Pickup resonance, Miller effect and tone booster

[tca]

Hi, I've been playing with a simple JFET booster:



Because I have a crappy SS amp my strat does not sound as I would like, and
because of that I always put some booster in front of the amp (with a high input
impedance). What I miss about the sound that I get is the strat quack. So I was
thinking that this could be recovered by some decent preamp.

Using the pickup simulator (taken from one of Gus posts) one sees that the freq
responce is flat up to 2kHz (it will depend on the pickup, obviously) then it
starts to decay and so the resonant frequency is, or can be, made to resonate if
the guitar is connect, for instance, to some  RC network. This is showed
in the previous pic. A similar effect occurs in a 12AX7 tube preamp with a 10p
capacitor from grid to plate to minimize the Miller effect.

The effect is subtle with and without the capacitor. I've seen other stompboxes
with the same idea, adding a capacitor to ground at the input (besides treble
roll-off), e.g., tone booster from Everyday Electronics 1978 (can't find the
link...)

Is it common to boost the resonant freq of a pickup? Any tube amp with a
correction of the Miller effect will do it (up to 5dB)?

Comments?

Cheers.

P.S.(edit)

Actually the tone booster that I've mention is not an good example, it boosts some freq interval but not by using a capacitor to ground at the input.

[gritz]

Don't forget to simulate the capacitance of the guitar lead - for a six meter cable it will be in the order of 1nF.

This is why I tend to use analysis of real signals, rather than simulations (which need all the relevant info if they're to be meaningful).

[tca]

Don't forget to simulate the capacitance of the guitar lead - for a six meter cable it will be in the order of 1nF.
This is why I tend to use analysis of real signals, rather than simulations (which need all the relevant info if they're to be meaningful).

Well, I've done a *real* analysis... with a breadboard, a cable and a crappy amp! Thanks for remind me of that. I'll redo my sim.

P.S.: (edit)

+ guitar

[Jdansti]

Sorry I don't have an answer to your question, but I notice that your preamp is similar to the Tillman preamp found here:  http://www.till.com/articles/GuitarPreamp/. I use the Tillman quite a bit with my SS amp and really like the way it enhances the sound of my Strat. I don't have any modeling software, but would be interested in how the Tillman configuration compares to yours.

[pinkjimiphoton]

tiago,

have you built a fuzz face? do you have access to decent ge transistors? they're not crucial, if you can get low gain si's.

you can use a fuzz face with your guitar turned down and get a "cleaner" than clean kinda sound, with that classic "quack" intact and a decent volume if
you bias the fuzzface a little hot...

i like 'em to be set so i get a nice even sweep from off to about 9 or so, and all out fuzz at 10...then i can dial in pretty much any tone i want on any guitar i want..

sometimes with the guitar way down, you get a nice "compressed" super clean tone that's really hard to beat..

good luck in the quest man... rock on!

[ashcat_lt]

You've also omitted the self-capacitance of the pickup itself.  It's usually very small and swamped by cable capacitance and other things, but...

[tca]

Found the tone booster schematic:



It works nicely with the 2N4401 (NPN) and the 2N4403 (PNP).

Thanks for all the replies. I know that the way the pickup is simulated is very
simple (maybe to simple) but it captures the main characteristics, as a first
approximation it is not bad, similar to the approximation of speaker behavior
using  Thiele-Small parameters.

@Jdansti
I've simulated the Till preamp, the bump only shows up if you add a capacitor to ground
at the input (could be a connection cable or other parasitic/put-on-purpose capacitance).

@pinkjimiphoton
I'll try that with some low gain Si's. There is still some things that I need to
look at with the FF circuit (just waiting for the signal generator to arrive,
the digital oscilloscope has just arrived!)

@ashcat_lt
Ok, can you point me to a more complete pickup equivalent circuit. A book
ref. would be ok.

Cheers.

[Gus]

The guitar or bass sim I post are simple ones.  I add a cable capacitance between the guitar or bass.

When you run your sims change the volume control settings and tone setting, note the change in the frequency response with the volume control turned down as you add series resistance (the volume control turned down).  Also change the input resistance of the gain stage.

A volume control is another tone control, some combinations of cable capacitance and input resistance of the stage after the guitar and cable can be another tone control




Also think about  "pickup simulators" that you can find on the web

Note when you disconnect the guitar from the input of an effect that has an interaction  you will not get the same response if the guitar goes to say a buffer then a pickup sim then to another stage. 

You lose the tone change from the volume pot and always have a high end bump from the pickup sim circuit.



I used LT spice to help a friend adjust parts in their highway P bass tone and volume control circuit to another circuit for what they wanted.
To do this 100% correctly you would need to model the pickup and input of the following active stage
However a model like this can help adjust a bass (this is not the circuit we ended up with)



Think about why people like different cables, it could be as simple as the capacitance of the cable interaction with the guitar and gain stage.

Also model different tapers and volume and tone control resistance values.

Note what happens above and below about 470K input resistance with guitar/bass input cable capacitance

[Digital Larry]

I thought the "Strat quack" was that special sound you got in switch positions 2 and 4, mostly due to phase cancellations of the two pickups.  But I don't own one so that doesn't make me the expert on it.  What you think of as the classic Fender sound may also include the effects of the Fender tone stack in the amp.  Of course this tone stack has been widely copied but that doesn't mean your amp has it.

I have a Tech 21 Trademark 60 and although this seems to be an example of a "good" SS amp, it still doesn't sound like a Fender twin.

I did some SPICE analysis of the wiring layout on my Yamaha AES-800 here.  While it probably doesn't help you in your situation, it's a pretty cool wiring trick for a dual humbucker guitar if you want some twangy single coil sounds that are unusual.

[ashcat_lt]

Once you add the capacitance across the coil you're pretty much as close as we can get with most sim programs.  The reason I mentioned it is pretty specifically because, as you said, the resonant peak only shows up with a cap.

I usually reference JohnH's GuitarFreak.   He's included "presets" with typical values of for a few different types of pickups.  He's also included a few of the typical "mods" that people add to their guitars, like treble bleed on the volume pot and I think even inductors based tone options.

This link is on that page, but it's a pretty good discussion of how the pickup circuit works.

It's worth keeping in mind that this models only the filter created by the pickup and its attached wiring, the basic frequency response of the circuit.  It does not take into account the characteristics of the strings, or the effect of pickup placement.  So it gives you an idea of what it might sound like to blast white noise through the pickup, but not necessarily exactly what you might actually hear from the guitar itself.  Not that it's not useful, just something to keep in mind.

[puretube]

puretube
Posts: 7130



but, err, I`m not really here...


            Re: JFET biasing ideas
« Reply #29 on: Yesterday at 00:36:10 »   



Quote from: WaveshapeIllusions on Yesterday at 00:17:48

Quote from: merlinb on 21 June 2013, 10:01:36
Has anyone decided if we're trying to bias all the FETs to:
the same drain current?
the same Vgs?
the same relative point with repect to cut-off / Vgs = 0?


Well, I guess we'd want at least two for the ability to do away with trimmers. We'd want them to bias to the same point relative to saturation and cutoff. That would put it at the same point in the curve, so the transfer function would be about the same. So the signal would have similar distortion. Ideally, thus would put the drain voltage at the same point, which is what they often instruct one to do when using trimmers. If it is biased into the middle of the curve, then the drain voltage would have to sit between cutoff and saturation, which would be middle of the supply, right?

Since devices vary in Idss though, middle of the curve would happen at different currents. So the drain current would have to differ. Which means even though they are at the same point in the curve, the output level would be different, right? Hmmm...

A lot of the suggestions posted use some degree of feedback. Amptramp's design looks like the simplest way to get them working. It basically amounts to negative feedback, but with the caps on the feedback point it is DC only.


tca, what you've shown there looks like a good way also. It's similar in that it uses feedback biasing, but the parts count is lower. So it's simpler to implement. The curves look nice, I'd go with the Schade since it appears to show more distortion overall. Now, I think somebody mentioned in the previous thread that higher values of drain resistance also helped deal with device variation? Would it be possible to increase that drain resistor to 10k and scale the source up accordingly?



Yes...



Quote from: WaveshapeIllusions on 20 June 2013, 12:54:57
As unproductive as that last thread was, it gave me some ideas.
...

Just for reference sake...

tca: glad to finally see someone else scaling UP the things, like WSI mentioned... 

[tca]

@ashcat_lt
Thanks for the refs.

[StephenGiles]

 Bismillah! No - we will not let you go - let him go
 

[tca]

> sometimes with the guitar way down, you get a nice "compressed" super clean tone that's really hard to beat..

@pinkjimiphoton
That's true, a little compression does make the quack more visible (2 and 4 position pickups).

@StephenGiles
> Bismillah! No - we will not let you go - let him go
I really don't have anything more to say but...