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Old 16th June 2014, 01:33 AM   #1
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Default Pot before a high pass filter

Dear all:

If I put a 10K pot before a High Pass filter (C1-R2 or C4-R4), will the pot create a Low pass filter formed by the Pot resistance and the input capacitor (C1 or C4)?

I would like to add a pot before my LM4880 input high pass filter, but i don't want to create a Low pass filter by adding a pot.

Regards.
Alfredo Mendiola Loyola
Lima, Peru
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File Type: jpg LM4880_VolumneControl.jpg (24.6 KB, 141 views)
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Old 16th June 2014, 01:52 AM   #2
DUG is offline DUG  Canada
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You have two high pass filter in the above schematic.(for each channel)

One on the input with C1 / R2+part of P1.

One on the output with C2 / R3//load

The position of P1 will affect the input high pass filter.

Assuming a low impedance signal source at CN1:

The 3dB with P1 wiper at the bottom or top is 15.4Hz.

At mid position the 3dB point is at 13.8Hz.

Hope that helps.
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Old 16th June 2014, 01:56 AM   #3
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Hi Lima

Yep, in this case pot will be a part of your HPF, and its position will affect the cut-off frequency. Although if you select pot value much lower than R2 R4, the affect will be pretty minor.
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Old 16th June 2014, 02:01 AM   #4
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Quote:
Originally Posted by DUG View Post
You have two high pass filter in the above schematic.(for each channel)

One on the input with C1 / R2+part of P1.

One on the output with C2 / R3//load

The position of P1 will affect the input high pass filter.

Assuming a low impedance signal source at CN1:

The 3dB with P1 wiper at the bottom or top is 15.4Hz.

At mid position the 3dB point is at 13.8Hz.

Hope that helps.
Dear Dug:

What do I have to do to include the pot resistance on the high pass filter cut off formula?

Is there a way to avoid a change in the high pass filter when I use a pot to control the input volume?

Mid position Pot.

Regards.
Alfredo Mendiola Loyola
Lima, Peru
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Old 16th June 2014, 02:09 AM   #5
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Quote:
Originally Posted by Sergey888 View Post
Hi Lima

Yep, in this case pot will be a part of your HPF, and its position will affect the cut-off frequency. Although if you select pot value much lower than R2 R4, the affect will be pretty minor.
I use this formula to get the cut off frequency:

fc= 1/(2*PI()*R*C)
= 1/(2*PI()*22000*0.47)
= 0.000015392161

How does R change when I use the pot with the wiper at middle position.?

Do I have to calculate the parallel value between 22K and 5K when the pot wiper is at middle position?

Regards
Alfredo Mendiola Loyola
Lima, Peru

Last edited by amendiola; 16th June 2014 at 02:12 AM.
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Old 16th June 2014, 02:14 AM   #6
DUG is offline DUG  Canada
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If there is a low impedance source (and using a 10K pot):

At mid position there is a 5K to ground and 5K to signal (lowZ)

The equivalent model of this is a source of half of the input voltage (makes sense, right) and a resistance of the two 5K "resistors" in parallel: 2K5

Adding 2K5 to R2 shifts the input high pass filter slightly.

As suggested by Sergey888, selecting a lower value for the pot will reduce the affect on the corner frequency.

Is this frequency very critical?
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Old 16th June 2014, 02:17 AM   #7
DUG is offline DUG  Canada
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Quote:
Originally Posted by amendiola View Post
I use this formula to get the cut off frequency:

fc= 1/(2*PI()*R*C)
= 1/(2*PI()*22000*0.47)
= 0.000015392161

How does R change when I use the pot with the wiper at middle position.?

Do I have to calculate the parallel value between 22K and 5K when the pot wiper is at middle position?

Regards
Alfredo Mendiola Loyola
Lima, Peru

0.47 is in uF or micro Farads

Put in your formula 0.47E-6 in stead of just 0.47.

Use the resistance of 22000 for low source impedance and 24500 for wiper in mid position.("worst case")
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Last edited by DUG; 16th June 2014 at 02:19 AM.
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Old 16th June 2014, 02:45 AM   #8
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Quote:
Originally Posted by DUG View Post
0.47 is in uF or micro Farads

Put in your formula 0.47E-6 in stead of just 0.47.

Use the resistance of 22000 for low source impedance and 24500 for wiper in mid position.("worst case")
Thank you for your help.

I understood that I have to parallel the wiper position resistors and add the result to the High pass filter R.

Example:

Wiper at 60% position:
R1= 6K (POT1 60%)
R2=4k (POT1 40%)
R3=22K
REquivalent= (6K||4K) + 22K = 2.4k + 22K = 24.4K

And with 24.4K and C=0.47uF then fc at -3db = 13.88 Hz.

Thank you very much for your help.
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Old 16th June 2014, 02:58 AM   #9
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Will POT position affect the input high pass filter of the CMoy Design? (cut off frequency variation)

Do I have to make the same calculation to get the cutoff frequency when the POT position is 50%?

Regards.
Alfredo Mendiola Loyola
Lima, Peru
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File Type: jpg CMoyHighPassFilter.jpg (49.6 KB, 130 views)

Last edited by amendiola; 16th June 2014 at 03:02 AM.
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Old 16th June 2014, 04:20 AM   #10
DUG is offline DUG  Canada
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2K5 compared to 100K is less change than 2K5 compared to 22K.

2.5% compared to 11% (more or less)

Tolerance of caps * tolerance of resistors?

If the corner frequency stability is more critical than noise then use a buffer stage between the pot and the high pass filter elements. Then pot position will not affect the corner frequency.
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