5

Some pedals, usually tuners (BOSS TU-3, Korg Pitchblack) have 9V dc out and the they take 9C DC in. I'm not sure how it may be beneficial comparing to doing daisy chain with parallel cable.

9

If you have a power supply with eight outputs, for example, then you can add the tuner without taking away from the ability to power eight other pedals. It gives you nine pedals in total.

An interesting follow up question is why more pedals don't have this. My guess would be that perhaps they require more current and the total draw would be greater than the typical power supply can provide. That suggests that tuner pedals draw a very small amount of current, so it makes sense for them to pass along the rest to a pedal that can really use it.

Interesting: The TU-3 normally draws 30 mA, according to the manual, which is low but not that low. In high brightness mode, it draws 85 mA, which is most of the capacity of a typical 100 mA supply.

So I have another theory: Normally the tuner pedal is off while you're playing, and when the tuner is on, you don't need any of the other pedals to be working, so the tuner "steals" current from some other pedal while you're tuning, and then let's all of the current through to another pedal when you actually need it.

  • I'm vaguely recalling something about using fuzzes at lower voltage -- this link seems to jive with that missionengineering.com/fix-your-tone-with-a-dead-battery/… , so it's not unusual to have pedals running with insufficient power input. – Dave Feb 5 '18 at 15:18
  • @Dave Voltage and current are quite different in this situation. The potential problem here is that the total current draw might exceed the current supply capacity of the power supply. That does not make pedals sound better or worse, it overheats the supply and can cause damage or a fire. – Todd Wilcox Feb 5 '18 at 15:24
  • What you say is true if the pwr. out is setup in parallel with the guts of the tuner pedal. I think that it is not if the pwr. out is in series. But now we're digressing into pedal circuit design, though it would be relatively easy to hook up a voltmeter across the pwr. out terminals and see if it drops when the pedal is engaged. I guess my point is that, assuming Boss and the rest have properly designed their circuit, then the worst that happens is the downstream pedal is underpowered, and running pedals underpowered is not unheard of; weakly/indirectly supporting the final paragraph. – Dave Feb 5 '18 at 15:36
  • @ToddWilcox the reason it's not common practice to plug daisy chains into power supply (which the tuner is doing in this case) is because daisy chains are not isolated, which can create noise problems. I think your right though; tuners do it because it mutes everything else when engaged, thus negating the noise problem. – William Perron Feb 5 '18 at 16:24
  • 1
    @supercat Yes, generally pedals are always powered when plugged in, as I mentioned in my earlier comment. The thing is, the amount of current drawn by a (analog) pedal when it is powered is not constant. It varies with the level of the signal. So when the pedal is bypassed, and therefore there is no signal, for most pedals the current draw is very small. Buffered pedals that don't use true bypass will draw more current than true bypass. Digital pedals might not even drop that much at all in current draw since the processors would be kept running. – Todd Wilcox Feb 5 '18 at 17:59
4

I believe the idea is that when you engage a tuner, your other pedals get no input (the tuner mutes it) so their power consumption is low at the moment. Therefore it's safe to use the same power line for a tuner and one other pedal.

3

Using this output, you can daisy chain pedals without having a special parallel cable: you can use just "simple" jack-jack DC cables.

2

If a performer is going to be using many pedals, there are a number of ways by which they might all be powered:

  1. Use a separate "wall brick" for each one. That may be a workable approach for 2-3 pedals, but would be less workable as the number increased, and could be pretty horrible once the number exceeded 5-6.

  2. Have two parallel-wired jacks on each device, plug the supply into one device, and then use an interconnect cable to connect other devices. This is nicely extensible to any number devices if they don't have conflicting ground requirements and none draws too much power. Downsides are that it adds extra expense to each device, and it may complicate efforts to disconnect a battery when an external supply is plugged in.

  3. Use a single "hydra-headed" supply with many output plugs wired in parallel. This requires that the supply make provisions for the number of devices one will be using, but reduces the number of discrete cables one would need. It also avoids the expense (and possible complexity) of adding extra jacks to each device. It still has the same grounding and current-consumption issues as #2.

  4. Use a hydra-splitter cable with one input jack and multiple output plugs.

  5. Use a supply which has multiple independent (ground-isolated) outputs. This would be the best approach, except for the expense. Approaches #2 and #3 are cheaper when they are usable.

If equipment designers were to consistently use approach #2 for equipment where it would be suitable, then the presence or absence of a second power-supply jack could serve as an indication as to whether a device would likely need to be run off its own supply or isolated feed. In practice, however, the market seems to have favored approaches #3-#5. Approach #2 requires two jacks and two plugs for each device, while approach #3 just requires one. Approach #4 requires an extra plug and jack beyond #3, but may be extended at the cost of adding another plug-jack pair. A hydra-headed supply could have almost twice as many plugs as needed and still come out cheaper than the total cost of using daisy-chained power connections; a hydra cable with one power input jack and four power output plugs would probably cost about the same as three plug-to-plug interconnect cables, but wouldn't require any of the devices to have two jacks.

  • A lot of this seems to assume that all pedals have the same voltage and polarity requirements and that current capacity of power supplies is not a concern. Neither of those assumptions are true. That's why #5, the most expensive option, is so popular. It helps make sure that current capacity is not exceeded and that each pedal has the required voltage and polarity (and type, in the sense of AC versus DC). – Todd Wilcox Feb 5 '18 at 17:49
  • @ToddWilcox: Many guitar pedals draw so little current that even a quarter-amp supply could handle a dozen of them, and connect the negative supply rail to ground (typically handling bipolar signals by AC-coupling their inputs and outputs). It might have been helpful for the industry to have standardized on a connector exclusively for use by pedals which could be used 9V negative-ground, and conspicuously labeled their maximum current draw near their connector (thus making it easy to determine whether any group of pedals could be serviced by a supply)... – supercat Feb 5 '18 at 18:03
  • ...but there's no convention I've been able to identify to distinguish devices that can share a supply from those that can't (beyond the fact that most devices that have two jacks could share a typical supply with at least a few other similar devices). – supercat Feb 5 '18 at 18:04
  • If you like germanium fuzz, high headroom boost pedals, BBD based delay, phaser, and flanger, and digital effects, then you're pretty much stuck with a lot of conflicting power needs. In my experience, only distortion, maybe compression, and cheap pedals of other kinds are the only ones that can be expected to have the same voltage and polarity needs and fairly low current draw. – Todd Wilcox Feb 5 '18 at 18:13
  • @ToddWilcox: Even a small supply can produce 100mA. There's quite a lot a pedal could do while drawing at most 50mA. While there are plenty of pedals that don't use a 9V center-negative-ground barrel connector, I'd guess that the fraction that do use that, and that would be likely used on a board with many other pedals, would be greater than 50%. – supercat Feb 5 '18 at 18:23

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