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Line level signal vs. Instrument signal, the difference?
jeremija
post Feb 17 2010, 10:37
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Can somebody tell me (or point at somewhere to read about) what is the main difference between the line level signals and instrument level signals?

Also, what should be the output impedance of line level signals generators and instrument signal generators?

All I know for now that the line level signals have a maximum voltage of 2 V peak to peak, and that guitar amplifiers usually have the input impedance around 1Mohm (because the pickups have the impedance at around 250500kohm). Does this 1M refer to the impedance that is measured with a test constant voltage of a single frequency which has the biggest current Z = U(f) / I(f) ?

I'm considering building a device which will transform the signal from my sound card sand send it to a guitar amplifier's input (something like a re-amp box). I have found a scheme here, but I would like to know more about the requirements of such device (what should be the input impedance, output impedance, output max voltage and such...)

This post has been edited by jeremija: Feb 17 2010, 10:38
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Iain
post Feb 17 2010, 11:43
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I don't know all about the details of the electronics but I have used a passive DI box backwards (with the 20dB pad switched in) to feed a line signal into a guitar amp. It worked a treat after I made up the right cables.
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Arnold B. Kruege...
post Feb 17 2010, 13:51
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QUOTE (jeremija @ Feb 17 2010, 04:37) *
Can somebody tell me (or point at somewhere to read about) what is the main difference between the line level signals and instrument level signals?


A line level signal may have the same voltage level as the signal from a electric guitar pickup (which is the most common audio signal from a musical instrument), but a guitar pickup is designed to drive a high impedance load, while most other line level signals are designed to drive a medium impedance load.

The classic "direct box" is an impedance matching transformer that sacrifices voltage to obtain the kind of current that is needed to drive long cables. Guitar pickups have such a high impedance that a reasonable length of ordinary shielded wire can easily cause audible differences. The cable's capacitance may even resonate with the inductance of the coils in the guitar pickup within the audible range. Thus, different guitar cables can make the guitar sound different.

QUOTE
Also, what should be the output impedance of line level signals generators and instrument signal generators?


Test equipment generally provides a medium source impedance, generally ranging from 72 ohms to 600 ohms.

A lot of modern musical instruments deliver signals from a medium source impedance. A good electric piano for example may provide a ca. 1 volt or larger signal into a 600 or 10K ohm load, just like an equalizer, mic preamp or console. From time to time I encounter electric guitars with batteries, which I presume powers an internal buffer that provides a signal with a medium source impedance.

QUOTE
All I know for now that the line level signals have a maximum voltage of 2 V peak to peak,


That would be a typical line level signal, perhaps on the low side. Line level signals can be as low as 30 millivolts and as high as 10 volts - both numbers would be RMS. 10 volts rms is almost 30 volts peak-to-peak.

QUOTE
and that guitar amplifiers usually have the input impedance around 1Mohm (because the pickups have the impedance at around 250500kohm). Does this 1M refer to the impedance that is measured with a test constant voltage of a single frequency which has the biggest current Z = U(f) / I(f) ?


The 1 meg is typically a resistance, not an actual impedance. You can bet that the 1 meg resistance is paralleled by some sort of capacitance, so the input impedance of the amp drops significantly at high frequencies. However, the capacitance of the guitar cable is likely to be even larger.

QUOTE
I'm considering building a device which will transform the signal from my sound card sand send it to a guitar amplifier's input (something like a re-amp box).


I don't see why a transformation would be required. Audio is not in general about matching impedances. It is rare to go wrong by driving a high impedance load with a low impedance source.

The most likely prolbem that people have when attaching computers to audio gear is hum due to ground loops. Radio Shack sells a little device called a "Ground Isolator" for about $17 that will resolve such problems in almost every case, should they occur.

QUOTE
I have found a scheme here, but I would like to know more about the requirements of such device (what should be the input impedance, output impedance, output max voltage and such...)


Looks like pretty conventional stuff, but I'm not sure that it will actually do anything that you need to do.

Don't think yourself into a hole. Try connecting your computer to your guitar amp and see how it sounds. If you have hum problems I already gave you an easy solution for that. If you don't get the tone quality that you want, the most logical way to address that problem is with an equalizer.
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neelX
post Feb 17 2010, 14:29
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QUOTE (Iain @ Feb 17 2010, 12:43) *
I don't know all about the details of the electronics but I have used a passive DI box backwards (with the 20dB pad switched in) to feed a line signal into a guitar amp. It worked a treat after I made up the right cables.


This is common practice on live stage.
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jeremija
post Feb 19 2010, 18:38
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Thank you for your replies!

QUOTE (Arnold B. Krueger @ Feb 17 2010, 13:51) *
Don't think yourself into a hole. Try connecting your computer to your guitar amp and see how it sounds. If you have hum problems I already gave you an easy solution for that. If you don't get the tone quality that you want, the most logical way to address that problem is with an equalizer.

I connected the output #3 of Terratec Phase X24 to a guitar amplifier VOX ac15cc1x and I had absolutely no unusual problems with hum. When I connect the guitar to the instrument channel and set the cubase to pass the signal to the output #3, the only hum I hear is from the single coil pickups - which is normal.

However, I do hear a loss of tone brightness (treble). Is this because of the:
- A/D and D/A conversion
- doubling of the cable lengths (I am using two 5m cables for this, as opposed to only one cable while connecting the guitar directly to the amplifier)?
- impedance mismatch?

NOTE: I only know that the impedance match is important for maximum power transfer and while connecting speakers to the output of a power amp, but this is another story. If somebody could explain how the impedance mismatch manifests in this situation, it would help me a lot...

And did you say to use the EQ if I get this kind of problem?

And another question: can this kind of connection cause damage to either the amplifier or to the sound card? IMHO, the input impedance of the guitar amplifier shouldn't cause any damage to the sound card, as it's equal to infinity when no cable is connected to the output. Lowering the output, but keeping it over the lowest supported impedance (for line level), no damaging current would start to flow. Is there something I'm foreseeing?

EDIT: There is no problem with treble, there was a EQ switch for a lowpass filter left turned on in Cubase. The only difference that I can currently hear is the difference in the volume.
Addition to the last question: Can this difference be harmful to the amplifier - for instance, if I send a signal with 2 V rms (which is maximum output from my sound card), can it "burn" something in the amplifier?

This post has been edited by jeremija: Feb 19 2010, 19:21
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DVDdoug
post Feb 19 2010, 19:42
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I wouldn't use a transformer (at least not as a 1st attempt). I'd first try a simple resistive voltage divider (or a pot). Transformers are an easy way to convert between balanced & unbalanced lines, and they can isolate grounds, but I'm not sure you need that here. Transformers don't require power, but neither does a resistive network.

QUOTE
However, I do hear a loss of tone brightness (treble). Is this because of the:
- A/D and D/A conversion
- doubling of the cable lengths (I am using two 5m cables for this, as opposed to only one cable while connecting the guitar directly to the amplifier)?
- impedance mismatch?
Hmmm... This connection should be more immume to high-frequency roll-off than the direct guitar input, even with longer cables. What is your source? Is your guitar plugged into the Terratec? Maybe the Terratec's instrument input has lower impedance than the guitar amp, and it's the input that's affecting the tone????

QUOTE
If somebody could explain how the impedance mismatch manifests in this situation, it would help me a lot...
As Arnold said:
QUOTE (Arnold B. Krueger)
I don't see why a transformation would be required. Audio is not in general about matching impedances. It is rare to go wrong by driving a high impedance load with a low impedance source.
Typically, outputs have very low source impedance... 1/10th or less of the input impedance of whatever it's connected to.

Sometimes the specifications list the impedance "rating", which is really the load it's designed to drive... And output might be rated at 600 ohms (designed to drive a load of 600 ohms or more), but it's actual internal source impedance may be much lower. This is also true of solid state power amps. Their source impedance is usually less than 1 ohm, but they are designed to drive 4 or 8 ohms. (If you connect a "matching" 1 ohm load, something bad might happen! wink.gif )

QUOTE
And another question: can this kind of connection cause damage to either the amplifier or to the sound card? IMHO, the input impedance of the guitar amplifier shouldn't cause any damage to the sound card, as it's equal to infinity when no cable is connected to the output. Lowering the output, but keeping it over the lowest supported impedance (for line level), no damaging current would start to flow. Is there something I'm foreseeing?
Correct! You are safe connecting a high impedance load to a low impedance source.

QUOTE
Addition to the last question: Can this difference be harmful to the amplifier - for instance, if I send a signal with 2 V rms (which is maximum output from my sound card), can it "burn" something in the amplifier?
This is generally safe. If you're driving the amp into distortion, and if turning the volume down doesn't eliminate the distortion, then you're overdriving the input stage. But, it's pretty hard to "blow" anything with 2V! Especially at high impedance (low current).


This post has been edited by DVDdoug: Feb 19 2010, 19:49
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jeremija
post Feb 20 2010, 09:50
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QUOTE (DVDdoug @ Feb 19 2010, 19:42) *
I wouldn't use a transformer (at least not as a 1st attempt). I'd first try a simple resistive voltage divider (or a pot). Transformers are an easy way to convert between balanced & unbalanced lines, and they can isolate grounds, but I'm not sure you need that here. Transformers don't require power, but neither does a resistive network.

Hmmm... This connection should be more immume to high-frequency roll-off than the direct guitar input, even with longer cables. What is your source? Is your guitar plugged into the Terratec? Maybe the Terratec's instrument input has lower impedance than the guitar amp, and it's the input that's affecting the tone????

As I said in the edited post, there is no problem with treble:
QUOTE (jeremija @ Feb 19 2010, 18:38) *
EDIT: There is no problem with treble, there was a EQ switch for a lowpass filter left turned on in Cubase. The only difference that I can currently hear is the difference in the volume.
Sorry for the confusion smile.gif

Thank you for the in-depth explanation of the impedance matching, it helped me a lot! As I have no problems with sound quality (at least I can't hear it right now), it works great! I was afraid that the sound would be muddy, like it was some time back when I used some digital guitar effects, which had no true bypass.

This post has been edited by jeremija: Feb 20 2010, 09:53
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