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 250÷500kohm). 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...)
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.
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.
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?
If somebody could explain how the impedance mismatch manifests in this situation, it would help me a lot...
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.
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?
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?
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?
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.