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billybudapest 1st October 2010 08:32 AM

Low pass filters for removing ultrasonic noise?
 
Any easy way to implement a decent low pass filter (an ultrasonic cut) for the output of a preamp? I'm concerned about potential buildup of stuff if there is already noise or other junk in 96khz material and would like to be safe just in case. Any common implementations? A cut off above 20k but under 30k would be preferable. Thanks!

AndrewT 1st October 2010 11:58 AM

I always recommend an RF filter at the input of every bit of audio gear, (except a tuner).
A competent source should not emit excessive RF.

billybudapest 1st October 2010 07:45 PM

an example circuit?

AndrewT 2nd October 2010 09:53 AM

Hi,
series R in the signal line followed by a C from line to signal ground.

Usual R values vary from 100r to 3k3.
Usual values for C vary from 100pF to 1n5F.

I find that I cannot hear any difference taking RC lower than ~0.7us.
Therefore I use 1k0 & 680pF giving RC=0.68us. This has an F-3dB ~230kHz
There is a big subjective variation in acceptable RC values. Experiment.

I also add a small 47pF across the input RCA phono socket to attenuate the VHF before it gets into the "box".

qusp 2nd October 2010 12:58 PM

does this not add to the effective output impedance of the preceding stage? or all is fine provided the next stage input is high impedance enough?

AndrewT 2nd October 2010 01:06 PM

At higher frequencies the input of the receiver sees the 680pF as source impedance.
At lower frequencies it sees the 1k0+Rs in parallel with 680pF.

Those should be low enough source impedances to satisfy most Power Amplifiers and low gain Pre-amps. If Rs=200r, then over much of the audio band the effective source impedance <=1200r

qusp 2nd October 2010 01:27 PM

right got it, so the simple answer is yes and yes ;) quite a pertinent thread really, as i'm toying with setup for the main output of my dac for active monitors, I cant find the spec online, but I would assume most active studio monitors will have a reasonably high input Z. I have been eyeing them off lately for an upgrade, so will find out for sure what is at the input soon enough

AudioEngineer 10th July 2014 10:33 PM

Quote:

Originally Posted by billybudapest (Post 2319393)
Any easy way to implement a decent low pass filter (an ultrasonic cut) for the output of a preamp? I'm concerned about potential buildup of stuff if there is already noise or other junk in 96khz material and would like to be safe just in case. Any common implementations? A cut off above 20k but under 30k would be preferable. Thanks!

If you need an ultrasonic filter which is 2nd order or above you might want to consider a bessel filter which is usually the best solution as it has excellent phase characteristics. Take a look at this datasheet:

http://www.ti.com/lit/an/snoa586d/snoa586d.pdf

On page 11 you will find an ultrasonic filter. You can also get filterpro from Texas Instruments which is free software for designing filters including bessel filters.

bear 10th July 2014 11:08 PM

Huh?

Why would you need or want such a thing?
The so called 96kHz source material that is commercial is going to be free of any high level "garbage". The other thing is what speaker do you have that will reproduce any of it to begin with?

It would take a rather high level signal above 20kHz to do anything much to your amplifier.

Seems like a non issue.

Have you found a problem that has been measured by you or anyone else, or are you just worried about it?

_-_-

DF96 11th July 2014 12:37 PM

NOS DACs often omit the required low-pass reconstruction filter so then you could get lots of ultrasonic image frequencies. Normally-engineered DACs will not, so no further filter would be necessary.


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