"Gain", "Distortion", "Output": What's the difference?

[Mark Hammer]

....aaaaaannnnnnnndddddddd.....he makes the tag.

As Commander Keen notes, a voltage divider is a pair of resistances.  Those resistances could be two physically separate objects, but could also be two sides/ends of one single object.  Every volume pot is a voltage divider, and as the wiper moves, the resistance between the wiper and the far end of the resistive strip on each side of it, changes.

How much the voltage you input is divided down will depend on the ratio of the resistance it encounters to the total resistance.  If we have a 100k pot, and feed a voltage to one of the outside lugs, the voltage you measure at the wiper will depend on how much of that 100k is on one side or the other of the wiper.  As you can well imagine, if the resistance between the output/wiper and ground is quite small, relative to the resistance between the input and wiper, then you won't get much voltage at the wiper.

In the case of the Distortion+ and so many circuits like it, a voltage divider is used to derive a "reference" voltage that is one half of the supply (9v/2 = 4.5v).  Most circuits will attempt to use this V+/2 as their "floating ground, because it allows the signal to fluctuate as much on one side of Vref as on the other.  To get an output voltage that is one half of what you put in, all you need do is two 2 equal resistances to make up the divider.

In some instances, for a bunch of reasons we won't go into here, sometimes you want a reference voltage that is not "half", but something else.  This is where you'll see things like 10k/15k pair used in the Zombie Chorus.  It's still a voltage divider; it just divides the voltage down by less than half.

One of the reasons why the 10k resistor just ahead of the diodes in the Dist+ "wants" a larger value volume pot is precisely because of how the resistor+pot divides things down.

As noted, let's try a 10k resistor followed by a 10k pot.  We turn the pot up full which means there is zero ohms between the input and the wiper, and 10k between the wiper and ground.  But just a second.  There is 10k leading up to the input of the pot, so there is actually 10k on one side of the pot and 10k on the other.  Ignoring the diodes and cap to ground for the moment, we have divided the op-amp output down by at least half, no matter where we set the volume control.

If we make the pot a larger value, the series resistor will comprise a smaller percentage of the total resistance.  There will still be some voltage division even when the volume is turned up full, but not as much as with a smaller value pot.

Can you just keep increasing the value of that output volume pot to 250k, 500k, 1meg, etc., and continue to enjoy the volume benefits?  No, because as RG wisely notes, that extra resistance will cost you in terms of current, and you need that current.

[Steve Mavronis]

Interesting discussion. I'm curious about how different stages of distortion combine, but not just in the same circuit. How about when using two pedals instead - say overdrive and distortion pedals combined? Maybe that's bad practice, and wouldn't know myself because I've only used one or the other. Could that be useful in a practical sense or would one just be dominating or muddy? I'd think unless you maybe toggled one at a time for rhythm (ex: Distortion+) or lead (ex: 250 Overdrive) for 'slightly different' kind of tone variations, there might not be any point.

[Mark Hammer]

Here I think we devolve into another thread.

You certainly ask some interesting questions, but I was intending to have this be something akin to a stickie.  Sorry.

[Steve Mavronis]

I was intending to have this be something akin to a stickie.  Sorry.

No problem, carry on! Aron may delete these 3 unrelated posts so as not to distract. Sorry my bad.

[ashcat_lt]

But Steve it was an interesting question, you should start another thread on that.

I don't think most of us consider filtering (in and of itself) to be a form of distortion, even though the output is clearly different from the input.  Maybe a bit nit-picky, but maybe requires a slight adjustment of the definition.

I'd also like to mention that a voltage divider does not necessarily reduce Output or Gain.  In the negative feedback path of a non-inverting opamp stage a voltage divider actually causes an increase in gain, which could lead to an increase in Output level and/or Distortion intensity.

[victor nery]

It's not as simple as I thougth... anyway considering that a pair of silicon diodes give a higher output than germanium ones, reducing the values of the pair of resitors in the voltage divider would increase the output as more current flows into the opamp, right? So this way I can go germanium diodes with more output? Did I get it rigth? Thanks a lot for all the explanation and the patience. I learned a lot with these posts. I'm also gonna look for the articles you suggested at GEOFEX.

[PRR]

"Buffer" has a billion meanings. The rubber bumps on a loading dock. Wood pilings around a bridge pier in a shipping channel. The iron contact points on UK railway cars. The antacid added to raw aspirin so it won't upset your tummy (Bufferin).

It's some kind of protection. Without specifics you can't know what or how.

The industry thinks that "buffer" means "has a high input impedance to not load a signal source, and has a lower output impedance than the source so it can drive more loading than the original source without being loaded down". ... Voltage gain is not an issue, and may be more or less than unity, although most people in the industry think of a buffer as having a nominal gain of one or very near it.  ... often unity, but does not have to be.

Right. In electronics, an amplifier that gives gain can can also be a buffer, but there are situations where a special-purpose nearly unity gain amplifier is cheaper than an any-gain amplifier.

In vacuum tube op-amps, minimum load was typically 100K. When you had the right gain/level, but needed to drive some lower impedance load, a Cathode Follower was cheaper than a special beefed-up op-amp. Later transistor opamps were rated 10K load, and a unity-gain booster was offered for lower loads. Cathode/Emitter Followers are simple.

There are also complex amps like the LM310, which is nearly a full op-amp, but hard-wired unity gain which allows a drastic (for the time) simplicity and optimization. (The unity-gain makes it easy to run the input transistors at zero Vcb, which not only reduces input leakage but also allows doping for extreme Beta; before simple JFET inputs, the LM310 was go-to for low-low-low source loading.)

At another extreme, there are buffers which will drive 50 ohms or even 10 ohms. Optimized for grunt with other specs allowed to suffer. Generally used under control of a normal (medium-grunt) op-amp to mask gain, offset, and input loading compromises. LH0002 will deliver 200mA (ten times more than small op-amps) and incredible (for the time) speed but relatively large input current. There was an input buffer with far lower input errors than common op-amps, and gain.

So "...often unity, but does not have to be." "Unity gain" is a significant sub-class of all amplifiers.

However around guitars, we usually mean input over 500K, output under maybe 10K, and no real voltage gain except what is gotten by reduced loading of the source.

[stringsthings]

I wish you luck, Mark, but I have this feeling that you're shoveling sand against the tide. 

when i hold a super-duper-monster-treble-fuzz-box up to my ear, i can hear the ocean 

[Mark Hammer]

"Buffer" has a billion meanings. ......................
However around guitars, we usually mean input over 500K, output under maybe 10K, and no real voltage gain except what is gotten by reduced loading of the source.

And I think one should distinguish between "buffer" as a thing and buffering as an action.  So many stages that provide buffering can have unity gain, while other stages accomplish the buffering function but throw in some gain to boot.

The problem comes in when one has a stage that provides both gain and buffering action, and newbies who think a buffer is a thing are sitting scratching their heads going "Well is it a buffer or is it a gain stage?".

[Johan]

"Buffer" has a billion meanings. ......................
However around guitars, we usually mean input over 500K, output under maybe 10K, and no real voltage gain except what is gotten by reduced loading of the source.

And I think one should distinguish between "buffer" as a thing and buffering as an action.  So many stages that provide buffering can have unity gain, while other stages accomplish the buffering function but throw in some gain to boot.

The problem comes in when one has a stage that provides both gain and buffering action, and newbies who think a buffer is a thing are sitting scratching their heads going "Well is it a buffer or is it a gain stage?".

the reason I brought it up in the first place was that I've seen several posts lately with a high Z  voltage amplifier refered to as a buffer instead of as an amp, when the output was to high to be considered bufferd, ...it was just voltage gain and still High Z.   of course an opamp with gain>1 doesnt need a buffer, but a single bjt-transistor or FET with a collector/drain resistor >10k is not a buffer in my book...not when the guitar pickup, naked, is  ~8k
J

[jcgss77]

Mark, I thank you for this thread.  It really puts into light what many get confused.  I definitely second the motion to make this a sticky.  Having a clear definition of some of the most important terms in our projects will help alleviate confusion and make the ship sail more smoothly.

[R.G.]

I'd also like to mention that a voltage divider does not necessarily reduce Output or Gain.  In the negative feedback path of a non-inverting opamp stage a voltage divider actually causes an increase in gain, which could lead to an increase in Output level and/or Distortion intensity.

That's a bit of sophistry that will unnecessarily confuse someone who doesn't know what "voltage divider" means.

A voltage divider, in and of itself, always reduces signal through it. Voltage -divider-

A voltage divider in combination with other things can influence gain, distortion, frequency response, and so on. But it's the other things influenced by the voltage divider that do that, not the voltage divider itself. And in fact, having to run through this gets the beginners lost in the semantics. It's like pointing out that opamps do not necessarily have gains over unity, and may be set up as active attenuators. It's true, but unusual, and confusing to the issue of trying to decide what opamps are for and do.

[Mark Hammer]

.....however, when it comes to comprehending how to alter gain in op-amps, understanding voltage dividers can be very helpful.  I've posted this explanation before, but it bears repeating for those who are joining the conversation late.

Imagine an op-amp to be like a car going down a steep hill with the gas pedal set to full speed and no way to ease up on it.  Op-amps "want" to drive full speed, with hypothetical gains in the thousands.  What prevents them from doing that is negative feedback from the output back to the input.  The negative feedback "applies the brakes", much as you would regulate your speed in the hypothetical runaway car by stepping on the brakes harder or softer.

The nature and amount of the negative feedback determines how much less than full speed the op-amp is going to drive.  So, in a unity-gain non-inverting buffer stage like this one in figure C, there is nothing between the output and inverting input to impede ALL the negative feedback, and no (visible external) path for any of that negative feedback to be lost to ground.

Look at figure B, though.  Here, some unspecified amount of negative feedback is coming through R2 back to the inverting pin.  But R2/R1 also form a voltage divider.  The junction between R2 and R1 at the inverting pin is EXACTLY analogous to the wiper of a pot.  And of course, the bigger R2 is, relative to R1, as with any voltage divider, the less of the input signal (in this instance, the negative feedback from the output) is "seen" at the junction/wiper.

So, you can adjust the amount of negative feedback - effectively adjusting how hard you step on the brakes, and how far from maximum speed you keep the op-amp - by either changing the value of either R2 or R1.  If R2 is smaller than R1 then, just like a wiper rotated towards the input lug on a pot, then more of the negative feedback is conserved, and the gain of the op-amp is lower.  If R2 is noticeably larger than R1, then it's analogous to the wiper being moved more towards ground.  And when you lose more of the negative feedback to ground, that's like taking your foot off the gas pedal as you careen downhill.

[PRR]

> the guitar pickup, naked, is  ~8k

Only at low frequencies.

Up around 2KHz-5KHz the L (winding) and C (coil and cord) resonate-up to much higher impedance, 100K-300K, before falling again.

That's why inputs with 68K loading make guitar mellow. The whole 2KHz-5KHz range is lower than when loaded with >400K (the usual guitar-amp input). Some simple pedals do that on purpose: excess highs, distorted, can be displeasing, and heavy load on pickup is the cheapest fix.

There's also typically a 250K pot in the guitar, so the wiper impedance could be as 63K. (Which means some of these loading guidelines change if you run full-open or backed-off.)