Ac power bick wall filter?

How can i "eliminate" any frequency above say 70 cps from my power supply to stereo?
Our power should be 60 cps at 120 volts and I want to provide that. Thanks
The only way to be even 99% sure of this is to use a power regenerator.  See PS Audio.

The next level below is an inductive low pass filter.  See ZeroSurge/Brickwall AC filters.

In the U.S., utilities regulate line frequency within a very tight tolerance - it's necessary to keep the electrical grid stable. Voltage is maintained to a much looser tolerance.
@cleeds is absolutely correct. The power generation Hertz (cycles per second) is very tightly controlled. It used to be that electric clocks were among the most accurate, given by law a day had to have a precise number of cycles. No longer the case I believe.
Still, cleeds is right BUT, eliminating anything not 60 Hertz is a good goal in eliminating noise and distortion that may appear long after the power company generates it.
Every wire everywhere is an antenna collecting radio frequency radiation. Also every device connected to the power grid is connected to every other device on the grid. Literally connected. Think about it.

Hardly any of these connected devices are drawing current at a steady rate. A lot are motors and things that draw in surges, in the process creating a back- or counter-electromotive force, EMF.  

These two things, RFI and EMF, account for the majority of what we call noise on the line. They are there in the form of high frequency but very low level waves riding on the main 60 Hz AC wave.  

So, a couple problems with the OP question. Eliminate is a pretty strong word. Eliminate means zero. You can never get it down to zero. Even if you did, guess what? Three feet past the conditioner and the wire has picked up RFI in the room, because RFI is everywhere. It's how your cell phone works.  

But let's say you somehow magically eliminate all this noise all the way down to zero. And wrap your whole room in a Faraday cage so no RFI gets in. (Impossible, I know- the lack of cell service alone could be fatal!) The line is a perfect sine wave perfectly free from all noise.

You turn your amp on and uh oh, voltage drop! Wait, no, its back. Whew! Play music. Dang! More voltage drops! What the....

So we go through the whole exercise all over again, only this time discussing all the many problems with power supplies. And on and on.

Either that or- shocker! - we find one we can afford that sounds good.

I know. Shocker.

+1 ErikI have a ZeriSurge feeding a apower Plant.
If you are anal put one of these in your service panel:
millercarbon, you sound vaguely like a paranoid schizophrenic. Your understanding of Voltage, impedance, transformers and power transmission needs some touching up. 
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