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Old 13th August 2019, 11:26 PM   #1
mashaffer is offline mashaffer  United States
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I wish to include an infrasonic filter in a preamp build and have been simulating a Steve Bench style 3rd order active but as a pipe organ fan I want to put the filter at 16Hz which makes the 3rd order Butterworth a bit more gentle than I would like. At such low frequencies it seems that a little bit of ripple in the pass band would be perfectly fine as a trade off for a sharper/steeper initial roll off so naturally Chebyshev comes to mind.

Now it has been several decades since I have been involved in LaPlace, Pole Zero plots, and the like so I am kind of asea at this point. It seems that it should be possible to implement something like that but not sure where to begin.

What I would like is a pointer to some tutorials to get me back up to speed on the S-plane filter design and maybe some kind of Chebyshev for dummies. Also ideas on realizing such filters with vacuum tubes.
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Old 13th August 2019, 11:38 PM   #2
Wavebourn is offline Wavebourn  United States
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Today I would google for online calculators that can do that for you. :-)
Speaking of vacuum tubes, you have to remember that the device must have pretty low output resistance due to phase shifts in the filter, and pretty low distortions driving such resistance up to the needed swing. People often forget about it getting a frequency dependent distorter instead of a true filter, even when use opamps with much better driving capabilities.
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Old 13th August 2019, 11:48 PM   #3
rayma is offline rayma  United States
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Is this for use with a turntable? If I were a pipe organ fan, it would be CDs, not LPs.
While LPs can be excellent, the bass is the worst area for both turntables and LPs.
A resonance peak around 10Hz, and mono bass only, are two of the biggest failings.
Plus, a sharp high pass filter will have bad group delay, which is again bad for the bass.

Last edited by rayma; 14th August 2019 at 12:13 AM.
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Old 14th August 2019, 12:09 AM   #4
mashaffer is offline mashaffer  United States
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It will be in the line amp. the phono preamp will be separate. In addition to TT rumble I have noted that some of my CDs have infrasonic noise as well.
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Old 14th August 2019, 12:28 AM   #5
rayma is offline rayma  United States
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You might consider using the third order high pass circuit in the Holman preamp, which is set at 15Hz.
It is designed for low group delay. LF noise on a CD is likely to be acoustical in origin, HVAC, traffic.
APT Corporation Holman - Manual - Stereo Pre Amplifier - HiFi Engine

Last edited by rayma; 14th August 2019 at 12:32 AM.
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Old 14th August 2019, 01:23 AM   #6
mashaffer is offline mashaffer  United States
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Am I in the ballpark? Using an online calculator from AnalogDevices.

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Old 14th August 2019, 01:44 AM   #7
Miles Prower is offline Miles Prower  United States
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Looks like a Butterworth characteristic, not a Chebychev
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Old 14th August 2019, 02:09 AM   #8
mashaffer is offline mashaffer  United States
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My previous butterworth results did not have this bump Which improves things slightly but I still end up with only 18dB/8va. Will work on it more tomorrow.

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Old 14th August 2019, 10:45 AM   #9
DF96 is offline DF96  England
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18dB/octave is set by having three poles. Chebyshev does not have a faster ultimate rolloff, but just a sharper transition from passband to rolloff. If you want faster rolloff then you need more poles.
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Old 14th August 2019, 12:16 PM   #10
mashaffer is offline mashaffer  United States
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Quote:
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18dB/octave is set by having three poles. Chebyshev does not have a faster ultimate rolloff, but just a sharper transition from passband to rolloff. If you want faster rolloff then you need more poles.
Yes, not sure what I was thinking last night. The above does give me a sharper transition rhan previous efforts. The hump is only 0.5dB so I would be willing to accept slightly larger hump but the above may be good enough.
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