How to calculate a subsonic filter for phono pre? - diyAudio
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Old 15th May 2007, 04:18 PM   #1
Kal is offline Kal  Germany
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Default How to calculate a subsonic filter for phono pre?

Hello,

can someone please help me calculating a first order subsonic filter?

I have a separate phono pre amp connected to my pre amp. The input impedance of my pre amp is 75 mV and 47 KOhm and I want to put the filter between my phono pre and the pre. I think about a soft first order filter around 20 to 30 Hz. What do I need and how do I have to connect it? Thank you!
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Old 16th May 2007, 09:52 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
your RIAA may already have the subsonic filter built into it's circuit.
The original RIAA was supposed to be flat below 20Hz but they brought out a new curve specification about 20years ago with a highpass roll off from about 30to 40Hz.
In my opinion, this killed the bass from records and as a result ruined the performance.
I reverted to the older specification.
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Old 16th May 2007, 10:02 AM   #3
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If your input impedance is 47k then the good old formula

C = 1 / (2 . pi . F . R)

is what you need

C is in Farads (divide by 1000000 to get microfarads)
F is in Hertz
R is in ohms (multiply kilohms by 1000 to get ohms)
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Old 16th May 2007, 12:08 PM   #4
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A subsonic filter with only a first order cut will not be very useful. If you put a 0.15 F capacitor in series with the signal, you will get a cut off below 22 Hz (0.1 F and the cut off starts at 33 Hz). A third order filter (18 dB/octave) at 25 Hz is better suited. Why do you need a subsonic filter?

Roger
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Old 17th May 2007, 09:31 AM   #5
Kal is offline Kal  Germany
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Thank all of you so far!

I know that a subsonic filter will harm the sound in some way therefore I only want to use a soft first order filter, since subsonics look so scary with my 17cm mid/bass making big jumps...
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Old 17th May 2007, 02:25 PM   #6
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Looks like your cartridge/tone arm is not damped well at subsonic frequencies. Typically the resonance should be around 10 Hz (?) and should be well damped. You can add a fluid damping device to your arm and reduce the resonance peak and its frequency. You will need a small container to hold the oil ( silicone oil or something similar ) and a paddle that is fixed to your tone arm. It also cleans up the bass very nicely. Some experimentation is required.

I used to have a whole set of Japanese articles on fluid damping. Can't find even one sheet
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Old 17th May 2007, 07:32 PM   #7
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Quote:
Originally posted by Kal
Thank all of you so far!

I know that a subsonic filter will harm the sound in some way therefore I only want to use a soft first order filter, since subsonics look so scary with my 17cm mid/bass making big jumps...
A first order filter is of very little use in your situation. If your bass drivers are pulsating, you probably have a bad combination of arm and cartridge. The frequency of resonance is too low and warped discs will not play back very well.

There are several ways of how to reduce this. One of them is mentioned by ashok, fluid damping of the arm. You can also connect the signal in mono below circa 50 Hz (it is already in mono on the disc anyway but means you need to modify your amplifier) or use a third or fourth order filter to cut the lowest frequencies. Such a filter will not harm the sound very much. It will certainly unburden your speakers and let them perform better.

My own RIAA amplifier, have these options included.

You can have a schematic if you like. Just let me know your E-Mail address.

Roger
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Old 17th May 2007, 09:47 PM   #8
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Although it would seem that a first-order filter won't be much use, in fact it does the job pretty well. My old Bryston .5B had a first-order infrasonic filter, -1 dB @ 31Hz according to the spec sheet.

At the time I was using a Grado Signature which had very little internal damping, a tonearm with no damping, and ported speakers. The woofers were flapping quite a lot, and the filter cut it out completely, so I suggest you go for it.
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Old 17th May 2007, 09:52 PM   #9
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Quote:
Originally posted by richie00boy
If your input impedance is 47k then the good old formula

C = 1 / (2 . pi . F . R)

is what you need

C is in Farads (divide by 1000000 to get microfarads)
F is in Hertz
R is in ohms (multiply kilohms by 1000 to get ohms)
I've always wondered about the F in this formula. Is F the frequency at which the filter starts to act, or the -3 dB point, or what? I've always assumed it was -3dB, but never seen it confirmed.
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Old 18th May 2007, 02:00 AM   #10
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Yes it is the -3dB point.
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