Impedance Matching etc. - Need a Primer from some Sound Experts

[Rob Gerety]

I have never understood this whole issue about what components can go with what components in a signal chain to a Mixer and PA. It would be hugely helpful if someone could explain the issues - especially impedance issues. My particular situation generally involves going from a pickup (various types including Schertler Dyn and magnetic sound hole and K&K types - sometimes blended - ) to a good PA mixer with all the stuff in between including pre amps, DI, eq, reverb stomp box, tuner, etc.

The main purpose of this post is to gain a basic understanding of the issues involved so that I (and others) will be able to assess options on my own - not so much to get advice about specific components. I have worked with this some - one issue that definitely causes a great deal of confusion for me is impedance - input and output. Also, if there are any good current explanations or instructional materials out there on the web links would be helpful too!

[foldedpath]

Short version:

Every audio connection has a "source" at one end, and a "load" at the other end. If you want to maximize power transfer (as with an amp driving a speaker), the source impedance should match the load impedance.

If you want to maximize voltage, which is what we do when we connect mics to mixers, and pickups to preamps, then it should be a "bridging" connection, meaning the load should be 10 times the impedance of the source. As long as the load is 10 times or more the source impedance, you're good to go.

For example, I plug a Shure SM57 microphone rated at 150 Ohms output impedance into the XLR input of my Soundcraft mixer, rated at 2,000 Ohms. 150 times 10 = 1,500, so I'm over 10 times the source impedance at that input, and I'm good to go.

On the other hand, a typical passive piezo pickup on a mandolin has a very high output impedance, so you wouldn't want to plug that into a mixer's mic input, or even into the "line" input in most cases. You'd want a proper bridging connection with something like the LR Baggs PADI preamp with a 10 MegOhm input, or an active DI box with a very high-Z (high impedance) input. Using a preamp this way is also called "buffering" a passive pickup signal. Look at the spec sheet for whatever gear you're using, and use that "10 times" rule for the impedance rating of outputs and inputs, and you'll know if you're doing it right, or not.

You don't worry about impedance when connecting line level gear (like a preamp output to a line input on a mixer), because line level gear is always designed to be a proper bridging connection.

Here are some links that discuss it in more detail (especially the second one), although neither covers the specific case of passive piezo pickups, which are the main type that need to see very high impedance at the first input in the chain:

http://www.tape.com/resource/impedance.html

http://www.soundonsound.com/sos/jan0...ceworkshop.asp

[Tim2723]

Hey FP, that was great. One of the best explanations I've heard that fits on a single screen!

If you were to amend that with an explanation of matching speaker loads (and why to never run an amp without a load), it would pretty much cover the bases.

[mandroid]

& how come when you daisy chain speakers the impedance goes down?
more = less

[rockies]

Mandroid, the reason is most PA speakers (unless they have been modified) have the two jacks parallel wired to the speakers in the cabinet. When you daisy chain them the impedance goes down in resistance. The formula is 1/Rt=1/R1 + 1/R2 + 1/R3 etc. So if you daisy chain two 8 ohm speakers the resulting impedance will be 4 ohms. If you add another one the impedance will be 2.66 ohms. If you have five 8 ohm speakers daisy chained the impedance would be 1.6 ohms. And on and on as you add speakers. The only way to avoid this is to wire some of the speakers to the jacks in a series configuration and then daisy chain in series / parallel configurations to maintain a reasonable impedance match to the PA output (within the specified ratings). Also be aware that in most PA's the two speaker jacks for the main speakers are paralleled inside the PA so hooking two speakers up you are now already at a load of 4 ohms. And ditto on the monitor jacks.
Dave

[pops1]

When you connect speakers in series the resistance go up and there are formulas for this and in parallel, but for simplicity lets use speakers of equal value ie- 8 ohms 8 + 8 is 16 ohms for series. In parallel if you are using equal values divide by two 8 ohms in parallel with 8 ohms is 4 ohms. if you are using an 8 ohm speaker and a 4 ohm speaker the total would be 3. something as the formula has to do with product over sum and square roots we won't get into that here. So in parallel it will always be less than the smallest load or 1/2 if they are equal. Thats why amps are burnt up as you parallel two 4 ohms you get 2 ohms and most amps can NOT handle a 2 ohm load.

[Tbone]

Would it be possible to build an impedance "matcher" or something of some sort, so I could go from my schertler, to a 1/4" plug, to a tuner pedal, out from the pedal, back to an xlr with a low impedance that I could run to the board?

[mandroid]

Why not just stick on a disc on a cord piezo, just to run the pedal tuner?,
It will work , even if its attached to the back.

Or, if you need the mute {I have a switch on a mic cable end. Neutrik .. solves that}

then I'd try a rewire on the tuner pedal , substitute TRS jacks, for the TR.

the + runs into the tuner circuit, cold and ground were together in TR jack , they are separate when you use 3 contact jacks.

so they connect direct from one jack to the other..

[lenf12]

The pre-amp I use (Art MP) has both 1/4" and XLR in/outs. Can't you go from your Schertler into a pre-amp to the mixer via the XLR's and run your tuner off one of the 1/4" outs? Is that doable? Experts?

Len B.
Clearwater, FL

[pops1]

I have used the Schertler with a 1/4, just made 1/4 to xlr adapter and plug it in. sounded fine. Schertler really doesn't need to go into the low impedance input. You could also use the line out from the board or amp to go to your tuner

[Tbone]

Problem is, I'd also like to run a booster pedal. Our guitar player got a sweet acoustic booster pedal for solos that sounds and works great....

[mandroid]

first its a tuner pedal now a booster pedal,
got an acoustic amp with an effects loop?
get one of those and you can add all sort of stuff in series in the effects loop. because thats, generically . what it's there for..

example : Roland AC[60/90] has a tuner /DI send but no return [& its not a proper DI because it's1/4" TR, basically,
just a dry signal out before any gain stages.

there a loop between the pre, and the power amp stage to run stuff that you may wish to put in the bypass. the return is parallel with ins for a CD player...2 RCA + 2 1/4".. (left only for mono)

thats what i gleaned from the manual anyhow...