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Old 02-28-2017, 12:50 PM   #9
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Join Date: Dec 2012
Posts: 4,224

I don't know how far I want to get into this right now.

The (at least) 10:1 Rule (of thumb) is good in most cases, but it's complicated by the inductive nature of the pickups. While 5K-10K isn't bad as a general middle of the road guesstimate for DC resistance of most pickups, their impedance gets much bigger at higher frequencies, and in order to keep as many of those higher frequencies as possible, we'd like the load impedance he as large as possible.

But the inZ of whatever you're plugging your guitar into doesn't really "set" the load Z so much as "modify" the load because most people have pots in their guitars which are loading the pickups. We have to look at the parallel total of all of these pieces to know exactly what's going to happen. That equation looks ugly, but it works out that the total of several impedances in parallel is always smaller than the smallest value. It also works out that the closer one value is to another, the more it tends to affect the total. 1M || 125K is 111K -very close to the 125K, the 1M can almost be ignored - while 125K || 125K is 62.5K - we definitely need to consider that kind of variation.

I used the numbers above on purpose 125K is the total of two 250K pots in parallel as you find on many single coil guitars. And it's also very close to that 130K number that Radial article mentioned. Now 62.5K does just make the 10:1 thing against the DCR of a typical single coil, but as the inductive impedance starts to take over, the ratio gets smaller fast, and you start to lose treble. When you halve the load impedance, you lose an octave, so I think the difference between a passive DI and a 1M active buffer is pretty apparent. Almost an octave, sort of...

Before I get to why that's only sort of true, I want to adress that 130K spec that Radial spits. The fact of the matter is that a transformer doesn't really have an impedance of its own. There is of course some resistance in the wire, and some inductance and capacitance from the coil of wire and all that, but the good ones are designed so that these values don't affect the rest of the circuit enough to matter. But when you plug the guitar into the DI, the impedance that guitar "sees" is not that of the transformer itself, but that of whatever mic pre the other side of the DI is plugged into "reflected" through the transformer. Basically, it'll be the inZ of your mic pre times about 133. So Radial must have meant that their transformers will look like 130K if plugged into a 600Ohm input. There aren't too many 600Ohm inputs out there anymore. 1K is pretty standard, but many very popular brands nowadays are bigger than that. Many are more like 3K and more and more are coming out at 10K. So that's kind of good news in that we get back some of the treble we might have lost back in the 50s, but it's going to be different wherever you plug it in.

So I guess that's how far I'm going to get into it. I'll just say that I prefer not to use a passive DI for a passive guitar as much because it can be a little low Z and because of the stepdown in volume. Guitars are already a little too quiet, and it doesn't make sense to turn them down and then back up. That kind of thing doesn't matter in a 64 but mix engine, but it sure does in meatspace! I always just plug my guitar into a pedal with a buffered bypass and connect that to a mic or line input on my interface.
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