Jaguar bass EQ mod

[akoure]

I'm a relative newcomer to electronics, I've built a number of DIY kits and I'd say I'm fairly competent with a soldering iron.  I want to get into analog circuit design and I've been trying to come up with a good first project which will be a good challenge without being impossible / dangerous for a beginner to the design side of things.

My tentative idea: I have a 90's MIJ Fender Jaguar bass which is a beautiful instrument with a garbage active tone circuit and an over-abundance  of switches and knobs. I thought it would be cool to scrap the active circuit and design a more flexible passive eq into the existing passive tone circuit.

I've been reading a lot about basic circuit analysis and my high schools physics unit on ohm's law etc. is all coming back to me. From what I've been reading online about basic filters, this seems like a reasonable first design project, I just feel like there's a million slightly different examples of simple filters and don't really know how to get started. Are there any definitive texts or starting places that you guys could recommend? For example I've been looking over some pultec schematics, and I'm wondering if I can just cop a specific frequency from the HF band and have boost/cut knobs? I'm sure it's not that simple I'm just looking for a way to connect my basic understanding of the theory to this application so I can't start trying to solve the puzzle.

Thanks in advance for your patience!

 

[PRR]

Welcome.

> passive eq into

Passive EQ right off the guitar pickup is tricky and often non-optimum. Signal level is low so any loss puts you closer to universal hiss. Pickup impedance is high and very variable (inductive to a point then capacitive) which foils many reactance-based EQ schemes. Inductors around a pickup must be VERY high impedance. Stray capacitance sucks the top away.

If you can support active, everything changes. But active is a hassle. And there must be an instant bypass for when the battery quits, or just gets loose, mid-solo.

The one thing which works dead-simple is a capacitor across the guitar to cut the high end some or more, for mellow rhythm vamping.

But it is an old-old goal and you can find several old passive EQ guitars. Some inordinately complex, expensive, and not in production long (which is a hint).

 

[abbey road d enfer]

I'm a relative newcomer to electronics, I've built a number of DIY kits and I'd say I'm fairly competent with a soldering iron.  I want to get into analog circuit design and I've been trying to come up with a good first project which will be a good challenge without being impossible / dangerous for a beginner to the design side of things.

My tentative idea: I have a 90's MIJ Fender Jaguar bass which is a beautiful instrument with a garbage active tone circuit and an over-abundance  of switches and knobs. I thought it would be cool to scrap the active circuit and design a more flexible passive eq into the existing passive tone circuit.

I've been reading a lot about basic circuit analysis and my high schools physics unit on ohm's law etc. is all coming back to me. From what I've been reading online about basic filters, this seems like a reasonable first design project, I just feel like there's a million slightly different examples of simple filters and don't really know how to get started. Are there any definitive texts or starting places that you guys could recommend? For example I've been looking over some pultec schematics, and I'm wondering if I can just cop a specific frequency from the HF band and have boost/cut knobs? I'm sure it's not that simple I'm just looking for a way to connect my basic understanding of the theory to this application so I can't start trying to solve the puzzle.

Thanks in advance for your patience!

The big issue with passive EQ is that in fact it needs some active electronics to provide low source impedance, high load impedance and some gain to compensate the loss of the EQ circuit. Typically, a Pultec EQ needs a 20-ish dB gain make-up amp.
There's only so much that can be done with passive controls on a passive instrument.
If you want to go passive I would think you can't do much more than what's done on the Fender standard Jaguar bass, i.e. two volumes and one tone.
If you really want to experiment, you may try one tone on each p/u, and if you want to explore every alley, you could experiment with a bass-cut. I have, and I put it in the category of things that may be useful once in a while, but not worth having constantly under hand. I must say that it's useful on a baritone guitar, though, in orderr to get the characteristic tic-tac sound.

 

[abbey road d enfer]

And there must be an instant bypass for when the battery quits, or just gets loose, mid-solo.

Please, it's a bass, don't encourage him to play solos.  ;D

 

[akoure]

There's only so much that can be done with passive controls on a passive instrument.
If you want to go passive I would think you can't do much more than what's done on the Fender standard Jaguar bass, i.e. two volumes and one tone.

Passive EQ right off the guitar pickup is tricky and often non-optimum.

So it sounds like passive isn't really an avenue worth pursuing. Would designing a new preamp (or improving the existing one) for the active circuit be a reasonable undertaking?  Even if I never use it I'm mostly looking for a project where I can learn something about circuit design.

 

[L´Andratté]

You actually should have learned something valuable already about circuit design from PRR and abbey...

I remember from ´stompbox cookbook´, there´s a passive bridged-T filter  scaled for roughly guitar impedances, it´s called axe-o-matic:
http://www.super-freq.com/wp-content/uploads/2010/06/Picture-1.png
The pot is a dual linear-
I tried many years ago, don´t remember how it was...

 

[abbey road d enfer]

You actually should have learned something valuable already about circuit design from PRR and abbey...

I remember from ´stompbox cookbook´, there´s a passive bridged-T filter  scaled for roughly guitar impedances, it´s called axe-o-matic:
http://www.super-freq.com/wp-content/uploads/2010/06/Picture-1.png
The pot is a dual linear-
I tried many years ago, don´t remember how it was...

It could work. I guess it is an attempt at a similar effect as the Gibson Varitone. Most of the guitar players I know leave it in bypass, but I know BB King was constantly fiddling with it...
OTOH, it's cheap to experiment.

 

[Tubetec]

Yeah its the simple top cut passive arrangement that rules the roost with most players .
You could try converting your active to passive ,but the thing is many pickups designed for active on board pres arent wound as hot as usual passive pickups ,so levels might not be great out of it . The kind of boost/cut tone controls on active bases these days isnt possible with a fully passive setup . There is a few guitars over the years with extra bits added to passive setups ,maybe inductors ,and switches to cut in and out different coil taps or tone cap selection , the fender Jaguar being one of the most elaborate of these. Might be a good plan to measure the dc ohms of the pickup coils to begin with 6-8kohms would be typical of fender pickups ,if you find there a lot less than that output could be on the low side in a passive set up .I generally prefer all passive sets ups for simplicity ,theres no chance of a battery going down on you mid gig ,and alot less other stuff to go wrong also ,down side is you wont have the range of control  like you get on active setups , still your amp will allow boost or cut ,at several frequencies ,so you might get the sound you want that way .I always found that with Fender basses a little notch at around 100hz evens out the levels on the lower registers as they can be a little peaky in that region .

 

[andyfromdenver]

Hi akoure,

you may know you have caught the attention of some amazing contributers here with excellent advice given.

my thoughts:

1: what if you/we (meaning them) explored what it is about your active guit set up you detest and perhaps there is an opportunity to make 1-2 small changes to steer it towards your liking.
would need problem, scheme, pics to help folks.

2: following with a posted sugg, there is the Craig Anderton passive tone shaper.

I made a box which has two 1/4" ins and one out. each in has passive lvl control and one of the channels contains the CA tone shaper.  I use this box all the damn time when I want to have two instruments into a one channel amp.
It has losses cause it's passive, but for a practical band/ show situation, it's a fun tool.
some of the scoopy settings sound cool on kybds and stuff, and if not desired, add a "none" selection on the switch or keep the depth control up. You can expand the range too with a different switch and more caps.

anyhoo, check out like a DOD passive mixer to get the right pot values and mix resistors, and sort out how to combine the two. it was a fun little project. no batteries, no wall wart

take care!
Andy

 

[andyfromdenver]

I made two, sold one to a good friend for $40, which paid for all the parts for both. ok, my time was a loss (maybe $1 an hour)....but was it?  ;D :. the power of diy...lol

Andy

 

[abbey road d enfer]

there is the Craig Anderton passive tone shaper.

that's exactly what the Gibson Varitone is, but with the addition of a depth control. On the Varitone it's fixed depth.

 

[akoure]

Hi akoure,

you may know you have caught the attention of some amazing contributers here with excellent advice given.

my thoughts:

1: what if you/we (meaning them) explored what it is about your active guit set up you detest and perhaps there is an opportunity to make 1-2 small changes to steer it towards your liking.
would need problem, scheme, pics to help folks.

Yeah its the simple top cut passive arrangement that rules the roost with most players .
You could try converting your active to passive ,but the thing is many pickups designed for active on board pres arent wound as hot as usual passive pickups ,so levels might not be great out of it . The kind of boost/cut tone controls on active bases these days isnt possible with a fully passive setup . There is a few guitars over the years with extra bits added to passive setups ,maybe inductors ,and switches to cut in and out different coil taps or tone cap selection , the fender Jaguar being one of the most elaborate of these. Might be a good plan to measure the dc ohms of the pickup coils to begin with 6-8kohms would be typical of fender pickups ,if you find there a lot less than that output could be on the low side in a passive set up .I generally prefer all passive sets ups for simplicity ,theres no chance of a battery going down on you mid gig ,and alot less other stuff to go wrong also ,down side is you wont have the range of control  like you get on active setups , still your amp will allow boost or cut ,at several frequencies ,so you might get the sound you want that way .I always found that with Fender basses a little notch at around 100hz evens out the levels on the lower registers as they can be a little peaky in that region .

Thanks Andy, really appreciate all the great input from everyone, thanks for all the insight and  wisdom! When I get home from work I'll take a look at the pickup output, upload a schematic and picture of the wiring,  and try to come up with a more well defined problem.

Tubetec, I don't disagree with you on passive in a live setting. Lately I've been doing more home recording than gigging so active could present some interesting tone options in the studio, but again this project is almost as much for my personal edification as anything else so I'm not too concerned if it doesn't wind up being the most practical thing in the world.  I'll get back to everyone with pictures, schematics, and meter readings this evening.

Thanks again to everyone for being so helpful and supportive!

Alex

 

[clintrubber]

Are there any definitive texts or starting places that you guys could recommend?

I recently stumbled upon this Nice & Massive Collection of Circuits, mainly for passive basses:

scroll down here:
http://161589.homepagemodules.de/t29f2-Cadfaels-kleine-Schaltplan-Sammlung-fuer-passive-E-Baesse.html

or direct link to the pdf:
www.ak-line.com/medium/Bassschaltungen417.pdf  (7.65 MB)

It's in German, but the schematics & diagrams will come across.

Enjoy!

 

[abbey road d enfer]

Interesting! How many variations of the same thing can there be? 
In pages 101 & 102 (GL1000/2000) there's a good example of the bass roll-off I mentioned earlier ( 1Meg/2.2nF)

 

[clintrubber]

Interesting! How many variations of the same thing can there be? 

Indeed ;-) 

Imho the listing of all permutations like on p449 could have been skipped, but hey, that's nitpicking... it's far easier for me to make such a comment than to compile al this...  massive amount of work, much appreciated!

The circuit like that in the Rick 3001 (p172)  I hadn't seen in a passive bass before.

 

[abbey road d enfer]

The circuit like that in the Rick 3001 (p172)  I hadn't seen in a passive bass before.

Yes, I noted that; it's a variation of the White EQ that was popular in HiFi stuff in the 60's. When I say a variation it's because the signal flow is reversed compared to the standard application. It was used in some early Fender amps (Princeton) in altered form.
In fact it is not unusual for guitar "electronics" to choose reversing the signal flow, since it creates less interaction.

I'm also curious about the Gibson EB2 at pg. 113. I tried to simulate it but I don't really see the action of the inductor, even with tweaking the parasitic values...

 

[Tubetec]

The gibson eb has a great tone ,think alright now by free ,
to me it always had a more even tone over the low range  than the fender setup ,
I think its a humbucker in the eb ,maybe stacked ,as I only see four pole pieces ,
Anyway , I wonder how a battery powered active boost cut  valve eq  would sound in a bass ,and maybe a cathode follower output stage , a 9volt battery with an attendant LT regulator  and HT generator circuit could power a valve onboard stage .
battery life would wouldnt be great,hours as opposed to months ,
you could do a baxandall with switched turnover frequency ,
http://drtube.com/schematics/marshall/2001pre1.gif,
dual subminiature double triode might just do the trick, but Di input would be best ,
A few hundred volts under the hood of your trusty instrument  sounds a bit scarey in a gig situation ,but in the confines of your home studio why not ?

 

[Tubetec]

https://youtu.be/gT-0us1Dyqg
I love the live studio feel of that
6122 might do the job ,relatively available cheaply
Soviet  era equiv  probably does the job just as good ,
Donnie boy is reminding me more and more of the guy in Dr Strangelove who rode the missile in ,just saying ......

 

[abbey road d enfer]

The gibson eb has a great tone ,think alright now by free ,

There are many incarnations of the EB bass, some with the LC circuit, many without, and even if it has, it may be disengaged.

 

[Gus]

Did you search for the schematic?
here is a link http://www.freestompboxes.org/viewtopic.php?f=3&t=19659

It does not look like a bad circuit, what don't you like?
You can change parts and/or change the EQ note the C to B feedback/bias resistor at the first stage. This makes the input a not perfect summing node and the RLC before it(pickups) is part of the feedback calculation.
Load the circuit into spice make sure you use a good pickup model and not just a voltage source when you run the sim.  Adjust the EQ to what you like.