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Author Topic: Custom LPF build is distorted  (Read 15773 times)

shinychrome0

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Custom LPF build is distorted
« on: August 05, 2010, 07:33:51 PM »
I've been working on designing and building a 5 cascaded stage low pass filter , and it doesn't exactly do what i want it to.  The schematic is attached.  There is some output, but it is sputtery and badly distorted.  Is there something in the schematic that shows a design problem?  I've been staring at this thing and prodding with my meter all over to make sure its built exactly to the schematic, and nothing is touching or shorted where it shouldn't be.  The schematic is attached.

J M Fahey

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Re: Custom LPF build is distorted
« Reply #1 on: August 05, 2010, 08:38:14 PM »
Well, yes, a couple ones.
1) It's not biased, and ground is unrelated to + and - rails.
You have an end to end 9V power supply, which is floating.
You fix that by wiring a , say, 10K resistor from the positive end of the battery to ground, and another from the negative side to ground.
*Now* you have +4.5V and -4.5V rails. Also wire 100uF capacitors from each rail to ground (pay attention to polarity).
*Now* U6, your first op amp, is biased to ground, *if* your sound source/generator provides a resistive path to ground and does not have a DC component.
I'd wire a 1 M resistor from U6 + input to ground.
All other Op Amps, being direct coupled, will follow it.
2) I have a doubt with the value of R2/3/nn/5 (nn=unlabeled) ┬┐are they 100 r (ohms) or you meant 100Kr?
R6 is labeled 10000 which I think means 10Kr.
3) Did you mean to wire R1 to -V (-4.5V)?
Good luck and correct those small doubts.

shinychrome0

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Re: Custom LPF build is distorted
« Reply #2 on: August 05, 2010, 09:24:09 PM »
1) well the battery is stupidity on my part.  I was planning on running off of a 9v DC adapter, but i was just using a battery for testing, and COMPLETELY forgot about the adjustments.  Would the 9v supply also need the resistors and capacitors to make it a bipolar supply, or can i just use it directly?  And what exactly do you mean about biasing?  Do i just need the 1 meg resistor to ground on the first stage to have everything covered?
2)the first 4 resistors are all 100r, and the 10k is a 10k log pot.
3)there is no r1.  I don't know what it was but its apparently no longer there.

J M Fahey

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Re: Custom LPF build is distorted
« Reply #3 on: August 05, 2010, 10:14:46 PM »
Hi Shinychrome.
1) Yes, *everything* needs to be biased, which really means "where do I stay when there is no signal present?"?
Op Amps are simplest to bias but even they need to be provided in certain approppriate pin a certain voltage.
Besides, voltages always exist referred to some other point, that's why meters have two probes which go to different places.
You always measure voltage (more precisely difference of potential) between two points.
For comparison, you measure temperature or pressure or magnetic field intensity or PH ... etc. on a single point.
2) This type of circuit calls for two voltage rails, referred to a third wire called ground.
If you do not use a battery but an external single supply, you will still need to "split" it into 2 rails.
3) Your input jack will have a hot pin connected to the input and another to ground.
Your output jack will do the same, but connected to output pin.
100r is the minimum resistor that can usually connected to a regular op amp output. It'll be happier with, say, anything above 470r or 1K. Your capacitors will be smaller (cheaper) too.
4) You need that 1M in the input for when you unplug your guitar or whatever.
5)Congratulations for getting your feet wet.

shinychrome0

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Re: Custom LPF build is distorted
« Reply #4 on: August 05, 2010, 11:07:55 PM »
Ok cool. So my last question then is how do i split that supply?  Something like this, with the 9v from the wall wart going where the battery is?

http://www.generalguitargadgets.com/richardo/distortion/bias2.gif

or this?

http://www.generalguitargadgets.com/richardo/distortion/bipolar.gif
« Last Edit: August 05, 2010, 11:43:55 PM by shinychrome0 »

teemuk

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Re: Custom LPF build is distorted
« Reply #5 on: August 06, 2010, 03:51:06 AM »
4 stages to achieve -3dB @ 32 kHZ and then the final 1st order filter stage where it all goes to -3dB @ 320 Hz.

This design doesn't make any sense to me. Not one bit.


What's the application? If the goal was achieving the steeper curve from the summing effects of series filters then why not just go straight to using second order active filters and cut down the component count drastically? If the goal indeed was the -3dB @ circa 320 Hz then you effectively do it in a single stage (pretty much with a curve of a plain 1st order filter to add), so what's up with the rest four stages that only introduce only -5 dB above audio range? They seem pretty pointless to me.
« Last Edit: August 06, 2010, 04:00:09 AM by teemuk »

shinychrome0

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Re: Custom LPF build is distorted
« Reply #6 on: August 06, 2010, 08:57:22 AM »
It was more of an experiment than a practical application, but i'm still going to use it.  And yes the steeper curve was the idea, using the simplest circuit possible.

However, i still have the same problem with distortion, even with the updated power supply, and 1 meg resistor to ground on the input of the first stage.  Could it have anything to do with using 100r resistors on the outputs?

phatt

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Re: Custom LPF build is distorted
« Reply #7 on: August 06, 2010, 09:55:33 AM »
Hi shinychrome0,
I'll take a stab here,
I'll bet it worked on the sim and you assumed it would work in real life. ::)

*** Be very aware Sims are not bullet proof ***
Quite often they give a result that might look really good on screen but
in real life are impossible. :'(
Very powerful tool but can easy lull a novice into a false sense of security.

You do need to do some homework and read up so that you have a reasonable idea of *basic circuit function*.

I'm saying this because Like You I was learning and making these kinds of mistakes not to many years back. Be patient, Google the terms you read so that you develop a solid basic circuit understanding.
(And of course ask these wonderful chaps who've posted above me as they are a treasure trove of knowledge :tu:)

As has already been said the circuit is way off and there are much easier ways to achieve what you wish.

I'll also assume you are trying to get a very sharp rolloff?
If so then Google  *Ed Rembold Marshall Simulator*
(Ed's circuit is a simlified version  found in Marshall JTM Amp schematics)

Or hunt down the *LXh2 cab sim* stuff.

LXH2 is complex but Ed's sim is about as easy as it gets and it does it's job well.

If you wish I will post my Marshall Cab sim circuit for you to work with,
you can even tweak some of the values to your liking on screen to see how each part changes the rolloff.

At least you know it will actually work when you put it together! ;D
Phil.

teemuk

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Re: Custom LPF build is distorted
« Reply #8 on: August 06, 2010, 10:01:01 AM »
e.g. 2nd order low-pass (a.k.a. "Sallen-Key")
« Last Edit: August 06, 2010, 10:02:43 AM by teemuk »

shinychrome0

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Re: Custom LPF build is distorted
« Reply #9 on: August 06, 2010, 10:02:14 AM »
Yeah i'd love to see your circuit of the cab sim.  And for the purposes of learning from my mistakes, i'd still like to make this build work, even if its not quite what i was going for.

shinychrome0

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Re: Custom LPF build is distorted
« Reply #10 on: August 06, 2010, 10:07:11 AM »
teemuk, what is the circuit you posted?  Is that the marshall cab sim?

teemuk

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Re: Custom LPF build is distorted
« Reply #11 on: August 06, 2010, 10:15:58 AM »
No, it's an active second order low-pass filter. You likely find two or more of these basic circuits connected in series from most cab sims.

Basic "workhorse" filter circuits such as this are pretty well covered in electronics literature, with all the applications and mathematic equations for component values included. There's no need to reinvent the wheel.

You can turn the same circuit to 2nd order high-pass like this:


And even combine the two in series just using a single OpAmp. You simply don't need buffers for every stage.

Here's the same thing with discrete components:

and a calculator for values:
http://www.vk2zay.net/calculators/sallenKey.php

Here's the OpAmp versions shown with neccassary biasing arrangements, if you plan to power them from a single-sided power supply such as battery:

Notice how the inputs need to be referenced to a DC potential that is exactly half of the rail-to-rail voltage (Vcc/2).

With a dual-rail supply you could just use ground as reference because half of the rail-to-rail voltage is zero volts.

You will need to provide this reference to the OpAmp stages because otherwise their inputs will "float" and the stages won't operate correctly, they may even clamp the output to one of the power supply rails. All AC coupled stages will need the reference, DC coupled can be series connected assuming you provide the reference to at least one of them. But in latter case you got to watch for DC offsets that real-life OpAmps generate. Those offsets may cause the reference to drift from the exact point and again compromise the operation.

Note that simulated circuits tend to work too well because they use ideal components and often they are not hindered by deficiencies that exist in real life. The schematic you posted works well in a simulation, badly or not at all in real-life.

To get it to work in real-life you need to address the issues of at least:
- Creating a properly working power supply
- Creating proper DC references for the OpAmps
- Handling DC offsets that series connected stages may generate

J M Fahey is right on the money with his previous posts.

If you plan to build something with OpAmps, even electronics stuff in general, I suggest you learn the theory behind before rushing straight into making something. You have much more chance of success. Most analog electronics theory books cover the 101 stuff in an easy-to-learn format. Plenty of learning material can also be downloaded for free.
« Last Edit: August 06, 2010, 10:33:29 AM by teemuk »

shinychrome0

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Re: Custom LPF build is distorted
« Reply #12 on: August 06, 2010, 10:20:42 AM »
Is there a way to make a filter like that adjustable?  Otherwise it won't help me out a whole lot.

teemuk

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Re: Custom LPF build is distorted
« Reply #13 on: August 06, 2010, 10:26:39 AM »
Adjustable, as in what way?

shinychrome0

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Re: Custom LPF build is distorted
« Reply #14 on: August 06, 2010, 10:29:35 AM »
adjustable cutoff frequency.  I found one article that says to use a dual ganged pot to keep the Q the same, but that means using two dual ganged pots to get a four pole filter circuit.

 

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