Building a simple inductor bandstop filter.

[stonerbox]

What's up all you wonderful, lovable crazy freaks!

I'm trying to build a super simple passive inductor+capacitor+resistor+potentiometer bandstop but it's really kicking my noob ass. I've been trying to build the Duncan Tone Stack Calc bench mid filter but modified for a narrow 200hz cut but all that I seem to end up with is either a LPF + volume drop or just a volume drop with really no notable change in tone (except maybe a tiny bit of loss on the high end side, like somewhere around the 8-15Khz area). I've searched the forum quit a bit but not found anything on these passive filters, except for the Varitone and lot's of active filters. So please, if you guys would enlighten me into new vast heights of consciousness it would really mean the world to me.

So what have I tried so far? Well, all of these, in this particular super failing order).

1st and 2dn attempt (fail)
10K LIN Pot + 68n Capacitor + Inductor 10Uh (10%) [Brown / Black / Black / Silver]


3rd attempt (fail again)


4th attempt - The one on the right (once again fail.. F[]CK!)


5th attemp - the one on the left but modified. Fail again, craaaap! I' need some assistance before I  (yes, in hyperbol) tear my @#$%ing head off.

[Mark Hammer]

Regards from somewhere much colder (we're -20C today, "warming up" to -12C)

Why, may I ask, do you want to use an inductor, as opposed to an RC-based filter?  Not a criticism, just curious about your motivation for not using something that can be more precise, and less costly.

[armdnrdy]

Regards from somewhere much colder (we're -20C today, "warming up" to -12C)

I have the door to my office open...and I'm freezing this morning! It's 57F! I can't wait for the weather to warm up a bit!

I believe that the subject of inductors in stompboxes has been brought up here a "few" times.

http://www.diystompboxes.com/smfforum/index.php?topic=107370.0

The reasons that Mark outlined and "noise" issues seem to keep them out of designs.

If you scroll down on this link: http://sound.westhost.com/dwopa.htm

You will find a simulated inductor that uses an op amp, capacitor and a resistor.

[stonerbox]

Regards from somewhere much colder (we're -20C today, "warming up" to -12C)

Why, may I ask, do you want to use an inductor, as opposed to an RC-based filter?  Not a criticism, just curious about your motivation for not using something that can be more precise, and less costly.

Hah! You poor sons of a bitches, stay brave! I miss that kinda weather here in sexy Sweden. Been (too) many winters since we got those kind of temperatures.. or at least here in the southern parts of the country. Up north it's pretty common, though.

No offense taken! I choose inductors mainly because I thought that they would have a much more narrower Q than e.g the big muff style tone setup (minus the pot) and that passive inductor filters are faster, easier to build because of less components then a gyrator circuit. EDIT: And also because I'm a badass who wants.. no needs to do everything different from everybody else.  

I laughed so hard at this:

They are
(1) big
(2) expensive
(3) not easy to standardize
(4) avoided like the plague by industry for reasons 1-3
(5) and therefore not available for DIY as leftovers.

............
I believe that the subject of inductors in stompboxes has been brought up here a "few" times.

http://www.diystompboxes.com/smfforum/index.php?topic=107370.0

The reasons that Mark outlined and "noise" issues seem to keep them out of designs.

If you scroll down on this link: http://sound.westhost.com/dwopa.htm

You will find a simulated inductor that uses an op amp, capacitor and a resistor.

I have searched and read a lot on inductor filters (mainly notches/bandstop) for the last two days, both here and on other sites but without any luck. I'll be sure to check out the links you provided, thank you Larry!  Yeah, I figured that this had been on topic a couple of times before on the forum (which is a grave understatement ).

[PRR]

> a much more narrower Q

Audio-range L-C filters will do "medium Q". The parasitic resistances do not allow for easy HIGH-Q filters.

There ARE hi-Q notch R-C filters, just not in the Muff topology. Bootstrapped bridged-Tee can notch as deep as you wish.

A high-Q notch filter is very nearly inaudible in general music. Your ear hears what is there, not what may be missing.

On one hand, all your "fails" are progress--- you seem to have eliminated many paths.

However "fail, fail again" is not good note-taking. Are the values even in the audio range? What came out... no sound, screech, or full sound with no notch? Are you sure there was no notch? What are your drive and load conditions? Many of these plans are sensitive to how they are used. Did you run sine-wave frequency sweep or just slap a guitar? Did you really have a 3.3H inductor, or does "modified" mean any-handy-value?

[stonerbox]

> a much more narrower Q

Audio-range L-C filters will do "medium Q". The parasitic resistances do not allow for easy HIGH-Q filters.

There ARE hi-Q notch R-C filters, just not in the Muff topology. Bootstrapped bridged-Tee can notch as deep as you wish.

A high-Q notch filter is very nearly inaudible in general music. Your ear hears what is there, not what may be missing.

PRR!

I do a lot of recording and first and foremost mixing and I strongly believe that the power of discreet/"invisible" notching is not to be taken lightly upon. It can make a world of difference if applied right and together with some "masking" equing (boosting the wanted freqs nice and wide in a noticeable manner). I don't want the notch as an effect but rather as a touch of improvement, so a narrow Q ( 7-10 ) is exactly what I'm after!

On one hand, all your "fails" are progress--- you seem to have eliminated many paths.

However "fail, fail again" is not good note-taking. Are the values even in the audio range? What came out... no sound, screech, or full sound with no notch? Are you sure there was no notch? What are your drive and load conditions? Many of these plans are sensitive to how they are used. Did you run sine-wave frequency sweep or just slap a guitar? Did you really have a 3.3H inductor, or does "modified" mean any-handy-value?

Yeah, I'm certain I'm learning something here.. as you said I do eliminate possibility. But I've started to wonder if I got the uH of the inductor wrong or just the basic layouts of the schematics with pot and everything.. I don't really know to be quite honest.
I've tried two different inductors. One small resistor-like with the colors of a 10uH 10% [Brown / Black / Black / Silver] and a bigger round black cylinder type with the marking 101 (100uH) on it. In order yo get both inductors to work at 200hz, but at separate times of course, I used a 68n cap for the 10uH and a 6.8n for the 100uH (which gave the correct response in the Tone Stack calculators). EDIT: Tested it with a guitar going straight into my board.

Right now I've given up on the hope of inductors and are trying to get this: http://www.divshare.com/download/launch/17679042-b87 spice simulation to do my 200hz cut. No too sure if I can get it to cut in a narrow Q though. I'm also stuck on the math (freq_filter=1/(2*pi*R11*C3) but I'll keep on it 'til it comes out right.  

[PRR]

Without doing the math.... "generally" inductors in uH sizes are useless in audio.

Ah, lets do math.

> 6.8n for the 100uH

6.8nF with 100uH is 193,102Hz. This may not be audible?

> correct response in the Tone Stack calculators

I just ran Duncan, Bench, with 6.8nF and 100uH in the mid-net. I get a high-shelf response controlled only by the 6.8nF, the 100uH can be varied a lot without any effect, and there is no resonant peak visible. When I change the 100uH by a factor of 1,000, to 100mH, I do get a 6KHz resonant peak. This makes sense: 193KHz/6KHz is about 30:1, a coil-only change of 1,000:1 would give a resonance shift of 31.6:1. The 6.8nFd+100uH case is far off-scale in Duncan.

Math.... uh...  100uH * 5,000Hz * 6.28 is, um, 3 Ohms.

A simple 5KHz high-cut would be 100uH series, 3 Ohms shunt to ground. Source must be much less than 3 Ohms, load much greater (or included). You could do a loudspeaker high-cut this way, but all other audio points will be much higher than 3 Ohms so there would be "no effect".

Let's use a Reactance chart.
https://pdf.yt/d/5zi3opizxxXfSpEC  330KB PDF
(Download it: the in-browser view is all smooshed.)

Follow the 100uH slant-line down to the 4000Hz vertical line. Put a dot. Read the other slant down to where the uF numbers are: 10uFd will resonate with 100uH at 4,000Hz. Read left, Xl = Xc = a little over 3 Ohms. For a Q of 10 the series resistance must be less than 0.3 Ohms, the shunt must be over 30 Ohms. Still silly-small impedances to work at.

Measure the DC resistance of that coil. I bet it is much-much more than 0.3 Ohms. it can not give a Q of 10 at 4KHz, and certainly not at any lower frequency.

The reactance chart is good for quick ball-parking values. "Audio processing" is around 1KHz and usually 1K Ohms to 10K. Find 1KHz at bottom, read up to 1K and 10K at left. Now read up-right to inductance scale. We find 0.1H to 1.0H. Checking 100Hz we find 1H to 10H. At 10KHz we need 0.01H to 0.1H.

We need inductors 0.01H to 10H, or a range around 0.3H.

Your 100uH is about 3,000 times smaller than it wants to be for audio.

It is surely scaled for use at frequency 3,000 times higher than mid-audio, above 1MHz.

(Even for AM radio we would use 1mH which is 1000uH.)

You want REAL coils. A 10H choke looks like a Power Transformer. The small choke used in many Fender tube amps is very-roughly 4H. The 120V side of a small power transformer may be roughly 2H.

[stonerbox]

Measure the DC resistance of that coil. I bet it is much-much more than 0.3 Ohms. it can not give a Q of 10 at 4KHz, and certainly not at any lower frequency.

The reactance chart is good for quick ball-parking values. "Audio processing" is around 1KHz and usually 1K Ohms to 10K. Find 1KHz at bottom, read up to 1K and 10K at left. Now read up-right to inductance scale. We find 0.1H to 1.0H. Checking 100Hz we find 1H to 10H. At 10KHz we need 0.01H to 0.1H.

We need inductors 0.01H to 10H, or a range around 0.3H.

Your 100uH is about 3,000 times smaller than it wants to be for audio.

It is surely scaled for use at frequency 3,000 times higher than mid-audio, above 1MHz.

(Even for AM radio we would use 1mH which is 1000uH.)

You want REAL coils. A 10H choke looks like a Power Transformer. The small choke used in many Fender tube amps is very-roughly 4H. The 120V side of a small power transformer may be roughly 2H.

Well, that explains a lot! Why it sometimes only resulted in a volume drop and made no change what so ever with in the 10hz-20Khz response, it was working way out of hearing range! And why I sometimes also just ended up witha a LPF ( probably working around 5 Khz, just like you said ).
Now, inductors are still somewhat kind of a new thing to me, need more reading and basic understanding of them, but you help me a lot on that front. A good thorough kick off. I'll leave them for now and especially when it comes to filtering out audio stuff.

Measure the DC resistance of that coil. I bet it is much-much more than 0.3 Ohms

The black cylinder type with the mark 101 on it and the resistor-like one (0000) both reads 2 Ohms.



Great, great input PRR! I'll send a bunch of love and some sexy Swedes your way!  

[tubesimmer]

There's an online LC filter value calculator at

http://www.1728.org/resfreq.htm

You can play with values of L, C or f to place yourself in the audio range - e.g., 0.7mH inductor, 22uF cap resonates at 1280Hz

I'm making one today to try in an OD box mod.

Fyi, new here & I'm -not- associated with 1728.org, I don't even know what the site is for!
I mention that site only because the other online LC calcs I've tried gave "NaN" results when I added values with decimal places etc., so I found them pretty frustrating.

[stonerbox]

There's an online LC filter value calculator at

http://www.1728.org/resfreq.htm

You can play with values of L, C or f to place yourself in the audio range - e.g., 0.7mH inductor, 22uF cap resonates at 1280Hz

I'm making one today to try in an OD box mod.

Fyi, new here & I'm -not- associated with 1728.org, I don't even know what the site is for!
I mention that site only because the other online LC calcs I've tried gave "NaN" results when I added values with decimal places etc., so I found them pretty frustrating.

Don't sweat it! I never stumbled upon that calculator the last time around but I'll definitely make use of it if I ever give inductors another shot. Thanks!


Even though this update may lean towards necro-posting it's all good, you're just trying to help out.

[punkbass]

How bad do you want the filter to be passive? , because you could just put a highpass filter, then an opamp with the +on the output of that filter, the - on the output of the opamp and the output of the opamp also into a lowpass filter, i'd make the resistors variable so you can tune and mix with the range.

[garcho]

^+1

[tubesimmer]

Just to finish off with my results - I wound up pursuing 2 bandstop circuits for their simplicity:

1) A simple LCR in series to ground  --- 0.68uF --- 50mH ---- 1K --->     this worked very well, love the sound and was the final mod.

2) a simplified 'DUAL' version of the BRIDGED T active RC notch found at http://www.geofex.com/article_folders/eqs/paramet.htm#bridged_t



The addition of the isolation resistor allows for a Depth bypass R to change the dB depth of the notch.

[axemanv90]

On your "4th attempt on the right" circuit.  The "R" is your pickup's DC resistance and will affect the Q of the filter.  You should not put a resistor here if this is connected to your guitar's pickups.   A 16K DC pickup will have a lower Q than if it and another pickup are selected in parallel.

To go with your 10H inductor, a 3300pF capacitor will give you a notch frequency of 876Hz.  That would be useful to add "quack" when coil-tapping humbuckers with both pickups on.  It would also be useful for a pickup with a higher DC resistance  (12K & up)  full on for a scooped midrange distortion grunge sound.  Quack is my name for a standard strat sound with the bridge and middle pickups selected.

A 560pF to 680pF capacitor with that 10H inductor will give you a notch at around 2Khz. 
That would be useful to add "cluck" when coil-tapping humbuckers with both pickups on.  Cluck is my name for a standard strat sound with the middle and neck pickups selected.

A 10H inductor might be a bit much and make too narrow a notch to notice much difference.  You might want to try 1-2 Henrys with higher value capacitors.


You can run this circuit off a standard tone pot circuit in place of the cap, but it won't do much more than turn it on and off.

I am always interested in getting compact 1-3H inductors, so if you know of any, please post!

[hymenoptera]

EDIT: Tested it with a guitar going straight into my board.

On your "4th attempt on the right" circuit.  The "R" is your pickup's DC resistance and will affect the Q of the filter.  You should not put a resistor here if this is connected to your guitar's pickups.   A 16K DC pickup will have a lower Q than if it and another pickup are selected in parallel.

I was going to add the same thing. You have to take into consideration the inductive nature of pickups. To eliminate this and get predictable responses from filter calculators will require buffering.

The only reason I haven't dug too deep into LC filters is because I still don't have an LCR meter.

[Granny Gremlin]

I believe that the subject of inductors in stompboxes has been brought up here a "few" times.

http://www.diystompboxes.com/smfforum/index.php?topic=107370.0

The reasons that Mark outlined and "noise" issues seem to keep them out of designs.

Old post I know but I thought this would be relevant to the discussion.

The noise issues can be eliminated by making a humbucking inductor (2 series inductors, reverse wound to each other).  Gibson did this in the EB2, EB3 and Ripper basses.  In the EB basses, pre70-72 they also included faraday shields (part# GA-90C or GA-90-1C pre 72).  All basses after that (Ripper existed only after) had pretty much naked coils (part# 70-442).  The inductance of this part (series 1 of the EB3 and EB2 at least) was 15 H and was used in a passive second order high pass filter (along with a 0.02uF cap) to tame (some would say emasculate) the sub-woofing lows of the mudbucker Sidewinder bass pickup.  I had calculated the rolloff freq for this at one point but totally forget now what it was.

Pre-72 EB3:


Post 72 EB3:


Ripper: