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Extended Steiner-Parker Synthacon-VCF

Status: Proposal
February 3, 2014

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This project is dedicated to Mr. Hans Camenzind and got inspired by his tutorials and teachings, especially on the herefore used "Doubled-Currentmirrors", wich I have implemented here now once more for this presentation.

In the year of 1970 Mr. Camenzind was the Inventor and Designer for the famous "Signetics NE555 Timer-IC", the most by Billions ever sold Semiconductor-Chip on the whole world and this until up today.

Mr. Camenzind was also the Autor of the E-Book: "Designing Analog Chips" - This book holds also the amazing story to the development of the 555-IC. The book is available for free download on the Internet. It is highly recommended to every person who is interested and learning anyhow on electronics!

Hans Camenzind died in August 2012 - RIP - ******************************************************************* ---------------------------------------------------------------------------------------

For the used Currentmirror-Techniques and the possible usages of OTAs, please refer for this items also to my other publications, here on the Elektor-Projects-Website:

"OTA-Overdrive with genuine Germanium-Sound"

"Enhanced 48dB-Ladder-Filters" (sorry - little lazy, but this gets completely updated in a few days, also with PCBs on mixed Conventionals and SMTs)

"Innovative White-Noise-Generator"

The Projects "Overdrive" and "Noise" are presented as outstanding individual modules with readymade Eagle-PCBs, for the possibility of giving the files directly to a PCB-Service (Of course Elektor-Europrint-Service recommended on a possible best quality for their PCBs).

The Noise-Project offers also the possibilities for interchanging the Noise-Transistors with variable and free selectable Impedances (Pickups??), also as Light-Sensors, or Photo-Diodes, etc., for an easy adapting this advantage to the usage in modular Synthesizers, and for Live-Events!

With the shown Currentmirrors in these projects, you can also very easy perform you own OTAs as SMD-Modules and what gets also explained by details in the book for "Designing Analog Chips".


Extended Steiner-Parker Synthacon-VCF - Preamble

The described VCF-Circuit, as my next published Synthesizer-Project here for the Elektor-Community, is presented mostly as an universal unit and it is therefore coming in several xx(in progress) different PCB-Versions. One of those Modules is planned with almost no Potentiometers and also no Trimpots, but with prepared connectors for adapting all additionally needed components from outside, where maybe this would be best possible - as I think so - by implementing it as a smart SIL-Module, with therefore used "Chippin-Leadframes", as those also are used and known for SIL-Resistor-Arrays and Thick-Film-Hybrids. The published material for the project will therefore get updated time by time, as I will find it, and it gets indicated to the last date of changes and also it got prepared by some lots of work in theory and practice over years, with the backgrounds for all kind of possible usages, hereby being either very small, or coming in a very handy size, but also thought to be suitable in the end to all kind of existing Synthesizers and also of course as a quite larger possible version, for big Modularsystems. In a second manner, the project is also meant as a trial for getting a more precise thing out of that special VCF-Topic in the end, what means for the Steiner-Parker-Concept as generally for being a little difficult and therefore this gets also somehow for a challenge, due to the used arrangement of all the parts in this kind of an electronic architecture, as also to say for this Filter-Topology in the case of a usage with a ladderlike and widely tuneable arrangement. Because of that facts, the Steiner-VCFs are often known for to fit only on a possible maximum of two, or maybe three useable, or let´s say playable octaves, by what they really could get tuned for, and this with respect of working without any problems. This fact is changeable, as I think so, and this needs maybe only some more research-working, but with the respect on modern possibilities and also maybe by using todays available SMDs and hereby also those supersmart matched parts, like Dual-Transistors on high performance, wich are mostly not too much expensive and wich got developed for automotive usages in harsh environments, where now the Synthesizer-Usages can get also a wide benefit onto that. Therefore also a part of my ideas goes onto the goal of maybe in the end coming to something like a "Steiner-Parker-Synthesizer-IC", or such like that, as a small and standardized module and hereby again fixing that without any problems into different DIY-Projects with universal abilities. As a possible outsourcing commercial product to this project, me and some friends (almost a group of 3 and sometimes more) have planned also a standalone useful audio-effectmodule, maybe also for guitar-usages and maybe performable as a special Wah-Wah-Pedal, or such like that, etc., with all additionally needed parts on it, like an input-mixer, maybe an envelope-follower, or pitch-dependent units or maybe a PLL, several switchable patchways and maybe additionally what would make any sense anyhow, but as I hope so in the end, not blowing the whole thing too much. The modules will come with a suitable mechanical performance on these backgrounds, generally with 4 standardized holes on the corners (- as used for the most things... so not laugh), for a possible fixing as needed and the PCBs will be routed by means in a 2,54 mm gridsize, so in mind for the user can fix them very easy and valueable on prepared veroprints, for an easy adapting to this standardized routing grid.

To all these facts, as I hope so, Nyle Steiner is also for favourite as the main leading Inventor to that thing, where I asked him for that in the last few days by an email, but until today I haven´t got an answer yet. (Maybe that planet is too large, - so let´s work - on the project.)

The Steiner-Parker Multimode-VCF

The project for this Synthesizer-Module follows with its main theme - and also all the presented circuit schematics - on the first basic principles of "Nyle Steiner`s Synthacon-VCF", with a performed "Sallen–Key Filter-Topology", for wich an early development with a published circuit was presented in the year of 1974 by Mr. Steiner in the "Electronics Design Magazine". This first circuit was quite simple, but showed in all manner all the possibilties and directives, as also some details and theory to that concept and it is in a special way implemeted with diodes as current-driven resistors, where two additionally diodes on the positive an negagive pole of that string are needed, to block the generator voltages of the filterdiodes and therefore, for not cancelling themselfs around and nulling the audio over the used "hangup pole" of the control voltage, and what here means also the true difference, compared to a ladderlike-construction. But right on this point the thing starts to get a real science, and where in the end it is just needed to mention, that applied currents over (let´s say for standard diodes and this differs only a little on several tested types) maybe around 30-50-80µA, won´t change anything useful on the frequency-parameters of the string anymore, and this almost due to the saturation effects on the PN-Regions of the used diodes, and now also to drive more current and different semiconductor-voltages over the diodes, doesn´t by fact make anymore sense!

Several voices on the whole internet (see here for instance some good informations on the website by Mr. Dieter Doepfer, or also Mr. Steiner reporting on these facts then on really much different experiences!) are herefore giving quite less data, but this basic circuit got afterwards used in quite a lot of known analog Synthesizers, until up today and with surely no less on actuality and also on several much modifactions for the basic thing. One of the main goals on Mr. Steiner`s filter-concept was the fact, that one could get from a 100mV squarewave-audiosignal, in the resonance-mode of the filter, produced signalpeaks around of 2 volts, or even more, without any problems. Another great benefit of the concept is its valuable and for not to say "inexpencive" structure, or better say organisation, what means for this next highlight of that thing, that it fits on a very good audioquality, with not too much money to spent on the herefore needed parts, as that fact is known for several other VCF-Topics in the wider Analogsynth-Area, so of course wich can differ much and quite often not giving any superbly results, as the Steiner-Filter does. A second background for this circuit is the quite large tolerance for mismatches on the herefore used parts, due to in parallel arranged Resistor/Capacitor-Filter-Units, so with short words to say, is here on the filterelement only the need for preselecting the diodes (easily possible by the use of the diode-tester on a Standard-DVM, or also quite often - on my personally experience: If you buy SMD-Dual-/Double-Diodes - for instance maybe inexpencive BAV99 - as whole film-stripes or chains and by several dozens, they come out there as mostly matched pairs over the whole chain). Finally, there comes only the needed use of half a dozen of capacitors on a good quality (here the best using is Styroflex) and then a few 1%-Metalfilm-Resistors. Also because of these facts, the concept is quite easy to perform for motivated electronic beginners by accurate working on DIY-Directives, and on a simple and small veroboard (those expoprints are very good for an easy usage of SOT-23 or SOT-143 SMD-Parts on the soldering side), but hereby also giving the possibilities for later expansions, or on maybe on highly matched parts if demanded and then to get a really great and outstanding device and therefore being not only some electronics, but in the end on the interest to get a real good "Musical Instrument", but then of course you should also match all parts as good as possible.

The main PDF-Circuit for this project is quite large (but still fits to one regular print-sheet!), for being as universal as possible, but I will update another post with a smaller schematic, especially upset for beginners and therefore also fully tested. Also a full universal version is therefore at my lab in progress and I will upset some photos and a test report as soon as available.

The used Techniques for the Synthacon as Invention, some Problems and the possible Sounds

The main topic to the hardware and the behavior of this filter is a real unique one, but also only on doing this like Mr. Steiner´s way, it doesn´t have any errors as much as of a possible Q-Damping on the Audiosignals, like this is well-known for the Moog-Ladders (what means lowering of the volume of the mainsignal in Lowpass-Filtermode, when increasing the Resonance of the Filter, but then also lowering the Signalvolume), or other disadvantages like that and quite often known also from other voltage controlled filtertopics. As another part of the very unique characteristics of the Steiner-Filter there is on the other hand the quite easy possibility to get the filter into a no more controllable "screamy Oscillation-Mode", what also could get a little problematic, because when this point is reached too soon on the wiping way of the Q-Controller-Pot, then the result can go that way, that it doesn´t satify anymore by the main performance of the whole device.

The whole background to this project addresses mainly these facts, with a later in detail described solution on the use of a special hardware enhancement, and to solve such problems, as the above told "Hang-Up in Resonance-Condition".

How it works on the main principle and the extensions

The basic corefunction to the systematic of the Steiner-Filter, as to say this for an herefore individually performed and physically Invention, there exists the so-called "Current-Tuneable-Diodestring-Arrangement" (this is so called, because it is not really, but quite often told for a "True Ladder Filter", like this is known for the Moog-Filters, but the Steiner-Filter is maybe more like a callable "Balance", wich got "hung up" on the positive Pole of an attached Voltage-Source and wich is "vented", or better say "biased by a positive working-current", wich sets the filters frequency-pole/resonant point), where the biased diodes are working as current controlled resistors, in conjunction with the non-variable acting filters capacitors, to perform a widely variable and possible combination of Low-Pass-, "High-Pass, Band-Pass- and All-Pass Filter-Characteristics", in a tunable Frequencyrange of 20 Hz to 20 kHz. By changing the insertion-points for the audiosignals, you get the several filter functions on the performed characteristics, wich therefore can be told as possible massive Basses in Lowpass-Mode (LP), but there could also come sounds in characteristic Bandpass- (BP) or Highpass-Modes (HP), like maybe coming out of a paperbox, or also as quite "bity" and maybe "metallic" and "radiolike", so therefore the told Multimode.

A needed additionally performancepart to these modes of the filter is also the fact, that each mode has an individual level for the given volume on the audio-output, wich therefore sometimes needs to get observed/controlled and adjusted separately, and what again leads to another consequence of some more and additionally needed electronics around of that filtercore at all.

But right on the view to this point, there is nothing new in this project, so here come the extensions: As a first enhancement to other modern known Synthacon-Circuits (for me up today, the most of those are using opamps inside of the Q-Control-Loop, to get the thing in a stable working condition), the here presented circuit has an additionally implemented electronic Q-Factor-Control, wich is done by an LM13700-OTA and wich gives the advantage for setting the resonance level for the filter, by applying a defined controlling current into that unit, the so called Iabc of the OTA. The OTA is on one hand of course also a true Opamp-Function, because of its differential Input-Transistors, but it is therefore also really different to that of an Opamp, because this is in the end a current-driven action of its current-output, where this extends the thing to control the above told diodestring as a driven Impedance and where the OTA loads the Capacitors of the Filter by a portion of Charge (Iabc) and what gets controlled by the applied Feedback-Loop for the Resonance. Hereby the OTA works unlike an Opamp without any directly applied Feedback-Control between its Inputs and Output, as this would be known as typical for the Opamps and what comes now a little funny, because this missing parameter in case of a voltage feedback gives now also the possibility to get separately modulated by a selective control-current, the said Iabc, coming from somewhere outside and from various possible modulation sources and therefore extending all the possibilties, but again in a really different manner, than this would happen instead within an Opamp´s control loop. But only this is just half the way, to get this to a real goal on the whole thing and for getting something advanced!

As a second improvement to Mr. Steiner´s early construction, there are additionally two Precision-Currentmirrors (BCV61/62, this means 4 additionally applied and manufactr.-matched Dual-Transistors and twice the needed working current of the device!) inside of the upper and lower biasing-/balancing-system for the diodestring, where these mirrors are monitoring the "Energystate" (= directly the Lowpass-Audio, on highest possible DC-Components on the point of the incoming AC-Signal and therefore giving another possible Audio-Out for the so-called "Lowpass-Boost" - Please refer for this also to the schematics and to the JPG-Diags) and therefore also the main feedback signals for the Q-factor controls onto the positive and negative Inputs of the used OTA and giving the possibilty for controlling the "Energy-Phaselag" inside of the Diodestring and for the whole "Feedback Loop" again in conjunction with the applied inputsignals and wich returns the reproduced Audiosignals back into the Diodestring. If this systematic, or respectively its whole product, gets highly unbalanced, then you have to deal with that uncontrollable "Hang-up in Oscillation-Mode" and no more getting in conjuncunction with any new input! On this fact there is very important to see, that the main scene and dynamic in the filter works only on positive polarized signal-waves, forced by the applied biasing of the diodes (in the main PDF-Schematic between Points A and B), where only the audiosignals from outside, or by the feedback-loop from the OTA, are serving electrically negative-potential-parts back into that construction and therefore affecting any possible limitation on the feedback-amplitude. The two current mirrors are working also in conjunction, where the upper positive mirror sources the current (containg hereby again the whole audio-dynamics, wich are performed in the filterstring!) downwards over a pair of clamping diodes (wich are performing on Schematic Point C a "virtual zero-pole" inside of that system) to the lower current mirror, wich sinks this working current. The said diodes are additionally trimmed on that point by a trimpot (Emphasis-Symmetry - could also get a useful big Pot on the outside) and therefore also nulling the OTA´s offset-errors, and splitting the whole circuit in its upper positive and lower negative parts.

Not really needed to say, but I think here to mention, that both current-mirrors reproduce the audio very fast and so to say promptly, and therefore keeping again the highly possible dynamics, for wich the Steiner-Parker-Filters are known for, and not lowering the audio anyhow, maybe by compressing the main audiosignal, or such, because this compressed signal passes only on somekind of overload and only for the controlling section in case of the feedback. The upper mirror serves hereby the signal inverted (mirrored) directly for forcing the active feedback onto the negative input of the OTA, where in this chain the lower mirror only comes to action, if the signal gets too much (and also if the control-voltage arises and therefore more current coming from the emitters of the LM3046-Transistors, but where this is quite less on the whole action, on only some microvolts, but enough to control this system to its zeropole), so that it can pass by the clamping diodes and reaching a limiting, over the positive input into the OTA and what gives therefore the closed loop in that thing!

For these lines also very important to say is, that the here performed signals are working only in the range of a few millivolts, but with enough level, to get this loop controlled onto the OTAs inputs (This conditions could get varied, if you want to experiment on that part of the loop, if you change the values for the Emitter-Resistors, maybe from 1:1 to 1:2, or 1:0.5, etc., at the Currentmirrors. I will place another schematic for such a modified circuit and with some changed values over all, in the next few days!). Additionally the OTA gets from another important point also activated for this feedback-loop, by the Signal at the High-Pass-Input-Node (HP - over encircled Resistor R40), to perform an active potential (on its Output-Node on point D) for the capacitors C12 and C14 and therefore to perform the main Filterfunction! Therefore the Q-factor should never get to a zero-state, what would mean no more control current into the Node Iabc of the OTA. Without this part of the whole loop, those two caps would hang electrically in the free air (floating) and then performing nothing! This means also in fact, that the Resistor R40 should be choosen and valued for almost the best performance and this should get observed very well and always stay in mind, if you want to do any changes in that loop. The value for R40 therefore also depends on the individual data of the OTA (this can vary perhaps a lot between LM13700/NE5517/NJM13600/CA3080/NTE996, etc.) and maybe this needs to get changed again or adjustable for working ok! Additionally and right on this point D, on the output of the OTA, this IC has its highspeed-abiltity of a 2 MHz frequency range, in conjunction with the working current-source of that device, what results in a very fast control on higher frequency phasing-effects with that filter (this is a part of the thing, I also haven`t tested yet in all manner, but this gets again really pregnant, if you insert different signals into that thing, on the separate HP, LP or BP inputs and only on these facts rules that different construction with the used Current-Mirrors for making any sense). Another background to such a choosen regulation and mechanism, is also the possible problematic for getting the OTAs in the right arrangement for properly working and hereby with no damping anyhow on the performed audiosignal, and resulting in decreasing any parameters of the possible dynamics, for wich the Steiner-Parker Filter is beloved and known for. This additionally means also the benefits from the other possibilty, to place that electronic diode-clamp into the feedback- and control-section within the OTA-Loop, for getting the whole construction to a more controllable resonance characteristic and therefore not to get too early in a state of a resonance-oscillation of the filter. Of course you can reach that "Point of no Return" and then get an oscillation within that circuit, but with much more possibility to control that onto the point to get in overdosis.

This works only tellable for right and then gets ok, when returning to non-oscillation by a small back-move on the Q-Pot, where here additionally is needed to say, that the amplitude of the resonance gets determined by the values of the filter-capacitors in conjunction with the driven current/charge dependent to the resistor R75. The smaller you select the value for this resistor, the smaller could act the resonance´s amplitude by its maximum. So another conclusive fact is here on these conditions, coming therefore very important and this is highly recommended to get observed!

In my simulation program (Electina V10) I have tested the whole loop in all manner, and also with severeral OTA-Models (also on really "harsh" selfmade parts, constructed with Current-Controlled-Current-Sources, instead of transistors) and always got the same performance, so this should work well without any problems! Old Standards with multiple Inputs for several Filter-Characteristics, on 2SK30A J-FET-Analogue-Switches The whole thing gets best performed by two Quad-Opamps (also a matter of taste, I would prefer LF347CJ, or TL074CJ-"Blacktop", if availbl.), one good matched Transistor-Array (by proper selecting for matching the Diffamp-Pair on LM3046, CA3046, or also specially selected UL1111 by the Ex-Russian Military-Surplus are quite good) for possible best frequency tuning, the OTA (LM13700N, or NJM13600, best would be CA3080A) and the Currentmirrors (also recommended matched parts - preferred BCV61A/BCV62A-NXP-Infineon for Europe, or DMMT847/DMMT857 manfctr. by Diodes-Zetex almost availbl. inside of USA, but ?). The shown JFETs 2SK30A are almost vintage & obsoltete and of course interchangeable to others, as needed and if present. The schematic shows all inputs switched on and therefore all the switch-jumpers should be set. Each input can then mix on 2 audiosignals (8 inputs on LP, BP, HP, AP)

Like in the 1974-Version of the Filter, the audiosignal gets best and very fast coupled out over two Transistors (with a fixed amplification by 2) and an additionally arranged Output-OPAMP, wich decouples and preamplifies the audio again and wich is performable by needed conditions, set by the user. In my debutant kit (just in progress) I have choosen for these Transistors two Russian Low-Noise Germanium-Parts (MP11 & MP15). Here in Europe those are still available, but if not, there could also come every other standard transistor for possible.

Some other additional parts and facts to the project and to the schematics:

Again very important to say is, as also a given Directive from the National-Semiconductor-Datasheets for the LM13700-OTA, to get all parts in that schematic best situated on performed Currents around 1 mA ,as this rules for half the maximum of the allowed value for that IC. In the following folders to the project I will also place again the old circuits, as they were performed in 1974 by Mr. Steiner, as soon as I have his OK therefore. Compared to the new circuit there goes now also a portion of the signal directly from the Buffer-Transistors back onto the BP-Input on the Filter-Caps (100 kOhms in serial with 3,9 or 4,7 nF) giving a little bit of a permanently fixed feedback, but what also cancels too much going high-frequency-components, wich could come by the OTA-Section and therefore this results in a more stable action. Dependent to a possible varying PCB-Layout and other possible individualities, this snubber has to get optimized and changed maybe in values again. Another change was done by decreasing the main query-current up to the section with the diodestring, where the original working current was around 2,5 - 3,0 mA (according to Mr. Osamu Hoshuyama´s directives, best performed by an applied JFET-Currentsource as shown in the schematic as alternative!) and wich got changed now to 1,0-1,35 mA (by referring to the Elektor-Formant-Books, here is also to say that the best Log-Conformance of the Dual-Transistor-Q3DT (LM3046) lies around the values 10-100 µA, with a maximum at 500 µA). A next step hereon was to lower all capacitor-values to 1 nF or 470pF, instead of 4,7 nF. In the Simulation-Prog I have tested this circuit with caps down to 180 pF and the stuff all together worked optimized very well, without any problems, so what to choose in the end, is by everyones selection variable and therefore to be tried and tested, as the user likes to.

And again: Resuming on longer made experiences, I highly recommend not to change the Currentmirrors to regular Transistors. Of course this would work, but if you want the best quality sound out of that thing, then use the BCM61B/BCM62B by NXP if possible (availbl. at Farnell or BCV61/62 availbl. at Reichelt/Germany). Those better matched Mirrors are also housed in SOT143-Packages like the BCVs and are also optimized for the best Temperature-Characteristics, this Manufacturer can do. Therefore they are only offered in that size and surely they won´t get packed in different cases/packages anyhow, because this doesn´t make any sense. As a conclusion to say is, that the presented schematic is arranged like that, for all best possibilities on experimenting with that circuit, so all values might get also changed a little.

The Circuit will not work without any Input-Capacitors, resp. on the LP-Input !!

Not to change is: R76, R86 (for not killing the OTA - !!)

Only carefully change: R58(working current for the whole string), R84,R5 (only for Voltage on PWR)

The 4 x 47 Ohms Resistors on the Emitters of the Currentmirrors are not directly needed but got choosen also for maybe varying in experiments and changing the transfer-ratios of the CMs.

With incoming more experiences on that project, I will update this in future by some more data. Also as a little resumee and a conclusion to this project again I have to say, that I am not really a big friend to only theoreticals and mathematics, and this especially on the calculations for filters (!!Skulls & Thunderbolts!!), where of course those parts are also nessesary in the end.

Because of this fact, and also on the new kind of the here used and described electro-mechanism for this circuits and that Feedback-Loop, I haven´t upworked the schematic for mathematics yet and therefore I cannot present here any working-formulas. My simulation program gives quite easy the possibilities therefore, but until up today I did not find the time to upset this, so I hope here for the readers pardons. Contributions and maybe some help from the community, and maybe also from the elektor-labs in this case, are therefore highly welcome! All the material for this project has got commercially copyright protected by myself in the mid of 2013 (Priormart-System: Basic Principles for the used Currentmirror-Loops on the Schematics/Circuits, PCBs & Text -, but is directly open for all kind of D.I.Y. and generally all usages in a private manner. Every Elektor-Reader is invited and allowed to copy all the material for his own private use! If you want to use this material for any commercial purposes, this is of course possible and please contact me therefore with an email to, instead of leaving any contributions on the project.

If you want to use parts of this material on Nyle Steiners copyrighted own, then please contact him directly, according to the directives given on his holding and on his official website at


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Project on Current-Mirrors perf. without OTAs
Circuit without OTAs as shown JPG-Schematic
Adjustment Performing on diff. Points shown for Highpass

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