Monday, 30 March 2020

Synthesizer Build part-25: DUAL BUFFERED MULTIPLE with LEDs.

A very practical module that replicates any voltage on the input and splits it 4 ways. Also known as a CV/Gate Expander but it's the same thing =) After seeing Sam Battle's version with the bi-coloured LED's I've added a second version of my own with the bi-coloured LEDs because I think it will be in demand. =) The version with LED's is half way down this article.

I have finished the case for the second stage of my DIY synthesizer so now I can continue building modules and writing about them here. So the first thing I wanted in the new case was a Buffered Multiple. I have built a number of filters that need a 1V/Octave signal but I only had one output to provide it so I designed this little circuit. You can connect anything you want to the inputs: Audio signals, LFO signals, 1V/Oct. CV, Gate signals, you name it. Any voltage presented at the input is replicated at each of the four outputs and if you need more you can connect one of the outputs of the first stage to input 2 and so get a total of 7 outputs for one input.

This module will work on both +/-15V and +/-12V.  Running it on 15V just means it can handle a bit higher voltages but as the average synth only uses control voltages with a maximum of +/-10V it really won't matter.

Here is the schematic drawing showing one stage. As you can see it's just 4 non-inverting buffers with all the inputs connected together. The module is just two of these on one piece of stripboard. I've put 100 Ohm resistors on the outputs for some extra protection. Because there's (practically) no current flowing these resistors won't influence the output voltage. I've tried 1K resistors but they influence the voltage and bring the notes down by a few Cents. I did a lot of testing on this and 100 Ohm is really the maximum. Of course you can use 1K and retune your VCO's but I want to keep it as clean as possible. If you don't feel comfortable with resistors on the outputs you can leave them out all together. In my own version I left them out too and the IC's have built-in short-circuit protection anyway.



Here's the verified stripboard layout. Just two IC's and some wire bridges and resistors. I recently added de-coupling capacitors for the chips because it was kindly remarked upon in the comments below that they were missing and it is good practise to include them although, if you use a normal linear powersupply, there's really no need for them. But many people use switch-mode power supplies and in that case it's good to have them included. Also if there is quite some distance between your powersupply and the module, so the wires can act as antennea and pick up noise. In that case it's good practise to use decoupling capacitors too.


(Last revised 14-April-2020: Added de-coupling capacitors for the ICs on the power rails. 18-May-2020 Added 1K resistors to the outputs. 24-June-2020 replaced some wirebridges with direct connections underneath the IC's, 28-June-2020 replaced 1K resistors for 100 Ohm ones.)

Print only:


Bill of Materials:


BI-COLOURED LED VERSION:


(Last revised: 24-June-2020: Replaced some wirebridges for direct connections underneath the IC's., 28-June-2020: Replaced 1K resistors for 100 Ohm ones.)

Print only:

BOM for the LED version:


Make sure you choose the bi-coloured LED's with two legs and not the ones with 3 legs. They won't work in this set-up. They must be two-legged. You may need to alter the value of the 2K2 resistors according to the brightness of the LED's you're using but I find 2K2 to be a nice middle value. Not too bright not too dim either. I've altered my own CV/Gate Expander to include the LED function and These 2K2 resistors work just fine with the Red/Blue LED's I'm using. Below note G2 the Red light, which is positive voltage, doesn't light up but above G2 you can see it light up. The higher the note, the brighter the LED gets and it can handle voltages as high as 15V without problems. The LED doesn't light up below note G2 because it has a threshold voltage of 1,76 Volt. So it doesn't matter what value current limiting resistor you use. No LED will light up below its threshold value. For the blue part of the light this threshold is even higher, about 3,5 Volt. That's just normal with this kind of circuit.

Of course you could also use normal LEDs. That will just mean that, in the negative cycle, the LED won't light up but other than that it'll work just fine. Or you can use two LEDs; one for the positive cycle and one for the negative cycle. It's up to you.

Here's the schematic for the LED version. I've connected it to output 4 of each of the two sides but you can choose any output that's convenient. Because the LED is buffered it takes no voltage away from the output. This schematic drawing shows only one side because the second side is identical to this one.


This module is very useful to have in your setup because sooner or later you're going to need at least one of these, just like you need at least one mixer/passive attenuator.

Here are a few pictures of the finished product. I made a little L-Bracket (also visible on the layout) so I could mount the print at 90° to the panel.


This is the version with the LED's installed. The right LED is displaying the voltage of the Gate signal which is +5V. The left LED is displaying the 1V/Oct signal which is a bit lower.


The print up close. As you can see mine has more wire bridges because I forgot that some connections can be made directly under the IC. So I did it the hard way. The first picture below is the original module without LEDs. The second one is after I converted it and added the LEDs.


Because I added the LEDs later I had to use some jumpwires on the print to connect it all together.



Okay, that's an other one done. Number 25! A bit of a milestone for my synth build :)
Got any questions or comments just leave them in the comments below please. See you on the next one!

Saturday, 21 March 2020

Synthesizer Build part-24: ADSR with 7555 (YuSynth design)

A great ADSR. Works perfectly and it's a  very simple design, no trimmers to set. With verified stripboard layout.

This is the fourth Envelope Generator I present on my website and I think this one is the first that worked as it should straight away. No trimmers to set in the circuit either. I just used the schematic from the Yusynth website made a layout and built it. On the website he has two versions, an old and a new one. I built the new one. I can say without any doubt that this design is perfect if you want a good and reliable ADSR to pair with your VCA or to drive a filter. And because the circuit is so simple, even a stripboard version like this one would be robust enough to put in a rig you take on tour with you because, providing the panel is sturdy enough, there's practically nothing that can go wrong on the print.

This is the schematic.



And this is the stripboard layout I made for it. It is verified, I used it for my build and it worked first time. I didn't even test the print after building it. I made a panel and wired everything up and then I plugged it into my synth and it just worked.



Print only:



Bill of Materials:



The wiring of the potmeters may look a bit strange with pin 3 left unused on three of the four potmeters, but I assure you that this is the way it should be wired up. Just follow the layout. It'll work fine I promise you. You can see in the schematic drawing that these pins are left dangling in the wind so that's what we do.
The ADSR triggers with a gate signal with a threshold of 2 Volt. The output envelope is 10Vpp. There's a manual trigger button on the panel (which is useful for testing). The envelope generator has two outputs. There's a normal output and an inverted output with a switch that lets you choose between +10V to 0V or 0V to -10V. There's also a switch to change the duration times with 'Fast' and 'Slow' settings. Use a DPDT ON-ON switch for the Fast/Slow function and a SPST (ON-OFF) or SPDT (ON-ON) switch for the Inverter voltage function.
In Fast mode the duration for Attack, Decay and Release can be set between 1mS and 1Sec. In Slow mode they can be set from 5mS to 10Sec. These times are generated by the 1µF and 10µF electrolithic capacitors C4a and C4b. In the text on the Yusynth website it says to use Tantalum caps for this but I used normal Electrolithic Caps and this works just fine. I hate Tantalum caps anyway, they always blow up on me, LOL. If you want longer times you can install bigger caps. You could even take a 3 position switch and add a third cap of, for instance, 47µF to generate really long times. I haven't tried this myself so I can not guarantee it works but I don't see why it shouldn't.
There's a LED to indicate the level of the envelope. I'm not sure why, but on my print the LED does not go fully out. It indicates fine but it keeps glowing faintly, if the ADSR is not used. There's no DC offset voltage on the output tho, so that's not it. It probably keeps glowing a bit to indicate the ADSR has power. Anyway, it doesn't matter.
Make sure you use three logarithmic 1 Mega Ohm potmeters for Attack, Decay and Release. Otherwise it will be difficult to set the parameters accurately. For Sustain we use a normal linear 10K potmeter.
You can run this envelope generator on a dual 12 Volt powersupply without any changes only the envelope output levels will go from 0 to 8 Volt instead of 0 to 10 Volt.

Here are some screen shots from the oscilloscope:

The normal envelope:

Inverted 0V to -10V:

Inverted +10V to 0V:


Well, that's all there is to say about this project really. A very satisfying build because everything worked as it should right from the start. I would say that this is the perfect ADSR. The panel potmeters work over their complete throw, unlike some other E.G.'s I built, and you can set all the parameters very easily. If someone would ask me what ADSR to build I would certainly recommend this one. You can easily add on extra outputs if you so desire. You can add a TL074 for instance and wire up some extra outputs and/or inverted outputs. That's easy enough to do.
Okay, to close off, here are some pictures of the finished product. I made a copper bracket to keep the print in place behind the panel. That way I could use just one M3 bolt. I soldered all wires straight to the copper side.




Okay, see you on the next one. If you have any questions or comments please leave them in the comments below.

Thursday, 19 March 2020

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. 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.)

Print only:


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. If you're going to build this filter for a monophonic modular synthesizer then build it without the "Fine" control and use just the "Coarse".  Fine is more for Polyphonic systems, it has only a very small influence on the Cut-Off Frequency. I'm telling you this now so you can adapt your panel design. Most filters have just one C.O.Freq control.
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:



Lastly I want to share with you the efforts of LookMumNoComputer 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 build 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. 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 and they will be answered asap.

Thursday, 5 March 2020

Synthesizer Build part-22: RING MODULATOR (Yusynth design).

An excellent ring-modulator to serve as an extra source of weird sounds. This one is simple to build and works very well.

The ring-modulator was something I always wanted to include in my DIY synthesizer and I was thinking of doing it the old fashioned way with audio transformers but they are very expensive. So I went looking for designs that used semiconductors and came across the Yusynth design. I ordered some MC1496N IC's from a shop near where I live because I didn't want to wait for components from China, what with the Corona virus going on etc.
I got the chips the next day and I set out to make a stripboard layout.

Click here to go to the schematic.  Here is the (verified) stripboard layout I made from it. The schematic is for a double ring modulator but I only built one. But it's just the same circuit repeated. I used this layout for my build so it is tried and tested. Note that the electrolytic capacitor on the AC Output has it's polarity reversed from those on the AC Inputs.


Print only:


Bill of Materials:



The circuit has two trimmer potmeters and the way I tuned it was to create a sinewave with the LFO of about 400 Hz and feed that into input B and a triangle wave coming from VCO-1 fed into input A and I had the little oscilloscope connected to the output. I turned the trimmers in such a way that I got the following picture on the oscilloscope:



The triangle wave appears inside the sinewave so to speak and you trim it until the low amplitude sides are flat and so you get a whole row of these modulated sinewaves. If you turn the trimmers you can see the symmetry changing and it becomes flatter at the top or bottom. So I tuned it until it was a nice symmetrical waveform. There apparently is also a DC-Offset that needs to be trimmed away (according to the text with the schematic) but I haven't tried that yet. For now I just use the AC inputs anyway. This ring-modulator can handle 10Vpp signals like all the builds on this website. I use the Moog 10 V peak to peak as a standard for my DIY synthesizer.

Ring modulation is a very interesting way of combining two frequencies and can get very complex very quickly if you use waves that have a lot of harmonic content like squarewaves. More on the theory involved is here on this Wikipedia page about ring modulation.

And that's it. I didn't think I could fit any more modules in, but this one was just small enough to go into the wood panel above the modules. Here's a look at the finished panel and how I fit the stripboard behind it with a little copper L-Bracket I made myself and soldered onto the stripboard.




Ignore the sawdust specks in the bottom picture, LOL. I took it just after putting the module in and the synth was covered in fine dust.
Okay that's another one done. This is getting to be quite a big and powerful "sound design machine." It's about time I made a new case that I can put on top of this synthesizer so I can keep building ^___^

If you have any questions please leave them in the comments below!



Wednesday, 4 March 2020

Synthesizer Build part-21: ARP2600 LOWPASS FILTER (4072).

The best sounding filter of any I built so far! With verified stripboard layout.

A word of warning right at the start; this is an advanced project, not for beginners. You need to know your electronics and you also need to have a good multi channel oscilloscope.

Apologies to anyone who tried to build this filter before the 24th of August 2020 and did not succeed. I discovered a mistake in the layout which I have now corrected. The 10K resistor from pin 5 of the LM3900 was not connected to the base of Q7 as it should have been. This fault is now corrected.

The ARP2600 has become one of my favourite synths lately and after tackling the ARP's Envelope Follower I thought it was time to try out the famous 4072 filter. ARP has had a number of well known filter types. The 4012 (4035 for Odyssey) which was a Moog type ladder filter over which they got in trouble with Moog for patent infringement. The 4023 two-pole filter of the early Odyssey synths. Then later came the 4072 for the later ARP2600's which had a fault at first which limited the bandwidth of the filter. And then there's the 4075 which was the filter used in the later ARP Odyssey's.
If you want to build this filter there's really only one schematic you can turn to and that's the Yusynth schematic. So I set to work making a layout. I first tried just starting at the lower left of the schematic and building the layout up from there. Within minutes it turned so complicated I couldn't make heads nor tails of it. So after an other unsuccessful try I came to version 3 of the layout and this time I decided to place all the semiconductor components neatly on the board first. All transistors in a row on top and the two chips in their own space underneath and wire it all up that way. This worked fantastically and after a days work I had a layout that looked really good and, more importantly, turned out to be faultless right from the get go.

However, like I mentioned at the beginning, this is not a project for beginners. It's reasonably complicated and you need to work very methodically and do things in steps. First map out all the cuts in the copper strips with a Sharpy and cut the traces accordingly. Then solder in all the wire bridges and then solder in the components. Keep counting the holes and make sure everything is placed exactly like on the layout, otherwise you will run into trouble with space on the board and things end up not being connected right. I worked from left to right soldering it all in and checking every connection with a powerful loupe. And in the end, of course, it didn't work straight away. It turns out I had forgotten to cut four copper strips near the 1V/Oct trimmer. After I cut those the filter suddenly sprung to life and started making sounds that instantly reminded me of the ARP2600.

Here's the Yusynth schematic. It looks a bit weird but the LM3900 really operates on negative voltage, in this circuit. I tested the filter on dual 12V and it works just aswell on 12V as it does on 15V without changes. I did notice I had to open up the Resonance potmeter more to get the same effect but it all works as it should.:



And here's the layout. Like I mentioned before, the layout is verified because it's the one I made and used for my own build. (All potmeters are shown from the front). [Addition: I've had confirmation from a reader that this layout has been used successfully. If the output volume is a bit on the low side you can try to increase the gain of the output opamp by changing R41 from 56K to 100K]:



(Last revised: 24-Aug-2020: Corrected a mistake in the layout. The 10K resistor from pin 5 of the LM3900 to the base of Q7 was not connected right.)

Stripboard only:



Sometimes you'll see a cut in the copper strip overlapping a component in the layout above. I've done that on purpose so the cuts are easily visible. The layout is pretty complicated especially for beginners so I want things to be as clear as possible. That's why I always advise to mark the cuts with a black felt-pen on the component side of the print.
To make things even easier, I have made a layout showing just the cuts you need to make in the copper traces. This is viewed from the copper side:


Bill of Materials:



If you don't trust yourself to build this on Stripboard then here's the PCB design for this filter. If you print this on A4 size paper it will be to the right scale. You can check by measuring the DIP14 IC package which should be 15mm from pin 1 to pin 7.


I refer you to the Yusynth website for the rest of the build information:
http://yusynth.net/Modular/EN/ARPVCF/index.html

There are 12 transistors in this filter and they need to be matched pairs but you don't have to use a curve-tracer or anything. I simply matched them with the transistor tester on my multimeter and that was close enough. They need to be matched over the value of Vbe, so if you measure the voltage drop over the Base-Emitter junction, and match them that way, that will be perfectly fine. The four 470pF capacitors need to be high quality and also closely matched in value. I used polystyrene caps for those. I even matched the 220 Ohm base resistors so they all had the same value. In my case they are all 216 Ohm.  The CV inputs all have 100K resistors on the inputs and a 150K resistor on the wiper of the Cut-Off Frequency potmeter. I didn't have room for them on the stripboard so I hung them over-board so to speak. In reality I soldered those resistors straight to the wipers of the potmeters and in case of the 1V/Oct. straight to the input jack. Then I put some heat-shrink tubing over them and after that I put some heat-shrink tubing over all the wires from one input together so there's never any tension on the resistor itself. This works fine. Of course, if you use a bigger piece of stripboard you can accommodate those resistors on the board itself. Or you can use a small piece of stripboard, solder the resistors on that and connect it to the main board with wires and then use some hot glue and a plastic spacer to glue it to the main stripboard. Lots of options :-)
The resonance potmeter needs to be a dual- aka stereo potmeter. I didn't have one but luckily my neighbour, who repairs audio equipment, had one laying around but it was a logarithmic potmeter. I put it in anyway and it worked like a charm. :) For the trimmer potmeters you can use a 50K for trimmer T1 if you don't have a 47K. In fact, it can be any value from 20K upwards because it's just connected between plus and minus 15V so the actual resistance isn't important for the working of the circuit. But don't forget there is 30 Volts across that trimmer so don't use a value below 20K to keep the current flow down. For trimmer T2 you can use a 2K instead of a 2.2K, but you must keep close to the recommended value for that one because it is part of the input bias for the transistor Q3. I used a 2K on my print and this works fine.
This filter has a 1 Volt per octave input connection to make the resonance follow the chromatic scale. The filter sounds better over all if you use that connection although it is not necessary for the filter to function. There's a trimmer (T2) for the 1V/Oct and the way I set it was to listen to the filter's response while going over the keyboard from low to high. If it is set wrong you'll hear the notes become all muddled up and out of tune at the higher end. If you set the filter potmeters in such a way that it self-resonates, then the resonance pitch will follow the keyboard scale. So you need to tune the filter so that the resonance is in tune with the keyboard notes if possible. I myself however did not tune it that way. I simply tuned it so the notes sounded ok over all the octaves and left it at that. That's good enough for me and the filter works fine.
The other trimmer is the Low Frequency trim-pot (T1). It needs to be set so that the output wave at the lowest end of the keyboard, and with the Cut-off pot turned all the way counter-clockwise, is a nice sinusoidal bass tone, at least, that is the way I set it. I'm not saying that this the way to do it. I'm simply saying, this is how I did it (and that goes for every article on this website too).
The frequency cut-off potmeter is wired up in such a way that it lets through the high frequencies when you turn it clockwise and when you turn it counter clockwise the sound gets very deep and low.
The values of the potmeters for CV IN and for the audio inputs are not critical and you can use anything from 10K to 1M for those because they are just level potmeters. For the audio potmeters the schematic says to use logarithmic ones but in reality linear will work fine too. It's log because it's audio. Like I mentioned before, I used a logarithmic stereo-potmeter for the Resonance control because that's the only thing I had but it seems to work very well eventhough the schematic says to use a linear type. It probably wouldn't matter what value you use for the Frequency Control either but I'd stick to the recommended 50K or 47K for that one. (I used 100K's for the CV level control potmeters.)
Don't forget to solder the 100K resistors, for the CV inputs, to the wipers of the potmeters or to the input on the stripboard, and don't forget either that the resistor on the wiper of the Frequency Control potmeter is a 150K and not a 100K one! (A mistake I initially made.)

Here are some pictures of the finished stripboard. This is an early version that has one more jump wire than the new layout. I realized I had a copper strip that was not in use so I used it to replace a jump wire. You can see I marked out the cuts in the copper strips with a black felt pen. I also marked out the 0V/Ground strip with a black line on the component side of the stripboard. I always mark out the ground. This helps to prevent mistakes.






Here's a little video with a demo of what the filter sounds like, taken right after I built it in. Remember when I filmed this it was the first time I played around with this filter so this is just a simple demo of the sounds it produces but I think you'll agree it sounds very much like the original ARP2600.



In this second video the filter CV-1 input is connected to the little 7555 AD/AR with the big arcade button. This kicks up the resonance of the ARP filter as soon as a key is pressed and then releases it pretty quickly thereafter. The AD/AR is set to trigger mode so it gives an Attack/Decay response.  The filter is fed with a single squarewave from the VCO. I think you'll agree it sounds amazing. Like a synth should sound. With apologies for my poor keyboard playing :p



This filter can also produce those helicopter sounds that you can hear in the beginning of 'Apocalypse Now'. (Francis Ford Coppola had an ARP2600 himself.) All you have to do is turn the cut-off frequency counter clockwise and connect an LFO with a sawtooth wave to the CV input, set to the frequency that the rotor-blades of the helicopter would have and turn the resonance counter-clockwise too.
One little attention point you must remember when using this filter. It's possible to overload this filter with audio in so much that the resonance won't work at full capacity. I had this happen to me where the resonance wouldn't produce the famous whistling sound and I had been trouble shooting for a day changing out the IC's, checking transistors, replacing the capacitors until I finally found out I had the input level set too high (The Audio-1 level potmeter on the front panel). I turned it back by a quarter and everything was back to normal. I'm telling you this so you don't make the same mistake. ^___^

This is what the panel looks like now. I've touched the lettering up a bit because it was all crooked (and it still is I guess, LOL) so it's good enough for me. What's important is what this panel represents; the best friggin' filter I've ever built!! :)



Okay that's it for now.
To finish off this article here's a fantastic new documentary about the history of ARP Instruments. Enjoy!


See you on the next one!