Looking for help Making a pedal adapter for analog Synthesizer - Page 1
Forum section: electronics / projects

author: ndzied1
author_rank: Contributor
timestamp: September 01, 2020, 05:41:31 am
content: Hello,I'm trying to make an adapter for my son who has an old analog Korg PolySix Synthesizer (http://www.korganalogue.net/korgother/Polysixmanual/PolysixOwners.pdf). The original pedal for adjusting the cutoff filter is long out of production and had a non-standard output of 0-4V DC. They can be had but they are very expensive.I'm trying to make an adapter for a readily available pedal, the Yamaha FC-7 (https://usa.yamaha.com/products/music_production/accessories/fc7/index.html) which is basically a 50k Ohm pot with an audio taper. I'm using an Arduino MKR Zero (https://store.arduino.cc/usa/arduino-mkrzero) which has a 5V source to power the 50k pot and run the wiper into one of the Arduino analog inputs. This Arduino has a true analog output but it is only 0-3V output. I'm needing a little help designing the part of the circuit that can amplify the 0-3V Arduino output and turn it into 0-4V for use in the Synthesizer. I have built up a proof of concept circuit with only the 3V output. I'm using the Arduino because I can linearize the pedal output and do other things to the signal by being in the middle between the pedal and the Synthesizer.Here is a sketch of what I have so far with a question mark where I'm not sure how to proceed.https://www.dropbox.com/s/1c88kfd7bhxuufr/PedalAdapter.jpg?dl=0Any help greatly appreciated.

author: Ian.M
author_rank: Super Contributor
timestamp: September 01, 2020, 07:42:32 am
content: Before you go putting an Arduino in there, at least breadboard a proof of concept. Stick a 12K resistor in series with the top end of the pot and feed it 5V from a micropower TO-92 package 5V regulator fed from a 9V battery to get a 0-4V control voltage. Use a RRIO single supply OPAMP as a unit gain buffer to buffer it.When you do put the Arduino back in the loop, it would be *MUCH * easier with a 5V Arduino. However if you *MUST* use a 3.3V Arduino, design to an output range of 2.5V so you can use readily available voltage reference and use an OPAMP to amplify it 1.6 times to get the required output range.

author: Renate
author_rank: Super Contributor
timestamp: September 01, 2020, 12:42:39 pm
content: I have a Yamaha FC7.I used to use it with an old style PC joystick 15 pin input.I remember checking it long ago for taper (and finding it linear).I see on various forums that everyone reports it as having an audio taper (i.e. modified logarithmic).So I went and measured it all (again).The "height" is from the lip of the leading edge of the pedal above the rim of the base plate.The angle is asin((height-0.75)/arm)The resistance is measured sleeve-ring.The "arm" is the distance from pivot to lip.The lip is directly on the line to the pivot, so that's accurate.It looks pretty linear to my foot!Edit: I see on the Yamaha link above that this is shown with a TS plug.Mine has a TRS plug (a molded plug that says "Yamaha" and has never been modified).Sweetwater shows it TRS:https://www.sweetwater.com/store/detail/FC7--yamaha-fc7-volume-expression-pedalThe instruction manual shows TRS:https://usa.yamaha.com/files/download/other_assets/3/865833/fc7_en_om_b0.pdf

author: Renate
author_rank: Super Contributor
timestamp: September 01, 2020, 03:20:40 pm
content: Though the pot is a 6mm/4.5mm flat standard size, it's not standard at all.It rotates about the usual 300°, but the active part is only about 90°.There's about 100° at the start where it's zero ohms to that side.Then at the other end there's a section where it's zero ohm to the other side.The "B" in "50KB" indicates linear.I had been thinking about putting in a rotary encoder, I've got bunches that are 6mm/4.5mm.But they all have detents and even if I get rid of that it would leave me with only 24 possible values over the 90° range.Moreover, I think that a crappy pot has a better lifetime than a crappy encoder.If you put in a high res optical encoder, yeah, maybe.In any case, I have more use for a USB pedal than either an analog or a MIDI pedal.It's a question of whether I integrate the µC as an separate adapter or internal to the foot pedal.

author: ndzied1
author_rank: Contributor
timestamp: September 01, 2020, 03:26:46 pm
content: I didn't do it quite as scientifically as you but Mine sure seems tapered although I didn't take it apart like it sounds Renate has. And the plug is indeed a TRS.I definitely want the adapter separate from the pedal. I envision (maybe just dreaming but ok) having some buttons to do things like change the slope of the output or even run wave forms on top of the main pedal signal for various effects." alt="" class="bbc_img" />

author: Renate
author_rank: Super Contributor
timestamp: September 01, 2020, 04:52:02 pm
content: Aw, be a sport and take yours apart too.Ok, I'm building a USB HID (joystick) adapter with a Teensy 2.0 (ATMega32u4). Done.Those 1/4" stereo jacks are humongous in comparison to anything modern.Now that I think of it, I have used the FC7 with the iRig Blueboard (it has two 1/4" inputs).Unfortunately the iRig firmware is a bit stupid, you can get updates but you can't poll it for initial position.

author: ndzied1
author_rank: Contributor
timestamp: September 10, 2020, 09:32:37 pm
content: Quote from: Ian.M on September 01, 2020, 07:42:32 amI haven't really used any opamps before so I wan to check if my test circuit matches your description. Does this look right?https://www.dropbox.com/s/2p7y12jlq8g2rfb/TestCircuit.jpg?dl=0

author: Renate
author_rank: Super Contributor
timestamp: September 10, 2020, 11:42:41 pm
content: Yes, that's fine.You could even feed the plus power to the op amp directly off the battery.You'd keep the regulator and the 12 k where it is.You could even skip the whole op amp and feed the wiper to the synth.(The input resistance of the synth might affect the curve, but it should at least nominally work.)

author: Renate
author_rank: Super Contributor
timestamp: September 11, 2020, 01:22:11 pm
content: You know, if you want to keep this simple you can just stay in the analog domain.A bit of bypass resistance on the top end will pretty much linearize this out.You'll need to switch the pot feed to 4V (instead of the 5V and a 12k) because the resistance varies.Why don't you try this and I'll bet that your foot will like it.

author: ndzied1
author_rank: Contributor
timestamp: September 26, 2020, 05:52:22 pm
content: Ok, I ordered some parts to make up the analog circuit but now my newbee status is showing full force.I purchased this board thinking it would be like the breadboard that I'm mocking everything up on but there are no connections; every point is isolated. How do you use a board like this? Do you have to solder wires between the components on the back side? Can I lay things out so that I can bend component wires to connect with each other? I feel like I'm missing something." alt="" class="bbc_img" />Thanks for any suggestions or examples of using boards like this.

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2020, 01:32:37 am
content: That stuff (matrix board) is a PITA to work with, although with enough patience very compact protoypes can be built on it. If you bend component leads to make connections its extremely hard to make any changes so I wouldn't recommend that. Caution: pad adhesion is rather weak so if you are at rough with it or resolder a joint more than a few times, pads will start dropping off.You need to usethinbare tinned copper wireon the track side to link the pads so you can desolder a component lead without ripping up everything else connected to it. The wire should be thin enough that you can get two through one hole! Either hook it through a hole if you've got a spare one next to the end leg in the group or tack it down at the end soldering that component leg. Run it past all the other component legs, making rightangle bends as necessary and fix the other end as you did the first. If you hooked the ends, form the hook end flush to the board on both sides and trim it topside so the end is shorter than the hole spacing then solder the pad the hook goes through. Check the wire is laying how you want it before soldering the other end. Finally you can go back and solder the component leads through the intervening pads, making sure the tinned wire is pushed right down to the pad as you do so. For corners on long sparse interconnects where there isn't a component lead to bend round, its probably better to hook the wire end through the corner hole and start a new wire hooked through the same hole.If that's too much of a PITA either get protoboards that actually have tracks that mimic a solderless breadboard layout, or get stripboard (aka: Veroboard, but that's the trademarked name of the original brand). See:https://www.nutsvolts.com/magazine/article/june2013_DratwaDon't be ashamed of using breadboard layout protoboard. You'd be in great company. e.g. Fran Blanche built herfirst prototype Frantone pedalon a cut-down Radioshack276-170board.

author: ndzied1
author_rank: Contributor
timestamp: September 27, 2020, 01:45:19 pm
content: Thanks for the detailed reply and link to the video. I bought a lot of spare Parts so I’ll have a go with the pads and see what I can get.