Low pass filter properties?
Low pass filter properties?
(OP)
I've got a circuit depicted below. Another, more colorful diagram can be found at http:// www.seymou rduncan.co m/support/ schematics /S_1single coil_1vol_ 1tone.html . The circuit has a grounded guitar string that vibrates over an inductor in a magnetic field. The inductor is actually a composite inductor in the sense that it is six little inductors each wrapped around a magnet. When the string vibrates the inductor generates a voltage.
The resistor is a variable resistor, an I suspect that the resistance and capacitance values are arbitrary. How can I determine the frequency response of this filter? I have the equation
Vout = [Zc/(Zc + Zr)] Vin
where
Zc = 1/jwc
Zr = R
I'm not sure if that's right though.
steel guitar string
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ground
| ||
inductor pickup | ||||
.---------------L/L/L/L/L/L-----'-----||||||
| M/M/M/M/M/M ||||
| magnets to generate ||
| field
|
|
|--------------------------------------------
| Vout
|
|
| \
| / R = 250 K Ohms
| \
'---------------->/
\
/ ground
\ C = 0.022 mcF ||
| |/ ||||
'----------||-------||||||
|\ ||||
||
The resistor is a variable resistor, an I suspect that the resistance and capacitance values are arbitrary. How can I determine the frequency response of this filter? I have the equation
Vout = [Zc/(Zc + Zr)] Vin
where
Zc = 1/jwc
Zr = R
I'm not sure if that's right though.
steel guitar string
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ground
| ||
inductor pickup | ||||
.---------------L/L/L/L/L/L-----'-----||||||
| M/M/M/M/M/M ||||
| magnets to generate ||
| field
|
|
|--------------------------------------------
| Vout
|
|
| \
| / R = 250 K Ohms
| \
'---------------->/
\
/ ground
\ C = 0.022 mcF ||
| |/ ||||
'----------||-------||||||
|\ ||||
||





RE: Low pass filter properties?
His circuit looks like a RLC filter, with a few caveats. For the math behind this circuit, check out:
http://en.
That said, it probably has a resonant (peak response) frequency above the audio band, so it looks like a high pass filter in the audio band (caveat #1 - if you hook the variable resistor up backward so counterclockwise is more resistance you'll think it is a low pass filter). So that means as you tweak the resistance you change the slope of the response vs. frequency (less resistance gives you more higher end response).
Caveat #2 - I'm not in a band so this is assuming that the load at the mono audio jack is much larger than 250K; if that is true than the frequency response is independent of the volume control. Sweet!
Good luck trying to copy this circuit... I'm sure there are many more caveats to this design than what they posted on their web-page. Also see:
http://en.wikipedia.org/wiki/Seymour_Duncan
RE: Low pass filter properties?
* Perhaps a slight exaggeration; it might have been Guglielmo Marconi instead.
RE: Low pass filter properties?
I think the only thing Seymour Duncan designs is electromagnetic pickups. I believe the basic circuit design was designed 40+ years ago by the Fender guitar corporation.
RE: Low pass filter properties?
IIRC (having had electric guitars myself), the ohmic resistance of the pickup is in the range of a few kiloohms = negligible.
Model it as 250k in parallel with 250k (variable) in series with 22n to ground.
Input impedance of the amp is usually around 1Mohm = beligible.
Benta.
RE: Low pass filter properties?
L------out
|
VR (0 - 250k)
|
C
|
ground
And if you turn the tone full CCW, then what you have is:
L------out
|
C
|
ground
The R part of the RLC is the series resistance of the pickup, which gives:
R-L------out
|
C
|
ground
For the R +L values, pull them off of Seymour's site. For example:
http:/
For this pikcup L = 4.08H (!) and the series resistance is 9.7k
For this pickup, with tone full CCW the peak is +3dB at about 480Hz, w/-3dB point at 750 Hz. With tone at 50% there is no peaking and -3dB is at 4.6kHz. (Thanks to Circuitmaker for the simulation.) This all omits the capacitive parasitics inside the pickup, which are significant, and will change the peaking depending on the value. Calculating backwards from the Q number of 2.52 on Seymour's website gives about 6800pF. I'm not sure how repeatable these specs are. You'd need to test a bunch of pickups.
Additionally, I'm simulating with lumped parts, and the parasitics are distributed throughout the windings, so that's another source of ugly reality creeping into nice simple simulations.
e2zn
RE: Low pass filter properties?
R-L------out
|
C
|
ground
the same as this
R--------out
|
C
|
ground
???
RE: Low pass filter properties?
Gunnar Englund
www.gke.org