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designing a lowpass filter for audio

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LvW

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Hi AUDIOGURU !

Quote:
An opamp has distortion as low as 0.00008%.

Question: Did you measure or simulate this extraordinary figure by yourself? Or where did you derive it from?
 

Audioguru

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The datasheets for good opamps show their distortion.
 

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snafflekid

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Hi shoe,

With any new design, start simple. I would suggest using a single Sallen and Key filter stage. Because you want to have a Butterworth response (maximally flat passband) this requires a Q factor=0.707

Sallen and Key will have two resistors and two capacitors. You could set one resistor as a ratio of the other, and one capacitor as a ratio of the other. Q factor depends solely on the ratio, so as you adjust the frequency, Q will remain fixed. Adjust the R and C values for the cutoff frequency.

Setting up a system that has a bank of R and C that can be switched in and out with analog switches might be easier than trying to find variable R and C components.
 

LvW

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Audioguru said:
The datasheets for good opamps show their distortion.

Thanks, Audioguru.
I doubt how and if this extraordinary figure could be verified. Do you believe such a specification?
 

FvM

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Yes, a specification like the said OPA134 THD value can be verified. The datasheet briefly tells, how.
The datasheets for good opamps show their distortion.
Not particularly, I think. Possibly the datasheets of dedicated audio OPs. Strictly spoken, THD is a circuit rather than a device parameter.
It can be valid for a specified application circuit only, including specified load and other operation conditions. But there are so much other
parameters that can be claimed essential for a "good" OP. Good for what? must be added obviously. From a particular application perspective,
I can tell you also, that a good OP must have at least 1 GHz GBW or 5V/ns slew rate, or ...

But if you're not targetting to high end professional audio equipment beyond usual CD audio quality, the discussion is a bit off topic in this place.
The original poster has clarified, that he's rather interested in a sweepable VCF of tb-303 type than in a fixed frequency OP based active filter.
Extending the dynamic range of a VCF would be challenging. But it's mostly a problem of finding suitable variable gain or resistance elements, as
long as we stay in the analog domain.
 

LvW

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Snafflekid is right, however some comments are in order:

Quote Snafflekid:
With any new design, start simple. I would suggest using a single Sallen and Key filter stage.Because you want to have a Butterworth response (maximally flat passband) this requires a Q factor=0.707

This applies for one 2nd order stage only. Higher orders require other pole Qs.

Sallen and Key will have two resistors and two capacitors. You could set one resistor as a ratio of the other, and one capacitor as a ratio of the other. Q factor depends solely on the ratio, so as you adjust the frequency, Q will remain fixed. Adjust the R and C values for the cutoff frequency.

This applies to the unity gain Sallen-Key stage. But it has not been mentioned that for pole shifting two elements have to be tuned simultaneously (for a constant ratio).

Added after 23 minutes:

Concerning distortion properties of opamps:
Quote Audioguru:
An opamp has distortion as low as 0.00008%.
Quote FvM:
Yes, a specification like the said OPA134 THD value can be verified. The datasheet briefly tells, how.

It should be added, that this value is specified by the manufacturer for 100% feedback !
 

FvM

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But it has not been mentioned that for pole shifting two elements have to be tuned simultaneously (for a constant ratio).
It has been already mentioned in this thread. Also, that you can choose a circuit with equal resistance values e.g. for a Butterworth characteristic. (You have to adjust the gain or select a different circuit to achieve a specific Q then). Equal resistance values are a practical requirement to apply some variable resistance options, e.g. tandem potentiometers, LEDs or FETs as variable resistors.
 

LvW

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More than that, I think (according to data sheet) the THD value is a calculated value for a gain=0 dB based on measurements performed with less feedback (factor 101).
 

Audioguru

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LvW said:
Concerning distortion properties of opamps:
Quote Audioguru:
An opamp has distortion as low as 0.00008%.
Quote FvM:
Yes, a specification like the said OPA134 THD value can be verified. The datasheet briefly tells, how.

It should be added, that this value is specified by the manufacturer for 100% feedback !
With a gain of only 1, the distortion is 0.00008% up to about 6kHz into 600 ohms or more at 3V RMS.
With a gain of 100, the distortion is 0.008% up to about 300Hz into 2k ohms.

Most people cannot hear any amount of distortion of fundamental frequencies above 10kHz.
Most people cannot hear 0.1% distortion of low frequency fundamental frequencies.
 

shoe

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Thanks for the help everyone! however, Im still no further forward. I've seen the schematics, filters can't possibly be as simple as those little patterns though?

Could somone provide the forumulae so that I can calculate the R/C ratio; becuase I can only find variable capacitors in the microfarad range, so i'll need to get some resistors to match and I don't know the math.

:)
 

LvW

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Shoe: Thanks for the help everyone! however, Im still no further forward. I've seen the schematics, filters can't possibly be as simple as those little patterns though?

Could somone provide the forumulae so that I can calculate the R/C ratio; becuase I can only find variable capacitors in the microfarad range, so i'll need to get some resistors to match and I don't know the math.


Hi shoe, I can imagine that you are "still no further forward". But don't blame us!
In this context, read again my comment from May 21st, 18:17.
As long as you cannot tell us what you need, I am afraid , nobody can help you.
(formulas for what? R/C ratios for what? You don't know the math for doing what?)
 

Audioguru

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Variable capacitors are in the pico-Farad (micro-micro Farads) range for radio frequencies, not audio frequencies.

In the "good old days" there were ICs made to do guitar effects but they are obsolete today when DSP is used and programmed to do almost everything wirh audio.

A transconductance amplifier IC was used as a voltage-controlled variable resistor and I think an LM13700 IC is still available today.
 

jamiewoody

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"shoe", i'm a musician, and looking to build some of these things myself. a VCF is what i want to try next. if you build one, please contact me.

i can make a basic oscillator using a timer or cmoss. you can just twist pots, etc and make all kinds of noise.

i am so-so at reading schematics, but learning. i can draw decently, so i usually make more of a block drawing of what i build. i know, that can get me killed in a forum like this! lol!
 

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