Hi everyone,

I've been tinkering around with the idea of making an active low pass filter for a DIY guitar pedal.
I made a basic test passive one which worked, but adding an op amp at the end of the circuit has left me confused.
In image 04 the gain of the circuit has gone down as it did in the passive circuit but it has boosted in voltage, is this normal? and would it affect the guitar signal at all?
I added a resistor chain using the formula Av=1+(R1/R2) but this seemed to decrease the gain instead of increasing it, although the voltage jumped back down.

How would I get this working? And if it's possible to do this would it be possible to add second and possibly third orders and control the cutoff with the same pot?

Cheers.

 

The Input-Impedance is an unknown under real-World conditions with your schematics.
This means that You should have a Buffer before the RC-Filter, as well as after the RC-Filter.
This will eliminate, or at least severely reduce, any apparent Gain discrepancies.

Also, each Op-Amp-Stage can have a Fixed-Feedback-Capacitor which will
further Filter extremely-high-Frequencies which You are sure that You will never want in the future.

Gain may be added before or after the RC-Filter(s).
It's best to have multiple Gain-Stages instead of one large one,
( they will Multiply together, not add to each other ).

For steeper Cut-Offs, the easiest way is to use a 2 or 3 Ganged-Pot controlling 2 or 3 RC-Filters.

The best way to find out what it is that You are really looking for is to
purchase a ~31-Band-Graphic-EQ and play around with it for several months.
Beringer-EQ
They also have 7-Band-Graphic-EQs in a Guitar-Stomp-Box format,
but this will limit what You can do.
Still a very valuable, and versatile, Stomp-Box type to have.

Don't forget that if You are using a
Musical-Instrument-Amplifier with a Musical-Instrument-Speaker,
that the "Tone" that You get is a fixed, and extremely limiting,
factor in your "Tone" that is not adjustable.

I recommend achieving your "Tone" Electronically,
and not by using Amp and Speaker model selection.
High-Power PA-Amps and Speakers will usually work just fine,
but I prefer custom-made High-Fidelity Amps and Speakers which do not
add their own questionable "flavor" to your "Tone".
This also has the bonus of allowing You to set your Volume-Level anywhere You want
without adversely affecting the "Tone" that You have worked so hard to create.
These Speakers will also spread their Sound-Output over a wider, more even area,
with equal "Tone" throughout the room, ( no bad seats ).
You will also have the option to have a Left and Right Stereo-Sound-Field,
( great for Reverb and Tremolo Effects ).

This scheme also works great for Bass-Guitar,
but the Speakers must be much larger physically, to handle the Lower-Frequencies.
.
.
.

 

Your confusing 1st schematic shows R1= 0%, Tot= 500 and Wiper=1.
Your 2nd schematic again shows R1= 0%, again shows Tot=500 and Wiper= 0.
But you do not show the resistance of R1 and its setting.

In your waveforms you say green is Vout but you do not say what is blue. Your input signal??

I do not know why the 2nd opamp has no output signal.

 

Your confusing 1st schematic shows R1= 0%, Tot= 500 and Wiper=1.
Your 2nd schematic again shows R1= 0%, again shows Tot=500 and Wiper= 0.
But you do not show the resistance of R1 and its setting.

In your waveforms you say green is Vout but you do not say what is blue. Your input signal??

I do not know why the 2nd opamp has no output signal.

the potentiometer has a resistance of 500 ohms shown by the Rtot=500, and the wiper at 1 means that it is at maximum position. i don't know what the % is for but I left it and it worked fine for the passive filter. I didn't mention any colours in the original post but the input signal is green and the output signal is blue

 

You can make a reasonable model of a guitar pickup for SPICE purposes but putting 6H and 6kΩ in series with your voltage source.
Then add some capacitance to ground to represent the cable. 100pF to 300pF should do, depending on the length of the cable.

 

Your circuit is not an Active lowpass filter. Instead it is a simple RC lowpass filter that is buffered so that a load does not affect it.
The opamp has an extremely high input impedance that does not affect the filter.

Your signal level was too high for lower frequencies causing the opamp output to produce severe clipping so in my simulation I reduced the input signal to -3dB (half the peak voltage level).

Edit: The filter results will be completey different if the signal soiurce has a high impedance like a gutar pickup.

 

Your circuit is not an Active lowpass filter. Instead it is a simple RC lowpass filter that is buffered so that a load does not affect it.
The opamp has an extremely high input impedance that does not affect the filter.

Your signal level was too high for lower frequencies causing the opamp output to produce severe clipping so in my simulation I reduced the input signal to -3dB (half the peak voltage level).

thanks for that information, but what is the difference between what I did and an active filter? and is there any benefit to having an active filter over what a passive filter that is buffered by an op amp?

 

Is there any benefit to having an active filter over what a passive filter that is buffered by an op amp?

The answer to that will be very obvious if you use a real model for the guitar pickup, instead of assuming that it is a real Thévenin source.

 

An active filter is 2 poles or more and has feedback that allows a different Q.
Your filter has a single pole and if it had a high input impedance then its simple gradual cutoff it will produce sounds the same as the treble tone control turned down on a sound system.