Subsonic Filter

[ChrisM91]

Are subsonic filters usually included with phono stages or the preamp (line) stage? I’ve been told they are usually paired at the end of the phono stage rather than the beginning of the preamp. That said I don’t see many phono designs with subsonic filters.

Though I don’t think my current setup is very susceptible or impacted by sub-sonic frequencies I would like to add a filter to remove them. As I understand first order filters have to slow (shallow) of a rolloff and aren’t effective. I need a filter of a high order to accomplish the task of cutting the sub <20Hz.

Any suggestions for what designs to check out and/or build?

Thanks!

 

[rayma]

I would like to add a filter to remove them. As I understand first order filters have to slow (shallow) of a rolloff and aren’t effective.
I need a filter of a high order to accomplish the task of cutting the sub <20Hz.

You could build an active second order high pass filter in a single stage, and switch it in and out as needed.
To be effective, set the corner well above the typical 10Hz phono resonance point. An example:
Sallen?Key topology - Wikipedia, the free encyclopedia
This type of filter has unity gain, and does not invert polarity.

 

[DF96]

As LPs don't have any stereo information at bass frequencies, and most unwanted subsonics are vertical and thus out of phase, you can get some benefit from filtering out-of-phase more than in-phase. There is a thread somewhere on this site about this.