Setting an amp flat

[getz out]

Jeremy,

 

Doesn't the Ephiphone Jack Cassidy signature bass have a VariTone?

[jeremy c]

You're right, Maury.

 

And Geoff, I have heard of has-sound, in fact I'm using some of their pickups right now. I'll check out their other offerings.

[TimR]

Changing the capacitor has the most effect, the pot will only allow so much tonal change.

 

Robb, I agree, however the point I was trying to put across, was that the pot works in reverse to how it would in a conventional RC filter. Also, if the resistor was in the conventional RC position, the whole signal would be subject to attenuation at the same time so it would work (undesirably) as a volume knob as well.

 

I would be interested to know what the difference in frequencies affected by this method is compared with a conventional RC circuit.

[Tenstrum]

Also don't forget the acoustics of the room you're in, the weather, and various other Gremlins that tend to affect tone...

[Eric VB]

Well I decided to go look into this some more and came back with a fairly good grasp of how this simple tone circuit works from an electronics standpoint. I came back here to see that robb. has described the whole thing from an audio point of view. That way certainly seems simpler to digest and directly applies to what we're ultimately interested in: how does the tone knob effect the sound?

 

One way to envision robb.'s explanation is to imagine a parametric EQ with sliders at octaves (doubling in frequency from left to right: 2 kHz, 4 kHz, 8 kHz, etc.), and each slider adjustable from 0 dB (top position) on down. The sliders would then make a slant (line) that decreases from left to right.

 

 

I came across one site that gave me some numbers so I could do some crunching. At least in the guitar world there are two fairly standard configurations:

(1) .022 mfd cap with 500k pot, to brighten up the sound of a dark Les Paul with humbucking pickups, and

(2) .047 mfd cap with 250k pot, to warm up the sound of a bright Strat with single coil pickups.

 

I assume that it is no coincidence that the product of the capacitance and resistance is kept nearly constant -- about 0.01 -- as this would be the time constant (in seconds?) if this were an RC low-pass filter. Is it possible to introduce a noticeable delayed effect if the resistance is kept constant and the capacitance is increased enough?

 

Treating the tone circuit as an RC low-pass filter -- which it technically is only when the pot is not engaged (or zero or whatever you want to call it) -- the above configurations have a cutoff (or corner) frequency of about 14 Hz. (Most pro audio equipment considers the range 20 Hz to 20,000 Hz to be a useful representation of our range of hearing.)

 

Ok, great. Now what?

 

Well, after looking at some voltage and impedance divider references, I came across a formula for computing the "gain" of the tone circuit, or Vout/Vin. That is, what fraction of the input signal (from the pickups) becomes the output signal (from the tone circuit). To keep things simple I used the no load version (so it's not 100% accurate, as Tim pointed out earlier).

 

This is probably another reason why robb.'s view is more helpful, as there was very little difference between the two configurations in terms of gain.

 

At 20,000 Hz (or 20 kHz), the tone circuit acts as a linear cut. In other words, (assuming you have a linear taper tone pot) setting your tone knob to 10 lets 100% of the signal through; at 9, 90% gets through; at 8, 80% gets through; all the way to 0, where none (0.00 gain) of the signal is passed.

 

At 2 kHz it's nearly the same story, except a little signal (0.01 gain) passes when the knob is at 0.

 

At 200 Hz the gain starts to curve above the line starting at the 5 setting and below, until about 7% passes when the knob is at 0.

 

At 20 Hz the gain is now quite non-linear (a curve) starting at roughly 60% (knob at 0) and increasing to 100% (knob at 10).

 

Now, even though these calculations suggest that setting the tone knob to 10 has no effect on the signal (all frequencies are passed 100%), as Tim points out this only happens in reality if there is a switch that removes the tone circuit completely.

 

When the tone knob is all the way the other way (at 0), everything gets cut. This makes sense because the cutoff (14 Hz) is below our range (20 Hz to 20 kHz). It's just that the high frequencies are cut quite a bit more than the low frequencies.

 

Now, this does raise the question of whether or not it is useful to set your tone knob below 10 if you only have a single pickup. If you want a deeper tone, wouldn't it be better to use some sort of EQ device instead? That way you can cut the high frequencies without reducing your low end.

 

OTOH, if you have two pickups and only a mono output jack on your bass, you're kind of stuck using the tone knobs to blend the neck and bridge signals. Unless perhaps you always set the tone knobs to the same setting (both on 3, for example)?

 

I'm sure this has been gone over before. I guess I should do a search.

 

Am I missing anything here?

[getz out]

Originally posted by Big Daddy from Motown:

Some may like that sound, but it's not the sound I hear in my head when I think of great tone.

I completely and totally agree it is subjective. It can also vary wildly with different types of basses. I equalize my Warwick differently than my Lull Jazz or my Rob Allen semihollow.

 

 

Originally posted by Big Daddy from Motown:

OK, since I've brought it up who has great tone?

I do. Seriously. My playing is another matter...

 

I was listening to Booker T. and the MGs on the ferry this morning, and that compressed sound that Donald "Duck" Dunn has is fantastic. To me, that is the classic "Fender bass" tone. I love it.

[josh a]

I am lost, but oh well. I know how to set an amp flat!

[Eric VB]

They say a picture is worth a thousand words. Hopefully this one is: http://www.geocities.com/ricbassguy/tone.jpg

 

The horizontal axis shows the position of a tone knob, from 0 to 10. The vertical axis shows the gain. A gain of one (unity gain) indicates the signal from the pickups was neither increased nor decreased. Since we're talking passive here, everything is a decrease (cut).

 

Curves for four frequencies are drawn: 20, 200, 2000, and 20000 Hz. An alternate 20 Hz curve is drawn using a different cap/pot combination.

 

Also included are curves for a true variable low-pass RC circuit.

[Big Daddy from Motown]

Duck is certainly the man.

 

By the way I think the Cassidy bass is supposed to be different taps on a transformer. I only liked the one sound on that knob the lowest gain had the sweetest tone.

[way2fat]

Originally posted by jeremy c:

More flatness confusion just to mess you up.

 

Fender-style amp controls...based on the famous Fender Showman, Fender Bassman, Fender Twin, etc. are not flat in the middle. Someone else will have to tell you where they are flat (or you could look it up yourself). Amps using this kind of circuit include Boogie and Alembic Preamps to the best of my knowledge.

 

With an active bass that has cut and boost controls there generally is a center detent or click in the middle of the pot. That is flat. If the active bass is boost only, than all the way off is flat.

 

Who has great tone is one of those questions where everyone will give you a different answer.

 

I could make a big list of people whose tone I like....and they all sound different from each other (and from me). And then of course there is live tone and studio tone which is not necessarily the same sound.

Here's a variation on the where is flat theme, tone controls on my pre-amp (Ampeg) say stuff like +17db/-13db, +15/-20db and there are no detents. I guessing that 12 o'clock should be close.

[rumpelstiltskin.]

Originally posted by Big Daddy from Motown:

Duck is certainly the man.

 

By the way I think the Cassidy bass is supposed to be different taps on a transformer. I only liked the one sound on that knob the lowest gain had the sweetest tone.

that is correct. part of how the cassidy gets its unique tone is by using a low impedance pickup (similar to an EMG), but instead of using an internal preamp, it has a tranformer with windings of varying impedances to induce different tones.

 

from what i'm reading, the ES has various capacitors, but the cassidy does not.

 

as a technical side note, in engineering, microfarads are denoted uF, and it means millionths of a farad, or 0.000001F (10 ^ -6). "mF" means millifarads, or thousandths of a farad [0.001F (10 ^ -3]), and "MF" means megafards, or millions of a farad [1,000,000F (10 ^ 6)]. in guitar world, it is common to abbreviate microfarad as mfd. sometimes it's capitalized. it confuses me, and now you are all likewise confused.

 

robb.

 

robb.

[soretro80s]

i like lots of bass lots of highs..not much mid.

[TimR]

Thanks for that graph Ric. Its more usual to show the gain in dB and plot this against frequency on a log scale for the different pot positions. But I think the basic understanding is there. That is that the tone control doesn't behave exactly as might be thought. If you draw the circuit out you will see that theoretically it doesn't work at all in isolation. Also interesting to see is the plot of impedance verses frequency for different pick ups. Most seem to have a fairly flat response at the low frequencies which sudenly rises to a peak as it approaches the resonant frequency of the pickup and then drops off sharply as the frequency increases further. This is due to capacitance and inductance contained within the pickup and obviously has an impact on how the tone control works, dependent on the note being played.

The other factor is the input impedance of the amp that the guitar plugs into.

[Rocky McDougall]

A Simple question from a simple mind. Most of the info on this thread is over my head but I still enjoy reading it. My question:

When wiring a pot, the incoming signal usually goes to the center post and the outgoing signal comes from one of the outer posts. What is the differentce of attaching to the left or right outer posts? Is it "full on" or " full off"?

Rocky

[TimR]

If you still connect one to the center then the only difference between the left and right is only whether it increases or decreases when you turn it clockwise.

I suspect what you mean is if you connect your leads to the Left AND Right. In this case you have the full value of 250K or 500K, this is fully off. ie minimum change in tone, minimum treble cut. This as we've discussed is still some treble cut which is why I say minimum. Less treble cut for a 500K than a 250K.

If you want no treble cut at all you have to remove the pot and the capacitor.

[jlrush]

by jeremyc:

 

And Geoff, I have heard of has-sound, in fact I'm using some of their pickups right now. I'll check out their other offerings.

 

Jeremy was the winner of the Has Sound monthly drawing in June '06. Way to go mang!

[jitter]

If your browser is capable of executing Java Applets you can play around with a RC low-pass filter here: http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/experiment/lowpass/lpf.html

 

I'll edit this message when I find a Java applet showing a Bode diagram, which is way more intuitive IMHO :-)

 

jitter

[Eric VB]

Originally posted by robb.:

as a technical side note, in engineering, microfarads are denoted uF, [...]

Thanks robb. I knew it didn't look right, but I saw it that way on another site so I just copied from them. I changed it to mfd because I figured that would work on everyone's system. Sometimes things can look funny with differences in the Mac and PC extended character sets. (I'm not a big fan of using u in place of mu. )

[Eric VB]

Originally posted by TimR:

Thanks for that graph Ric. Its more usual to show the gain in dB and plot this against frequency on a log scale for the different pot positions. [...]

Yeah, that'd make a good graph. It's still not first nature for me to think in decibels, but I'm working on it. Maybe next time.

[TimR]

You could just use log() of the gain, for this example it would be fine.

 

Here's a diagram of the differences, to make sure we're singing from the same hymnn sheet.

 

http://www.timr.cd2.com/bassplayerimg/filter.jpg

 

For the less technical. The capacitor allows higher frequency current to flow. The higher the frequency the easier it flows. In the first case high frequencies try to flow down to earth (The bottom line) through VR1, but VR1 will try to stop them. So the smaller VR1 is the more of the higher frequencies will go down to earth and not out to the amp.

The low frequencies will find it harder to go through the capacitor so more will flow out to the amp.

 

If you used the low pass filter as shown, VR2 would try to stop all the frequencies so it would effect the overall volume as well.

 

Theres a lot more going on besides this, I'll try to find a impedance/frequency plot for a pickup.

[Big Daddy from Motown]

Sorry I'm not buying that one, check out this link for a basic tone control low pass filter.

 

http://www.seymourduncan.com/support/schematics/schematicspdf/std_pbass.pdf

 

The output comes before the resistor not from the junction of the resistor and the cap.

[rumpelstiltskin.]

correct. the pot is connected to the pickup output, not the cap. one end of the pot is floating, so there is no voltage divider, and only a variable frequency LPF (low pass filter).

 

robb.

[TimR]

Thanks guys, I think we're now arguing over final intricacies.

 

1) For the VR in the UK if we only use two wires of the pot, we use that symbol. I guess you must always use the same symbol for a three wire pot.

2) the position of C1 and VR1 are irrelevant, either way round they will still do the same job as this an AC circuit and they are in series.

3) The first diagram labelled 'Tone Control' is the tone control. The second labelled 'Low Pass Filter' is a low pass filter and the discussion was whether a tone control was a simple RC low pass filter. I show them both so you can see the differences. The second circuit is not used because as Robb says there would be a drop in voltage or level due to the voltage divider effect. This is why the tone circuit has different characteristics to that of a simple low pass filter.

 

Sorry for any confusion, I'll mod the drawing when I get a chance.

[Eric VB]

Ok, my chart is off then. I based it on this diagram (the "jaguar" drawing) and my interpretation of it (assuming the middle #2 pot terminal to be the wiper).

 

The curves I called "true variable low-pass RC filter" were based on the output coming from the voltage across the cap only (as show in this Wikipedia article ), so that's not even quite right, as the "standard P-bass" drawing shows that it's the voltage across the series variable resistance and cap.

 

Would this be a reasonable schematic for the standard P-bass drawing, or is it incorrect to assume the "output jack sleeve ground" is the same as "ground wire from bridge"? (There's no evidence from the drawing that they are the same.) I separated things into functional blocks to be more clear as to what is doing what. The inputs and outputs line up, but I didn't explicitly draw the connections.

 

http://www.geocities.com/ricbassguy/tone_diag.jpg

[Big Daddy from Motown]

That's correct.

 

The grounds are the same.

[TimR]

Picture updated to eliminate ambiguities

[TimR]

This link is really good. Although based on changing cable length, shows pickup impedance and resonance.

 

http://terrydownsmusic.com/technotes/guitarcables/guitarcables.htm