Passive mid boost/cut circuit questions

[jerrybass1955]

Hi Folks,
I am new to this forum and look forward to future interaction.

I have installed a Torres style mid control on a Frankentele that I put together and am having poor results with it.  I saw several suggestions on various forums to build the circuit around a 500K linear push-pull pot, which I did.  The results are, not too surprisingly, that all of the action happens in the first few degrees of the pot rotation.  When the mid control is not engaged, it should act like a normal tone control, but again, since I am using a linear taper pot, all the action happens at one end.  The schematic I am using is attached.  mid_tone_control.pdf (15.3 KB)

Some questions:
*    any ideas on how to improve the adjustability of this circuit?
*    Seems like it would work better with a log/audio taper pot, but only at one end of the range.  Am I missing something?
*    When fully engaged, it doesn't sound much like the mid-cut I was expecting.  Seems to mostly roll off the highs (although rotating it while playing sounds like a wah).  I am not sure that the inductor is actually 1.5H - I measure just under 1H, but I am not sure my meter is trustworthy for inductance.  But adding capacitance doesn't seem to act as if it is lowering the resonant frequency as I would expect.  Instead, it seems to just roll off more highs.

I am considering doing away with the .022 cap and 1 Meg resistor and rebuilding it using a 250K audio taper pot in hopes of an improvement in adjustability.

Any thoughts or advice would be appreciated.  Seems like a fun mod that would have a useful sound, but it isn't working out that way for me yet!

Thanks,
Jerry

[JohnH]

Hi jerrybass1955, welcome to GN2.

Those midshaper circuits are interesting and you might find it helpful to be able to see how frequency response changes as you tweak values. I wrote a spreadsheet called GuitarFreak to do this, and the current version 4.03 will deal with various inductive and mid-shaping tone controls, and it will also do bass cut as on your other thread.

GuitarFreak

Its based on modelling the whole electrical system within a guitar, from pickup to amp input and using classical theory to see how the parts interact at each frequency

It needs excel to run it, any version from 1997 onwards, or apparently libreoffice also works. If you have trouble, Id be happy to run some scenarios.

I have played on it to try to see what the Torres system might do, but did not find a compelling reason to want one!

John

[jerrybass1955]

Hi John,

I actually did some simulation using your spreadsheet just before joining the
forum!  Nice job.  It actually confirmed that the all of the
excitement is in the first 10-20% of the pot's range.  It also leads me to
wonder why I see posts suggesting using large value linear pots.  Since I
have mine on a switch, seems like a 50K pot would be fine (similar to Craig
Anderton's passive tone shaping circuit).  Since I can switch back to
normal tone control, there would be no issue with extra load.  Perhaps my
next experiment will be to put a 47K resistor across the pot from wiper to LC
tank and see what happens.  If only there was a way to easily simulate
that .........

Jerry

[jerrybass1955]

Sorry about all that garbage.  I sometimes have problems on another site with the site losing my post before I post it, so I often write in Word and copy it.  Guess that isn't a good idea here!
Jerry

[JohnH]

Oct 28, 2014 15:12:06 GMT -5 jerrybass1955 said:

Sorry about all that garbage.  I sometimes have problems on another site with the site losing my post before I post it, so I often write in Word and copy it.  Guess that isn't a good idea here!
Jerry

Hi Jerry
You should be able to edit your last post to remove the excess html text. As a mod, I can too, but not off this cell phone!

If your idea with the 47k amounts to putting it across one side of the tone pot, you can probably capture it by using tbe 'custom' tone option and putting a suitable formula in the relevant cell for resistance. Or, to take these studies further, get hold of a free sim program such as 5Spice.

[newey]

jerrybass '55-

Hello and Welcome to G-Nutz2!

I edited out the Word coding for you.

To copy/paste text into posts, I use either MS Notepad or Wordpad, either will output into .txt or richtext which will play nicer with the BB software.

My inclination was to move your thread to the general wiring section, as this section is devoted to JohnH's tone control discussions, more of a reference than a place for general questions. But since it is germane, and since John hasn't objected to it being here, I decided to leave it as is. JohnH is free to move it if he sees fit.

[jerrybass1955]

Thanks for the fix and sorry for the newbe errors. I played around with the circuit this afternoon without much headway. But I built a new circuit that I hope will work better.  There is a small range that sounds good, so I am switching that in and using a 250k log pot.  I'll post my results after I test it out. Sorry if the post is mislocated, so feel free to move it if appropriate.

Jerry

[sumgai]

jerry,


That's an interesting circuit, the one you linked in your OP.... it has no input signal!  Hint: the only thing that should ever be implied in a circuit is ground - all other connections must be explicit in order to avoid ambiguity, or worse; incorrect assumptions.


Also, FWIW, most tone-stacks that incorporate a Mid control (this includes others than those that just mimic Leo's classic tone-stack) use 10-25KOhms pot values.  Where a control is not made available to the user, a fixed resistance value is used, often less than 10KOhms.  I have to add that while I always applaud experimentation, I often want to insert myself into a conversation in a negative way... meaning, I want to shout "Don't do that!" whenever I see a boatload of GeFooeySpeak (that's where an engineer encounters a layman's explanation that is so far off the mark that it requires a double-facepalm).  In this instance, we're not quite to the point of blowing the whole concept of a Mid control out of the water, but we're certainly dangling one foot off the dock and over the water... and standing on a banana peel. Let me elucidate:

I'll make the admittedly rash assumption that you are incorporating this circuit inbetwixt a standard guitar's volume-and-tone circuit (meaning a treble-cut device), amiright?  In that case, half of your circuit is duplicating/interferring with that treble circuit.  You did mention something about how the circuit seems to be rolling off the highs, in your opening post.  Since you show a proclivity for circuit analysis, I'll leave it to you to determine which components I'm speaking about.

OTOH, if you have implemented this contraption all alone, no other tone controls, then I can see why you might want to try both a standard treble-cut and a mid-cut, via the push-pull switch.  But as I note above, this flies in the face of normal circuitry design - you sacrifice controlability for one by optimising the other. (In this case, you lose control over the mids in order to have control over the highs.)  In short, regardless of what you read on the 'net about how to do this, it's not that easy.  By that I mean, there's either gonna be some compromising going on, or else there's gonna be some more complicated circuitry getting installed.  And you know as well as the rest of us Nutz, the more you put in there, the more likely it is that Mr. Murphy will be paying you an unwanted visit!  Or in other words, the KISS principle applies, in spades.

My advice: First, get rid of the notion that one control can do two jobs, especially those so closely related.  Figure this out: you've got the treble dialed in just right, now you want to fiddle with the mids.  Err, by switching from one to the other, you just destroyed your treble setting, didn't you?  Sorry, but I don't see much profit there - you wasted time getting one thing right, only to flub it up as soon as you flicked the switch.  Not a user-friendly world, IMO.

Second, and assuming that you want to go with the Mid control, crank back on the pot's value.  Find a 25KOhms, or even a 10KOhms pot, and stick it in there - now test for effect.  (And harkening back to my first sentence, I'm assuming that you make no other changes to the circuit, and least not yet.)

Third, give a moment's cogitation to Gibson's Vari-Tone circuit, using the search engine of your choice.  In there you'll find that they use a choke that's quite a bit higher in value (on the order of 3 to 6 Henrys, depending on age and axe in which it was installed).  I think you can eliminate several components from your design, and come up with a workable circuit.  In fact, if you use this search term: site:guitarnuts2.proboards.com as part of your query string, you'll find that we've done quite a bit of that work on that particular setup, right here in the NutzHouse.  But I don't want to prejudice you by linking to our stuff first, I want to leave it up to you to seek for yourself all the myriad other answers out there in the big, bad, Internet.    This is how you learn, by doing it yourself - not by having it spoonfed to you by a bunch of Nutz.

I hope this little epistle didn't leave you all cross-eyed and muzzy-headed, it was simply intended to point out some things that we've learned over the years, here in the NutzHouse, that's all.  By all means, keep plugging away, and when something goes bump in the night, you know where to find us.

HTH




sumgai

[jerrybass1955]

Hi Sumgai,
Thanks for your thoughtful post.  Basically, this is an experiment mostly for the fun of it and to see if I like the results enough to keep it installed or even spread it around the herd!

I should have been more explicit in the drawing in that the "out" line is also the "in" line.  I made the poor assumption that describing it as similar to the Torres control would clarify (note to self - no more assumptions!).  Per your "OTOH" this device is in fact intended to completely substitute for the normal tone control in a two pup tele style guitar.  So, the "out" would be connected to the top of the volume pot like a standard tele tone control.  The circuit is pretty much what I gather is almost exactly what the Torres switched mid-control circuit (and others) looks like.  When the p-p switch is down, the circuit will operate exactly like a conventional tone control with the .047 tone cap engaged (but for the linear 500K pot) and the mid control is out of the circuit.  The 470K resistor was added by me in an attempt to provide a resistive portion of the load of about 250K when the pot is fully engaged.  When the P-P switch is pulled, the normal tone control is removed and the mid control is engaged.

I gather, from looking at the circuit and from the simulations without benefit of other explanation of operation, that when the pot is pulled to engage the mid-circuit, the idea is to allow for swinging from a tone control with a .022 tone cap at one extreme and the mid filter at the other.  Near the center seems to be relatively pretty neutral over a wide range (Torres uses a special pot with a center detente I believe).  A little odd, but it does seem to work that way, except that the range of control is at the far ends of the linear pot with a large range that has little perceivable effect.  It is difficult to adjust, but there is a bit of a sweet spot there that I hope to exploit in Rev. 2.

So, this would not be used in addition to the normal tone control as a mid control in a tone stack, but in fact is THE tone control that switches from a normal tone control to a mid control.  Your comments are well taken with the assumption that this is to be used along with a tone control.  I agree that that makes little sense and over-complicates and duplicates circuit functionality.

I have been looking at the vari-tone circuits and Craig Anderton's passive mid control as well as the various incarnations of the Torres circuit for inspiration.  As a result, Rev. 2 of the control which I built last night but haven't installed in the guitar yet is pretty similar to what I think you are suggesting.  Here is what I currently envision for the whole circuit to look like.  Tele_Mod_Rev2.pdf (32.53 KB)

The 50K pot is a 10 turn pot that I had handy (Anderton uses a 50K pot in a similar location).  I have it initially set it to about 5K and can tweak it to change the Q of the tank to see if I can find a sweet spot that I like.  In this circuit, my concept is that when the switch is in, I have a normal tone control using a 250K audio pot.  When the switch is out, the RLC tank is engaged with a fixed resistor (50K pot) that I will tweak to see if I can find a nice sound.  Also when the switch is out, the tone control still works as a tone control, but with a .022uF cap instead of the .047uF cap.  I kept this concept so that I still have tone control when the tank is engaged and the tone adjustment is a bit farther removed from the frequency range of the mid-cut.  I'll experiment around with these values (and the value of the .039 cap) to see what I like best (if anything).

I haven't installed it yet, and it will likely on hold for a few days.  But I'll let you know what happens.

BTW, the 1.5 H inductor is the primary of a 42TL021 transformer (Mouser or Small Bear).  I have two devices for measuring inductance - neither of which do I have info on what frequency is used.  One reads out of range on the 2H scale and .9H on the 20H scale!  The other reads about 5H.  Obviously not able to determine what the actual inductance is for myself, but there is lots of stuff out there suggesting that at audio it comes in at about 1.5H.

Hope that clarifies.

Jerry

[b4nj0]

If you have access to a GDO or its modern FET equivalent, (from your posts I suspect that is possible?) you could dip it in parallel with a known capacitor? Lacking an inductance scale on any of my meters, that's how I do it. Does rely on a close tolerance capacitor that will fall into a range with sufficient band spread to give a sensible reading, but it will separate a 1.5H from a 3.0H etc.
e&oe

[jerrybass1955]

Good idea.  I don't have one, but may be able to get hold of one from some friends who are hams.  Barring that, I may just rig up a white noise generator and see where the dip occurs on my scope (that has an FFT mode).  I do have a friend with an audio generator that I can borrow that would probably do the trick to just look at the frequency where the output takes a dip.  It is an old HP with a frequency dial.  Mine is a cheap digital generator and won't do noise or a sweep.  I am also thinking of just getting a noise generator app for my phone and hooking that up to look for the dip.  I would like to know what the value is for future reference.  1.5H seems to be the word on the street, but who knows if the part changed to lower cost or whatever.
Jerry

[sumgai]

b4nj0,

A grid dip oscillator? Who woulda thunk it?   A nice throw-back to the days of yore, but as jerry points out, there's probably an app for that!

73's OM!

--------------------------------------------------------------------------------


jerry,

I like how you're thinking, this just might prove to be a workable design for more people than just you... take notes, pictures, and sound clips along the way!





sumgai

[jerrybass1955]

Turns out, I already had an app for generating noise, but never got around to making a suitable cord to use it as a source.  Maybe can do that this weekend.  I was able to work on a test fixture to try out things like this, and have it about 1/2 done.  I figure since this circuit is basically right across the volume pot when the volume is on full, I can make a little rig to just plug into the guitar jack with a short cord (with known capacitance) and experiment with the values without even having to open up the guitar (Knowing that the current tone pot will be a bit of a load, but I should be able to generalize fairly well).  Here is the fixture I am making.  Very simple, but with a short connection to the guitar, it might be a useful tool (probably something similar posted on the board somewhere else - I doubt I invented the idea!!!!).  Also, a 1/4 inch plug with alligator clips can be used to easily tap something at the binding posts into the circuit for other testing.

Here is the test box.  I'll take pix of the switched pot as I wired it up and post later.



Sorry about the image quality.  I'll have to figure out a better way to insert images.
Jerry

[jerrybass1955]

Finished up the test box last night and did a little testing.  Box works great!  Here are a few pix:




And a few more next post

[jerrybass1955]

More pix:



Last picture is the potential home for this circuit.  Just finishing up this build and pretty happy with it.  Used a Warmoth conversion neck and I like it!

Note there is a minor error in the latest schematic/wiring diagram for the mid control that I posted.  The blue wire to the outside pot terminal should go to the other side of the pot (unless you like the tone control to work backwards).  Maybe I'll post an update when everything is in final form.

I did some testing of the beast yesterday as shown in the middle picture above between guitar and amp.  I played around with the .039 cap value and by ear, the value appears to be about right.  I want to see what frequency is tuned on the scope though and see if I can get some improvement.  I tweaked the trim pot a bit.  At lower values, it seems that the highs disappear along with the mids.  By ear, around 40K ohms (30 to 50K is not bad) sounds pretty good without sucking out too much of the highs.  When I am able to look at the FFT on the scope, I hope I can see what is going on better and tweak it in more intelligently than just by ear.  I will need to pull the tone control out of the guitar completely to see what changes, if anything.

Basic evaluation: If I made no changes to it now, I think it would be a reasonable sound to have as a switchable option.  Will probably sound pretty good for a rhythm sound and certain leads.  Mids are definitely scooped out a bit.  I wouldn't replace a conventional tone control with this particular circuit though.  I don't think it is the sound you want to be stuck with, but it is a pleasant and somewhat unique sound.  The .022 cap for tone when the tank is engaged seems about right.

I'll post more when I am able to do a bit more scientific evaluation - maybe this weekend.

Jerry

[sumgai]

Jerry,

Oct 31, 2014 11:25:20 GMT -5 jerrybass1955 said:

..... and tweak it in more intelligently than just by ear.

I'll leave any remarks about particular juxtaposition to others, I ain't gonna touch it!

I will need to pull the tone control out of the guitar completely to see what changes, if anything.

Not very much, but probably a little bit. This is why there's such a thing as a no-load pot for the tone control circuitry. To my ear, it doesn't make much difference, but I'm only one out of... how many guitar players are there, again?

Mids are definitely scooped out a bit.

I wanted to point out another 'gotcha' that's gonna getcha - the amp itself always affects the tone, and absolutely always, in a different way than any lab equipment is gonna be able to predict. 5Spice and others are fine for comparing guitars to guitars, but when it comes to the real world, your ears, and only your ears, are what's gonna count. (OK, you can use other people's ears too, and I personally consider that a good thing, but what if the owners of those ears report different likes/dislikes... are you prepared to put up with that?)

Speaking of amplifiers.... Fender's tube-amp tone control circuitry is known for "notching" at about 300Hz. Their solid-state stuff is usually more flat, especially the later bass amps with tweeters. Marshall's tone stack tends to go the other way, seemingly to emphasize the mids, due to their chosen pot and cap values. Ditto for Vox amps. I can't speak for Gibson amps overall, but I know their Lab series specifically emulated Fender's notch, I'll leave it to you to guess why.

When it's all over but the screaming and hollering, your ears will be the deciding factor. My advice: don't get too wrapped up in the "ivory tower" world, looking at graphs and specifications and all that. Use them maybe as a starting point, but don't go overboard, and then forget why you're draining the swamp in the first place!

HTH




sumgai

[jerrybass1955]

I'm with you all the way Sumgai.  I hope to figure out if the mid scooping is happening at a higher frequency than I think is should (thus sucking out a bit of the highs).  That would confirm that the value of my inductor is on the low side.  Also seems to me that it should sound better with less resistance than 40K or so.  This also suggests to me that the highs are coming back as I reduce the Q of the tank.  If that is the case, I can see about tuning it lower and hopefully regain some highs with a higher Q and lower resonant frequency.
Tried to test it out with a phone app for generating a signal, but the output from the phones jack was too low.  So, I have to either boost that up or build a noise generator.  THe fun continues!
Jerry

[nikogo]

Hi jerrybass1955.
I would suggest some modification for your interesting all-in-one push-pull schematics.

1. Remove the 0.022 cap and 1M resistor from upper right contact and connect there inductance about 2H. The second end of coil connect to ground.
2. Remove all stuf from upper left contact and connect top left and top right contacts  with capacitor 0.1 to 0.3uF. 
3. Replace the 0.47 capacitor with 0.022uF, reconnect it to bottom right contact and remove 470K resistor.

Than with pot value about 250K in pull up position clockwise rotation will give you bass cut filter,
In upper CCW position will be mid-cut filter. In the middle top the filter influence will be insignificant .
In push down position CW rotation will give you a treble-cut filter, and CCW will bring to near unfiltered tone.

[nikogo]

The inductance coil can stay connected to the upper left contact with the second lead on ground.
Than CCW will be bass-cut filter, and CW a mid-cut one.
Bigger capacitor (0.1 -  0.3uF) will cut more mid-low tone leaving brighter tone with lower bass.
For single coil pickups better have bigger capacitor and smaller inductance (lower impedance).
For humbuckers the filter needs higher impedance, so cap = 0.1uF and higher inductance. (consider a coil from signal relay 24 - 48VDC) 

[Deleted]

very interesting thread, haven't read it all yet. I was just wondering, I ebay'd for some inductors in the 1-1.5H range and found little. What's the deal with those? Also I'd like your opinion, would such a circuit work with an active pup, like the Seymour Duncan livewire metal, which was designed and produced back in the 80s?

[nikogo]

The mid-cut filters should have resonant frequency in 150 - 500 Hz range.
So for any inductance could be found right capacitor. But impedance of such a filter would be different.
So its efficiency. The filtering effect depends very much on the pickup output impedance. 
For active pickups, my guess, described filter would have too high impedance and be not very effective.
At least in my guitar it does little job after the piezo pickup amplifier.

[nikogo]

The Phoenix Contact relay on DigiKey costs $22+

www.digikey.ca/product-detail/en/2961121/277-2390-ND/349672 

[Deleted]

Feb 23, 2015 7:39:09 GMT -5 nikogo said:

The mid-cut filters should have resonant frequency in 150 - 500 Hz range.
So for any inductance could be found right capacitor. But impedance of such a filter would be different.
So its efficiency. The filtering effect depends very much on the pickup output impedance. 
For active pickups, my guess, described filter would have too high impedance and be not very effective.
At least in my guitar it does little job after the piezo pickup amplifier.

thanx.