Wednesday, 1 April 2020

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. 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 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.

Here's a link to the schematic drawing I used from the Yusynth website. This is the revised re-designed version like I mentioned before.
Here's the stripboard layout and wiring diagram I made for my specific needs with the somewhat odd switch arrangement:

The Steiner-Parker filter will work fine on a dual 12V power supply. No extra changes are needed.
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 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 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 2-April-2020: Added Audio output jack and normal 4-way switch and made more room around the caps on the left plus L-Bracket for panel mounting. 4-April-2020: Added earth connection to pin 3 of TL074. 28-April-2020: Labeled rotary switch with filter-modes.)

All my stripboard layouts are made on 24x55 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.

Print only. (Print this out and use it for your build. Don't forget the cuts underneath the 4th wire-bridge from the left and underneath the chip.):


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.

Bill of Materials:

(Last revised: 15-5-2020 Added extra potmeters and input jacks that are not shown in the layout.)

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.
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' print. 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 MaxiBrute:
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.)

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.:

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.

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 build all the modules in.

If you have any questions or comments, please leave them in the comments below or contact me on Facebook. See you on the next one!

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