overall, the Steiner-Parker Synth VCF tribute module represents a sophisticated approach to voltage-controlled filtering, with a focus on both sound quality and user interaction, making it a valuable addition to any modular synthesizer setup.This module is a "tribute" module, based on the awesome Steiner-Parker Synth VCF. Those who know me will know I`m not a big VCF fan. Nonetheless, this VCF really appeals to me. Its sound is quite unlike the Moog ladder, has a lot of character. Resonance is a bit unusual in its behavior. Once it starts oscillating, the resonance pot needs to be bac ked off a fair way to get it to stop again. It can also be frequency dependent. Connect the CV input to a voltage source such as a keyboard, envelope generator or sequencer. Connect the output to a VCA or amplifier. Feed the signal to be filtered into the high-pass, band-pass or low-pass input. Unlike the original, this version allows signals to be fed into all inputs simultaneously. If the same signal is used in all inputs, the result is reminiscent of a phaser. The real fun starts when you feed different signals into each input, then you get a frequency based "interpolating scanner", where panning between different sound sources is possible, though also subject to the frequency at which they are running. I have never heard an effect like it before. The circuit uses a standard, non-inverting amplifier configuration. The three modes (HL, BP, LP) are obtained by injecting the signal into three different points of the circuit.

An increase in the gain of the amplifier increases the filter`s Q. The Q remains almost constant as the filter is swept across the audio spectrum. In the circuit, diode strings are used as voltage controlled resistors. The differential-amplifier transistors apply the bias voltage to the parallel diode string RC networks in opposing phase. The opposing phases cancel the control voltage so that none appears at the output. The final stage is a simple gain stage, as I found the original was too quiet for my needs. In the first version of the PCB, this stage was inverting, while on the Rev 1 PCBS, this stage is non-inverting.

The LEVEL pads are to allow for a level pot to be installed. Use a pot with a value of 50k or 100k (going even higher if needed, e. g. 250k), depending on the overall gain required. Alternatively, a fixed resistor could be put here, or even a link, if the 47k in series with it is increased to compensate. Note that the level pot does NOT allow the output to be reduced to zero, as this is impossible to do with a variable resistor in the feedback circuit of a non-inverting buffer.

It will adjust the gain from the default 5:1 up to 15:1 assuming a 100k pot is used, effectively allowing you vary the output from standard to three times that level. The following four point apply only to the VERY early PCB runs, and are here only as a reference to those with these old PCBs.

REV 1, VER1. 1 and later boards can be built as per the overlay. (historical) On both the first and second runs of the PCBs, there are also some suggested changes: the input electros are backwards in my design. It would seem I put them on the wrong side of the input dividers! This affects the three 1uF input capacitors. (historical) On both the first and second runs of the PCBs, the 10uF between the output transistors and the input of the op-amp buffer is correct on the circuit diagram, and reversed on the PCB.

Please install it backwards with respect to the PCB artwork. It can`t have been my day. (historical) The next modifications actually removes the input capacitor in the HP input. Put a link there instead. Also, do not install the associated 390k pull-down resistor (leave empty) on the High Pass input. You may find that the levels and responce to the CV vary depending on the transistors used. As such, the "tweak" values given may not be appropriate for your filter. Do not be afraid to experiment. The 2N2222 and 2N2907 were chosen as they had gains closer to those used in the original design than other frequently used transistors such as the BC547. 2N3904 and 2N3906 have been successfully used, though they need to be i