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Author Topic: Custom LPF build is distorted  (Read 15774 times)

teemuk

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Re: Custom LPF build is distorted
« Reply #15 on: August 06, 2010, 10:42:20 AM »
I don't think there's an easy way around it. It's an RC filter after all, so for four of them it makes sense you need to vary four "R" 's.

Possibly you could replace the ordinary resistors with a resistive element that you can control "remotely" (e.g. LDR, solid-state device, etc.) with just a single control but that will make the circuit rather complex.

Some parametric EQ circuit tweaked to operate only at proper low frequency, on single band, and only in "cut" mode, likely is a better alternative than the convential filter circuits. Those will often feature a "gyrator" circuit - a simulated inductor - that allows to tune the L of a LC filter, hence making possible the adjustment of both frequency and Q of the filter.
« Last Edit: August 06, 2010, 10:51:10 AM by teemuk »

DJPhil

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Re: Custom LPF build is distorted
« Reply #16 on: August 06, 2010, 01:42:57 PM »
Just wanted to throw in my 2pf.

I've been neck deep in filter literature lately for one reason or another. It seems that as a general rule analog filters represent a trade-off of adjustability, performance, and component sensitivity. As I learned more I remember thinking that the engineers were crazy, after all, a simple tone stack is a bare handful of (usually cheap, low performance) parts. They were lamenting the need for several precisely matched capacitors and resistors per circuit. Ok, we don't need that level of precision, but something's off.

In studying some of the passive tone stacks I found that often the settings were interdependent, i.e. one filter's settings could change the response and basic function of another. The engineers would find this messy heaving of load around a circuit to be insanity, but the early amp designers didn't care about precision. They just wanted a low parts count circuit that'd be as flexible as possible. I suspect a lot of their work was experimental 'see what it sounds like' sort of exploration, and when you could get six components and two pots together to make an interesting range of tone it was time to celebrate.

It was a bit of a revelation to me, but in hindsight it should have been obvious that these two camps had different goals. I'm sure I've read exactly this sort of breakdown before, but it didn't sink in for me until a few days ago as I was trying to make different values of pot work in a tone stack.

Hope that helps. :)

shinychrome0

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Re: Custom LPF build is distorted
« Reply #17 on: August 08, 2010, 04:37:39 PM »
How about a design like this then?  I'll play with values on the first stage on a breadboard first, and then just have two knobs on the second stage(the 10k resistors) for adjusting that one.

shinychrome0

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Re: Custom LPF build is distorted
« Reply #18 on: August 08, 2010, 05:33:01 PM »
ok new problem now.  I layed everything out on a breadboard just to test, and i didn't have the right value caps, so i made it according to the schematic below just to test with.  Apparently, i'm getting some sort of HF oscillation, as the opamp quickly became to hot to touch and stopped making any sound.  It worked right for about 5 seconds though.  How do i fix the oscilation?

shinychrome0

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Re: Custom LPF build is distorted
« Reply #19 on: August 08, 2010, 08:47:12 PM »
ok i put  68 ohm resistors in the feedback loop to kill the feedback(and replaced the chip, which was apparently fried) and now it works great!  I'll just have to play with some capacitor values to find the right cut off point, and then i'll be ready to solder!

DJPhil

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Re: Custom LPF build is distorted
« Reply #20 on: August 08, 2010, 08:58:53 PM »
ok i put  68 ohm resistors in the feedback loop to kill the feedback(and replaced the chip, which was apparently fried) and now it works great!  I'll just have to play with some capacitor values to find the right cut off point, and then i'll be ready to solder!

Excellent! Here's a calculator that might help. Sallen-Key Low-pass Filter Design Tool

There's a bunch of good filter calculators on that site.

Hope that helps.

J M Fahey

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Re: Custom LPF build is distorted
« Reply #21 on: August 09, 2010, 08:49:43 AM »
Well, just off the top of my head, no calculators, your close shave3 looks like an 8KHz, 24dB/oct lowpass.
Is that what you are trying to achieve?
By the way, it should not oscillate, being that each stage is unity gain, so your problem must be another one.
Using 68r resistors in the feedback loop , or a piece of wire, is the same.
If anything, you can use that 68r (the "standard" one would be 100r) between the last op amp output and the output jack, as to isolate it from output cable capacitance.
Active filters are fascinating subjects, study them because they are one powerful mean of tweaking sound, specially in "speaker simulators"
There's an old, yet "the Bible" type reference book: "Active Filter Cookbook" , which has it all between 2 covers; you can buy it cheap, used, from Amazon.
There is also another similar reference book: Op Amp Cookbook.
Best stuff. :tu:

phatt

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Re: Custom LPF build is distorted
« Reply #22 on: August 09, 2010, 08:56:54 AM »
hi Shinnychrome,
                Once you grasp what is needed I doubt you will have any need for adjustable stuff.

Once the toneshape is created the rest can be done with simple stuff.

ToneBrake circuit;
 I built 6 of these some years back and sold all bar 1 unit which I still have.
This one worked wonders after my old Quadraverb unit.
Worked best in a recording situation. Most of the big name rack stuff will benifit greatly when recoring.

On clean sounds it's hardly inspiring,, but gezz once you get into heavy OD's it really becomes obvious.
You need to insert this **AFTER** Dist or raky gear, not before.

I found this thing to be a great learning tool ,,not only for the electronics but you start to realise just how critical tone shaping is.
As one chap stated with eyes wide open after using it for some recordings.
"WOw I never realised just how much Hi Freq crap just destroys good guitar tracks,
an that magic box cleans up the mess".
This chap owned Masses of expensive recording gear BTW.
Needless to say he asked me to build one for him.:)
One thing I would change (Now that I've learnt a little more) is the output really needs a buffer.

If you wish for more things to look at check my Schmo page here;
http://www.ssguitar.com/index.php?topic=1648.0
The *DDC* there is both an OD unit and Cab sim all in one neat little circuit,
though a little more complex.
Cast an eye over the PhAbbTone circuit ,,delivers up to 30Db notch cut @ 400Hz  8)

One thing you might find useful is wedging in a simple Graphic EQ after the Cab sim.
This will give you access to many of the sounds you may wish to nail.
GEQ without the Cabsim box *first* will not be as convincing.

GEQ before cab sim will help but I've found it much simpler to run an old fashioned passive EQ stage before dist/OD >> then Cab sim >> then GEQ.

Truth is there are just so many ways to do it and some will be better than others but for me the home made equipment I've assembled/ collected over the last 10 years is all basically cheap and simple gear.

I personally refuse to lay down big money on gear that is often only marginally better than what I've built for very little outlay.

BTW I'm not alone in this manner of thinking as there are many pages on this at Amptone;

Try starting here; http://www.amptone.com/index.html#eqconcepts

It will take forever to read,, ALLLL TEXT (sorry no fancy stuff) and I must say hard to keep track of,
a little too wordy in places but a 100 ideas to give you insight as to how to approach things.


Heres' a little snippit I took the liberty of lifting for you.
________
EQ concepts and pre-distortion EQ
John Murphy, chief engineer for Carvin Corp., wrote "the pre-clipping frequency equalization and post-clipping EQ are absolutely critical adjustments. Once you have a well-behaved clipper -- even if it's just simple diodes, as in the stomp boxes -- it is the precise combination of pre- and post-clipping EQ that mostly determines how an amp sounds. The 'secret' of the best sounding guitar amps lies in the pre-clipping EQ response curve."

Van Halen's guitar tech recommends an EQ pedal above all, as the most valuable pedal, in his book Guitar Gear 411: Guitar Tech to the Stars Answers Your Gear Questions, pp. 75-76.
________

Now I'll just add this to give you a head start;
The trick is NOT about making the BEST tone circuit (i.e. I've just built the best EQ circuit on the planet)
and expecting it to deliver gold.

The best results will be found in a combiation of different circuits and shapes.
What you hear from the speaker is combination of many shapes all imparting their particular alteration to the sound produced.

Hence my simplemans approach is old styled passive tone in front> Dist> Cab sim> GEQ > Main Amp.
Have Fun,, Phil.
« Last Edit: August 09, 2010, 08:59:10 AM by phatt »

shinychrome0

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Re: Custom LPF build is distorted
« Reply #23 on: August 09, 2010, 09:18:40 AM »
For the moment i've got all the tone shaping i could ask for.  I use a pod xt live as the heart of my rig, and run it through a power amp into an actual cabinet.  However, i prefer a very smooth mellow sound, Carlos santana style, and love the effect i can get with a low pass filter after the POD.  I just have a passive single pole filter i use now, with some caps on a six position switch.  I don't need much adjustability, its mostly a set it and forget it effect in my rig, but sometimes for recording i'll sweep it around while layering guitars, some with more grit, some with hardly any.  So if i can just find a cut off point that is slightly below where i want it, i can then back it down by turning the two 10k pots, and find the absolute tonal sweet spot.  So adjustability is more for the sake of having it than for any actual necessity.

phatt

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Re: Custom LPF build is distorted
« Reply #24 on: August 10, 2010, 08:24:38 AM »

I don't have a Pod handy but I doubt a passive setup could even come close to what the above circuit (or Similar) can do.

I'm only offering advice having been there done all that,,if you catch my drift? 8|
Phil.


shinychrome0

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Re: Custom LPF build is distorted
« Reply #25 on: August 10, 2010, 02:56:19 PM »
Yeah the passive filter was a good start, but its not quite good enough. 

Ok so here's where i'm at.  I just finished getting everything transferred to a board, and i must be close but not quite right.  There is a big volume drop when i kick in the filter (probably from those 68r resistors that i put in.  I'll be removing those shortly) and it does not appear to be cutting anything.  Its passing everything as far as i can tell.  It should be fairly obvious if it is cutting or not.  The cut off point should be at about 3000 hz with both knobs dimes.  Any suggestions?  I'm confused how it could be just acting as a buffer without also acting as a filter.

shinychrome0

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Re: Custom LPF build is distorted
« Reply #26 on: August 10, 2010, 08:54:44 PM »
Here's the updated schematic by the way.  Just the Capacitors and the power section have been changed to make use of a DC adapter.

There's a considerable volume drop when the pedal is first engaged,(even without the feedback resistors) and then it comes back up slightly after a second or two, but not back to full volume.  Isn't this supposed to be a unity gain circuit?  And shouldn't the maximum setting be pretty obvious?  With a cutoff of 2.3khz, i would think it would be.  I can tell there is a slight cut, but it sounds like its way higher up than it should be.  I'm a little lost here.
« Last Edit: August 10, 2010, 10:05:45 PM by shinychrome0 »

phatt

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Re: Custom LPF build is distorted
« Reply #27 on: August 10, 2010, 10:49:57 PM »
Hi SChrome,
                You have no gain block only a filter which is lossy by nature.

Take a look at the Tonebrake, the 2nd stage is where all the gain happens.
Without that it's worse than a passive setup :(
Phil.

shinychrome0

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Re: Custom LPF build is distorted
« Reply #28 on: August 10, 2010, 11:24:57 PM »
Wow.  Well thanks for mentioning that now that i have no space left on the board.  It would have been nice to know something like that a week ago.  Another project for the scrap bin.  Its useless as it is now.
« Last Edit: August 11, 2010, 09:48:43 AM by shinychrome0 »

J M Fahey

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Re: Custom LPF build is distorted
« Reply #29 on: August 10, 2010, 11:41:46 PM »
Hi shynicrome.
Your schematic is basically right ... maybe the Protoboard real world version has some real mistake.
Some doubts (I understand you drive it with your Pod, it's impedance is low for a passive guitar connected straight and that would cause an important level loss)
1) The frequency is right, around 3400Hz, which is very audible, specially on distorted sounds.
At 24dB/octave even more audible.
Will sound similar to using a 15" bass speaker for your guitar.
2) To "engage it", it should always be connected to the Pod output, and a switch should select between taking signal from the input jack or from the output of the filter.
3) For now forget about making it tunable, just get it working and after that, change whatever capacitors you need, but leave it for a second version.
4)Get it working on Proto first.
Good luck.

 

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