Hi Rob. Lots of things in your post.

Firstly, impedance. If you need to transfer power, you use matched impedances, as that transfers the maximum power. So a source with s 600 Ohm output will deliver maximum power to its load if the load impedance is also 600 Ohms. The voltage seen across the load under these conditions will be half the voltage seen at the source with no load. These days, power transfer is less necessary, so matched loads are not ofen used. Instead, loads are bridging, that is, they have a relatively high impedance relative to the source output impedance. So even if the source is 600 Ohms, it would be common to use a 10K Ohm or higher load to get maximum voltage rather than power at the load.

Now for things like piezo guitar pickups that inherently have a high output impedance you need an even higher load impedance, and this is where DI inputs come in. They are typically 0.5 to 2 MOhms to avoid loading the piezo, and this is a much higher impedance than a typical mixer line input. If you use long cables from a piezo pickup back to a remote mixing desk at that sort of impedance level, the capacitance of the cable plays a big part and will cause a roll-off of the high frequencies. Both to avoid this effect and to make use of existing low-impedance balanced cables from stage (or studio) back to the mixer or control room, a DI box is used, positioned close to the instrument. You can thus utilise short (and hence low-capacitiance) cables from the guitar pickup to the DI box, minimising HF roll-off, and then treat the output of the DI box as a standard low-impedance balanced microphone source feeding though the cable looms back to the microphone inputs of the mixing desk or pre-amps.

Apart from signal level loss and HF roll-off, the other factor is the sonic quality of the signal if mismatched. A piezo transducer when loaded with a relatively low impedance does not just sound like a quiet pickup, it has an unpleasant nasal quality to it. The load impedance thus has a big influence in several aspects of recording or amplifying an instrument like an acoustic guitar. However, many guitars nowadays are fitted with battery-powered active pickups that buffer the high-impedance transducer and present a lower-impedance output.

Hi, thanks for your quick response and the explanation.

Boswell, post: 391921 wrote:
The voltage seen across the load under these conditions will be half the voltage seen at the source with no load. These days, power transfer is less necessary, so matched loads are not ofen used. Instead, loads are bridging, that is, they have a relatively high impedance relative to the source output impedance. So even if the source is 600 Ohms, it would be common to use a 10K Ohm or higher load to get maximum voltage rather than power at the load.

So the higher the impedence, the lower the current and the higher the voltage? And the higher the voltage the better the signal?

Yes within reason, but it's usually not a large effect amongst standard devices. Even if the load impedance were infinite, you would not get any more voltage at the load than the source puts out, and the difference in voltage between infinite load impedance and matched impedance is only a factor of 2, so the signal-to-noise ratio would not change by more than 6dB.

There are secondary effects that are worth mentioning. On the advantage side, these include the ability for a source to drive two or more loads in parallel if they are bridging loads. On the disadvantage side, not all sources can drive all types of load, so you still have to be careful about exactly what you connect to what.

Piezo pickups tend to be special cases because of the tonality change with load impedance. This can be used creatively (with care), so is not necessarily looked on as something that has to be avoided at all costs.

WatchTower, post: 391915 wrote: Talking of DI boxes, I understand from some research that they take an unbalanced instrument level signal and convert it to a balanced mic level signal that you can plug into a pre-amp or similar destination. I get the unbalanced/balanced thing, that's great, but why would you lessen the signal from an instrument level down to a mic level, surely you're losing something there? I would have thought it more appropriate to go from instrument to line level...? Then you would just plug it into a line in on your recording desk rather than preamp in. Is it something to do with the fact that if you had a DI box with a preamp in it would be more expensive, bigger, more fragile?

Because XLR is the standard professional balanced connector used for long lines, and once the signal is put down a line via XLR it will almost certainly be connected to a mic preamp on the other end. Using the mic pres that are already connected beats digging around in the back of the console to find the right XLR plug, attaching an adapter to TRS and putting that into the line input, especially when time is pressing. A lot of instrument pickup levels are not much hotter than mic level so they will need amplification anyway.

If you want to find a DI with a built in preamp just look for it to be called a preamp with a built in DI. For example check out the UA Solo 610. It's a mic preamp and an instrument preamp/DI. It has XLR output that's switchable between mic and line level. It's big enough to need a handle, needs its own A/C power for the tube preamp and it costs $750.

Thanks both for your great responses, that really helps.