The electronics of amp & guitar volume controls

Factual Questions
1

My question is about how the combination of a volume control on a guitar and the volume control on an amp contribute to the final amplified signal. There is a lot of setup here with the questions at the end. (I once took EE 101 and that’s as far as I got–Kirchoff’s current law, transistor analysis. So answer like I’m a dumb musician.)

A guitar pickup produces a signal induced by the vibration of the conductive string in the magnetic field of a magnet wrapped with a wire coil. The signal is modulated by the volume control, which is a potientometer used as a variable resistor (I think). I guess the way it works is to add resistance to the raw signal, thereby reducing the amplitude of the signal.

I have no idea what the volume control on an amp does, whether it limits the final signal, limits the input signal, or whether it somehow boosts the power being delivered by the amp, all the way up to 11.

I have noticed that if I crank the amp volume up and reduce the volume at the guitar, the signal coming out of the speakers is very clean, and has a more acoustic quality to it. If I max out the volume on the guitar, and reduce the volume at the amp correspondingly to produce the same volume at the speaker, the sound at the speaker has more of an electric quality with more distortion.

This particular rig is a carved-top archtop guitar (Hofner New President) and a Roland Cube XL80 (solid state) amp.

  1. Does a volume control on a guitar change the signal characteristics other than the level of the signal? Is the signal coming past the volume control distorted in any way that would be detectable to a listener? Seems odd that *reducing *the resistance would *increase *the perceived distortion.

  2. Seems counterintuitive that I could get a cleaner sound from an amp by turning the volume up. What happens electronically when I turn the volume up on an amp? Does the input signal level somehow “drive” the amp more?

This is a factual question about electronics rather than a question about artistic aspects of music so I put it here rather than Cafe Society. I’m open to a change in forum if the mods so deem.

2

I gotta run out, but here are a couple threads:

Tube vs. Transistor amps:

Both discuss what the guitar’s volume and the amp’s various volumes can do and how they interact. Take a look (if you haven’t seen them already) and see if it helps.

I have to head out to run an errand in Brooklyn - hopefully I can stop by RetroFret and check out the vintage archtops they have in stock :wink:

3

Older guitars all had passive controls. Changing these varies the loading on the pickups, so can change the “color” (frequency response) of the sound. It also changes the source impedance seen by the amp input, but this usually has almost no effect.

It is my understanding that some modern guitars have active circuitry…preamps, effects, and such. In this case changing the volume would not necessarily change the coloration, but it is entirely possible that deliberate changes are made to make these behave more like classic guitars.
Beyond that, most distortion (which can be desirable in some music styles) is mostly the result of over-driving the electronics, causing clipping and worse.

But a guitar amplifier is actually several electronic amplifier stages one after the other…two at a minimum, a low level amp (pre-amp) and a Power Amplifier (PA) Usually everything ahead of the PA is lumped into being called a pre-amp even if it is in fact several amplifier stages. The amp volume control is typically between the last two stages, that is to say just ahead of the PA.

Distortion in the pre-amp stages can have a very different character than PA distortion, especially with transistors. How it differs depends on the amp design.

When you run the guitar at low levels, you avoid over-driving the pre-amp stages. When you then crank the amp to 11, you are still able to over-drive the PA and get some distortion, but it sounds different from how an over-driven pre-amp sounds.

If the amp were only one stage, then the two controls would have a pretty similar effect, except for the loading effects I mentioned at the start.

4

Not really. For reasons to do with impedance matching (don’t ask) passive controls tend to lose top-end as they’re turned down. Some guitars (Fenders) have a ‘bypass’ capacitor that leaks some treble past the volume pot, so a Tele actually gets toppier as you turn it down. Also a volume potentiometer is not simply putting more resistance into the circuit, what it actually does is take a portion of the available signal (which can go down to zero). If it was simply a resistor it might get a bit quieter (depending on what’s next in line - impedance matching again) but it wouldn’t go to nothing.

See above, There’s nothing distorted coming out of the guitar, what causes the distortion is overdriving some stage of the amplification.

Where to start? If the distortion is because the pre-amp stage is overdriven you’ll get a cleaner sound by turning down the guitar (or using a lower gain input) and turning the master level up to end up with the same overall volume.

The amp’s master volume controls how much signal is fed to the power stage. At some point feeding more signal in at this point can’t make the power stage deliver any more output and it will start to distort. What happens at this point and what it sounds like is a whole new question

5

The above pretty much nails it. There are however a whole lot of variations, and a critical one is whether you have a solid state or tube based amp. Real guitar amps are of course all tube. :smiley:

The variations you see in different amp designs include the location and number of gain controls. (They are not really gain, they are attenuators, when turned up full they don’t attenuate, and at any lower setting they reduce the signal passing through.) Early guitar amps tended to have a volume control reasonably early in the chain. When at full volume a typical guitar amp has vastly more gain than it needs. A common tube design will cascade 2 or 3 pre-amp stages with 12AX7 tubes. Each stage gets you a gain of about 60 = 35dB, so flat out that is 3600 (70dB), or 216,000 (105dB). Even with rails of a couple of hundred volts on the anode there is no room for this amount of gain and the amp overloads into glorious distortion. The gain/volume controls dial this back. A three stage pre-amp is pretty designed from the outset to overload.

Early guitar amps were never designed with distortion in mind, and the volume control was there to dictate how loud the amp played. Typically they didn’t have huge amounts of gain in the pre-amp, but they could overdrive the power amp into creating a great sound.

A later modification was the addition of the “master volume” which is placed after the pre-amp stages - right before the power amp. This allowed the pre-amp stages to be maxed out and the volume dialed back. So you got a quiet amp with arbritrary amounts of distortion from overloading later stages of the pre-amp. This pre-amp distortion being controlled with the earlier gain controls. Additional gain stages in the pre-amp make clear sense here as they are expected to be overloaded.

The location and setup of the tone controls (often referred to as the tone stack) is also critical. The tone controls act as frequency response shapers and gain controls - and where they are placed in the flow will determine whether they are shaping the signal pre or post clipping - and thus whether they are shaping the signal to be distorted, or shaping the distorted signal.

A complication is that clipping the pre-amp stages sounds different to overdriving the power amp, and the various sounds became identified with different amps and different genres of bands. Master volumes amps are associated with more thrashy sound, whereas amps that overdrive the power stage are a more traditional rock and roll distortion sound.

Distortion in the power amp can come in different guises, and some of it depends upon the topology of the amp. Simple cheap singled ended amps sound different when overdriven to push pull amps, (the preponderance of even versus odd harmonics is fundamentally different) and another critical thing is the setup of a negative feedback loop around the power amp. (Once the power amp is clipping the feedback loop ceases to reduce distortion and actually starts to make it worse, and just how much worse and in what manner creates a big part of the sound as well. This is a big factor in Bassmaster 5F6-A vs Marshall JTM45 sound.)

6

Great post, tiny nitpick. That’s a Bassman circuit.

7

Yuk, of course it is. In my defence it is 1am here. It is time for some sleep.

8

It is a great post - why don’t you hang out in the Great Ongoing Guitar Thread again?

ETA: Bassmaster - heh. The first memory I have in a guitar shop is the reaction I got when I mentioned humdinger pickups…

9

Some of the questions you raised I know the answers to. Some I don’t so I’ll do the best I can.

Um, I’d never explain it that way. The volume knob controls an audio taper variable resistor wired in as a potentiometer. Think of it as a variable voltage divider. The raw input goes to the top of the resistor, the ground to the bottom of the resistor and the output is taken off the wiper.

[QUOTE=CookingWithGas]
I have no idea what the volume control on an amp does, whether it limits the final signal, limits the input signal, or whether it somehow boosts the power being delivered by the amp, all the way up to 11.
[/quote]

It boosts the amplitude of the signal being delivered by the amplifier.

[QUOTE=CookingWithGas]
I have noticed that if I crank the amp volume up and reduce the volume at the guitar, the signal coming out of the speakers is very clean, and has a more acoustic quality to it. If I max out the volume on the guitar, and reduce the volume at the amp correspondingly to produce the same volume at the speaker, the sound at the speaker has more of an electric quality with more distortion.
[/quote]

Clean vs Distortion. Think of a clean signal as a nice sine wave. This is the input to the amplifier device (tube or transistor) if you’re not using any stomp boxes. If the output from the amplifier device is a nice sine wave (larger, of course) then you’ve got a “clean” sound. If the input has a high amplitude, you might drive the amplifier device into “overdrive”, where the output waveform is a sinewave with the negative and positive peaks are clipped (flat topped). And if the input signal has a VERY high amplitude, you can drive the amplifier device into “distortion”, where the output signal resembles a square wave. Some amplifiers have a “gain” control. These allow you to adjust the sensitivity of the tube/transistor, so to speak and allow changing the output waveform shape without necessarily changing the input volume. The waveform for an overdriven or distorted output will be the same amplitude as the one for the loudest clean output. This all in the preamp where you’re just “shaping” the wave. The power amp is where your real volume occurs.

So yes. When you reduce the volume at the guitar you’re sending a lower amplitude signal to the preamp so it’s probably going to come out clean. Crank the volume on the guitar and you’re sending a larger signal to the preamp that’s more likely to overdrive.

[QUOTE=CookingWithGas]

  1. Does a volume control on a guitar change the signal characteristics other than the level of the signal?
    [/QUOTE]

No. All it changes is the amplitude of the signal. The signal from the guitar is clean. Overdrive and distortion comes from the amplifier.

[QUOTE=CookingWithGas]
2. Seems counterintuitive that I could get a cleaner sound from an amp by turning the volume up. What happens electronically when I turn the volume up on an amp? Does the input signal level somehow “drive” the amp more?
[/QUOTE]

Low volume from the guitar + high volume on the amp = clean
High volume from the guitar + low volume on the amp = overdrive.
Overdrive/distortion boxes can change these formulas, but I took a quick look at the controls on your amp. It looks to me like you’ve got all the onboard OD and DIS you’ll need and won’t need any pedals to get more.

I think I already answered the rest of your question.

Oh, it looks like you’ve got a nice amplifier. I’d rather get my distortion and effects from the amp rather than an array of foot pedals other than just a strip of foot switches to turn stuff on and off on the amp.

10

Thank you all for great answers. I understand a lot of it but not 100% so I’ll do a little light research to dig down just a little.

11

Now I want some humdinger pickups and a bassmaster amp to play them through. :wink:

12

All of the above posts are spot on. If you’re not a tech minded person…this is how I explain it to my non-techie students:

Guitar > (Effects) > Amp Input Stage/Preamp > Amp Tone Controls > Amp Output Stage > Speaker

Some amps have all kinds of controls, some have a single knob. Usually you have a GAIN control and a MASTER output.

There a lots of different pickup configurations as well. This concept just assumes you’ve a signal coming out of the jack, and that you’ve found a combination of pickups and phase (the switch) you like.

Not including fX, there are three points of control…OUT of the guitar, INTO the amp, and OUT of the amp. The things you can adjust to shape your tone are simply what level they all have proportionally. Volume is irrelevant - if you threw a switchable power sink between the amp output and the speaker, you’d find the combination is pretty much the same at any volume. (note to EE/audio eng types…I know, non-linear scaling and additive subharmonic distortion, it doesn’t make a difference at this point… :slight_smile:

On the amp…higher master plus lower gain gives a cleaner tone. The higher the volume vs the input gain, the more the sound of the amp conveys the sound of the guitar pickups. That is because there is less coloration from the input preamp - which conveys the ‘color’ and ‘sound’ of the AMP. The output stage is (usually) designed for clean power. Clarity is also a function of HEADROOM…that is how much available EXTRA power is there, to handle the fast attack transients of a signal without additional load on the amp stage.

Higher gain pushes your signal through the preamp/input stage with more vigor. There is more signal to work with and more of the amp coloration is conveyed. A hotter signal coming through the amp means more ‘grit’ and ‘character’, from a slight crunch to raging distortion.

So on the amp: HIGH GAIN and LOW MASTER OUT give more amp color on the sound, and more crunch/overdrive/distortion. LOW GAIN and HIGH MASTER OUT convey more of the clean guitar tone.

The more signal you feed the amp in general, the more you get the positive effects of the gain/master dynamic. To a point. You need to give the amp enough to work with - but experimenting with various levels at all points will show you the sweet spots for your rig.

One thing to note is that what you need varies with what you’re doing. I’ll throw everything to 11 on a Mesa Triple Rectifier or SVT IV in the studio because I’m looking for a particular tone. That would be disastrous on a small club gig…define what you’re looking for/what you need to control and go from there.

The other factor is dynamic range - that is the difference in dB between the softest and loudest attack in your playing. This is much trickier to explain vis a vis the different controls. Best to experiment and find what is useful/you like because there is a huge difference between say, humbuckers, single coil passive and various active setups and the vast array of amps, wattage ratings, impedance and esp tube v solid state.

13

Yea, this is how volume controls usually work.

This is overly-simplistic description of what’s going on, but it essentially works like this:

The “sound signal” (guitar, voice, Slayer, whatever) is represented in the electronics as a voltage signal. In other words, the voltage signal in the electronics perfectly mimics the sound signal, whatever it may be.

It starts off as a “small” voltage signal in the input stage of the amplifier. The amplifier is a voltage amplifier, and thus boosts the amplitude of the signal. It is then fed to the speaker. The output of the amplifier is also able to supply a lot of current to the speaker on an as-needed basis. So not only is the amplifier boosting the voltage of the sound signal, it is also able to deliver a lot of power to the speaker.

The amplifier, by default, is always set to a volume of “max” or “10.” (Or 11 for Spinal Tap.) So you adjust the volume by *reducing *(not increasing) the output. Since the sound signal is represented by a voltage, the overall volume at the amplifier’s output can be reduced by simply using a voltage divider at the input. A volume control is a potentiometer configured as an adjustable voltage divider.