Impedance - When and when not to use HI-Z inputs?

[dannybuoy]

I have a Mackie Onyx Satellite audio interface that has regular line ins, mic XLR ins, and HI-Z (high impedance) inputs specifically for guitar and bass.

Now, if I'm recording raw guitar/bass I should plug into th HI-Z input. Plugging into a regular line input is meant to suck some of your tone because the impedance is not matched (as I understand).

But what if using various effects pedals? I'm considering getting a DHA VT-1 bass overdrive, and Dave Hall himself suggests you can use this pedal as a pre-amp, plugging directly into the line input of a mixing desk OR the instrument input of a bass amp.

My question is, does a high-impedance instrument level signal get converted to a low-impedance line level signal at the output of some FX pedals and should therefore be plugged into the line input and not the HI-Z input?

Also, I have a Aphex Bass Exciter on the way through ebay, and then I have another pickle - do I use the DI output on that to the line input of my interface, or use the regular output to the HI-Z input?

I realise that everything may well sound the same no matter which way I do it, but I keep hearing that impedance matching is important so I'd like to hear what some of you have to say on this one!

[seanm]

Most mixers idea of high-Z is not a bass instruments idea of high-Z. It is only high-Z compared to a 600ohm mic input It is cool that the Mackie gives you inputs for instruments.

If you plug a passive bass straight into a normal mixer input, then you will lose some high end to the impedance mismatch. This is not always a problem, some people like it. A DI solves this problem.

Active basses are always low-Z.

Non-true bypass effects are almost always low-Z.

True bypass effects while on, are almost always low-Z, while off are whatever the bass input is.

So if you have a low-Z output, you can use either input. If you have high-Z, you should use the instrument input.

I hope this helps, rather than confuses

[projectMalamute]

Originally Posted by seanm

Active basses are always low-Z.

I wouldn't count on this at all, for instance most EMGs have an output impedance of 10,000 ohms. Microphones seldom get much above 1200 or so, some dipping as low as 100 or 200.

[galder]

The rule of thumb is that input impedance should be at least five times greater than source impedance. Other than that you're just fine. It doesn't matter if input Z is a hundred times greater than source, the only thing that matters is that source shouldn't be higher Z than input, otherwise you get a mismatch.

DI output is good if you need low Z output, a balanced line or earth isolation.

- K -