Synthesizer Build part-26: STEINER-PARKER VCF (Yusynth Design).

An awesome sounding filter with LP, BP, HP and AP modes and a resonance that can be very agressive! (Beware your speakers, LOL). It's way up there with the ARP filter as one of the best sounding filters I ever built on stripboard.

Let's see, we've done the Moog, Prophet One, Korg, Dual Korg, ARP and Digisound so this is filter number six not counting the dual Korg. This is an other Yusynth design and once again I managed to make a Stripboard layout that turned out to work straight-away.

With this filter we get all the different options for filtering sound in one module; Lowpass (LP), Bandpass (BP), Highpass (HP) and Allpass (AP) which is more a phase shift effect although not very pronounced. The filter is self-oscillating in all modes (!) which is not usual for a 24dB/Octave filter I believe.
This filter has a very distinctive sound so it's an excellent addition to the synth because I want as much diversity in sound as possible. Especially the Bandpass and Highpass can have a very cool distortion if you turn up Resonance so it almost sounds like a heavy metal guitar. The filter was re-designed by Yves Usson (YuSynth) to do away with some of the erratic behaviour of the original design, but I noticed he didn't manage to get rid of it completely. The resonance can give a very loud whistle and if you turn it past that, with cut-off freq turned down, the filter stops working with a deep pop that can't be good for the speakers. I've included a 5K trimmer to replace the 3K9 resistor, R18 in the schematic, so I can set the reach of the Resonance control a bit better. This works up to a point but it's not possible to tame the resonance completely this way. The resonance gets better as you reduce the resistance of the trim pot but this reaches zero Ohm before resonance is completely tamed. But then again, that's the character of this filter so you don't want the agressive behaviour to disappear completely. 

One way to tame the resonance is a method mentioned in the comments below and recently tried out by one of our Facebook group members. It is to put a 10K resistor from pin 3 of the resonance potmeter to ground. In other words, you put the 10K in parallel over the potmeter, from the left pin to the right pin. You only have to make this change if you find the resonance potmeter to work too aggressively. If it's not a problem for you then you don't need to install the 10K resistor. (I myself don't use it).

Here is an illustration of where to put the 10K resistor. All the other connections are left as they are.

The panel potmeter needed for Resonance is a 50K reversed logarithmic one but in the Yusynth article he explains that you can use a 100K linear type with a 100K resistor soldered between pins 2 and 3 of the potmeter, so that's what I used and it works just fine. That's also the way I drew it in the layout because I figured not many people would have a reversed logarithmic 50K potmeter lying around. If you do have one though, use it.

I didn't have a 2-pole 4-way switch but I did have a 4-pole 3-way switch, a vintage 1970's one too, which I was dying to use in a project. To compensate for missing that 4th position of the switch, I added a second DPDT switch so I can switch from High-pass mode over to All-pass mode. In the layout it's a SPDT switch but I used a Dual Pole DPDT switch and I used the left over pins to switch on some 3mm LED's to indicate visually if the filter is in HP or AP mode. This works very well. The LED's are normally off and only if the main switch is set to HP does the 2nd switch get power to light up one of the LED's. I used one of the left over poles in the 4-pole switch to switch the power for the LED's on or off. This looks really cool and it works perfectly so I now have all 4 options available. I did not add the LED's etc to this layout because I added it as an after thought but it's easy enough to add something like this yourself. Anyway, you would normally use a 4-way double pole switch, so there would be no use for LED's then.

One thing to be aware of, make sure the TL074 you're using is not a cheap fake from China. I'm becoming aware now that I've got a lot of fake opamps installed in my modules. I recently got a batch of good ones from a reputable source and it makes such a difference in the sound this filter makes. The fake ones will work and for low frequency stuff like LFO's you won't notice the difference but the fake ones can not handle the high frequencies that resonance produces very well. So make sure you invest in good opamps.

One other thing I'd like to advise is measure the value of every component before you solder it in. I always do this myself too. Some components, like resistors but especially capacitors, can be way out of spec, especially if you use a second hand stock de-soldered out of other PCB's. This can present you with big problems when you need to troubleshoot the filter. So save yourself a headache and measure before you solder. This goes for all the projects on this website naturally. 

One more important issue I need to address with this filter is that it is susceptible to hum from your powersupply if you are using a switching powersupply. In my DIY synth I use two linear powersupplies and they deliver a noise free Direct Current but switching supplies can have a lot of high frequency noise on it which this filter doesn´t like. A good solution is putting some big electrolytic capacitors on the powerrails, on the stripboard. I myself used 1000µF caps for the filter I built for my Eurorack system (which does have a switching powersupply) and the hum is gone!  

Here's a link to the Yusynth Steiner project with the schematic and panel design. 
Here's the stripboard layout and wiring diagram I made for my specific needs with the somewhat odd switch arrangement. This is posted here for my own reference. Use the other layout beneath this one:

The Steiner-Parker filter will work fine on a dual 12V power supply. No extra changes are needed.

LAYOUTS:

Here is the version with a normal 4-way double pole rotary switch. These layouts are verified, I used them for my own build and they have been successfully used by many others, so you can print this one and use it as the wiring diagram. Please note in this layout I left out the second audio input jack and potmeter, the second CV input jack and potmeter and the 1V/Octave input jack, so remember to put those in! They are just copies of the first inputs with a level potmeter and an input jack for each. Except for the 1V/Octave input which is just an input jack. So in total we have 6 jack-sockets and 6 potmeters.:

(Last revised 03-Nov-2021: Cosmetic changes)

All my stripboard layouts are made on 24x56 stripboard and usually I use the whole board. Because I use the Kosmo format of 20cm high panels I have room enough to accommodate them behind the panels I make. So the builds on my website are not very Eurorack friendly. I keep the components spread out to make troubleshooting easier should that be necessary. There is now also a Eurorack version of this filter available on my website. Just click here to go to project 45.

Stripboard only. (Print this out and use it for your build. Don't forget the cuts underneath the chip.).

Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Schematic:

You can see from the schematic drawing how simple the design is. It's in fact just a diode ladder with 4 capacitors and a few buffers around it. A very effective set-up as you will notice after you successfully built this filter.

An overview of the cuts and wirebridges seen from the component side:

And here's a look at just the cuts that need to be made. This is viewed from the COPPER SIDE!!

Bill of Materials. The diodes and transistors need to be matched so I suggest buying a batch of 100 of each (or at least for the transistors. I didn't match the diodes because they were all from the same batch and close enough). They only cost pennies on eBay anyway. That way you have some stock from which to find matched pairs:

(Last revised: 15-May-2020 Added extra potmeters and input jacks that are not shown in the layout. 14-Jan.-2021 Corrected spelling errors.)

Calibrating the filter:
There are three trimpots on this circuit and the way to set them is just to turn them and listen to the filter's reaction. They mostly influence the cutoff point of the filter. The 5K trimmer, as mentioned before, is one I added myself. On my print it is turned to zero Ohm to give the Resonance potmeter as much throw as possible. So you could actually replace it with a wire bridge but I advise to just put in a 5K or 2K trim potmeter.

Just turn the trimmers for best sound is all I can say.
For the 50K logarithmic pots for audio in, you can use linear types if you wish and the value for the potmeters for audio and cv level is not important either. You can use any value from 10K to 1M for those potmeters. I put in 10K ones myself because I have a lot of those in stock. But do use the right value potmeters for the Cut-Off Frequency and for Resonance. (I guess you could use a 100K for Cut-Off but then the 47K resistor needs to be changed to a 100K type but I have not tried this and I don't guarantee that'll work. So just keep to the recommended values for those two potmeters.)

Here are some pictures of the finished module:

The 'finished' stripboard. Notice anything wrong? Yep I managed to forget 4 components, one 220K resistor, two 10µF caps and the earth connection for pin 3 of the TL074. So when I first tested the filter it didn't work like it should. (Even without the input caps I was still getting audio in. I was very surprised because without those caps the inputs are effectively cut-off from the circuit.)

In the first instance I didn't notice a big difference between the different modes of the filter. That was until I discovered I had forgotten to earth pin 3 of the TL074. Now that I've done that the filter sounds even better than it already did. I noticed that with this filter you can hear the original sound coming through the filtered sound if you turn Cut-Off and Resonance way back. I've done some research and this does seem to be exactly how this filter should sound so my build is spot on. Here's the filter in action in the Arturia MatrixBrute:
https://www.youtube.com/watch?v=7ul95vmNFwM
The Arturia MiniBrute was designed by Yves Usson aka Yusynth with help from Nyle Steiner, the original developer of this filter from whom it gets its name..

Here's a video demonstrating the sounds this filter can produce. It's my usual 'ploink-ploink' on the keyboard, but even so it sounds just amazing! I especially love the Bandpass mode. You can instantly see the squarewave disappear when switching from lowpass to bandpass and you're left with all the harmonic content on top. I LOVE this filter. (Perhaps even more than the ARP filter because of the these different modes.). You can hear my comprehensive conclusion at the end of the video; a single word... WAUW!! (<- That's Dutch for WOW!! ^^)

In case the video below doesn't show up on your mobile device, here's the link: https://www.youtube.com/watch?v=kGf4HB5miJY

Here's an other video, NOT by me, with a very methodical demonstration of all the functions of the filter. Even-though this filter was built with a ready made PCB, it reacts exactly like the the one in this article coz it's the same Yusynth circuit: -- LINK TO VIDEO --

And here's a YouTube video by Alan Wolke (W2AEW), demonstrating how to match diodes. Btw, his YT-channel is full of interesting electronics videos. They're not synthesizer based, more radio frequency stuff, but very interesting none the less. (He mentions a DMM a lot. That stands for Digital Multi Meter, just in case you didn't know ;):  -- LINK TO VIDEO --

Finally I want to share with you a picture by a member of the LookMumNoComputer Forum, Duane Kelly (Doolang), who successfully built this filter using my layout only he made it Eurorack size by cutting the print in half and connecting the necessary copper strips together. He did the same with the Digisound 80 filter which also worked perfectly.

As of December 2021 there is now a Eurorack version of this filter available on my website. Just -- click here -- to go to project 45.

Okay, that's an other filter to add to our collection. I have a few more on my wish list and now that I have the second case finished I have enough room to accomodate all the modules.

If you have any questions or comments, please leave them in the comments below or post them in the special Facebook Group for this website.

Synthesizer Build part-23: DIGISOUND 80.6 LOWPASS FILTER.

A very cool AS3320 design that sounds amazing! With verified stripboard layout and new schematics.

After having taken out the Sequential Pro One lowpass filter to make room for the Korg filter, I needed a new use for the AS3320 chip that was inside it. I found the Digisound 80 point 6 lowpass filter module on this awesome website that has all the schematics for the entire Digisound 80 modular synthesizer.
You can configure the filter for any type you want (it's all in the original text) but we are going to build the lowpass filter because for subtractive synthesis the lowpass is the best sounding and most useful of all the filters in my opinion.
I first made a new schematic drawing because the original had those zigzag lines for resistors and I find the rectangular way of drawing resistors easier and you can put the value of the resistor inside the box. Makes it less complicated to look at imho.
Anyway, here's the new schematic drawing:

So after that was finished I made a stripboard layout. It is verified because I used this twice and both times the filter worked perfectly. Furthermore it has been used successfully by others in their builds. Make sure you work accurately though because I wouldn't consider this a beginners project. The layout includes a second audio output with 3 times the gain of the original output. This is of my own design and is not included in the schematic drawing. It is this output that is wired up to the output jack-socket in the layout below. The original output is marked on the layout too. More about this further down the article:

(Last revised: 19-March-2020. Added a second audio output with 3 times gain compared to the normal output. 25-5-2023: Removed colour codes from resistors, added colouring to wirebridges.)

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Cuts and wirebridges seen from the COMPONENT SIDE! As always, mark the cuts on the component side with a Sharpie and then stick a pin through the marked holes and mark them again on the copper side and then you can cut them with a sharp hand held 6 or 7mm drill bit. 

Bill of Materials:

The panel potmeters used are all 100K linear types but the value isn't that important. Since they are all connected to either a powersupply voltage or an audio signal you can use any value you like from 10K upwards. I myself used three 100K potmeters for the Coarse, Fine and Resonance and I used three 10K potmeters for the audio and CV level controls. This works just fine. 

You can choose to include the Frequency Fine control potmeter or leave it out to save more room on the panel. I personally never use it but it is there if you want to play the filter as an oscillator when it is in full self-oscillation mode with the Resonance turned up full. You'll need to tune the self oscillation pitch to the chromatic scale of notes so in that case a fine tune knob will be very useful. But I personally never tried this so I don't know how well this filter responds to that. If you have any experience with that then please put it in the comments below so I can share it in this article.

This is a 24dB/Octave, 4-pole LPF and it is self oscillating unlike the Prophet One filter I used this chip in earlier. That one refused to self oscillate. I used simple ceramic capacitors for the 220pF caps and this works fine. There's no need for fancy polystyrene caps ^___^.
The 1µF electrolytic cap C7 at the input may seem to have the wrong polarity. Usually a cap like that would have the positive pole connected to the point where the signal comes from and negative to where the signal needs to go. In this case it is mounted correctly because the input opamp is an inverting buffer with a negative gain reducing the amplitude of the 10Vpp input signal by a third to an amplitude the chip can handle. In the output buffer the signal is then inverted again to a positive signal with a gain of 3 to give us the original amplitude.

Calibrating the filter:
There are three trimmer potmeters on this print and you can set them as follows:
RV8, the 100K trimmer, is used to trim away the DC voltage on the audio output. Measure the DC output voltage with nothing connected to the input and turn RV8 until it reads zero.
RV7, the 20K trimmer is an interesting one. It's used to have the filter track 1V/Octave oscillators correctly but I simply tune it for best sound. If you have a squarewave on the input and you turn this trimmer you can clearly hear the over-tones, the harmonics, change in pitch. You should be able to hear the frequency beating effect of the note from the VCO against the tone of the resonance. Trim until there's no frequency beating but also listen to the tone while changing the cut-off frequency and trim until it sounds right to you. There's a full description of the proper way to calibrate this filter in the original text, which is in the link I mentioned earlier in this article.
The last trimmer is the one in series with the current limiting resistor for the AS3320. Simply measure the resistance and set it so the total resistance of the trimmer with the 1K resistor equals 1,5K. You could also just put in a 1K5 resistor but turning this trimmer does have a little influence on the sound but you'll have to try it to know what I mean. I just set it to 1K5 and left it at that. Turning this trimmer all the way to zero resistance won't damage the chip though, eventhough it needs a 1K5 current-limiting resistor, 1K won't hurt it. The one thing I learned building this filter is that the AS3320 is quite a robust chip. I made a few mistakes building it the first time and the chip has had voltages (through resistors) placed on the wrong pins, short circuits and all sorts of other mishaps but it survived all that without a scratch. Thank goodness because I only have one of them at the moment :) Luckily I was able to test if the chip still worked by placing it back inside the old Prophet One filter and seeing if that still worked. That was very useful.
Anyway, you can use this filter with a dual 12 Volt power supply, but in that case the current limiting resistor should be 1,2K in total. But it's really not that important. Simply connect it to +/-12V and it should work fine.

This filter sounds amazing! It has its own distinctive sound and I can not say it sounds like the Korg or the ARP or the Moog Ladder filter. It sounds like a Digisound 80 filter, although it comes veeeeeeery close to the ARP in sound. This one sounds a bit more well behaved, if you know what I mean. The sound of the Resonance is clearer than in the ARP which has a Rensonance that is sharper and rougher in sound. But that's the only  difference I could hear so it occupies a solid second place over the Korg-MS20 and the Moog Ladder filter in my personal top 5. The ARP filter is still number one because it's a real rebel and I love it. But hey, remember, this is all just my personal preference. You may judge it quite differently. Actually, I find myself using this filter more often than the ARP filter somehow.
The output from the Digisound 80 LPF is a bit more attenuated than the other filters I built and that's why I used the left-over opamp in IC-1 as an output buffer with a gain of 3. There's a 150K resistor from pin 6 to ground and double that value, 330K, as feedback resistor from pin 6 to pin 7. (You can use any value over 10K for both resistors, as long as the feedback resistor is twice the value of the resistor to ground.) This brings the volume up to the same level as the other filters I made. As I mentioned earlier, the amplitude is first divided by 3 and then multiplied by 3 again in the output opamp but, at least in my filter, I found the sound still lacking in volume compared to the other filters. That's why I wired up the left-over opamp as an amplifier with an extra gain of 3 to bring it up to normal. I'm not sure if it's just my filter or if this is normal, that's why I left the original output un-touched so you can use it if you think my solution is too loud. You could also install a potmeter of 500K instead of the feedback resistor so you can manually set the gain. (Put a 50K resistor in series with the potmeter so the feedback loop can't go to zero Ohm.)
The original audio output is marked on the layout so you can choose which one you want to use.
The first time I build this filter I had used a coupling capacitor of 4,7µF over the audio output because I measured a big DC offset voltage on the audio output, but then I read the original text and found out you can trim that away with trimmer RV8 so I took out the cap and trimmed the DC away and now it's all as it should be.
This filter has an input for 1V/Octave but unlike the ARP filter it's not necessary to use this. The filter will work fine without it but if you connect a 1V/Oct. source to it, the filter will track the octaves better. The sharp synthesizer sound we all love, will be more prominent if you use the 1V/Oct input. It actually makes the filter sound better.
Like I mentioned before, this filter has 2 potmeters for the Cut-Off Frequency but I advise to only use the 'Coarse' control. Fine is only for Polyphonic synths. I included it in my build so I could hear its effect and write about it here, but I normally don't use it. It stays in the middle position because turning it just changes the sound a tiny bit. It can be handy though to tune it into a certain harmonic frequency because this filter brings out the harmonics of a square wave really well, but all in all; leave it out.

Here's a picture of the finished panel built into the synth.:

As you can see in the picture I also included a bypass switch on my panel so I can put other filters in series with this one and if I only want to use one filter I can bypass this one and send the signal straight to the next one without having to change the patch cables. For instance, having this filter in series with the Korg MS-20 in High Pass mode sounds pretty amazing too! That way you have a Band-pass filter made up of two different filters. The bypass switch is only connected to the 'Audio-1' input though. If you want to see the wiring diagram for this switch, you can find it in the article about the Moog Ladder Filter, in which I also installed a switch like this.

Here's a little video with a demonstration of the sound of the Digisound 80.6 LPF (EDIT: at this stage in building my synth it still hadn't occurred to me that you need to connect an AD or ADSR to the CV input to get that typical synthesizer sound. #facepalm (We live and learn LOL):

Lastly I want to share with you the efforts of LookMumNoComputer Forum member Doolang who successfully built this filter using my layout. He made it so it fits the Eurorack standard by cutting the print in half and connecting the copper strips together with wire. This works like a charm and he did the same with the Steiner-Parker filter which also worked fine.

So that's the Digisound 80.6 lowpass filter done. I can really recommend building this. It has some very recognizable synthesizer sounds that you should really have available in your synth. Make sure you use good quality stripboard though. The first one I built had problems because strips of copper would become loose and break. So I rebuilt it with better quality stripboard. Make sure you use the filter with the 1V/Oct connected to get the best out of it. This will make resonance follow the notes you play (filter tracking). It'll also work with out 1V/Oct of course.

Okay, thanks for being here and if you have any comments or questions just put them in the comments below or in the special Facebook Group for this website.

Synthesizer Build part-15: DUAL KORG MS-20 HP/LP FILTER.

This is just two Korg MS-20 filters behind one panel with switches to go between using them as individual filters or switching them in series.  This is one of my early projects so the implementation is a bit clumsy with how I use the switches etc. but I'm keeping the article up for archival reasons plus this dual configuration actually sounds pretty awesome. But I wouldn't be surprised if you found a better way to make a dual MS20 VCF than this one. 

I wasn't too pleased with the performance of the Prophet 5 lowpass filter so I decided to remove it and put a new filter in its place. I've seen lots of videos about the Korg MS-20 and really like the sound of it. I noticed that synthesizer has two nearly identical filters next to eachother; the highpass- and the lowpass-filter, so I wanted to emulate that in my own synth. So I set out to build two of the 'Late MS-20 filters' by Rene Schmitz, and fit them behind a single panel that was the size of the old Prophet 5 filter that I took out. It was a tight fit to put all the knobs and switches on but it worked out beautifully in the end.
The schematics and layout I used are just the same as the ones I used in article 12 of this blog, so if you want to build your own dual filter arrangement you can go to the Korg MS20 filter page and build TWO of those. Build both filters with the HP/LP switch but do not include the bandpass switch. You build two MS-20 HP/LP filters and put them behind one panel. Then, as extra, you add switches to the inputs so you're able to put them in series or use them independently of eachother. The wiring diagram for those switches is further down the article.

Here's a picture of what that looks like. You can see I have filter one on the left side and filter two on the right. Each has its own Cut-Off Frequency and Resonance controls and each has audio and CV-in level controls and each has it's own Highpass/Lowpass switch. Beneath those switches you see two more switches which enable me to switch both filters in series without using patch cables to connect them to eachother.:

The wiring of these two stripboards was a bit of a nightmare but I got it done in the end. I made some initial mistakes and had to re-wire some potmeters so that's why the wires look like such a mess. Luckily it doesn't affect the working of the filter.

In the first filter I used the LM13600 chip and in the second the LM13700 chip. And having them side by side is a good opportunity to compare them and the LM13600 is a bit tamer than the 13700. So if you have both chips in stock you can decide whether you want your filters to sound aggressive or a bit less aggressive. It's not a big difference though.

You can see in the pictures above that it's a tight fit but I did managed to include two volume or level potmeters for the audio inputs, which are not included in the original build but are very useful to have. I'm going to make sure that every filter I build in the future has input level control.
Beneath the HP/LP switches are the two switches that give you the option of using these filters as two stand-alone filters, so not internally connected, or if you want to put them in series so the output of filter 1 goes into the input of filter 2.
One other weird option would be to switch the output of filter 1 to the output jack but leave the input switch for filter 2 as is, so filter 2 has no input. Because these filters are self-oscillating you can now use filter 2 as an oscillator. Put the output into the 4 channel mixer described in article 17 and put the resulting wave through filter 1 or wherever you like. Connect a 1V/Oct voltage to the CV input. Just an idea, but you see there are endless possibilities. That is the beauty of modular synthesizers. :)
However, it would be better to use a single DPDT switch here and use it just to switch between two filters in series or both separate. That makes it easier to switch but that way you can not use one of the filters as stand-alone oscillator, but you won't use that function much anyway I'll bet.

Here is the wiring diagram for those two switches. By all means try and think of a better way to do this. This was one of my early projects so not everything is perfect. I still get confused by these switches when I use this module after 1,5 years of using it LOL

It would be better to use one double pole switch and connect the output of filter 1 to the wiper of one pole and the input socket of filter 2 the wiper of the second pole. With the switch set to the left the output of filter 1 goes to the output socket 1 and the input of filter 2 goes into filter 2. Set to the right the output from filter 1 goes into the input of filter 2 and the input socket of filter 2 is left disconnected.

Finally I want to show you a little video I made which demonstrates the sound of these two filters in series with eachother. I filmed this just after I had finished the build and I was still figuring out what the filter could do but it shows the added benefit of having two of these in series. It can do really deep and full sounding bass tones and it can also scream and distort and sound really weird. I am glad I fitted these and they are certainly a big improvement over the Prophet 5 filter although I will use the AS3320 chip inside it for a future build.
Plus all those knobs and switches so close to eachother look really cool I think ^_____^

Here's a look at the different sounds this filter can produce. (The phaser effects come from the special effects unit and not from this filter):

Okay, that's it for this one. If you enjoyed this article please check out the rest of my synth build and leave me a comment if you have any questions, or even to just say hi. Please also subscribe to my YouTube channel. That would be a great help. THANK YOU!!

Other websites that deal with the DUAL MS-20 configuration:

https://www.modulargrid.net/e/befaco-sallen-key-filter-bf-22

https://www.perfectcircuit.com/signal/korg-ms-20

Synthesizer Build part-7: THE MOOG LADDER FILTER.

The iconic Moog Ladder Filter. This version is built with transistors only, not transistor array IC's. This is an early project of mine so please bear that in mind when reading the article.

A note before we begin: I have PCBs available for the Moog Ladder Filter using the CA3046 transistor array chips. Just go to the 'PCB Service' option at the top of the menu for more details.

Since this article was written I have made a new version of this filter, this time including the CA3046 transistor arrays and it works very well and no need to match transistors with that version. If you want to build this filter I would really advise you to use that layout instead of this one (although this one works fine too of course). You can find it in Chapter 39 (click here).

I used the schematic from Yusynth's website.

Before we start. Most people always want to know if it works on 12V. I tested the filter on dual 12V and it works just fine.
In this schematic the top and bottom transistors are applied in the form of a transistor array chip, the CA3046, but I couldn't get hold of that quickly enough and this early in the build I didn't really trust myself to design a layout including those arrays, so I decided to use all transistors and that works just as well. It makes the layout a lot easier. It is always mentioned that you must use matched pairs of transistors for this filter but really, that's a throw-back to the early seventies when transistors were not as consistent and reliable as they are now so if you have transistors from the same batch they will probably be matched well enough but put them through the transistor tester on your multimeter and match them on hfe value. The only place where the transistors must be matched well is on the place in the schematic where they use the CA3046; the top and bottom of the filter and the output on the side. I personally matched all my transistors by using the Transistor Curve Tracer I described in an earlier article on this website.
I built a second ladder filter as a test for the layout below and I used all unmatched transistors. The layout works fine but using unmatched transistors did not turn out well. I could not get the resonance trimmed correctly and there were enormous differences in volume when using the resonance potmeter. I used a squarewave for testing and the top of the squarewave had an angle to it instead of being horizontal. So you must used matched transistors!

This filter has a few quircks that you need to know about but which are normal for this design.

- The Resonance potmeter has only a small area of influence. For most of the throw of the potmeter you will hardly notice anything. This is normal for this design. That's why we need a reversed logarithmic potmeter for Resonance. To stretch out that last bit of the potmeter.

- When the Resonance is fully open, the output volume drops. This too is normal for this filter and even the original Moog ones have this. Yusynth also talks about this on his website.

- If the audio input level is too low you can loose the self-resonance on the bottom parts of a squarewave. (If you use a squarewave as input wave of course). Again, this is a known quirck of this filter type. It needs decent level of audio input.

The build is quite straight forward but you need to be very accurate. The 50K anti-logarithmic potmeter for the Emphasis or Resonance control was an other thing I couldn't get a hold of so I made my own by using a linear potmeter with a 5K resistor between pins 1 and 2. This works very well, In the layout I used a reversed logarithmic potmeter and I show the alternative that I myself used, next to it.

The input level potmeters are 50K logarithmic ones but if you don't have those just use linear ones. They don't even have to be 50K. You can use 100K or 1M or even 10K if that's what you have available. They're just audio input level pots so they act as attenuators or voltage dividers and the value has no impact on the working of the circuit. This goes for all the level potmeters in all the projects on this website unless it is mentioned otherwise on the layout.

The Frequency Cutoff potmeter however must be a 10K!

I made a layout for stripboard including the wiring. I used this layout to build a second filter and it worked straight away so this layout is verified. (All potmeters viewed from the front):

(Last revised: 24-June-2020: Corrected polarity of C3. 15-July-2020 added alternative for reversed potmeter.)

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Here are a few pictures of the finished circuitboard:

As you can see in the pictures, I added two trimpotmeters which are not on the stripboard layout above. These are two 200K trim pots and they go over pins 1 and 2 of each opamp, to make the gain adjustable. It says in the schematic to 'adjust the value of the feedback resistors according to audio level'. These trimpots make that possible without having to use the soldering iron. It's a bit awkward with the wires but I had to put the potmeters on the print where there was room enough to accommodate them and the wires. Plus I added them as an after thought, so after I made the layout. At least they are all neatly in a row. :-)
For clarity I made a second layout which includes these alterations. If you decide to replicate this then don't forget to remove (or not solder in) the original resistors over pins 1 and 2; the 56K on IC-1 and the 120K on IC-2 because these are replaced by the trimmers, as the layout below shows. Lay-out is verified not only by me but I heard from many people who built this filter successfully using this layout.:

I lowered the Control Voltage input resistors from 100K to 5K6. In the schematic they are 100K but after installing the filter in my synth set-up I noticed that the CV hardly had any effect at all when I connected a LFO to it, so I lowered these to 5K6.

After finishing the build I tested the filter on the oscilloscope first and set all the trimmers to the right positions. It's easy enough to do, you just watch the scope for the best response. Then, after installing the filter you can adjust the trimmers to get the best sound out of it. This filter is self-oscillating, meaning that if you have nothing connected to the inputs and you turn Frequency Cut-off and Resonance all the way up the filter will oscillate of its own. There's no 1V/Oct. input though so resonance won't keep track with the notes on the keyboard. An other frequently occurring problem is that the self resonance only occurs at the top of the squarewave and not the bottom part. When you start it up it will self oscillate on both the top and the bottom but as soon as the transistors warm up, about 20 seconds, the bottom oscillations disappear. This happens when the transistors are not perfectly matched. Using the CA3046 transistor arrays will solve this and that's why I made a second Moog Ladder Filter project using the CA3046 chips. You can find that in chapter 39

The first all transistor filter I built didn't have this problem so if you're careful with matching the transistors you should be fine. Please note that the input volume is also an influence on this. If the level is too low you can also have the bottom oscillations disappear.

Here are some oscilloscope images to illustrate what I mean:

Self oscillation when filter is just switched on:

And here's the situation after about 20 seconds. The bottom oscillations are gone. The filter still sounds pretty cool though:

This is something I only discovered a short while ago but well matched transistors or using the CA3046 chip should solve this issue. The first Ladder Filter I built using all transistors didn't suffer from this problem because I was careful to match the transistors accurately.

MATCHING TRANSISTORS:

 I used to advise people that matching transistors on their Hfe value was good enough but I have found an easy way to do it properly which I advise you to use. This is the Ian Fritz method. We set up a small differential amplifier on a little breadboard and measure the voltage between the two emitters. If the transistors are matched that voltage should be zero. Here's a schematic of the test setup:

The diode is a 1N4148 or any silicon diode will do really. It's voltage drop ensures both transistors get the same base-collector voltage. Make sure the 100K resistors have exactly the same resistance value!

Beware this setup uses a dual 12V powersupply.

VIDEO DEMO:
Here's a video showing the test results on the oscilloscope so you have an idea of what the waveforms should look like. In this first filter I built and which I demo in this video the transistors seem to be reasonably well matched because I do retain some self oscillation on the lower parts of the squarewave:

       

A demonstration of the sound of the filter:

I recently made a Falstad simulation of the ladder filter and it works reasonably well. It shows the resonance and the trimmers work as expected. It even shows the amplitude getting smaller with resonance and self oscillation. You can view the simulation by --- CLICKING HERE --

Here's a look at my PCB for the filter. This is what is actually in my synthesizer and I build it from a different layout that I made earlier especially for the Eurocard format of stripboard:

Here's the panel I made for it:

As you can see in the picture, I've installed a bypass switch for this filter. It's great to have this filter in series with the Korg MS-20 or the ARP2600 filter but sometimes you want to be able to easily switch to one filter. With a bypass switch there's no need to constantly connect and disconnect patch cables so this is really helpful. The switch only works with audio input 1 and it sends the signal straight to the output jack and disconnects the in- and output from the in- and output jacks at the same time. Here's the wiring diagram for the bypass switch:

Okay that's it for this project. Way more synthesizer build articles to find on this website and while you're here leave a comment please!

Synthesizer Build part-6: THE SEQUENTIAL PRO ONE LOWPASS FILTER.

The first filter I built and not one of the best in my opinion. Skip to an other project. I'm keeping this up for archival purposes.

[EDIT] Before we start: I'm writing this about 7 months after I published the article below and I can tell you now that I removed this filter from my synth. I couldn't get it to self-oscillate and I was not impressed with the sound of it. Maybe that's due to the fact that I was just starting out with synth building when I made this and I made some mistakes, but if you have the AS3320 chip I advise you to use it for the Digisound 80.6 lowpass filter. That is an amazing sounding filter!
I'm keeping this article up because this website is an archive of all my builds and that includes the ones that were not a succes (That's only this project anyway. The rest all works fine.)

Okay on with the original text:

I've considered many filter designs, and there's a lot to choose from if you search for schematics on the internet, but I've picked out 3 filters that I want in my synthesizer. Two of those are LowPass filters and one has the choise between Lowpass and Highpass. That's the Korg MS20 filter We'll get to that one later.

Now we will concern outselves with the Sequential Circuits Pro One (and many others) Lowpass filter based on the AS3320 chip. I went to the ElectricDruide website and found a page full of filters and amongst them I found this circuit:

There are also versions which are re-configurable with the flick of a switch but they were a bit too complicated for my taste and I thought this would be a great project to start with. Well, it was a good project to learn from but the result was nothing to write home about.

I made a layout on stripboard which you can find in the picture below. All potmeters viewed from the front.:

(Layout revised 15-Feb-2021)

Print only:

Bill of Materials:

The first version I made of this had capacitors that were 220pF instead of the 150pF as seen in this schematic. I thought that would give me more control over the low frequencies and I was right, LOL! It brought the top cut-off frequency down to about 200Hz. Way too low. So I soldered in the 150pF and everything worked after that.

The three inputs in this schematic have different values but I did some testing and the 120K resistor worked the best on the input so I put those in all 3 inputs. I think you should also do this if you decide to build this filter because these schematics reflect the circuit that is used in the synthesizer itself and that's why the inputs have different values because they receive signals with different amplitudes. In our synth all the VCO's produce signals with a set amplitude of 10V peak to peak, therefore the input resistors need to be the same value.

There's not much more to say about this. The opamp in the schematics can be any low noise audio type. I used the TL072 for that because I have a lot of those in my components collection and they are the opamps most used in synthesizer projects. But you can also use the TL082 or NE5532.

You can use this filter with a dual 12V powersupply without any changes except for the current limiting resistor to pin 13. Change it from 1K5 to 1K2 for -12V operation.

This filter sounds like a lowpass filter should although it is not as versatile as the Moog or Korg filters. For one, this filter is not self oscillating (at least, the one I build isn't) so you can't make it scream but it does produce that resonant synthesizer sound, if you know what I mean.  It's just a different filter to the Korg or the Moog Ladder Filter which is an other filter I've put in my synth build. It's more a filter to round off certain sounds you've created on your synth, to make them fuller. I don't know, you have to listen to it to know what I'm on about. It should be capable of self oscillation if you read the text that goes with the schematics. The fact that mine didn't self oscillate is probably due to my inexperience at the time I built this.
EDIT 20-March-2020: I just finished work on the Digisound 80.6 lowpass filter using the AS3320 chip and it sounds fenominal. If I were you I would build that filter instead of this one. 

Here are some pictures of the PCB and the panel I made:

The filter is the one on the right. The other one is the VCO. I had yet to add the text to the panels.

You can see the inputs for oscillators one, two and for noise on the left and below that the resonance control voltage input (for the LFO for instance). That input has a level control and there's a switch for internal or external resonance control. On the right is the audio output. The top potmeter is for the Cut-off frequency and the one below is, as mentioned, for resonance.