bubbagump said:
The thing I don't understand is how impedance and voltage relate in this equation. I would imagine, looking at Ohms law (yeah yeah, DC I know) that the two (voltage and impedance) are linked.
The important thing here is that the relationship between output impedance and input impedance in the circuit determines the voltage a device is able to put into a given input. You could use Ohm's law and suss out the current flows and stuff, but this is about signal (voltage) transfer, not power. You can pretty much disregard the currents involved, and just look at the effect the circuit has on the voltages.
Here's some stuff to think about. A source, a destination, and a cable form a circuit. Mic-cable-pre=circuit. Guitar-cable-amp=circuit. CD player-cable-mixer=circuit. And so on.
All these circuits have a source with an output impedance and output voltage, a destination with an input impedance, and an interconnect as their components.
The output impedance of a device is considered to be in series with the output voltage. Any input has an input impedance. When you hook them together, the output and input impedances form a simple voltage divider network. The output voltage of the source is divided between them.
Plug an electric guitar pickup with 10,000ohms output impedance into an amp with 1Megohm input imedance, which is pretty typical for guitar amps. The input has much higher impedance than the pickup, and the vast majority of the output voltage of the pickup is developed across that input. Now plug that same pickup into a 10,000ohm line input. Only 50% of the pickup's voltage will be present across the input. It's easy to see if you draw it and do the math.
For best signal transfer, the input impedance should be about 10 times the output imedance. This is called impedance bridging. Typical mic impedance is about 250ohms, typical mic pre is 2500ohms. Typical line outputs are 1000ohms, typical line inputs are 10,000ohms. OK?
You might ask why a guitar amp has a 1Megohm input, when the bridging formula would seem to indicate that an input of 100,000ohms would suffice for a pickup of 10,000ohms output impedance.
The circuits like these we use in audio are considered to have an overall impedance. A mic-pre-cable circuit is considered to be low-impedance. Mics are around 250ohms, pres around 2500ohms. A guitar-cable-amp circuit is said to be a high-impedance circuit. Source and destination both have high impedances. The higher the impedance of the circuit, the more sensitive it is to noise. I don't know why, but that's the way it is. So for a high-impedance source like a passive pickup, a much higher input impedance than you would expect is used to keep the signal to noise ratio high. Also, magnetic pickups have rapidy rising impedance at higher frequencies, up to a peak, after which the output drops off. The impedance at this peak can be 30-40,000Kohms, which also means a very high-Z input is needed for best tone.
bubbagump said:
So what does the signal really look like going in and what does it really look like coming out? Then there is all this phase crap and I get totally lost.
Skip the phase, it doesn't matter for this.
Let's look at Farview's answer a little bit more (Hey, Farview, whatup?
)
Circuits have impedance, not signals, he said. Absolutely. And it's the ratio of output to input impedances in those circuits that matters.
Here's how it happens. I'm going to stick with the electric guitar, since it's what the original poster was asking about, and it also shows what is going on pretty well.
First circuit:
guitar-cable-line input. Let's make the output impedance of the pickups and the input impedance of the line input both 10,000ohms to make it easy.
So we have a high-impedance source connected to an input of equal impedance.
The equal impedances means the voltage of the guitar across the line input will be very low compared to when it's plugged into a high-impedance input on an amp, because the output and input impedances form a voltage divider. This low level will raise the noise floor of the signal.
Add to that, the high source impedance of the pickup makes the circuit more susceptible to noise overall. So basically you get a weak and probably noisy signal.
With me so far? That's just a rehash of the last section.
Plus- high-impedance and non-bridged circuits are also more susceptible to capacative losses in cables, which roll off high end, further degrading the signal.
OK- add the DI to get guitar-DI-2500ohm mic input.
This is really two circuits. One is guitar-cable-DI input, and the other is DI output-cable-mic pre.
The DI is an interface, and has no impedance of it's own. It's input and output impedances are derived from what is hooked to it and it's turns ratio.
Let's use a passive DI with a typical 10:1 turns ratio. We have our guitar with 10,000ohms output impedance on the DI input, and mic pre with 2500ohms input impedance on the DI output.
The input impedance of the mic pre is multipied by 10 and appears on the DI input. So the DI input has 25000ohms impedance. The output impedance of the guitar is divided by 10 and appears on the DI output. So the DI has 1000ohms output impedance. And signal going through the DI is divided by 10, for a 20db drop at the output.
Now the guitar sees a 25,000ohm input, which is not great, but better than the 10,000 it saw pluggin into a 10,000ohm line input. It can generate more output, and of better quality, and the mic input is more sensitive, and needs less gain. The mic pre sees a 1000ohm source on it's 2500ohm input, which is not great, but a better ratio than the 1:1 ratio of the guitar into the line input. This DI-cable-mic pre circuit is balanced and lower-impedance, making it less susceptible to noise. Balancing is the other big benefit of using a DI.
Neither of these come close to being bridged circuits, though, and can be noisy, are less-than optimal quality, and still can't deliver the full signal from guitar to input. And though the pickup can deliver more signal and the input is more sensitive, there is the 20db level loss with a passive DI. So you could easily end up in just as bad a situation as you had plugging right into the line input. You've gone from one circuit (guitar-cable-line input) with really crappy impedance characteristics to two circuits (guitar-cable-DI input and DI output-cable-mic pre) with only mostly crappy impedance characteristics, one of which is balanced and relatively low-impedance.
The DI doesn't change the signal, just the impedance characteristics of the circuits, letting them interface and pass signal better. For impedances typical of line gear, the numbers work a lot better. A keyboard with 1000ohms output impedance hooked to a 2500ohm mic pre with a DI will see a 25000ohm input. The mic pre will see a 100ohm source. Both are bridged, and will be very happy. The circuits on both sides of the DI are pretty low impedance, and will be very quiet.
Active DIs have fixed input and output impedances. 250K in and 600 out are common, 1Megohm inputs aren't unusual. They can easily interface any source with a mic pre, impedance-wise.
Seeya.
