How do I find out the impedence of different jacks?

[DarkHorseJ27]

I want to find the impedence of the 1/4" jack and the XLR jacks on my guitar, as well as the impedence of the input jack on my amp. I e-mailed the manufacturers several weeks ago, but no answer. How do I find it out myself?

[WRGKMC]

I'm curious to know why you would want to know. Knowing they are high and low is usually sufficiant.

 

Low impediance jacks are normally 600 ohms. A guitar will vary all over the place depending on the coil wraps on the pickups but normally range from 4K ~13K ohms. Its easy to measure. Just hook an ohm meter up to the guitar plug, and turn your volumes all the way up. This is actually an internal resistive measurement of the coil windings.

 

It would be difficult to computate an actual impediance because the magnet strength and number of wraps as well as resistance are factors here. It really doesnt matter though because all the computations are done with the strength of the signal output and the resistance of the coil. You can figure out all your other specs from thet using ohms law and trigonometry.

 

As far as the amp goes, Check the specifications of the amp. They're usually posted on the MFG's website or in a manual. Most amps are normally 500k to 1MEG ohms or higher to match the volume control and pickups on a guitar. Tube amps usually have the highest impediance and will give the best amplification of a guitar pickup. If its too high it will pick up excess noise and RF signals so having it 10~20% above a guitars impediance is the best match usually. Solid state amps can vary alot but tend to be in the same general ranges and tube amps. The main thing is going too low redices volume and turns the sound to mud. Too high it picks up too noise.

[LeftCoast]

It's not the "Jack" -1/4 inch or XLR that has an impedance, it is the nature or the load or source that has an impedance. Because an amp or a guitar are both complex impedance (inductive, resistive + capacitive) you can't really measure impedance with an ohm-meter, that will only give you the DC resistance of the load.

 

To measure the input impedance (at some frequency of reference) you need a signal generator, an oscilloscope and a variable or step resistor box. The technique IIRC is to put a sine-wave signal into it at a known level, with a variable, purely resistive load in series with the signal generator. You then increase the series resistance until the signal at the (amp) input is exactly half the output of the sine-wave generator. What you have done is create a simple voltage divider, so the impedance of the load (amp input) will now be equal to the value of the series resistor. An XLR input is usually a low impedance - 600 Ohm input.

 

 

To measure an output impedance, you first measure the unloaded, open-circuit signal level with an oscilloscope (with high impedance outputs this is tricky because the oscilloscope leads and input will load the output). Your oscilloscope has a very high input impedance. Then put a high impedance load across the output and reduce the resistance until the signal level is exactly half the open circuit signal level. If you model the device under test as a signal generator with a series resistance (output impedance), then your load resistor once again forms a simple voltage divider. When the signal across the load is exactly half the open circuit level, the resistance value is equal to the output impedance. Guitar output impedance should be in the neighbourhood of 15K Ohm.

 

Of course since both the input impedance of the amp and the output impedance of the guitar are complex (L-R-C) impedances, your measured value will only be valid at the frequency of measurement.

 

To connect a guitar (15K Ohm) to a low impedance (600 Ohm) input, you should use a DI box that has either a transformer or an active (amplifier) for impedance matching. Otherwise, you end up dropping most of your signal across the output impedance of the source rather than the input.