Buffers, impedance and other internet lore
What's a buffer? What's an impedance? What's an internetz?
We've all heard about the benefits of a "buffer" and how fuzz pedals don't "like" buffers, right? Your best buddy insists of having a buffer at the front of his pedalboard-- after his favorite fuzz pedal of course, and even recommends putting a buffer at the end of his pedalboard to "drive his cables" and "maintain his signal strength." So, the question(s) is/are: WHY do we want these buffers, and do we even need them at all? Pop a pop, crack a beer, put on a record and lets'd debunk the mystery of the buffer and talk a little about impedance and how it relates to "buffers."
A guitar "buffer" circuit is generally (and I am making a sweeping generalization here) a high input impedance, low output impedance, unity gain current amplifier. These terms may sound familiar, no? The copy that follows this often goes something like this: "Super high input impedance to preserve your signal strength," or maybe "... to prevent loading of your guitar pickups." While both of these statements are true, this is only one small part of the bigger picture. We gotta start by talking about impedance, after all, this is apparently what we are so concerned with.
A little about impedance
Just like its fraternal twin resistance (symbol "Ω") is applied to DC (direct current), impedance (symbol "Z") can be thought of as resistance to AC (that's alternating current). So where the hell are you going, Jack? Here's where:
Guitar pickups output an alternating current waveform, representative of the movement and tonality of the strings of the guitar. Why would we care about the resistance of an input if it doesn't apply to the signal we're concerned about? We wouldn't. We are concerned about the impedance of the input. Input impedance is, after all, the input's resistance to alternating current.
It may seem obvious: we want low resistance, right? Let the signal flow freely and powerfully, right? Wrong. As counter-intuitive as it may seem, we actually want high resistance; at least on the input of the circuit. Why you may ask? Lets take a look at what is going on and the situations that arise when a signal sees both high impedance, and low impedance.
Play this scenario out with me:
We have a signal coming out of our guitar. Since we've got guitar pickups in our guitars, not nuclear steam turbines, generating the signal (current), we've got a small signal. That is to say there isn't a whole ton of current backing our signal. Its weak. But it is strong enough to make our amplifiers work, and here's where it gets fun.
Lets say our guitar is connected to a circuit with a low input impedance. The signal can flow in freely right? Right! But everything at its cost. The signal flows so freely into the input that the input actually starts to put demand on the signal, making it even smaller!
Imagine your guitar cable is a hose carrying your signal. There is a changing pressure in the hose. This changing pressure is your signal. The hose is connected to a big bottle, with contstant low pressure inside. Lower pressure than what is in the hose. Nature being natural, everything wants to seek equilibrium and enlightenment, so the pressure in the hose moves rapidly to fill the big bottle. This lowers the pressure in the hose and begins to put a demand on whatever is supplying the pressure, as if to say: GIMME MORE PRESSURE!
This is the "loading effect" we want to avoid, so to speak. At least with a buffer, or current amplifier.
Now play this scenario out:
We have the same signal coming out of our guitar. The same weak signal.
We connect our guitar to a circuit with a high input impedance. Now the signal can't flow so freely into the circuit. It would follow that we are just getting a weak little signal into our amplifier, right? Well, you are right, but we don't need the whole thing. In fact, we don't need any of the actual current. All we need is the voltage; the change in pressure. Not the actual pressure.
Because there is no decrease (loading) in the changing pressure in the hose, we can see the full change of pressure, or the full signal.
Hopefully, these examples help to illustrate why we want a high input impedance (or resistance to current) in our buffer circuit.
Lets look at output impedance now.
As you may suspect, output impedance is the resistance to current on the output of the circuit. This one is a little easier to understand.
We're gonna look at both extremes (high and low output impedance) before drawing what would seem to be an obvious conclusion.
Lets say we have a high output impedance. A quick translation of engineering terms means we have a high resistance to AC at the output of the circuit, which means only a small amount of current can come out of the circuit. This behavior is similar to how our pickups work which, quite frankly, is not why we want a buffer in the first place.
Now lets say we have a low output impedance. The translation says: we have a low resistance to AC at the output of the circuit, which means LOTS of current can come out of the circuit. That means the circuit can push whatever it needs to hard, and hard is good. Ask your girlfriend.
So cool, Jack. Now we understand a little more about input and output impedances, but what about bufferz!? Okay. Lets talk about buffers and why we do and do not need or want them.
A buffer, by definition, is an impedance transformation circuit. In the guitar world, they can be loosely defined as current amplifiers. They amplify signal strength (current), but ignore signal amplitude (voltage). This is where the terms "unity gain buffer" and "strong signal" come into play when buffers are marketed.
So why would we want, or not want one, two, three, four, or more buffers in our guitar rig?
The answer lies in the common copy: Stronger signal.
While it would be super cool to have a guitar tone strong enough to move a fucking locomotive, there are limits to how strong the signal can get without diminishing returns. The biggest expense in a well designed guitar buffer is current or current draw. That is how much power is required to make the thing work. Lots of current coming out of the circuit may mean virile, well-endowed tone, but if a huge demand is put on the power supply (be it battery or outboard DC supply) we may limit the life of our power supply. Furthermore, if we only need 1mA drive current, why supply 1A? That additional 999mA of current is being unused and just adding to the heat generated by our circuit, and if we wanted more heat, we'd buy a space heater. Not a guitar buffer, dig?
So what are some figures and "tech specs" we should look for when selecting a buffer?
Its best to think about our application and needs.
Lots of people will tell you that a guitar's output impedance is in the order of 500KΩ, well, I'm here to tell you that is bullshit and whoever told you that just doesn't know what they're talking about, but we'll save that explanation for another edition of Straight Jive. Realistically, you're guitar's output impedance is somewhere between 10KΩ and 40KΩ with your volume and tone at maximum, but these figures change as you change the controls, pickup selection, etc. (innit fun how there are so many variables and considerations?).
There is a general rule of thumb when it comes to circuit interfacing in electrical engineering and it goes something like this: for small signals, the driven circuit's input resistance (impedance) should be over one order of magnitude larger than the driver circuit's output resistance (impedance). For those of use who don't use terms like "order of magnitude" and "sopping dripper" in our daily vernacular, this means the driven input resistance (impedance) should be >10x bigger than the output resistance (impedance) of the driver.
Lets look at a simple example:
If a given small signal source (i.e. your guitar) has an output resistance of 25KΩ, the safe bet is to have it drive a circuit with a 250KΩ or higher input impedance. Pretty simple, right? 25,000 x 10 = 250,000. Add some weird greek symbols in the mix and we're fully fledged engineers, doing calculations and math and stuff.
So what's ideal?
Well, I can't say what is best for you specifically, but I think you'll find some pleasing results if you have a well designed circuit meeting these criteria:
Input Impedance: 500KΩ - 1MΩ
Output Impedance: 1KΩ - 10KΩ
What about buffers with "Super High Input Impedance for maximum signal integrity and strength?"
Everything at its cost. Mega-super-crazy-extreme-ultra-tops high input impedance may be seductively alluring, but with high input impedance, comes high responsibility, grasshopper. The higher the input impedance, the greater the sensitivity. In short, this means as your input impedance gets ultra mega high, you will start getting more and more noise in the signal, and hey man, if noise is your thing, look no further.
That about wraps it up for today's edition of Straight Jive. Feel free to share this info with whomever you like. You can manipulate the numbers and language as well. Everyone else does!