Can anyone help me understand impedance?

Mingus Addict · #1 01-11-2008, 12:39 AM

I hear a lot of talk about impedance; especially in reference to fuzz. Fuzz is the effect I most use on bass so I figure I should try to understand this impedance stuff. I'm pretty sure impedance has something to do with resistance, and I want to say that it is specific to circuits with Alternating Current. I also know that "impedance matching" has to do with getting the output impedance of one device to work well with the input impedance of another.

Can someone fill in the blanks for me? Can you also help me understand how it affects the actual sound? Can mismatching impedances ever create a desirable effect? What happens when the output impedance is to low to for the input as opposed to too high?

I tried researching this a bit but I thought maybe someone here at TB could put it in layman's terms for me. My understanding of electrical circuits is limited.

Dave Muscato · #2 01-11-2008, 12:57 AM

I've never heard about impedance specifically in reference to fuzz.... anyone else want to post? lol

Mingus Addict · #3 01-11-2008, 01:03 AM

I often hear that impedance matching is important to making a fuzz sound good and many people seem to think that this particularly an issue with active basses and fuzz, and perhaps with other pedals too. Again, I don't really understand it so I won't be surprised if someone tells me I'm all wrong about what I think I do know.

ehque · #4 01-11-2008, 01:46 AM

If i am not wrong, input impedance is the resistance between the signal and ground. Since op-amps/transistors have (technically) infinite resistance, a resistor is wired to let the current bleed to ground while still mantaining the voltage "seen" by the first component in the effect. Too low and too much current flows to ground, causing a dip in voltage supply-side, but too high and the current might not be able to bleed out (im not sure what effect this might have, residual charge on the cable, perhaps)

output impedance gives a rough idea how low an input impedance it can drive without changing the sound. The lower the output impedance, the more current is available, maintaining the original sound at lower input impedances.

I've never heard of matching impedances except in head-cab combinations. I'd always want a low output impedance into a high input impedance, but sometimes, in the case of active basses, i would suppose some more bleed would be beneficial to the sound.

If i am not wrong.

**Rune Bivrin** · #5 01-11-2008, 02:06 AM

The issue could be that a passive bass (or guitar) has a fairly high output impedance, particularly for higher frequencies, since the pickup is essentially a big coil, which has rising impedance with frequency.

Feeding this into a fuzz box with low input impedance will remove a lot of treble, which will affect how the fuzz sounds. It may be good, and it may be bad.

ehque · #6 01-11-2008, 03:19 AM

Reading up about it, the general idea i get is

For maximum CURRENT transfer (which is roughly = power) - impedances should be matched
For maximum VOLTAGE transfer (which i read as "signal" or "info") - output impedance should be as low as possible and input impedance as high as possible.

bongomania · #7 01-11-2008, 07:27 AM

In layman's terms, impedance is best thought of as "efficiency". For example if one device is sending signal to another, they need an optimal impedance relationship or the signal will actually have to "work harder" to transmit from one to the other. A classic illustration is bicycling uphill- if you're in the right gear it only requires normal power, but if you're in too high of a gear you'll be struggling and straining and it will take you longer to get to the top. In signal terms we typically hear that struggle as a loss of highs and a reduction in signal level.

But with the whole fuzz pedal deal, we look at it the other way around. Most fuzzes work best with a passive instrument with a (relatively) high z out just due to the nature of the particular circuit design. In this case, it's like bicycling on flat ground in the lowest gear- sure it works, but there is a narrow "optimal" range where energy transfer is most ideal for the task at hand.

fretlessrock · #8 01-11-2008, 08:00 AM

Impedance (Z) is resistance as measured in the the alternating current domain, measured in Ohms. Resistance (also Ohms) is resistance to current flow in a DC circuit, and is linear. Impedance is resistance to current in an AC circuit (audio is a/c), and is not linear. Impedance changes with frequency, and the sensitivity to frequency change is called reactivity.

In terms of musical instrument circuits you have to consider the output impedance of a device, and the input impedance of the device it is sending signal to.

Typical output impedances:
Active electronics* (onboard preamp): Low Z (hundreds of Ohms)
Passive electronics: High Z (thousands of Ohms)
Piezo pickup: Very High Z (millions of Ohms)

Input Impedances:
Mic Pre: Low Z (Tens of Ohms)
Stomp Box: High (Hundreds or Thousands of Ohms)
DI Input: Very High (Thousands or Millions of Ohms)

*many active devices, like operational amplifiers used in active circuits, have high input impedances and low output impedances, and that can be useful.

When you connect two A/C circuits, you have each acting as a "load" on the other. When the loads are matched you get the best transfer of power (V and I). But in cases where the power level is very low (guitar outputs) there are advantages to a mismatch: a low or high output impedance feeding a circuit with a very high input impedance. The source (bass/guitar) doesn't load the destination (preamp, buffer amp...) and the circuit behaves in a more ideal manner. Also, that non-linearity of the impedance changing with frequency is minimized when the circuit is designed properly.

What you want to avoid is high output impedance into low input impedance. The result is poor frequency response and poor transfer of power. Since passive basses are fairly High Z, they want to see a similar High Z input, and in some cases, the higher the better. That is why you might use a line driver or a buffer amp as the first device in a pedalboard, for instance. You might think of it as the high impedance source (passive bass, piezo...) "loading" the low impedance input.

Lastly, a transformer is an impedance converter. It uses electromagnetic coupling to either raise or lower impedance. Again, not linear, and that can be useful if you know how it works, and you like the way it sounds.

On a gain sensitive device like a fuzz, a filter, a compressor... you may not get the intended result, or may need to work out all new settings, if your instrument is different than what the designers expected. This usually has more to do with gain than impedance, but since there is some correlation between them in typical devices (i.e. low-Z circuits providing more signal), and that could lead to a confusion between gain-related mismatches and impedance-related mismatches.

Mingus Addict · #9 01-11-2008, 04:32 PM

Thanks guys! Very helpful

niftydog · #10 01-13-2008, 03:31 PM

I'll have a bash...

Resistance and impedance are related, but distinctly different. Resistance is for direct current (DC) only. Impedance is for alternating current (AC). Your bass signal is AC, so we deal with impedances, not resistances. Input impedance is the impedance to ground that is "seen" or "encountered" by a signal as it enters a circuit. Output impedance is similar, but obviously at the output of the circuit.

When most electronic engineers talk of matching impedances, they mean making them equal (eg: 50=50) But this is done for reasons (max. power transfer) that are not applicable to audio signals. We are only interested in maximum voltage transfer. In this case the ideal situation is to have zero output impedance, and infinitely high input impedance - which is impossible to achieve, so the next best thing is a very low output impedance and a very high input impedance.

When you have the situation where you have a high output impedance and a low input impedance, your signal is effectively attenuated at the junction between the two circuits. (There can also be extra high frequency attenuation due to capacitance in the circuit.) Fuzz pedals, especially ones based on "classic" circuits, have lower-than-optimal input impedance. For some, this is a defining characteristic, lending a certain flavour to the fuzz. But, this character is highly dependent on the output impedance of the bass (or the next pedal up the chain) connected to it. Also, the fuzz circuits themselves are designed to, if you will, compensate for this by adding high frequency content.

If you counter the effect of the low input impedance by connecting an active bass (which has a much lower output impedance), the result is often unpleasant and harsh. This is because the active circuitry can handle the lower input impedance better without loosing high frequency content. Add this to the compensated treble of the fuzz pedal and the sound can rip your ears off you head! So, long story short, interaction between circuits becomes more evident when you're dealing with low input impedances, hence why fuzz pedal enthusiasts deal with these issues.

ehque · #11 01-13-2008, 10:16 PM

Agreeing with pretty much everything that niftydog has said.

Quote:

Originally Posted by niftydog

When most electronic engineers talk of matching impedances, they mean making them equal (eg: 50=50) But this is done for reasons (max. power transfer) that are not applicable to audio signals.

Except in amp-speaker connections, but you already knew that.

Mingus Addict · #12 01-26-2008, 01:11 PM

Wow nifty that's the best explanation I've heard yet. I would have have thought that lowering the output impedance of your instrument would always be good and that was confusing me. I'm glad I checked back on this thread now. I was intending to post the address of an article I found that I thought explained this well but nifty might have done better. I'll post it anyway.

http://www.muzique.com/lab/imp.htm