Dumb D.I. question.

[paul_c2]

2 hours ago, itu said:

When you actually meant to say that the Z is low, not high.

High

[itu]

It may be, that my poor english is the reason for not understanding the note. I just do not get the term "high impedance" as balanced lines are lo-Z. The balanced signal can be created by a transformer only, or a preamp only, or using a preamp to drive a transformer. All in all, all outputs are lo-Z.

Line level unbalanced signals can be quite high in impedance but then the signal is prone to electromagnetic issues. Especially if the line length is long. Lo-Z balanced lines on the other hand can easily handle 100 meters with negligible or no issues.

[paul_c2]

Balanced = 3 wires. Ground, + and -. Ground is at 0V, when the + line has (say) +0.001V the - line has -0.001V, thus any interference which alters those values (it will add the same to both) acts upon both the + and - and the circuitry can cancel it out.

Lo-Z. The pickups of a passive instrument are, by their physics, high resistance, due to the windings around the poles. For this reason, and so as not to cause alteration to the sound, the input (eg of a mixing desk or amplifier) is high too - convention says it should be at least 10x higher. This is deliberate. Put simply, the high impedance means very little current actually flows, which means there is very little loss of signal over (long) distances.

A passive DI box can't create energy on its own, its transformer is there merely to 1) tap off the - as well as + signal and 2) isolate the electrical path (by its transformer converting the energy to magnetic, then back to electricity for the line). Thus, the actual power transmitted down the wire is still very very low; so irrespective of the DI box's output impedance (which is around 600ohms) or the input's impedance (which will be very much higher),  it will still be low currents and low voltages too. 

https://www.soundonsound.com/techniques/understanding-impedance

It can be a confusing topic but you don't really need to know all the details. The main thing to "take home" is that when you plug a passive pickup guitar/bass into something, the "input impedance" of that thing needs to be high. If its not, it will alter the sound. DI boxes, by their nature (because they're designed to have instruments plugged into them) will have a very high input impedance.

[paul_c2]

ETA, the main reason a balanced line is (almost completely) immune to interference is because its balanced.

It doesn't need to be a high(er) signal level (Although of course, physics would say that a line level is less susceptible to a mic level. But its not the main reason - we don't all run active DI boxes and have shares in Duracell or need the phantom power). Passive DI is perfectly okay, and has the advantage of not needing power (batteries or phantom power or even a mains adaptor etc etc). So long as the thing its plugged into has enough gain on the pre-amp to utilise the signal properly.

[itu]

I see, you actually meant that the desk input impedance that needs to be high. Yes, this is true. But the line needs to be lo-Z. This was my point but your text was a bit long and I missed the mixer/desk comment in the beginning. My bad. But back to the basics:

Bass output (high or low impedance) - hi-Z input of the DI box and balanced lo-Z output - balanced lo-Z line - hi-Z input of the mixer.

[paul_c2]

At the lengths likely used in audio, the impedance of the cable itself is negligible. Its not like its an impedance-matched circuit; or power transmission (where losses are important - although there are v little/none in lengths likely used for audio); or signal runs of an appreciable distance eg 10s or 100s of miles.

To put some details: the impedance of a guitar pickup is typically around 10-100k; an amp input 1meg (1000k); and the cable itself 45 ohms.

[itu]

OK, I have to be more accurate:

Bass output (high or low impedance signal) - hi-Z input of the DI box and balanced lo-Z output - balanced lo-Z signal - balanced hi-Z input of the mixer.

So here we have two types of impedances: the equipment inputs (preferably high) + outputs (preferably low) AND the signal. The difference is a bit like we would be discussing about the right size plugs + jacks AND the cable types that fit to the hardware mentioned. 

[paul_c2]

On 17/01/2020 at 02:01, paul_c2 said:

............ 2) the impedance is high, so long runs aren't going to degrade ................

For 100% clarity, the impedance of the entire circuit (DI box --> cable --> mixing desk input) is high because the mixing desk input is (deliberately) very high, around 100k-1M. The line itself (around 45) and the DI box (around 600) are negligible here.

Its high relative to lines which do degrade, for example long runs to speakers or mains power.