Does an active bass need DI?

[noisedude]

Simple question really, I just got an active bass for the first time and someone mentioned to me that it doesn't require DI'ing because the output current is higher. Does this mean I can plug straight into a line input on a preamp without an impedance mismatch? I'm just wondering if I can cut my crappy Behringer DIs out of my signal chain for recording bass.

I know, I know, I should try it and see what my ears tell me ... but I'm full of cold and anyway I want someone to explain the science of it to me!!!

Nik

 

[ggunn]

Simple question really, I just got an active bass for the first time and someone mentioned to me that it doesn't require DI'ing because the output current is higher. Does this mean I can plug straight into a line input on a preamp without an impedance mismatch? I'm just wondering if I can cut my crappy Behringer DIs out of my signal chain for recording bass.

I know, I know, I should try it and see what my ears tell me ... but I'm full of cold and anyway I want someone to explain the science of it to me!!!

Nik

The output will be hotter than that of a passive pickup bass, but it's still not a full preamp level. Part of DI'ing, too, is to convert the unbalanced to a balanced signal so that you can take advantage of common mode noise rejection.

 

[noisedude]

Gordon

I understand what you mean about noise rejection - however, plugging straight into a preamp would be a shorter run than going into DI and up to balanced line level ...

 

[giraffe]

yes,
impedance matching.

a di box is not about level, if so a passive one would be useless.

 

[noisedude]

Can someone actually talk me through some facts and numbers? My understanding is that the impedance would at least be less 'mis-matched' ....?

a di box is not about level, if so a passive one would be useless.

Eh?

 

[metalhead28]

Well, I couldn't give you any specfic numbers or anything but even an active guitar or bass will still be high impedance in order to work well with an amplifier. The level of the signal won't change the impedance (I'm no electronics expert but I think that is correct). A DI box will convert the high impedance output from the guitar to a low impedance line level no matter how hot the signal is. I'm sure it would work plugged straight into a line-in but you probably won't get the fullest signal or the cleanest sound. I would agree that unless the pre-amp has a specific instrument level input, I would use a DI.

 

[easychair]

Can someone actually talk me through some facts and numbers? My understanding is that the impedance would at least be less 'mis-matched' ....?


Eh?

Impedance mismatching is much more critical with passive sources, like mics and passive instruments. Active ones suffer as well, but can handle a much wider range of mismatching.

Impedance mismatching has two basic effects. The source may not be able to function right, and the signal becomes more susceptible to capacitive losses from the cable, rolling off the high end.

Outputs and inputs both have impedance.

The idea is low output impedance into high input impedance gives the best signal, with no frequency loss from the source, and minimal effects from the cable.

The common standard is input impedance should be around 10X output impedance.

Typical mic- 150-300ohms output impedance
Typical mic pre- 1500-3000ohms input impedance

Typical passive instrument- 7,000-125,000 ohms output impedance
Typical amp input- 250,000- 1 megohm (one million)

Typical line-level output- 1000ohms
Typical line-level input- 10,000ohms

Even if proper impedance matching is followed, the higher the impedance the circuit operates at, the more susceptible to capacitive cable losses it is. That's why you need to keep guitar cables short and of low capacitance, even if the impedance is right, but you can run hundreds of feet of mic or line cable with no problems.

Active sources are generally lower-impedance, and they can interface with more types of gear without help. Whether or not you need a DI for your active bass will depend on it's output impedance.

Passive sources also need the proper input impedance to work right. Mics and passive guitars, for example, will put out a crappy signal if plugged into an impedance that is mismatched too much, in addition to the cable losses. Active sources don't suffer this way as much, but a severe mismatch either way will still cause sonic issues.

Passive DIs have limited input impedance, and so may not be enough for a passive instrument, but will be fine for most active ones.


And, of course, your ears are the final word.

 

[noisedude]

Now THAT'S interesting stuff. So let me see if I've got this straight:

The balanced line level output of a DI box will be at around 1000ohms, looking into around 10000 on my mixer? But most DIs are plugged into mic inputs, which at 1000:3000 would be way less than the 1:10 ratio being aimed for ....

I don't know a way of finding the output impedance of my bass, but this subject is turning up some other little questions for me. I can't have someone tell me what to do, I need to understand it and work my way through it from the basics. Hopefully I'll pick up less rubbish advice that way!!!

 

[easychair]

Now THAT'S interesting stuff. So let me see if I've got this straight:

The balanced line level output of a DI box will be at around 1000ohms, looking into around 10000 on my mixer?

Typical actives have around 150ohms output Z (impedance)

Passives have typically around 150, plus the output Z of the gear attached to the DI input, divided by ten. In the case of a piece of line gear or your typical active instrument, this means about 250ohms total. No problems.

 

[noisedude]

Ah, ok. I'm starting to get it now. Thanks very much! So the difference between plugging a passive instrument into a line input, and putting an active DI box inbetween, is anything up to 125k-150=124850ohms? I can see why that would be important to get right then ...

 

[snow lizard]

There's a tech article here that talks about impedance.

This is the mother article.

What kind of bass did you get? More importantly, does it have a cool paint job?

I'm not sure what the impedance of the line input on your preamp is, but if there's any way you can get a spec on it that might help. The line in jack on my SRC console is 10k ohm, while the XLR mic input is 2k ohm. The line in might be a bit low for a guitar with a passive pickup, but I'm guessing that active is ok. Active guitar pickups and preamps are low impedance. I'm not sure if it's possible for the input impedance to be too high for your bass or not. I'd probably just plug it in and see if you get a pile of hiss or a quiet signal.


sl

 

[Halion]

Question: what happens when I plug a low Z device (keyboard?) into a hi Z input (guitar amp?). In theory, that should sound fince because the input impendance on the guitar amp is so much higher than that coming from the keyboard?

 

[ggunn]

Question: what happens when I plug a low Z device (keyboard?) into a hi Z input (guitar amp?). In theory, that should sound fince because the input impendance on the guitar amp is so much higher than that coming from the keyboard?

To a lot of us, myself included, low impedance and balanced are very nearly synonymous, as are high impedance and unbalanced. Most gear that is low impedance uses XLR or TRS connectors, while high impedance stuff uses mono 1/4" or RCA connectors. Unless your keyboard has TRS (balanced) output connectors, it is considered high impedance even though its actual impedance may be lower than a passive electric guitar.

But keyboards usually don't sound that great into guitar amps anyway because guitar amps put out most of their energy in the midrange, where keyboards need more lows and highs.

 

[Halion]

Actually, I was just using those examples to simplefy the question. Here's the real situation in which I needed to know this:

A couple of weeks ago, I did a gig with Roland Grooveboxes and a synth, plugged into a really old mixer with HiZ or XLR inputs only. The grooveboxes and synth (JP8000) had dual TS outputs. We plugged them into the HiZ inputs. It sounded less than half good, but the FOH was terrible aswell and we were monitoring through small Yamaha (studio) monitors (I still hope we didn't fry them, but we might have), so I couldn't really judge what the cause was. I also plugged my passive bass into a HiZ input and that worked fine (level wise, not quality wise, it was just as terrible).

 

[ggunn]

Actually, I was just using those examples to simplefy the question. Here's the real situation in which I needed to know this:

A couple of weeks ago, I did a gig with Roland Grooveboxes and a synth, plugged into a really old mixer with HiZ or XLR inputs only. The grooveboxes and synth (JP8000) had dual TS outputs. We plugged them into the HiZ inputs. It sounded less than half good, but the FOH was terrible aswell and we were monitoring through small Yamaha (studio) monitors (I still hope we didn't fry them, but we might have), so I couldn't really judge what the cause was. I also plugged my passive bass into a HiZ input and that worked fine (level wise, not quality wise, it was just as terrible).

Well, I wasn't there, but I suspect that the "really old mixer" was the root cause of your problem rather than impedance mismatches. Overpowering the studio monitors coulda had something to do with it as well. In my experience, impedance mismatches usually take care of themselves when you balance your gain structure for good signal to noise ratio and headroom.

 

[noisedude]

What kind of bass did you get? More importantly, does it have a cool paint job?

It's an obscure Chinese brand ... however, running a guitar shop means I get to try a lot of obscure stuff out and choose what I like. This is a REALLY cool Warwick-style look I guess, mahogany, two covered humbuckers, active top and bottom boost and cut .... just really really cool. We sell them for around £200 or a little less .................... but obviously I didn't pay that.

Thanks for your help guys, this thread has some excellent info in it now!

Nik

 

[Halion]

Well, I wasn't there, but I suspect that the "really old mixer" was the root cause of your problem rather than impedance mismatches. Overpowering the studio monitors coulda had something to do with it as well. In my experience, impedance mismatches usually take care of themselves when you balance your gain structure for good signal to noise ratio and headroom.

I'm fairly sure the problem at from the FOH (the speakers were simply overpowered, I think) but the mixer could be the problem, I really don't know to be honest. It sounded very much like speakers clipping (I know, that's really bad). I didn't actually think the impendance thing was gonna be the biggest problem, but it just crossed my mind when I read this topic.

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.

 

[easychair]

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.

Yeah, most likely. Way too much impedance has a bad effect on microphone sound. They need a certain range to function well, it's not just because they are lo-Z that they have problems. An active lower-z source won't have the same problem, like active pickup instruments, drum machines, effects pedals, etc. They can operate into a much wider range of impedances than sources like dynamic mics and passive instruments.

 

[snow lizard]

To a lot of us, myself included, low impedance and balanced are very nearly synonymous, as are high impedance and unbalanced. Most gear that is low impedance uses XLR or TRS connectors, while high impedance stuff uses mono 1/4" or RCA connectors. Unless your keyboard has TRS (balanced) output connectors, it is considered high impedance even though its actual impedance may be lower than a passive electric guitar.

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

 

[snow lizard]

It's an obscure Chinese brand ... however, running a guitar shop means I get to try a lot of obscure stuff out and choose what I like. This is a REALLY cool Warwick-style look I guess, mahogany, two covered humbuckers, active top and bottom boost and cut .... just really really cool.

Awesome... sounds nice.

I miss the days when I worked at the second hand gear shop. I was always broke, but it's a great way to find deals.

(These days I have to ban myself from going into a music store for most of the year...)


sl