SH-101 Style Portamento

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SH-101 Style Portamento

Post by medbot » Mon May 04, 2020 9:40 pm

Hello, I'm trying to build a glide/portamento circuit similar to the one on the SH-101. Looking at the schematic, I see a standard gated-glide circuit with a FET similar to plenty of others I've seen. What I'm really after isn't just gating the glide, but to have it work similar to the 101 in "auto" mode, where it only glides when played legato.

I can almost get there by just plugging my gate into the glide input, but because the gate opens the glide switch simultaneously with the gate firing the envelopes, there is always glide present at the start of a note, even when playing well-separated staccato notes. I guess the questions I have are 1) is this done in software on the 101 (the gate also goes into the main IC through the "porta off" connection on the schematic), 2) if it is done in software, is it possible to emulate this behavior in hardware, and 3) can someone explain what is going on with the transistors on the right side of the schematic? I'm really lacking in understanding on most transistor functions :/ Is it a comparator to switch the FET?

If it's all done in software, would something like a gate delay work? I'm imagining it delaying the gate just enough so that the keypress activates the note without glide, but the glide circuit activates very quickly after for legato playing. I'm also imagining it not working because of variables I'm missing. Thanks for your input and help!
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Re: SH-101 Style Portamento

Post by guest » Tue May 05, 2020 6:19 am

yes, the transistors are essentially a comparator to turn the glide on and off. i

is the behaviour you want the following?

1. keydown, glide is off
2. keyup, glide is off
3. keydown on a new note before keyup on old note, glide is on
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Re: SH-101 Style Portamento

Post by medbot » Tue May 05, 2020 11:59 am

guest wrote:
Tue May 05, 2020 6:19 am
yes, the transistors are essentially a comparator to turn the glide on and off. i

is the behaviour you want the following?

1. keydown, glide is off
2. keyup, glide is off
3. keydown on a new note before keyup on old note, glide is on
Yes, that sounds right.

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Re: SH-101 Style Portamento

Post by guest » Tue May 05, 2020 12:19 pm

ok, so i can think of 2 ways of doing this. the first is as you described, where there is a delay after note-on, and then the slide is turned on. the other is to turn on based on note changes. the former sounds easier. it looks like the circuit shown above implements this, but in a reverse sort of fashion. the slide is always on, except during rising gates. so the note comes on, the slide is turned off for a short bit, and the capacitor quickly charges to the new CV value with no slide. then a short bit later the slide is turned on, so if any new CV changes occur, you get slide. the circuit above keeps slide on during note-off.
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Re: SH-101 Style Portamento

Post by medbot » Tue May 05, 2020 12:41 pm

Thanks for the help. Just to make sure I'm clear - in the 101 schematic above, are you saying the gate comes originally from the I/O labeled "Porta Off," then splits after R16, with one fork going to the transistor comparator to deactivate (?) the glide and the other going through C11, into I/O "Gate In," and on to the envelopes or wherever? I'm not quite sure I understand how that does the trick, but it's easy enough to add to my breadboard (if my assumption about signal flow is correct).

Would I need to replicate the transistor section or can I keep with the opamp comparator I have set up now? It's currently set up pretty much as seen here, minus all the switches for slope shape and direction.

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Re: SH-101 Style Portamento

Post by guest » Tue May 05, 2020 12:52 pm

you can use the opamp, but i think youll have to invert the inputs. right now, when a gate comes in the opamp output goes low, enabling the slide. you want the inverse of this. and you will have to add a capacitor at the input so it only deactivates the slide on the edge, and not hold it.
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Re: SH-101 Style Portamento

Post by medbot » Tue May 05, 2020 12:55 pm

Awesome, I'll give this a try later this afternoon and see how it goes!

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Re: SH-101 Style Portamento

Post by medbot » Tue May 05, 2020 6:23 pm

Well I gave your suggestions a shot and they came close but I can't quite get it perfect. Swapping the comparator inputs did the trick as far as getting the glide to activate only during keydown, so that was nice and easy. Then I put a cap in series with the gate input, so it's jack>cap>comparator +in. I wasn't sure of a good cap value, so I just tried a bunch. Too low (below say 220n) and the glide never turns off, too high (above say 4.7u) and the glide never turns on. I wound up doing most of the testing with 220n, 470n, and 1u film because those were the values I have on hand in that range.

The first problem that arises is that the cap introduces a sort of "ghost gate" on keyup. It doesn't retrigger my envelopes, so it's not a proper gate. The best way I can describe it is that it sounds like I've got a really subtle, really quick slapback delay. It seemed to get more noticeable with caps above 1u, which is why I didn't do much testing with higher values, but if it was more noticeable it was only a small amount more. I built the 101-style transistor comparator as well and had the same problem except it was more pronounced.

The second issue I noticed was that there was still a very subtle amount of glide even when the FET was switched off. Like teeny tiny, but still there. It only really stuck out to me when I played 2+ octave spans on my keyboard. Just a little chirpy warble on keydown, not the whole slide.

Hopefully I'm just overlooking something simple. It's so close!

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Re: SH-101 Style Portamento

Post by guest » Tue May 05, 2020 7:26 pm

turn the slide time knob all the way to zero, and see if the subtle slide is still there when the slide is turned off. if it is, then its no the JFET. if it is not there, then you need a lower on-resistance JFET.

do you hear the ghost note becuase the envelope is still in the release phase? one thing to try, is to put a 100k resistor between the opamp output and the diode that goes to the JFET.
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Re: SH-101 Style Portamento

Post by medbot » Tue May 05, 2020 8:51 pm

The subtle slide is not there when the pot is at 0, so I tried the only other FET I had on hand, a J201, and it made it worse. I've been using 2n5485, and for some reason I have a bunch from many manufacturers. No difference between them. I added the 100k between the opamp and the diode and I'm not sure if it made a difference; it was difficult to A/B.

I do only hear the ghost note when the envelope starts the release phase (so with low/no release, there's no audible issue). It's like the release voltage bumps up a hair above the sustain on keyup. It's almost so subtle that you might not notice it if you weren't looking for it, but now that I've heard it I can't miss it.

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Re: SH-101 Style Portamento

Post by guest » Wed May 06, 2020 7:41 am

you can try putting the JFET across the 10ohm resistor as well, so it spans both that resistor and the pot. if that doesnt fix it, try a smaller resistor for the 2.2M.

whats the envelope generator like? im trying to think of a way that this circuit could effect the envelope generator. at first i was thinking it was a slight pitch shift right at note off. try connecting your VCO to a fixed voltage, rather than running the CV through this glide circuit. but, leave the gate still plugged into this glide circuit, and see if the ghost note is still present. that will differentiate between it being a gate issue or a CV issue.
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Re: SH-101 Style Portamento

Post by medbot » Thu May 07, 2020 6:17 pm

An update to this: I replaced the electro cap I was using in the glide circuit with a gigantic 1u film cap and that cleared up just about everything :oops: :bang: It's now working really well, so thanks again for your help guest. You were right, by the way, about the 'ghost note' being a subtle pitch artifact rather than anything to do with the envelope.

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Re: SH-101 Style Portamento

Post by guest » Thu May 07, 2020 6:23 pm

glad to hear its working! and good to know about the electrolytics. they exhibit "soakage", which i could see effecting things here.
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