Modular synthesis intro, part 4: Some pitfalls

2018-01-07 shop intro

This is Part 4 in a series that started with Part 1.

This week I continue the series on how to get started with modular synthesis by looking at some common pitfalls beginners encounter, and mistakes I hope you can avoid making.

Beware of "passive" modules

Some modules are powered entirely by the incoming signals on their inputs. They don't require a separate connection to a power supply. I'm not sure why, but a lot of people think that's a really, really, cool idea. It often isn't.

I'm putting the word "passive" in quotes because that word has a specific meaning in electrical engineering and the way synthesizer people use it, is not what it actually means. But I don't want to have to go into a lot of detail on that here, and it is the standard term people use for this concept in the synth realm, so we're kind of stuck with it. I'd really prefer to call them "signal-powered" modules.

The power for "passive" modules has to come from somewhere. So if your power supply is close to overload, you're not going to avoid overloading it by adding a "passive" module - it'll just (by virtue of its low input impedance) cause the module that's really powering it to draw that much more from the power supply. Running out of power isn't a big issue for most people, but this point seems worth mentioning. The laws of physics, conservation of energy in particular, remain in force. All you're saving is the need to connect a cable.

Whenever two pieces of electronic equipment are connected together, there's an issue of how much of the signal between them is supposed to be carried by current and how much by voltage. This ratio is called the impedance of the connection. Different kinds of interfaces place different requirements on the impedances of the inputs and outputs. In radio systems it becomes especially complicated because the impedance ends up being a complex number, with real and imaginary parts; people usually try to make it pure real (with imaginary part zero), and the same for both input and output to prevent signals from reflecting off the ends of the cable, but that can be complicated. Audio is usually a little easier.

Modular synths attempt to keep the impedance situation as simple as possible by demanding that outputs should always have very low impedance and inputs very high impedance. As long as those demands are approximately satisfied, we can approximately assume that the information on the cable will be carried entirely by voltage and the current will be negligibly small, and that makes a lot of interconnection issues simpler.

"Passive" modules mess with the assumptions. By having an unusually low input impedance (to draw more current from the upstream modules) and an unusually high output impedance (because they have no power of their own to supply to downstream modules), these modules are not playing by the rules. They usually work pretty well as long as the other modules in the system continue to pull their weight, but you end up having to face some of the issues that the standard way of doing things is meant to avoid.

With a lot of passive modules in your system, you'll run into unexpected behaviour. For instance, trigger voltages might not trigger envelopes when they should, if they've been through too many "passive" logic modules; and oscillators and tracking filters may end up flat and mis-tracking if their control voltages are being used as power supplies. You may find that there are restrictions on what you can patch into what, which (depending on your style) may break the "anything into anything" artistic flow of modular patching. Just consider: what if you want to patch module A into module B and they're both passive? Do you think that's likely to work?

Now if there's a module you're sure you want to use and it happens to be "passive," I wouldn't treat that as a dealbreaker. There are some circuits, traditional diode ring modulators being one good example, where it just happens to be the case that the circuit you would want for other reasons doesn't have a power connection. If you want one of those, fine, get it. Attenuators, if they are true attenuators and not mislabelled variable amplifiers, can also quite reasonably be made "passive": their only purpose is to absorb power and they don't need an extra power input to do that. Passive multiples (no quotes, they're the real deal) are a glorified type of cable adapter and don't need power. Some kinds of interface modules fall into the same category, including USB interfaces which take their power from the USB bus like other USB devices. And some highly-regarded low-pass gate modules are also traditionally built in a "passive" format, although for my own money I'd consider that design to be borderline and I would want to pair such a module with a buffered multiple to restore the level. I'm not saying you should never buy a "passive" module; only that I think you should regard a module's being "passive" as usually a disadvantage, not a positive selling point.

Don't buy an output module

Every new user thinks they need a module specifically for connecting the modular synth to wherever the sound is going (often speakers, headphones, a mixing board, or a computer interface). I bought one as part of my first purchase of modular gear myself. Getting one of these is usually a mistake. Popular unnecessary output modules include the Pittsburgh Outs (which is the one I got, US$100); the Makenoise Rosie (US$140); and the Intellijel Interface (US$260).

Most audio gear other than modular synthesizers expects an input voltage level considerably lower than the native level used within the synthesizer. You don't need to boost or preamplify the modular signal to make it suitable for your other equipment; instead, you have to attenuate it, and you can usually do that just by turning down a knob on one of your existing modules (such as a VCA). Some of your outboard equipment (most pro and semi-pro mixing boards, in particular) will already have enough headroom that it can take a modular signal at modular level without any special provisions for interfacing.

Speakers as such have a very low impedance, and synthesizer outputs (although they have plenty of voltage) may not be able to supply enough current to drive speakers. So if that's what you're doing, then okay, you need an amplifier somewhere. But it usually makes most sense to use speakers with the amplifier built in, such as people plug into PCs, rather than having a speaker amplifier module in your synthesizer to use with unpowered speakers. One reason is that because of mass production, using speakers with built-in amplifiers means you'll either get the amplifier for less money, or a better amplifier for the same money.

Driving headphones directly is an application where it may possibly make sense to have an output module in your modular. You can get acceptable results plugging headphones directly into many, but not all, of the signal-producing modules in your synth; it's a question of impedances again, because the voltage/current ratio the phones demand is a lot different from what the synth modules are built to expect, and not all outputs react the same way to the mismatch. So having a headphone amplifier module would remove some uncertainty. If you think you need one, I like the Doepfer headphone amp (US$60). But don't buy it first thing. Wait until you're sure you need it.

People who intend to do live performances are sometimes attracted to the MakeNoise Rosie in particular because it's marketed as being specifically designed for live. I am unconvinced that it's necessary or valuable. I'd rather do the "smoothly add and remove signals without startling the audience" function using a Doepfer A-138e crossfader module (US$92), which can also do many other things, and add a headphone amp module if I really think I need a separate output for my headphones. The A-138e is a large module and it may not be appropriate for a small starter system, but I use mine in almost every patch since I bought it. There is a consistent pattern here worth noting: modules sold for a very specific purpose (like the Rosie) are less appealing to me than modules that have a more general function capable of being turned to that purpose or many others.

"Stereo mixer in Eurorack" = not a good idea

You don't want a stereo mixer built into a Eurorack module, either. You may think you want that instead of a standalone mixer for connecting your other gear. It's frequently at the top of people's "I wish someone would start manufacturing this kind of module" lists, and some manufacturers are starting to oblige with stereo mixer modules, but it really seems to be a bad idea.

One reason is that because they're mass-produced for a much larger market than any synth module, standalone mixing boards can offer lower prices for the same quality and feature set, or better quality and features for the same price, compared to what it costs to do this sort of thing in a Eurorack module. The constraints on panel size and shape for Eurorack, and the power supplies available, don't work for the usual layout of a mixing console and the signal levels it needs to handle. Another issue is that everybody has a feature they think a stereo mixer must have, and a feature they think it must not have. It's not enough to just choose not to use an undesired feature, because people don't want to pay the money or panel space costs. And your must-have feature will be someone else's must-not-have - which means any stereo mixer module can only satisfy a small fraction of the market, requiring many different models for different customers and further driving up the cost.

I do my final mixing using an external Mackie ProFX8v2 mixer; it was about US$200 and for that money I got two mono and three stereo main input channels with decent preamps on all of them, real slider controls, an external effects loop, some internal effects, a USB audio interface, a bunch of EQ features, and so on. Just a significant fraction of that in Eurorack modules would be over US$1000, and this is not even a high-end external mixer as external mixers go. And it handles modular levels on the line inputs without any special conversion, just a 1/8-to-1/4-inch adapter cable.

You do sometimes need mixing inside the modular synth too as part of a patch, but the traditional four-knob mono modular mixer, as sold by Doepfer, Pittsburgh, and many others (North Coast will be introducing one very soon, too), is much more appropriate for this purpose. If you want stereo, use two of them. If you want "sends" and "returns," use a matrix mixer (US$150). Each of these simpler modules can be used in several ways, whereas the "stereo mixer" as most people imagine it would be useful only as a substitute for a better and cheaper standalone board. After you have a final instrument signal from your modular synth, you can plug it, and the signals from whatever other studio equipment you have, into a traditional standalone board to do your final mix and panning, and that works a lot better than trying to do the final mix inside the Eurorack.

Power connections: the red line

Traditional Eurorack power connectors are not polarized. It's easy to plug the cable into the module or the busboard backwards. Then you end up with +12V applied to the -12V bus and vice versa. Some modules will be instantly destroyed if you power them up in that condition, which can be heartbreaking in the case of modules costing hundreds of dollars each. Others contain one or another form of protective circuitry, which may prevent or at least reduce the damage. Some styles of module-protective circuitry (parallel diodes, in particular) may pose a threat to the power supply in case of a backwards connection, even though the module is protected.

All my own designs use series diodes for protection. If you try to power up a North Coast module with the power reversed, then it will not power up, but it should not suffer or cause any other damage, and everything should be fine once you reconnect the cable properly. I can't promise, though, that it'll be immune to cases of misconnection more complicated than simply reversing the plug. Many other manufacturers do it the same way I do, but this isn't a no-brainer; there are tradeoffs involved, and other manufacturers who don't use protective circuitry, or use more dangerous types of protective circuitry, sometimes have important reasons for doing things the way they do.

Nowadays some boards and most cables are built with polarized connectors, and if all your power connectors are polarized correctly then it should be difficult or impossible to plug them in backwards. But it's also been known to happen for a cable or board to be made and sold with polarized connectors that are themselves installed backwards, so that the polarization causes the problem it should prevent. There is also one well-known manufacturer (Cwejman, a reclusive one-man operation based in Sweden) who have standardized on a power-cable stripe convention opposite to what everyone else uses. There's no real substitute for careful human attention to the power connections: whenever you plug and unplug power cables, make sure you have all the polarities right before you turn anything on.

Power connectors have a standard convention by which one pin is designated as "Pin 1," the reference pin. It will often be marked with a little triangle molded into the plastic body of the connector, for connectors that have a plastic body. There is also a standard convention regarding where the keying slots are in relation to Pin 1, so if you have polarized connectors, they will keep everything in the right relation.

The usual kind of ribbon cable used for Eurorack power is grey with a single red wire forming a stripe along one edge, and the red wire should be connected to Pin 1. You may rarely see grey ribbon cable with the special wire coloured blue or dark grey instead of red. There is also rainbow ribbon cable, where Pin 1 should connect to the brown wire; starting from one edge the wire numbers go brown, red, orange, and so on like the resistor code. Wire number 1 is one of the two wires carrying the -12V negative supply. On circuit boards, the connectors are usually oriented so that Pin 1 is at the bottom; and it is usually marked on the silkscreen art of the circuit board, either with text like "-12V" or "stripe here" or with a little line or bar; and if you look closely at the solder pads attaching the connector to the board, often Pin 1 will be different in some way. For instance, on my boards it has a square instead of round pad.

In general, then, you can usually get everything right by keeping all things that distinguish one end of the connector (markings on boards, markings on connectors, and the cable stripe) lined up with each other, and the stripe and the special end of the connector will usually be at the bottom, and any connectors that are polarized will agree with these clues. But double and triple check if you're not certain! It is unfortunately possible for any part of the system to have been manufactured wrong, in a way that will conflict and cause trouble if you trust a single indication when it disagrees with the others.

If you come into possession of an incorrectly-made ribbon power cable where the connector polarization doesn't match the stripe at both ends, throw it away! (Unless you're trying to follow the Cwejman convention, in which case, you're responsible for exercising appropriate caution.) A properly-made replacement doesn't cost enough to be worth the risk of trying to keep the bad cable in service while remembering that its marking is backwards.

Continue to Part 5 of this series.

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