What is the advantage of DC out in tuner pedals?
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.
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If a performer is going to be using many pedals, there are a number of ways by which they might all be powered:
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.
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.
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 .
Use a hydra-splitter cable with one input jack and multiple output plugs.
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.
Using this output, you can daisy chain pedals without having a special parallel cable: you can use just "simple" jack-jack DC cables.
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.
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.
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