What's up with the resistor to ground on the Smokeburst schematic?

[Richard Lainegard]

Was looking through the various wiring schematics on the support page
(because, well, I'm curious like that )
Came to the SMokeburst (or rather 2009 Nitro models):
https://www.prsguitars.com/csc/schematics/schem08/2009_nitro_models.pdf

What's up with the 3.3M resistor from the input lug of the V-pot to ground?
Never seen that before. Or rather, only time I've seen a resistor in parallell
with the pickup hot wire to ground would be where you want to change
the value of the vol pot that pickup "sees" (like Tom Anderson/SUhr etc does
on a HSS guitar with 500K volume pot).

But this is a dual Hum-guitar, so what's the purpose of such a high value resistor?

 

[John Beef]

Well, now you've got me curious too.

 

[Twinfan]

Looks like a tone-shaper to me.....

 

[John Beef]

But how would that shape the tone? I'm no electrical engineer, just a guitar wiring hobbyist, but if the electricity wants to take the shortest path to ground, seem like that path would be through a 500K volume pot and not through a 3M resistor.

 

[aristotle]

Naw....the 3M resistor isn't in parallel with the pot (it would be across terminal 2 and 3 if that were the case). If you draw the circuit out, you'd see that the pot is in series with the signal, and the 3M resistor is to ground. So you've got 3M to ground before the pot, then you have a .22uf in series with the tone pot to ground on the output. The volume pot is in line with the output, and not a resistor to ground.

 

[Twinfan]

Yep, so some frequencies are tweaked before the signal hits the vol pot...

 

[garrettb]

The resistor is adding more parallel resistance, which will take some more of the high end off, so it is the same concept as with Suhr, etc. just with a much higher value. The value is so high in order to keep from trimming off too much high end.

Using parallel resistance calculation, you'll see total resistance for a typical PRS (500k volume, 500k tone) is 1/(1/500+1/500)=250.

Add in the 3.3M: 1/(1/500+1/500+1/3300)=233

To further illustrate the relationship, if you used a 470k resistor instead: 1/(1/500+1/500+1/470)=167

Apparently, the guys at PRS felt like this model was a tiny bit too bright and therefore add the resistor. It's going to be a very small difference, though. But Paul specializes in small differences. :wink:

 

[John Beef]

That's an interesting concept.

One thing I have noticed with humbucker guitars especially, if the neck pickup is clear and bright and throaty, the bridge pickup can be painfully bright at times. If the bridge pickup is fat and round and warm, the neck pickup can be pure mud. This is not always true, of course, but it happens frequently. So, say you have one of the guitars where the bridge pickup is too bright, you could add this resistor to ground right at the switch and potentially roll off some high end. But why a resistor, when you use a capacitor to roll off high end on the tone pot?

(like I said I have absolutely no electrical training whatsoever)

 

[aristotle]

I don't think it's quite that simple guys. The resister in question isn't in parallel with either pot. It's off of the hot end of the pickup to ground at the input to the volume pot. Knowing how the resistor affects the tone would require a decent model of the pickup itself. If the pickup were an ideal voltage source (which it is not), the resistor wouldn't do anything at all, and certainly wouldn't do anything that's frequency dependent. But a pickup is much more complicated than that. It may in fact take off high end, but if so, it's not for the reasons described....in fact, without knowing how to model a pickup (or in my case, being too lazy to do so) it could actually add high end. But from a circuit point of view, that resistor doesn't do anything relative to what's downstream of it. The main thing it's doing is loading the pickup a little bit...which would presumably do something, and in this case, presumably does something good.

 

[garrettb]

That's an interesting concept.

One thing I have noticed with humbucker guitars especially, if the neck pickup is clear and bright and throaty, the bridge pickup can be painfully bright at times. If the bridge pickup is fat and round and warm, the neck pickup can be pure mud. This is not always true, of course, but it happens frequently. So, say you have one of the guitars where the bridge pickup is too bright, you could add this resistor to ground right at the switch and potentially roll off some high end. But why a resistor, when you use a capacitor to roll off high end on the tone pot?

(like I said I have absolutely no electrical training whatsoever)

Capacitors don't actually roll off high end, that's the role of the resistor (variable resistor in the case of a tone pot). The capacitor changes the resonant peak, which effectively shifts focus to mids.

I don't think it's quite that simple guys. The resister in question isn't in parallel with either pot. It's off of the hot end of the pickup to ground at the input to the volume pot. Knowing how the resistor affects the tone would require a decent model of the pickup itself. If the pickup were an ideal voltage source (which it is not), the resistor wouldn't do anything at all, and certainly wouldn't do anything that's frequency dependent. But a pickup is much more complicated than that. It may in fact take off high end, but if so, it's not for the reasons described....in fact, without knowing how to model a pickup (or in my case, being too lazy to do so) it could actually add high end. But from a circuit point of view, that resistor doesn't do anything relative to what's downstream of it. The main thing it's doing is loading the pickup a little bit...which would presumably do something, and in this case, presumably does something good.

You're over thinking it. It bleeds highs away to ground and leaves the rest to go into the pot.

 

[aristotle]

Resistors don't have anything in them that is frequency sensitive. When combined with a capacitor (or an inductor, or other reactive elements) the combined system is frequency dependent, and can be configured (as in the case of a guitar tone circuit) to pass more, or less, high frequency components depending on the value of the resistor. The tone pot setting, the volume pot setting and the capacitance value all contribute to the shape and location of the filter roll off. The capacitor value doesn't have anything to do with a resonant frequency in this case as this isn't a resonant circuit. But it definitely has to do with how much highs are cut.

Adding a resistor to ground at the input doesn't interact at all the way it's being described above. If you take a bench source, hook it up to the input of the circuit and look at the output over frequency, with and without that 3M resistor, there'd be no difference whatsoever. It's put there because somebody at PRS liked the way that a 57/08 sounded better in this particular guitar when it was loaded with a 3M resistor...and again, that may have absolutely nothing to do with taking off highs (it could be the exact opposite.)

 

[justmund]

I find this stuff fascinating, I've never gone very deep into the effects of resistors/caps in guitar circuits and just experimented when I've needed to.

Would be very easy to test this mod of course!

While we're here, I've always wondered about the cap going from the volume pot to the toggle switch in the Sig Ltd. I know PRSh talks about this making the circuit a little bit "sweeter" and Brent Mason didn't want it in his guitar. Any ideas?

https://www.prsguitars.com/csc/schematics/2012/signature_2012.pdf

 

[aristotle]

This topic got me to take a quick peak at all of the schematics posted on the tech site. Surprising how many different ways they choose to wire these things. I'd bet that's because of the differences in the pickups, or maybe they just like to tinker. In any case, regarding the signature limited schematic, the .022uf cap in series with the hot signal from the selector is acting as a DC block. All else being equal, and assuming very little extremely low frequency content, it wouldn't have much of an effect...but I'm sure that the pickup itself has some series resistance, and that, in combination with the 180pf across the volume pot does who knows what. In general though, I'd expect that it would remove a touch of the lows.

The more interesting one is the 180pf cap. The volume pot in this case is in-line, and the 180pf cap would tend to let high frequency components past the volume pot more so than low frequency components. So, the volume control would make it seem like the tone gets brighter as you reduce the volume control. Maybe this is the cap PRS was talking about? This is probably all old hat to somebody in the guitar business, and these tricks of the trade might be well known. Very interesting though. None of these circuits resemble what I had thought (and probably read somewhere years ago) the "traditional" guitar circuit is. I "thought" that the tone cap followed by the tone pot to ground hang off of the pickup output (after the selector if there is one) and the volume pot was wired across the top of the tone cap to ground....just acting as a simple voltage divider. These schematics are all different.

 

[redmax61]

Yeah, you're right Aristotle. The 180pF cap is known as a Treble Bleed cap, and it does exactly what you state. The cap lets higher frequencies escape so the low frequencies drop off before the highs do. This keeps the guitar from getting muddy as you turn the pot down.

The .022 cap in series with the hot signal to the selector is an extremely common value in guitars and amps. It acts as a high pass filter to, again, keep the guitar from becoming muddy.

 

[aristotle]

Thanks for the reply Red. I figured that this was all standard stuff in the guitar world. Interesting to the uninitiated (like me) nonetheless!

 

[Richard Lainegard]

Lol, thanks for all the replies guys! I am now more confused than ever :wink:

 

[rugerpc]

I always find this stuff very interesting.

For the non-electronic-minded, here are some things the guys are NOT talking about here...

Cap:

Pot:

Resistor:

 

[Ralph Ballinger]

Thanks for the reply Red. I figured that this was all standard stuff in the guitar world. Interesting to the uninitiated (like me) nonetheless!

I just stumbled upon this forum doing a wiring search on google. I’m not sure if anyone came up with the answer about the resistor on the volume pot or not? 4 years have passed since the last post here. The resistor if I’m remembering correctly was the vintage voicing. 500k pots are great for Humbucker but no so much with single coils. The resistor essentially turns the 500k pot to a 250k pot for the single coils like on a Strat. I remember the day that Roy Fought and Dave Schecter figured that one out. We were all standing in the assembly dept. of Anderson Guitarworks.