Low pass filters for removing ultrasonic noise? - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st October 2010, 07:32 AM   #1
diyAudio Member
 
Join Date: Dec 2007
Default 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!
  Reply With Quote
Old 1st October 2010, 10:58 AM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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.
__________________
regards Andrew T.
  Reply With Quote
Old 1st October 2010, 06:45 PM   #3
diyAudio Member
 
Join Date: Dec 2007
an example circuit?
  Reply With Quote
Old 2nd October 2010, 08:53 AM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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".
__________________
regards Andrew T.
  Reply With Quote
Old 2nd October 2010, 11:58 AM   #5
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
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?
  Reply With Quote
Old 2nd October 2010, 12:06 PM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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
__________________
regards Andrew T.
  Reply With Quote
Old 2nd October 2010, 12:27 PM   #7
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
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
  Reply With Quote
Old 10th July 2014, 09:33 PM   #8
diyAudio Member
 
Join Date: Oct 2011
Quote:
Originally Posted by billybudapest View Post
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.
  Reply With Quote
Old 10th July 2014, 10:08 PM   #9
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
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?

_-_-
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 11th July 2014, 11:37 AM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Filters For Low-Noise Measurement Preamplifier bryman79 Solid State 63 25th July 2011 06:24 PM
DIY Low Pass and Mid/High Pass (Bass Blocking) Filters runslikealpaca Everything Else 1 19th June 2009 10:59 PM
Need formulas for multi-poles RC low pass passive filters gaetan8888 Digital Source 9 13th March 2008 03:59 AM
A question on low pass filters Vaughan Subwoofers 15 10th October 2007 04:23 AM
Ultra low noise filters tiroth Solid State 14 21st May 2002 10:19 PM


New To Site? Need Help?

All times are GMT. The time now is 05:30 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2