I'm going to set C3 filter at ~800hz.
I think adding back some low mids and cutting some lows with C4 may get me where I want to be. A little more beef and less honk.
RAT - What's with these filters?
- Thread starter jwin615
- Start date
I think adding back some low mids and cutting some lows with C4 may get me where I want to be. A little more beef and less honk.
YourGuitarist
Member
The LM308 in this configuration does not invert. Some op-amps do invert once voltage swing reaches rails such as the TL072. Its notable that the R3/5 & C2/3 are not true hi passes as the non-inverting configuration has a minimum gain of 1 so the LF blocking behavior of the caps acts like a shelving filter.
As for the other stages, a lot of builders specify their filters outside of the audio range because there is attenuation approaching the -3dB that stacks when multiple filters are used. This is especially true for first order filters as are most commonly used in pedals. The effective system corner frequency rises with successive filters.
Also the inversion might just be the cumulative phase shift from the filters, each one contributing 90 deg individually.
In other words, several 1st order hi passes at Fc=10Hz will produce a total hi pass with say an Fc=~20Hz etc.
Just for reference.
As for the other stages, a lot of builders specify their filters outside of the audio range because there is attenuation approaching the -3dB that stacks when multiple filters are used. This is especially true for first order filters as are most commonly used in pedals. The effective system corner frequency rises with successive filters.
Also the inversion might just be the cumulative phase shift from the filters, each one contributing 90 deg individually.
In other words, several 1st order hi passes at Fc=10Hz will produce a total hi pass with say an Fc=~20Hz etc.
Just for reference.
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YourGuitarist
Member
Here's a graph of input response, it seems I *may* be wrong concerning the low / high passing attenuating below unity in the op-amp. The HF shelving effect is however visible. A pole (low pass) is formed by the gain pot and and C3 in the sim. That pole moves down in frequency as gain increases. This stacks with the famous slew rate limiting too (which is not modelled here). V1 is the input filtering. V2 is the op-amp output.
Note that FR changes with gain so you may be able to get different sounds by hitting the front of the pedal harder and keeping the gain control lower.
I did some more simulation and yes, the shelving hypothesis is correct. The op-amp does not produce any true hi or lo passes. If the bode plot is extended far enough in each direction, and the input bandpass is widened substantially, its visible that the gain provided by the op-amp flattens at unity. This is not shown in the graphs here.
Note that FR changes with gain so you may be able to get different sounds by hitting the front of the pedal harder and keeping the gain control lower.
I did some more simulation and yes, the shelving hypothesis is correct. The op-amp does not produce any true hi or lo passes. If the bode plot is extended far enough in each direction, and the input bandpass is widened substantially, its visible that the gain provided by the op-amp flattens at unity. This is not shown in the graphs here.
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cdwillis
Well-known member
In regards to the RC filter on the input 1k/1nf, I put a similar filter on the input of an overdrive I made except I bumped the 1nf up to 10nf thinking it would lower the filter knee to around 15khz. What I didn't consider is the effect the guitar pickup and cable would have on that filter and the input impedance. The circuit was built on a breadboard which worked fine and I added that filter in on a pcb I made without testing it. As soon as I built one it sounded like a cocked wah. I ended up removing the cap and leaving the 1k resistor for now. I'm sure the 1nf would be fine. Hell, I'm sure10nf would work fine too if there was a buffer in front of that.
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