Zelenka Music Press
San Miguel de Allende

Impedance

This page is about "impedance" as it applies to signal processing on a nylon-string guitar using piezo pickups.

A piezo pickup has an "output impedance" somewhere in the range of 10 to 20 mega-ohms. What's impedance? I don't know, really: it is an electrical "quality" somewhat resembling resistance. I treat it as a "black box" concept: I don't know what's in that black box, I just read the directions, push the button, and it works! So, what you have to know about a piezo pickup is that it has an "ultra-high impedance output". You don't have to know what impedance is to know this, but you do have to know that there are basically four different impedance level standards in sound equipment: in brief, piezo pickups, magnetic pickups, line-level signals, and microphone-level signals, in that order from high to low. An electric guitar has an impedance output of about one mega-ohm. This is considered "high" impedance - but not "ultra-high" like a piezo. The electric guitar's output impedance level is the standard for compatibility with the input impedance of electric guitar amps and of many signal processing devices, which nowadays come in the form of a "pedal." To use the piezo pickup with a signal processing pedal, its impedance must be stepped down to the same level as that of the magnetic pickup of the electric guitar, because that is the standard for signal processing. There are two additional "low" impedance levels to know about: "mic level" at 600 ohms, and "line level" at one kilo-ohm. You cannot plug an electric guitar or a piezo pickup into a device which is made for these low levels. The input impedance of the receiving device must be higher than the output impedance of the signal going into it. In fact, for some reason it can be much higher, but it cannot be lower. If you can remember this, and read the specs in the manuals, then you can figure out what devices are compatible with what signals. Many devices have specifications for both input and output impedances.

So the very first signal processor in the "signal chain" after the piezo pickup itself is a device for transforming the impedance. If you plug the raw piezo into a one-mega-ohm input impedance device, you will get a squawky sound with harsh trebles and no bass, and no amount of equalization will fix it. The first necessity is to step down the impedance. This is just a few electrical components soldered together in the right order, and if you are a die-hard DIYer then go for it. But otherwise go buy a pedal. But watch it! There are pedals and pedals.

If you own a storebought under-saddle "system" from L. R. Baggs or Fishman, which requires a 9-volt battery that you have to remember to check, and has a little plastic box with it that gets mounted in the side of the guitar, then it - probably most likely - has an impedance transformer built into it, and typically it also has a 3-band EQ and a volume knob, and so, it is appropriate to say that your guitar has a "built-in preamp" which does three of the necessary functions: it transforms the impedance, and it adjusts the relative levels of the bass and treble frequencies, and it adjusts the volume. The output impedance will typically be one mega-ohm and can be plugged into any standard device that accepts an electric guitar signal. Now I personally don't like any device which requires a 9-volt battery, and for that reason I don't use a built-in preamp, which means I have "pedals" for those functions instead.

An impedance transformer may also be built into some kind of pedal which is called a "pre-amp" or "preamp." But here's where we get into trouble: there are lots of "preamps" without impedance transformers. So what's a "preamp?" Duh - it's ANYTHING that somebody decides to put in a box to put between the pickup and the amplifier to make the sound better - and that they decide to call a "preamp" for lack of anything better to call it. But what's in that box may not be for your particular case. So a lot of preamps are made for electric guitars, and they have a one-mega-ohm input impedance, and no impedance transformer, and if you plug a piezo pickup into them, it's just going to sound crappy. And, a lot of preamps called "acoustic preamps" don't have them either, because they assume that you have bought some kind of "system" like the Fishman and Baggs devices which already contain "built-in" preamps. So if you have a built-in preamp, why do you need another one anyway? Ahhh - cuz it makes it "sound better." Oh. Well, it probably does. You can buy a "system" very much resembling the Fishman, made in China, for one-third of the price - but it doesn't sound as good as the Fishman, and if you run it through a good "preamp," chances are pretty good that it will sound a lot better. Here I am actually thinking about a friend who said he was going to buy a new pickup, and I said, why don't you try the one you have through a preamp? - and I lent him one to try (I have several) and yes, it made the sound a whole darn lot better. So, yes, players with built-in preamps do also buy "acoustic preamps" to go between the built-in preamp and the amplifier. But "preamp" is such a loosely-defined word that you have to read the specs carefully to see what you are getting and whether it will be useful.

If you have a raw piezo pickup - and this is not a bad thing, understand, because not everybody plays a six-string, and I have three piezo discs glued to my guitar top - then the preamp which you buy has to have an impedance transformer built into it, and this is something you have to check carefully. If it does, then it will, most likely, also have a switch on it with two options: "piezo" and "magnetic." If it has this switch, then you are home free. If it does not have that switch, then you have to read the specifications in the back of the manual (you have to read the manual) and see if it doesn't say, input impedance one mega-ohm. So, you can't use that, unless you put another impedance transformer in front of it. And NOBODY makes a stand-alone impedance transformer. Why not? No market. They will only sell you a "preamp" which does something else too.

There is a device which CAN BE CUSTOM MADE to accept an ultra-high impedance, and I have one, and it is first in my signal chain: this is the Buffer-Bandwidth-Amplifier "pedal" made by Broughton Audio. Yes, it is a type of "preamp" but an unusual one. The standard model has a one mega-ohm input impedance and is made for electric guitars, but Broughton offers to set it to any custom specification for users of ultra-high impedance piezo pickups, which includes a lot of bass players. I had mine set to 10 million ohms, and it works well enough; I might try 15 if I get another one. This device does three other useful things. (a) It filters the frequencies to remove ultra-high and ultra-low sounds which are beyond the range of normal human hearing. This improves the sound in an interesting way: the speaker is no longer working so hard to reproduce those sounds that people can't hear, and so, all it has to do is reproduce what the people CAN hear. (b) It "buffers" the signal, so that the output signal is consistent - I can't explain this any better, but it's supposed to be a good thing! and (c) it amplifies the signal by up to 20 DB - it has a knob for this - BEFORE sending the signal on to the other processors. Because the piezo pickup has a very weak output, this is a very good feature, and I have this boost knob cranked all the way up. Now, I currently use this first in my signal chain, and after it I use a compressor pedal and then an "acoustic preamp" pedal which has a one mega-ohm input impedance, and which I would not be able to use with a raw piezo direct. That preamp pedal, however, has many excellent features which I take full advantage of.

The simplest pedal that I have owned that has an impedance transformer is the Boss AD-2. It has a 10 mega-ohm input impedance, and so it is a great pedal to plug a raw piezo pickup into directly. It has three knobs, two of which are worthless. The third knob is a notch filter - this is a really good thing and makes the pedal worth while. It has two useful features: it steps down the impedance and has a notch filter. That's great! You can't buy either of these items as a stand-alone, and here Boss has put them both in one pedal. All acoustic guitars have boomy frequencies, and most typically it is the open A string that is boomy. A notch filter takes that right out. Now technically, this is part of "EQ" or "equalization." But it requires a special circuit with a dedicated knob. If you don't have one, you are going to miss it. It is one of the good things to look for in a preamp. The Boss AD-2 does not stand alone. You are almost certain to need some additional equalization. The two worthless knobs are called "ambience" and "acoustic resonance" and they are lame gestures towards reverb and equalization, but - as I say, you will want some more processors. You can put as many pedals as you want in a row all connected with quarter-inch cables, and that's the game. But this is about impedance.

So now you know pretty much what I do: if you are going to use raw piezo pickups on your instrument, then the first signal processor has to have an input impedance rating of 10 to 20 mega-ohms. Most likely, this device is going to be called a "preamp." After that comes the question of how to further improve the sound. In my case, I use compression, equalization and reverb, and that's all. These are all pedals, and the signal flows from one to the next, and finally to the amplifier. I will write about that somewhere else.

Here I will review some of the preamps that I have or have used.

First up is the Zoom A3. This could be a perfect pedal, but it wasn't for me, because it doesn't have enough EQ. It only has a three-band EQ, and I couldn't get my mids right, so I don't use it. It's a very difficult pedal to use on-stage!!!! In fact, it's sort of insane until you have learned to use it well, and for that, you really have to be a pedal nerd, because every knob has several different functions, and some can be both turned and pressed. But it does have an ultra-high-impedance-input setting, and it has all the effects that I use built into one pedal. The fact is, you can only use two effects, and I use compression and reverb. This could be a really good pedal for some players who don't have problem mid-ranges.

Next up is the BBE Acoustimax. It does not have an ultra-high-impedance input. That's one strike. Strike two is, it runs on 12 volts AC, instead of the standard 9 volts DC for about 95% of common pedals, which means you can't put it on a nine-volt daisy chain with the other pedals, and it has to have its own wall-wart; and you had better not get the wall-warts mixed up or you'll fry a pedal. But the Acoustimax doesn't strike out completely: it has a very good EQ, and a two-knob notch filter! which is quite a luxury. With the two knobs, you can adjust both the frequency and the depth of the cut, which is the way it ought to be, but most notch filters have a standard non-adjustable depth of cut and only one knob for the frequency. This is something that BBE got better than anybody else. It's a very decent EQ pedal. The Sonic Maximizer feature, however, is very dubious. I (along with many others) have never been convinced that it does anything at all, and I was reminded of somebody's You-Tube review of the "Placebo" pedal, inside of which the input jack is wired direct to the output jack, and the two knobs - "dosage" and "side effects" - are not connected to any wires. The Acoustimax gets one further half-strike against it: the effects loop has only a single jack, which means you have to wire up a y-jack. And for two hundred bucks used? hmmm. Anyway, I have one and I am not using it.

I had and sold the NuX Stageman. I didn't like it. It does have a switchable ultra-high impedance input. It had only a 3-knob EQ, not enough control for my purposes. The effects loop had only one jack. It had two built-in effects: reverb and chorus. I don't like chorus, and I want more control over the reverb than the Stageman offers. So there. OK, it works.

I should explain about effects loop wiring: if it has a single jack, then it requires a 1/4" TRS jack, with three contacts, output, return and ground, which have to be wired up to two 1/4" TS jacks, one for output and one for return. Now, this is not a big deal to wire up for a DIYer. But it happens that nowadays we can buy 1/4" TS flat ribbon cables for pedals which are as short as possible and noise free. If there are two jacks for the effects loop, then two of these fancy cables do the trick: and the flat ribbon cables are much less noisy than cheaper cables. The DIY y-cable is likely to be just a little bit noisier. When I bought ribbon cables for my pedal rig, it significantly reduced the white noise at the output.

I am currently using the Mesa Rosette preamp. No link - they are all gone like the dinosaurs! The guy who founded Mesa (who made the famous Mesa Boogie amp) retired and sold Mesa to some lucky opportunist, and the Rosette is no more, even used. It was a sport, a side product - the main product was an acoustic amp called the Rosette, and the preamp is a spinoff using the EQ section of the amp. The Rosette does not have an ultra-high impedance input - a remarkable oversight which I was very disappointed about, because that was a learning experience for me - read the specs!!! Mesa assumed that you have a Fishman or Baggs system in your guitar - their marketing horizon was not very wide. But it has a lot of nice features. It has an EQ with two sweepable mids, a one-knob notch filter, and three different output impedances: electric guitar level, line level and mic level, each with a dedicated jack, and you can use them all at once. The effects loop has both input and output jacks. And it runs on nine volts and requires almost no juice at all. So my signal goes first through the Broughton Audio Buffer-Bandwidth-Filter pedal, and then through a DemonFX Call76 Compressor pedal, which is a Chinese knockoff of a much more expensive compressor, and then into the Rosette. Then I have the reverb in the effects loop, using a very complex loop in which I compress the signal again before putting it through the reverb, and then mix it back as a side chain. I will write about that somewhere else.
More to follow...