Does an active bass need DI?

[noisedude]

Yes! I don't intend on staying in retail forever, partly because there's other things I want to try and partly because spending all day in a sweet shop means I seem to end the month without much pay, but with plenty of new toys!!!

 

[easychair]

Actually, the first article I posted above from Whirlwind claims that keyboards, active pickups and stereo equipment are all low impedance, and that impedance and balanced are two separate issues. (about half way down, past all the math junk)

I'm not sure about very low feeding very high (eg. the mic to the guitar amp).


sl

The actual impedance of those things may vary, especially active instruments, which can be hi-Z. But compared to a passive instrument, they can mostly be considered to be lo-Z. Lo-Z usually means the circuit (two pieces of gear hooked together by a cable is a circuit, really) is lo-Z enough that it can pass signal over long distances with no loss. In this sense, most everything but passive instruments and a few hi-Z active ones is considered to be lo-Z, even line-level stuff which usually sits around 1000ohms, high compared to mics, which are usually around 150ohms or so. A hi-Z circuit, like passive/hi-Z active guitar>cable>amp, is very susceptible to capacitive cable loss, so cable runs need to be short.

Impedance and balancing are indeed separate issues, though lo-Z and balanced have kind of become linked in people's minds due to common audio practice of using lo-Z runs for long distances to prevent loss, and balancing them for noise rejection.

As far as the mic into the amp- Input impedance affects the frequency response of the source. Passive sources like mics and pickups are very sensitive to the wrong impedance. They don't work well, the response suffers. This applies to active sources as well, but in general active sources are much more immune (impedance independent), and work well over a much wider impedance range. They also generally don't mind extremely high input impedances at all. So while an active bass with 2kohms impedance will work fine into a one-megohm bass input, a 150ohm mic won't.

 

[ggunn]

So to get back on my question, does a low z source, like a mic, connected to a hi z input, like a guitar amp, cause any problems impendance wise? I'm purely asking this as a technical and not a practical question, I know the sound will probably suck in this example.

Well, as you pointed out, there's balanced/unbalanced and there's low/hi Z. Yes, they are separate issues, but quite often they are linked, as in your example. You wouldn't be able to plug a a low Z mic into a guitar amp, since a low Z mic runs on a balanced (3 conductor) line and the high Z guitar amp input is unbalanced (2 conductor).

Distortion, if what it is that you are talking about ("sounds terrible" is pretty general) is most commonly generated from one gain stage overdriving the next one, i.e., clipping. In a signal chain from mic to speaker, there are several gain stages, and if any one of them is overdriven, there's garbage from there on; there's no way to "unclip" a waveform by running a later gain stage cooler.

When you set trim pots on a PA to balance the gain, you are effectively compensating for any impedance mismatches you may have at that point. Your prob could have been something as simple as the trim pots being set too hot on the input channels of the PA. That wouldn't be an impedance prob per se, though impedance is part of the equation.

You may be looking for a rule like "you can't plug a low Z signal into a high Z input", but it's not that simple. What you really should be looking at is balancing gain stages.

 

[Halion]

Thanks for the input, but I doubt gain staging was the problem. I go to a music production school and we learn about audio engineering aswell. Not to boast, but all of the people in that particular act had done some live mixing before, and all have done alot of studio work. All that stuff about gain staging and balanced/unbalanced lines is very important, I know. It wasn't really an "arg what is wrong?!?" kind of situation either, the gear was just half broken and/or overpowered (at the last stage, the power amp being driven too hard). It just suddenly crossed my mind what would happen if it were possible to connect a low z device to a hi z input and see what happens.

But anyway, I got my answer, thanks people

 

[ROblows]

Math is hard.

Mostly from trial and error (and a little understanding of the technology), I've figured out that active pickups usually work best through a passive DI and passive pickups work best through an active DI. Neither perform optimally plugged straight in.

 

[easychair]

Math is hard.

Mostly from trial and error (and a little understanding of the technology), I've figured out that active pickups usually work best through a passive DI and passive pickups work best through an active DI. Neither perform optimally plugged straight in.

Pretty much always works.