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Simple 555 synth idea

BobK

Jan 5, 2010
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Active filters can be high-pass, low-pass or bandpass, depending on how they are connnected. A bandpass will have a resonance.

Bob
 

Yoa01

Jun 18, 2012
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All filters have some semblance of resonance, though, right? If not, how hard would it be to add it?
 

BobK

Jan 5, 2010
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No, a pure low pass or high pass filter does not have a resonance. A resonance would be a peak or valley in the response curve, this makes it a band-pass or band-reject filter.

Bob
 

Yoa01

Jun 18, 2012
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Sorry, as a synthesist I work primarily with resonant filters, and assumed that since the two filter types can produce the same response curve that they would be resonant.

I keep seeing online that an inductor is needed to make a resonant band pass (or reject) filter. I was thinking there would be a way to incorporate it into a low pass filter; however, looking at resonant low pass filters I do not see an inductor anywhere. Aside from your standard Ladder, MS-20, and 303 filters, I'm looking at these two: http://www.diystompboxes.com/smffor...d80ac8ec568cab0bccf0dcd9529d43f&topic=73021.0 and http://thesquarewaveparade.com/images/srlpf.jpg .

I have a theoretical idea. Knowing that resonance is amplification of the threshold frequencies, would it be true that one could wire an amplified band pass filter in parallel to the filter (or output), then use the main Low Pass Filter's cutoff potentiometer output to control the position of the band pass cutoff frequency pot? You could then control resonance amount by a volume pot on the amplifier, I presume.
 

BobK

Jan 5, 2010
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You might want to look at state variable filters. These can be used to make bandpass filters that are adjustable by a single pot (each) both in center frequency and width of the passband.

Bob
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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I just thought I'd post this to follow up on your questions regarding LC filters. Shown is a low pass filter with a -6dB knee @ 17KHz. The source impedance of VG is 600Ω and RL = 600Ω. As far as resonance is concerned, this is a low Q circuit so the curve looks virtually flat until cut off freq. However, if the load (RL) is lifted a visible resonance peak will occur at about 18KHz but passive filters are designed around a known source Z and load Z.

Chris
 

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Yoa01

Jun 18, 2012
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So, that same schematic but without the RL would in fact be a simple inductor-driven resonant low-pass filter?

I think I will stick with an active filter, but this certainly opens some possibilities.
 

CDRIVE

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So, that same schematic but without the RL would in fact be a simple inductor-driven resonant low-pass filter?

I don't think you understand. RL is not part of the filter. RL represents the load that the filter sources. RL is representative of the input impedance of the next stage that the filter supplies signal to. The source impedance isn't part of the filter either. It represents the output Z of a previous stage. Admittedly this stuff is not novice level study. ;)

Chris
 

Yoa01

Jun 18, 2012
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Oh, no I get it. I was thinking that VG was input and VF was output, but I see what you're saying now.
 
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