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Thread: Passive mid boost/cut control using Mouser audio xfmr

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    Supporting Member Steve A.'s Avatar
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    Passive mid boost/cut control using Mouser audio xfmr

    In my guitars I used the Dan Torres mid-boost/cut control based on the same Mouser audio transformer that Craig Anderton used in his book on projects for the electric guitar:

    Mouser Audio Transformer #42TL021
    Outer P(rimary) terminals 1.5H

    I removed the boost portion (on top of drawing) and wired the midcut to a pot in my 1978 LP 25-50 for a really slick control that cleaned up the sound. (I need to reverse engineer that mod to see what value resistor and cap I used.)

    Wired as mid-boost/cut in my strats there was considerable signal loss in supposedly neutral middle position.



    Steve A.

    PDF of circuit:

    dual_mid.pdfClick image for larger version. 

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    Last edited by Steve A.; 06-11-2018 at 03:48 PM.

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    Old Timer J M Fahey's Avatar
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    Yes, passive can only attenuate sound, so itīs reasonable to use them in attenuating some "ugly" frequency.
    When trying to "boost" anything, it only means "attenuate everything else".

    Just checked, https://leleivre.com/rf_resonance.html (very useful page) says you are attenuating 658Hz, a midrange frequency if I ever saw one, very reasonable.

    Congratulations.

    That said, active Electronics are so small and low powered (and have been so for decades, just check MM and their LM4250 based preamp) that they become cost effective to use for most everybody.

    Cost effective here meaning not price but small space and low power consumption.

    LM4250 which is still available is so "economic" on that account that it can be powered by a couple watch/calculator button batteries, if you canīt fit a 9V alkaline anywhere and of course donīt want to route a pocket for it.

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    Last edited by J M Fahey; 06-08-2018 at 01:10 PM.
    Juan Manuel Fahey

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    Supporting Member Steve A.'s Avatar
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    Thanks for suggestions and frequency analysis. Actually by mounting the mid boost/cut control on a push-pull pot it is pretty much out of the circuit when set to 10 in the mid-cut mode so I don't personally see a need to add a preamp to it. With the mid "boost" engaged it smooths out overly-bright bridge pickup nicely as needed.

    HOWEVER... I really do like the EMG After Burner boost circuit, but not at its retail price of $80! I picked one up for $40 and mounted it in a 2 control PRS SE about 10 years ago... I set the the pot to the desired amount of gain and used a pp tone pot to switch it on.*** (It was my stealth control very handy at blues jams where FX pedals were frowned upon!)

    http://www.emgpickups.com/ab.html

    http://www.freestompboxes.org/viewtopic.php?t=18479

    Can you recommend an LM4250 circuit with a trimpot for 0-20dB gain that could be mounted to a push-pull pot for volume or tone? Or alternately one that could be mounted to a pot like the AfterBurner?

    *** Afterthought: There is enough room in typical PRS SE control compartment to add a 3rd pot between the volume and tone controls.

    Steve A.

    P.S. A search for the MM preamp schematic turned up this thread with posts from you...

    http://www.diyaudio.com/forums/instr...nd-preamp.html

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Yes, passive can only attenuate sound, so itīs reasonable to use them in attenuating some "ugly" frequency.
    When trying to "boost" anything, it only means "attenuate everything else".
    Well, not exactly... I hate to oppose the mighty JM, but there is a circumstance in this circuits implementation that wasn't considered in your response. And that's the resonant peak frequency. It's the reason the "tone control all the way down" tone works for neck humbuckers. It cuts treble, yes, but it also lowers the resonant peak of the overall circuit and there is an actual boost to other midrange frequencies. Remember that the pickup is an inductive element.

    And to stay on the right side of the issue I will simultaneously back your advice by acknowledging that much is lost in the exchange. Indeed the center position IS NOT the stock sound of the pickup. And never could be for the reasons you stated.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by Chuck H View Post
    Well, not exactly... I hate to oppose the mighty JM, but there is a circumstance in this circuits implementation that wasn't considered in your response. And that's the resonant peak frequency. It's the reason the "tone control all the way down" tone works for neck humbuckers. It cuts treble, yes, but it also lowers the resonant peak of the overall circuit and there is an actual boost to other midrange frequencies. Remember that the pickup is an inductive element.
    FWIW I seem to get that effect only with certain humbuckers and certain value tone caps. With others I just get the treble cut without much of a boost at all.

    And to stay on the right side of the issue I will simultaneously back your advice by acknowledging that much is lost in the exchange. Indeed the center position IS NOT the stock sound of the pickup. And never could be for the reasons you stated.
    It was a real "holy shit!" moment when I tried bypassing the mid boost/cut control in the center "neutral" position with a jumper and realized how much signal I was losing! "Dan Torres lied to me!" But that was 25 years ago before I discovered AMPAGE!

    Steve A.

    P.S. It looks like the current incarnation of our forum software does not support BBS code in our signatures, Chuck. Your hanging [B ]s and [/B ]s look terrible...

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    Bent Member Chuck H's Avatar
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    I hadn't noticed that before you pointed it out. Interestingly the current font modifications still use [B ] [/B ] to induce bold script. Like systems that aren't compatible are stuck together.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Bent Member Chuck H's Avatar
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    FWIW I just ran some ltspice stuff with the circuit shown. The losses are bad even with the switch. But here's an idea. Buy a 500k center detent pot and then scratch through the trace connection on either side of the detent. Then the control is out of the circuit unless you rotate it into mid cut or mid boost territory and the detent tells you by feel (for stage use) that you turned it off. But really, the mid boost or cut only "work" at damn near full rotation anyway so why have them on a potentiometer? There's almost no pot range for useful adjustment as in partial cut or partial boost. Just varying degrees of other signal loss. I've used the mid cut circuit and thought it was fun and novel, but I don't think I'd install one again. I might install the circuit on a switch though. Like a standard tone control with a pull pot that engages the cut hardwired and leaves the tone control in place.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  8. #8
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by Chuck H View Post
    FWIW I just ran some ltspice stuff with the circuit shown. The losses are bad even with the switch. But here's an idea. Buy a 500k center detent pot and then scratch through the trace connection on either side of the detent. Then the control is out of the circuit unless you rotate it into mid cut or mid boost territory and the detent tells you by feel (for stage use) that you turned it off. But really, the mid boost or cut only "work" at damn near full rotation anyway so why have them on a potentiometer? There's almost no pot range for useful adjustment as in partial cut or partial boost. Just varying degrees of other signal loss. I've used the mid cut circuit and thought it was fun and novel, but I don't think I'd install one again. I might install the circuit on a switch though. Like a standard tone control with a pull pot that engages the cut hardwired and leaves the tone control in place.
    To heck with that spice stuff... I just disconnect the pot and listen for a pronounced change in volume. BTW have you checked out the new pp pots with PCBs and full sized pots? Another slick item is the 250k Emerson Pro Blend/No Load Tone pots...

    https://www.stewmac.com/Pickups_and_...-load_Pot.html

    Hey, I just checked out the tone control on my brand new 2006 Epiphone jazz box and it boosts the bass on my neck pickup really nicely! Hey it took me 25 years to learn to turn my volume controls down from 10 and another 25 years to do the same with my tone controls... Give me another 25 years and I might even figure out how to use the middle position on my Les Paul selector switch!

    Steve A.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Steve A. View Post
    Thanks for suggestions and frequency analysis. Actually by mounting the mid boost/cut control on a push-pull pot it is pretty much out of the circuit when set to 10 in the mid-cut mode so I don't personally see a need to add a preamp to it. With the mid "boost" engaged it smooths out overly-bright bridge pickup nicely as needed.

    HOWEVER... I really do like the EMG After Burner boost circuit, but not at its retail price of $80! I picked one up for $40 and mounted it in a 2 control PRS SE about 10 years ago... I set the the pot to the desired amount of gain and used a pp tone pot to switch it on.*** (It was my stealth control very handy at blues jams where FX pedals were frowned upon!)
    A friend of mine who runs "the Blues Club around the corner" ... literally , has a silver "Eric Clapton" Strat, with built in "mid boost" ... killer sound, both classic Strat "qwack" and powerful Humbucker style fat mids with gain and power to sparte ... and nobodyīs the wiser

    Can you recommend an LM4250 circuit with a trimpot for 0-20dB gain that could be mounted to a push-pull pot for volume or tone? Or alternately one that could be mounted to a pot like the AfterBurner?

    *** Afterthought: There is enough room in typical PRS SE control compartment to add a 3rd pot between the volume and tone controls.
    Ok, a custom design just for you:



    Now that I notice it, one drawing mistake: Gain does not go from 0 to 10X but from 1X (unity gain or 0dB) to
    10X or 20dB .

    Some details:
    1) drawn just using the NS LM4250 datasheet , no simulators involved (which I canīt use in any case, just paper, pencil and calculator)

    2) on purpose, didnīt even look at the Stingray schematic (which I havenīt seen for years) just to avoid being "influenced" ... will do that later.

    Step by step (sort of) design procedure follows:

    3) started by calculating a 9V alkaline battery self-discharge rate.

    LM4250 can be turned down to really really low consumption levels, .... fine if you are designing a 25 year long NASA Mission to Pluto (no kidding) where a palm sized solar panel must feed 200 LM4250 fueled micrometeorite sensors or whatever .
    But here itīs pointless making it consume less than battery self-discharge, so ....

    Official Energizer data : 5 year shelf life , 680mAH capacity.

    So 5*365*24H=43800 Hours.
    Self discharge: 600000uAH/43800H=14uA

    So choosing a very very very low consumption will needlessly compromise Op Amp operation ... but battery will die anyway in 5 years, so ......

    My circuit above "eats": 2uA across input bias resistors, 1uA through current setting pin 8 and about 8 to 10uA for the Op Amp itself. Try that Tubeheads

    Since I am doubling self discharge current battery life in the Instrument will be half shelf life, so some 2.5 years (30 months) even if always on .
    So you neednīt provide for a power switch.

    OR you may choose to power this with a stack of 3 button cells in series, CR32 come to mind, which will last over 156666 continuous hours or 1 Year 10 Months continuous use.
    These are worth a power switch, which will extend life to 8 years.

    As you see even the laziest most careless Guitar Player will still get a long life out of this.
    I suspect Music Man Stingray and such also enjoy such a long life.

    4) so from datasheet tables and graphs I chose/calculated and considering pre-amp should work fine even with battery voltage down to 6V:

    * 2 uA through biasing resistors. (9V across 4M4)
    * 8/10uA through Op Amp itself.
    * so 1uA through Pin8 so 560k from Pin 8 to ground.

    Preset current **heavily** influences parameters, in a bad way, so itīs better not to go too low unless indispensable.
    10uA is long life as shown above but op amp is still usable for Guitar duty ... forget Hi Fi of course.

    5) from graphs open loop gain is 40/50k ... for DC (Instrumentation), not a reference for us.

    Gain Bandwidth GWB product is 50kHz, so for 10X gain we have 5kHz bandwidth: just enough.

    6) it can easily drive 100k which is perfect, and start having problems with a 10k load, so we try not to go below 22k in any case.

    7) I chose 220k/22k gain resistors, so if 220k is a preset and is set to0,Op Amp sees 22k which it can manage.

    8) with 9V battery, preamp can put out 6Vpp which is fine, at least as good as a TL072 (if not better) so on that account output is not compromised by the low idle current.

    Ok, designing a PCB/perfboard layout, guitar installation and obligatory MP3s are on you

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    Juan Manuel Fahey

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    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by J M Fahey View Post
    A friend of mine who runs "the Blues Club around the corner" ... literally , has a silver "Eric Clapton" Strat, with built in "mid boost" ... killer sound, both classic Strat "qwack" and powerful Humbucker style fat mids with gain and power to sparte ... and nobodyīs the wiser
    I put that Fender mid-boost circuit board in a lapsteel with a Burstbucker HB, but with an important mod I found on-line which bypasses the board to get the stock sound of the pickup with regular guitar pots. Best of both worlds! And for your friend no embarrassing sound of silence when the battery eventually dies in the middle of his big solo for the night...

    I'm pretty sure that I posted the instructions for that mod here around 2011 but with the limited search functions good luck finding it... (I hope that tboy can find a plug-in with a more functional search engine which would be a big upgrade for MEF. One advantage of using forum software with a big customer base... as you might remember tboy had written the forum software for AMPAGE which was head and shoulders above the canned software 20 years ago. Eventually the other guitar and amp forums started using features that tboy had introduced here.)

    I was going to buy one of those EC strats maybe 15 years ago but got a Washburn instead with a Wilkinson vibrato and put boutique pickups in it... and never looked back.

    Ok, a custom design just for you:
    Now that I notice it, one drawing mistake: Gain does not go from 0 to 10X but from 1X (unity gain or 0dB) to
    10X or 20dB .
    Oops... my bad! I did want it to go from unity gain to ~20dB.

    Ok, designing a PCB/perfboard layout, guitar installation and obligatory MP3s are on you
    Huh?!? I thought you were going to mail me a custom PCB board...

    The circuit looks simple enough to mount on my favorite: the Radio Shack pcb for DIP ics and relays. Or on veroboard. BTW on the modded EHX Soul Food klone I bought the guy used a small piece of veroboard to wire up the added bass control. Pretty slick!

    One more question... I was reading how the LM4250 has been discontinued and that it goes back to the 70's. Is there a newer chip that might work better in your circuit or better to stick with the tried and true?

    Thanks for all of your help on this!

    Steve A.

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    Last edited by Steve A.; 06-13-2018 at 07:45 AM.

  11. #11
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Steve A. View Post
    Oops... my bad! I did want it to go from unity gain to ~20dB.
    Well, thatīs what it does

    Huh?!? I thought you were going to mail me a custom PCB board...
    I DID make one for you, of course, solid gold tracks and embedded diamonds in the corners .... only condition is you have to *swim* to Argentina to pick it up.
    Iīm barely half a block from the old Buenos Aires river port so Iīll be waiting there for you.
    The circuit looks simple enough to mount on my favorite: the Radio Shack pcb for DIP ics and relays. Or on veroboard.
    It IS a simple circuit , Iīm a Minimalist, remember? Just check the FET VCA in the other thread ... 1 aadded resistor and 1 added Fet per channel. Oh, and 1 modded value resistor to restore unity gain.
    One more question... I was reading how the LM4250 has been discontinued and that it goes back to the 70's. Is there a newer chip that might work better in your circuit or better to stick with the tried and true?
    I designed around an LM4250 because thatīs what you mentioned, personally I am a fan of TL06x because they are "almost normal" Op Amps with 1/5 standard TL07x needs, always available, and cost pea nuts.

    Power consumption is not *that* low (I think itīs 200uA per Op Amp) that you can leave them continuously ON but with adequate switching you again get years ... again up to battery shelf life.

    I have seen instruments with embedded rechargeable batteries which can be topped up when needed ... in my book BAD idea, when needed they will invariably be discharged and not usable until next day.

    I guess there must be some modern equivalent or substitute of LM4250, should check around.

    Thanks for all of your help on this!

    Steve A.

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    Juan Manuel Fahey

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