FALKEN
*************************
the original poster didn't understand any of this and still doesn't.
DonF
Call Me "Grandpa"
If you think the 20 - 25 dB of loss (typical) through the transformer is a small signal loss, well .... I guess it's a matter of opinion whether that is significant.
I agree with you that level matching is not the main function of the passive DI. I suggest we just leave it there.
Don
I agree with you that level matching is not the main function of the passive DI. I suggest we just leave it there.
Don
ROblows
New member
FALKEN said:
There are three independent variables that people seem to confuse with each other because they often correlate. You'll probably understand better if you understand the difference between these three concepts: signal level, impedence and balanced/unbalanced.
Signal Level: Guitar pickups and microphones put out fairly low levels, although some condensor mics can be on the "hot" side as can some active pickups. These devices generally need some kind of a preamp (instrument or microphone, respectively) to bring their signal to "line level". Most effects processors and mixers operate at "line level", which also covers a range. There is no absolute standard for nominal line level. Some units operate at a nominal level of -10dBv, some at +4dBu, and some at 0dBu. Those are just nominal levels; actual signal strength of any particular signal will vary. Of course, all of this bears no particular relation to...
Impedence: The key here is you have to match impedence. Some devices (like guitar amps) are designed to generally operate best with high impedence sources (like guitar pickups). Microhone preamps generally operate best with low impedence sources (like most professional microphones). If you are going to plug a high impedence source (like a guitar or bass pickup) into something that wants to see a low impedence source (like a mic pre on a mixing board), you're usually best off converting the high impedence source to a low impedence source. That's what a DI does, though different DI's accomplish it in different ways. Of course, all of this bears no particular relation to...
Balanced/Unbalanced lines: OK, I've got to go, but suffice to say that whether a cable is balanced (3-conductor, XLR or TRS) or unbalanced (2 conductor, or TS) bears no relation to the signal strength that passes through them. Nor does it necessarily relate to impedence.
Later.
E
easychair
New member
NKJanssen, a passive DI changes level, from higher to lower. It has to, by the way it is made, with a step-down transformer. The level drop from the transformer serves the original purpose of the DI box: To match shitty line/consumer level gear with the most common input on the shitty pro PAs of the day: a fairly low-impedance mic-level XLR input. These needed both impedance and level matching, which the transformer does very well.
And passives don't really impedance match, they basically just get things into the ballpark where they don't sound like complete crap, if you are lucky.
DonF, most of the level change in DI is from the transformer's turns ratio, not insertion loss. Most DIs have about a 10:1 turns ratio, which along with a couple db of insertion loss gives the typical 20-25db level drop most passive DIs have. But you are right, any transformer has insertion losses.
And a DI in reverse will boost a signal, but not as much as it drops it in regular use, due to insertion losses. A DI that drops a signal 20db won't boost a signal 20db if used in reverse, in other words.
FALKEN, your initial guess was correct. Rather than thinking strictly in terms of mic or line or whatever level though, just realize the passive DI will drop whatever signal you put into it by 15-25db if used the regular way. And boost it by some amount if the DI is used in reverse. Both of these facts can be helpful in figuring out what is happening in your signal chain, for regular DI use, and reverse DI use, such as reamping.
Your sansamp is an active box, and does not use a transformer. It doesn't have the same limitations as a transformer-based passive DI, as far as level goes. So unlike a passive, an active can have zero level drop at it's outputs.
Sansamps are actually more than DIs, as they are capable of boosting signals above their original level. This by definition makes them a combination device, an active eq/preamp paired with an active DI.
Here is a deeper look, if you want. It might take some time to get it, and you don't need to to use your DI. No offense to anyone, because I think it's a good thing, but these days anyone can buy a pile of gear, hook it together with only the vaguest idea of what it does or how it works, and all is well.
DIs change signal impedance, so they say. But really they don't. Signals don't have impedance, and DIs are just an interface between pieces of gear with incompatible impedances. Two pieces of gear connected together need to be at the correct relative impedance for good signal transfer. A signal is happiest going from a low impedance source to a higher impedance input.
This is like the level thing, don't think in absolute terms of high and low impedance. It's all relative. A guitar is low impedance compared to an amp input. A mic is low impedance compared to a mic pre. But a guitar/amp circuit operates at a high impedance compared to a mic/pre circuit.
SO:
Say you want to plug your high impedance guitar into your low-impedance mic pre. So you hook up a passive DI. Most would say the signal is converted to low impedance. Uh-uh. The guitar and mic pre look at each other through the interface of the DI. Think of it as a window. Now, the guitar "sees" your mic pre as a higher-impedance input than it was. And the mic pre "sees" your guitar as a lower-impedance source than it was. Nothing has changed, really, but the signal is much happier.
Why say the guitar and mic pre see each other, instead of saying the DI has a high impedance input for the guitar, and low impedance output to go to the mic pre?
Because although the transformer has a small inherent input and output impedance, the final DI input and output impedance are determined by what is hooked to the DI.
The mic pre on the DI output sets the final value of the DI input impedance. The guitar on the DI input sets the final value of the DI output impedance. The DI interface makes them see better impedances than if they were directly connected, but they still affect each other through the DI.
This is due to how transformers work. This also explains why passive DIs don't have enough input impedance to satisfy most passive sources like bass and electric guitar.
Actives are a different ballgame. In actives, the input and output of the DIs are totally independent of what you plug into them. True impedance compatability can be had with an active DI, in accordance with today's standards. Again, passive DIs, while very useful tools, are often a compromise with respect to impedance. And again, these days it doesn't matter as nuch as it used to, except with passive instruments.
If you want, I will post about the transformer action in a DI.
And passives don't really impedance match, they basically just get things into the ballpark where they don't sound like complete crap, if you are lucky.
DonF, most of the level change in DI is from the transformer's turns ratio, not insertion loss. Most DIs have about a 10:1 turns ratio, which along with a couple db of insertion loss gives the typical 20-25db level drop most passive DIs have. But you are right, any transformer has insertion losses.
And a DI in reverse will boost a signal, but not as much as it drops it in regular use, due to insertion losses. A DI that drops a signal 20db won't boost a signal 20db if used in reverse, in other words.
FALKEN, your initial guess was correct. Rather than thinking strictly in terms of mic or line or whatever level though, just realize the passive DI will drop whatever signal you put into it by 15-25db if used the regular way. And boost it by some amount if the DI is used in reverse. Both of these facts can be helpful in figuring out what is happening in your signal chain, for regular DI use, and reverse DI use, such as reamping.
Your sansamp is an active box, and does not use a transformer. It doesn't have the same limitations as a transformer-based passive DI, as far as level goes. So unlike a passive, an active can have zero level drop at it's outputs.
Sansamps are actually more than DIs, as they are capable of boosting signals above their original level. This by definition makes them a combination device, an active eq/preamp paired with an active DI.
Here is a deeper look, if you want. It might take some time to get it, and you don't need to to use your DI. No offense to anyone, because I think it's a good thing, but these days anyone can buy a pile of gear, hook it together with only the vaguest idea of what it does or how it works, and all is well.
DIs change signal impedance, so they say. But really they don't. Signals don't have impedance, and DIs are just an interface between pieces of gear with incompatible impedances. Two pieces of gear connected together need to be at the correct relative impedance for good signal transfer. A signal is happiest going from a low impedance source to a higher impedance input.
This is like the level thing, don't think in absolute terms of high and low impedance. It's all relative. A guitar is low impedance compared to an amp input. A mic is low impedance compared to a mic pre. But a guitar/amp circuit operates at a high impedance compared to a mic/pre circuit.
SO:
Say you want to plug your high impedance guitar into your low-impedance mic pre. So you hook up a passive DI. Most would say the signal is converted to low impedance. Uh-uh. The guitar and mic pre look at each other through the interface of the DI. Think of it as a window. Now, the guitar "sees" your mic pre as a higher-impedance input than it was. And the mic pre "sees" your guitar as a lower-impedance source than it was. Nothing has changed, really, but the signal is much happier.
Why say the guitar and mic pre see each other, instead of saying the DI has a high impedance input for the guitar, and low impedance output to go to the mic pre?
Because although the transformer has a small inherent input and output impedance, the final DI input and output impedance are determined by what is hooked to the DI.
The mic pre on the DI output sets the final value of the DI input impedance. The guitar on the DI input sets the final value of the DI output impedance. The DI interface makes them see better impedances than if they were directly connected, but they still affect each other through the DI.
This is due to how transformers work. This also explains why passive DIs don't have enough input impedance to satisfy most passive sources like bass and electric guitar.
Actives are a different ballgame. In actives, the input and output of the DIs are totally independent of what you plug into them. True impedance compatability can be had with an active DI, in accordance with today's standards. Again, passive DIs, while very useful tools, are often a compromise with respect to impedance. And again, these days it doesn't matter as nuch as it used to, except with passive instruments.
If you want, I will post about the transformer action in a DI.
Last edited:
E
easychair
New member
ROblows said:
Thumbs up on the signal level thing, they are all the same, just different levels.
And you are right, impedance is important.
But people usually speak of the mic and guitar needing to see the proper impedance, not the amp or preamp, because it is the signal from the guitar or mic that suffers, not the performance of the amp or the pre, if the impedance is wrong.
This wording is also used because some sources are pretty much impedance-independent, and will plug into almost any destination with no signal degradation. A lot of line-level gear, for instance, is low-impedance, but doesn't care if it plugs into a high or low impedance input.
Your wording brings up an important point. In reality, source and destination need to be considered together as a circuit. Both need to be at the proper value for things to work exactly right, especially with very impedance-dependent sources, like mics and guitars.
