KSS Diode Patch Cords. Keep it Super Simple. Get -or make- some!

[KSS]

Another snippet from a directed reply that deserves wider exposure.

I'll use this as an opportunity to again espouse the "quintessential" value of diode patch cords. Easy to make, *everyone* should have a few.

Single diode -any* type- in series. Mark the plug end attached to the diode's banded lead <--Called the cathode. So you know which way it does its jobs. I use a red shell or heatshrink as identifier. Red because this is a one-way patch cord, and this end is the equivalent of a stop! wrong way road sign.

*Any type diode will work, but there are differences, and it can be useful to have Diode patchcords made from regular signal diodes like 1N4148, Rectifiers like 1N400X, And Schottky types like a 1N5819 too, due to their lower 'activation' level.<--AKA forward voltage.

Even LEDs of various colors make sense. They may not light up -that's not why they're used- but they do 'activate' at different voltage levels and so you have 'comparators' built into one-way patch cords.

So what can you do with diode patch cords? And why do you need them?
-Remove upper or lower half of an AC signal. Which also divides its p-p voltage by half.
^IOW, 50% attenuation EDIT: assumes signal centered on GND, like most VCO outputs. Signals with DC offset will be variably clipped, which can be useful too! /edit
^Increased modulation waveshapes from common -and not so common- LFOs. Great for providing variety in small or LFO-limited systems.

-Selectively trigger modules from more than one source without using module-rack space to do so.

-Provide interesting feedback options.

-Using several along with a mult or 0HP "star" type mults, to make diode ORs for gate and trigger combining.

-Perform boolean logic. Creative patching allows more than ORs. Diode resistor ANDs, for example.
^Especially true if you also have -and your format allows- the Vtrig to S-trig patch cords. AKA inverters. Inverted OR = AND.

-As laid out above, create different "ON" voltage levels. KSS 'comparators'!<--Not true cmparators, but can achieve similar results.

-Distortions of various types that don't use up module resources. EDIT: Note this is NOT the same as Rex Coil 7's reply below. Here the diode is in SERIES. His reply has a diode or two going from signal to GND. Useful, but different. /edit
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The first use I saw for this technique was back in the early 70's, where it was sugggested as a way to trigger an EG from two different sources. But since then, it's been easy to see the more general use possibilities are worth having and should be more widely known.

[Rex Coil 7]

... tie a diode from ~hot~ to ~ground~ inside of a plug and it creates asymmetrical clipping. If you can manage two diodes (arranged with opposing polarities) inside of a plug you'll get symmetrical clipping (as long as both diodes are the same, otherwise it will still create asymmetrical clipping ... just ~different~ asymmetry).

[DSC]

...

[sduck]

Brilliant! I'll have to make a few.

I knew that diode patch plugs were part of the deal with the EMS VCS3 and similar, but didn't have much experience with them back in the 70's when I was working with them. About 5 years ago I was sent the parts of a unbuilt phutney to build for someone, and included was a bag of patch pins. So I got the thing bult, and it's working, but the patches weren't really working the way they should have been. It turns out that all of the pins in that bag were diode pins. A bit of work, and I was able to convert most of them to the usual 2.7K resitor pins and things started working normally.

[col]

If you don't want to have the confusing patch cables, you can build one into a Star Mult.

[Rex Coil 7]

... this fella built a Low Pass Gate into a cable ... sortof ..

LINK: viewtopic.php?f=17&t=107006&p=3344247&h ... e#p3344247

... end result ....

[Addam]

Rex Coil 7 turned me onto using diodes for passive clipping/waveshaping in another thread and since then I've been wanting to hack up a 19" rack 1U panel and put a bunch of passive simple circuits in there. Maybe a guitar pedal circuit or 2 that could be powered off 12V.

Diode OR is an easy one but I'm curious how you get a passive diode AND. Also diode feedback techniques.. you mean basically running a signal into a diode for clipping (or rectification) and then adding that signal back into itself or do you have something more sinister in mind?

[KSS]

I've attached a diode AND circuit.
Most modules have an output resistor. Patch any of these to a mult with at least 4 jacks. One open jack is now your AND output.
Add two -or more- diode patch cords with the red end free. Schottky type are best, but the others work too. These are your AND inputs.

When the inputs are high their diodes are forward biased and do not pull the multed signals down. If either is low it *does* pull the mult down. So only if input 1 AND input 2 are *both* high will the mult be able to output. We have the truth table for an AND: LL=L, LH=L, HL=L, HH=H.

Remember you can use more than two. And remember that different diodes have different 'activation' voltages. You can get close to what used to be called fuzzy logic with this patch. Or at least variable -yet deterministic- results.

Note that this does depend on the patch points being used for the AND inputs. It's an 'analog' form of AND. Functionally it's often identical to a digital AND, but things like sourcer and sink capability of the chosen inputs, and forward voltages and voltage levels in general mean results can be variable. But that's a good thing!
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Savvy readers will notice this^ is a simplified version of the SERGE Trough circuit. Which is nothing more than a diode AND with some additions to make it more predictable in use. It's paired with the PEAK circuit, which is a diode OR -again with small additions for more predictable results. I've included Serge's orignal note showing both circuits. Ken Stone's CGS84 is basically the same thing with a couple more additions to increase utility. Worth having one of these in yor system and an easy build with or without a PCB.

But until then, you can do most of what those modules do -and a few things they can't- with KSS Diode Patchcords and creative patching!

[KSS]

Also diode feedback techniques.. you mean basically running a signal into a diode for clipping (or rectification) and then adding that signal back into itself or do you have something more sinister in mind?

Depends whether you're after 'accidental' or intentional results. Both are useful and I'd daresay either could result in 'sinister' sounds..

As an example of intentional, take an FFB with individual outputs and choose some to use the plain diode patchords from output back to a multed input. depending which bands -and which direction- you place the various feedback diode cords you can *greatly* modify the result in what will become predictable in a relatively short time.

But if you instead go for accidental patching and include the various types of diodes it gets 'interesting' and can break patch boredom or ingrained thinking while patching.

[KSS]

If you don't want to have the confusing patch cables, you can build one into a Star Mult.

Confusing?
Maybe it's just me but that looks a *lot* more confusing than a patchcord with one red end.
Harder to make too.

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But is IS a good example of another way to put diodes into patches. Which is a good thing! While I'm kinda halfway between kidding and serious above, I DO appreciate you showing the modded star mult.

[col]

OK, confusing was a bad choice of words. It's just that I could imagine being in a live situation with a big patch on the modular and something not working because I'd accidentally swapped in a one-way patch cable.

The Star Mult looks like this with the cover on, white marks the output:

[Addam]

I've attached a diode AND circuit.
Most modules have an output resistor. Patch any of these to a mult with at least 4 jacks. One open jack is now your AND output.
Add two -or more- diode patch cords with the red end free. Schottky type are best, but the others work too. These are your AND inputs.

Thanks for the info. When you say most modules have an output resistor - do you mean the resistor around the power section (+12V) of a module? Is this what determines the point at which a signal comes through (or is this dropped) in this particular circuit (or should I be considering current and not voltage necessarily)? Basically wondering if an lfo instead of a static voltage applied here could add a little variation.

[KSS]

Many modules have a 1K resistor in series at all module outputs.
You'll see it regularly discussed here and elsewhere online as a bad thing -primarily for pitch. But ever since ARP used it on the 2500 and then Electronotes in the 70's made it part of their "standard practice" it's been common on modules near universally.

Yes, the LFO can be your source instead of a static voltage. Mentioning the typical series output resistor to say that the resistor in the diode AND circuit can BE this typical 1K output resistor.

The Diode AND is based on the patch points you choose for its inputs -wherever you patch the RED ends of your diode patch cords- being capable of sinking current.

[Bernie Hutchins]

by KSS » Fri Oct 30, 2020 12:18 pm
"Many modules have a 1K resistor in series at all module outputs.
You'll see it regularly discussed here and elsewhere online as a bad thing -primarily for pitch. But ever since ARP used it on the 2500 and then Electronotes in the 70's made it part of their "standard practice" it's been common on modules near universally."

About those 1k resistors:

The 1k series output resistor was not really for protecting the outputting op-amp from damage (very unlikely), nor was the automatic “mixing” (often touted) of much importance. [It did assure that something well-defined happened contrary to an ill-designed chance occurrence of two op-amp outputs being directly connected! Also, a somewhat similar moderate output impedance (600 ohms) was common in audio signal work. And it was obviously never suggested for the lines carrying a main-control-voltage (volts /oct.) .]

Two useful things: First, the SDIY experimenter frequently (typically) has his/her ”synthesizer” (finished modules) driving an external breadboarded module under test (MUT). For example, a finished VCO might be modulating a new VCO MUT. Things are not working – no surprise. Then to your horror you see that your good VCO has also now stopped - what have you done!

Well, if you had the 1k series resistor your finished VCO would be running happily regardless of what is going on in the connected VCO MUT. (Op-amps generally drive anything if isolated by 1k). Without the 1k, a wiring error (perhaps a breadboard short to ground) may well get back into your finished VCO, causing confusion as well as anxiety.

The second useful function of the 1k series resistors is that they “decouple” capacitive (typically shielded cables at perhaps C=100 pfd/meter) loads from op-amps thus preventing high-frequency (MHz) oscillations WITH ASSOCIATED DC SHIFTS. Connecting a cable (even to a scope) directly to an op-amp output forms an RC low-pass (R being the inherent op-amp output resistance of perhaps 100 ohms). Such an oscillation is HF but low level (slew limited) and non-symmetric (non-symmetric slew limiting). The result is a fuzzy looking scope trace (looks out of focus) and is only there when a cable is attached, and as a VCO control can cause a small but noticeable pitch shift.

The oscillation occurs because the R is internal to the op-amp and the RC is INSIDE any feedback loop and contributes excessive phase shift. With the 1k series the RC (R now 1k) is OUTSIDE the op-amp’s feedback loop.
Fuzzy trace and pitch shift gone.

-Bernie

[KSS]

Thank you Bernie!

[Addam]

Excellent, thanks for the info KSS and Bernie.

[neil.johnson]

by KSS » Fri Oct 30, 2020 12:18 pm
"Many modules have a 1K resistor in series at all module outputs.
You'll see it regularly discussed here and elsewhere online as a bad thing -primarily for pitch. But ever since ARP used it on the 2500 and then Electronotes in the 70's made it part of their "standard practice" it's been common on modules near universally."

About those 1k resistors:

The 1k series output resistor was not really for protecting the outputting op-amp from damage (very unlikely), nor was the automatic “mixing” (often touted) of much importance. [It did assure that something well-defined happened contrary to an ill-designed chance occurrence of two op-amp outputs being directly connected! Also, a somewhat similar moderate output impedance (600 ohms) was common in audio signal work. And it was obviously never suggested for the lines carrying a main-control-voltage (volts /oct.) .]

Two useful things: First, the SDIY experimenter frequently (typically) has his/her ”synthesizer” (finished modules) driving an external breadboarded module under test (MUT). For example, a finished VCO might be modulating a new VCO MUT. Things are not working – no surprise. Then to your horror you see that your good VCO has also now stopped - what have you done!

Well, if you had the 1k series resistor your finished VCO would be running happily regardless of what is going on in the connected VCO MUT. (Op-amps generally drive anything if isolated by 1k). Without the 1k, a wiring error (perhaps a breadboard short to ground) may well get back into your finished VCO, causing confusion as well as anxiety.

The second useful function of the 1k series resistors is that they “decouple” capacitive (typically shielded cables at perhaps C=100 pfd/meter) loads from op-amps thus preventing high-frequency (MHz) oscillations WITH ASSOCIATED DC SHIFTS. Connecting a cable (even to a scope) directly to an op-amp output forms an RC low-pass (R being the inherent op-amp output resistance of perhaps 100 ohms). Such an oscillation is HF but low level (slew limited) and non-symmetric (non-symmetric slew limiting). The result is a fuzzy looking scope trace (looks out of focus) and is only there when a cable is attached, and as a VCO control can cause a small but noticeable pitch shift.

The oscillation occurs because the R is internal to the op-amp and the RC is INSIDE any feedback loop and contributes excessive phase shift. With the 1k series the RC (R now 1k) is OUTSIDE the op-amp’s feedback loop.
Fuzzy trace and pitch shift gone.

-Bernie

This post should be a sticky in the DIY forum.

Neil

[clickmrmike]

^this

Clickmrmike

[KSS]

^That would be Fine. But please don't move this thread to DIY. I posted it here in general synth on purpose.

[neil.johnson]

I took the liberty of putting Bernie's post on a webpage, together with some explanation:
https://www.njohnson.co.uk/index.php?me ... submenu=16

Neil

[KSS]

I took the liberty of putting Bernie's post on a webpage, together with some explanation:

nor was the automatic "mixing" (often touted) of much importance
To do this mixing you need a way of connecting multiple cables together without any intervening electronics. This is typically done with passive multiway adaptors ("mults"), originally designed to allow one output to drive multiple inputs. It is possible to use them in reverse, combining multiple outputs, but this has several pitfalls: the mixing depends on all outputs having this 1 kΩ resistor, the mix ratios depend on the output resistance of each source, which is not guaranteed to be exactly 1 kΩ so it's a bit of a crap shoot as to what you end up with, and it results in significant currents flowing around the patch cables, which can inductively couple into other patch cables.

You should add a note that in the ARP 2500 the 1K were *also* there for mixing. Call it a unique case, but don't leave it out.
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I don't think you can say it was *only* used for module development. Would need to go back to more Electronotes issues than the single quote in this thread which prompted your page.

While it's definitely true what's been written here, it's not the only or final word on the subject. To say so means that all the vintage synth designers who *did* include 1K output resistors on products -and *not* just those in development- got it wrong.

I'm not sure I'd want to say that about Dave Rossum, Serge, and others who were reading and contributing to the newsletter back then.
More to the point, who are still making modern modules. Do current new Serge R*S designs and Rossum Euro modules still have a 1K output resistor? I'd say that was telling.<--I don't know if they do or don't.

But I'd hardly lump DR or Serge into a cargo cult.

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FWIW-Couple typos crept in your page wrt it's v its.

[neil.johnson]

I took the liberty of putting Bernie's post on a webpage, together with some explanation:

nor was the automatic "mixing" (often touted) of much importance
To do this mixing you need a way of connecting multiple cables together without any intervening electronics. This is typically done with passive multiway adaptors ("mults"), originally designed to allow one output to drive multiple inputs. It is possible to use them in reverse, combining multiple outputs, but this has several pitfalls: the mixing depends on all outputs having this 1 kΩ resistor, the mix ratios depend on the output resistance of each source, which is not guaranteed to be exactly 1 kΩ so it's a bit of a crap shoot as to what you end up with, and it results in significant currents flowing around the patch cables, which can inductively couple into other patch cables.

You should add a note that in the ARP 2500 the 1K were *also* there for mixing. Call it a unique case, but don't leave it out.
------------------
I don't think you can say it was *only* used for module development. Would need to go back to more Electronotes issues than the single quote in this thread which prompted your page.

While it's definitely true what's been written here, it's not the only or final word on the subject. To say so means that all the vintage synth designers who *did* include 1K output resistors on products -and *not* just those in development- got it wrong.

I'm not sure I'd want to say that about Dave Rossum, Serge, and others who were reading and contributing to the newsletter back then.
More to the point, who are still making modern modules. Do current new Serge R*S designs and Rossum Euro modules still have a 1K output resistor? I'd say that was telling.<--I don't know if they do or don't.

But I'd hardly lump DR or Serge into a cargo cult.

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FWIW-Couple typos crept in your page wrt it's v its.

Thanks, I added some counter arguments, and info about the ARP2500's various output resistance values (they're not all 1k).

Neil

[DSC]

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