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Line level signal vs. Instrument signal, the difference?
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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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.

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????

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 )

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.

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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