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 19th April 2013, 11:41 AM   #1
elmura is offline elmura  Australia
diyAudio Member
 
Join Date: Aug 2012
Default Need an Active 2nd Order Low Shelf Filter aka Low Shelving Filter

Hey Guys,
I've been trying to design a simple, linear, active, second order low shelf filter with around 6dB attenuation without much success. Research on the web has proved a massive time consumer without luck.

The design of the low frequency shelf cut specs are around 5-6dB between shelves and a 1kHz corner. Needs to have low phase shift, and low group delay variation and have low noise & THD.

I've got a high pass shelf working based on Linkwitz Shelving Filter, but I need a low shelf cut.
Thanks
Attached Images
File Type: jpg 1st order Shelf.JPG (50.1 KB, 98 views)
  Reply With Quote
Old 19th April 2013, 12:38 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
that is a first order filter.
__________________
regards Andrew T.
  Reply With Quote
Old 19th April 2013, 01:21 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by elmura
Needs to have low phase shift, and low group delay variation and have low noise & THD.
You can't independently choose filter slope and phase shift, as causality links them. For low phase shift you want a first order shelf.

The circuit shown in post 1 does not seem to go with the plot. Circuit is low pass, plot is high pass.
  Reply With Quote
Old 19th April 2013, 01:28 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
plot and circuit are both high pass. They match each other, F-3dB = 8kHz
__________________
regards Andrew T.
  Reply With Quote
Old 19th April 2013, 01:45 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Sorry, yes you are right.
  Reply With Quote
Old 19th April 2013, 03:34 PM   #6
diyAudio Member
 
Join Date: Mar 2007
Location: California
See if the attached circuit works for you...

You can design this yourself, using the LT designer I wrote:
CFL Linkwitz Transform Designer with Monte Carlo Sensitivity Analysis

There are other ways to make the second order shelving filter. You have to implement a biquadratic filter, and you can do that using various "biquad" circuit topologies.

As others have said, phase shift and group delay are a direct consequence of the frequency response and are independent of which circuit you choose. The distortion is dependent on what amplifier (op amp) you use and the resistances in the circuit, etc. and is something that you can tweak.

-Charlie
Attached Images
File Type: gif LT cut circuit info.GIF (23.3 KB, 89 views)
File Type: gif LT cut circuit plot.GIF (29.3 KB, 87 views)
  Reply With Quote
Old 26th April 2013, 05:12 AM   #7
elmura is offline elmura  Australia
diyAudio Member
 
Join Date: Aug 2012
I've tried the Linkwitz Transform circuit and yes, it works quite well for the response curve. Nice spreadsheet CharlieLaub. Very interesting and looked like some work to put together.

! However ! Regarding the Linkwitz Transform:
- There are too many components in the signal path
- It inverts the signal
- Lots of noise & distortion added by the network

Is there a simpler, cleaner way? Similar to the high pass circuit I posted? Even if it is 1st order.
  Reply With Quote
Old 26th April 2013, 10:21 AM   #8
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes. Non-inverting opamp, with CR in parallel in the feedback arm and R to ground from the inverting input. Make sure the opamp is unity-gain stable. Unity gain at high frequencies; gain set by resistors at low frequencies, rolloff set by C (in conjunction with R's). Should be in any decent electronics textbook, or Google.
  Reply With Quote
Old 26th April 2013, 12:30 PM   #9
elmura is offline elmura  Australia
diyAudio Member
 
Join Date: Aug 2012
I believe you just described a low shelf boost, not cut.
  Reply With Quote
Old 26th April 2013, 12:55 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You want HF cut, don't you? That is what I described. Two equal resistors, with a cap across the feedback one, will give you a first-order HF cut shelf of -6dB.
  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
Second order shelving filter Buriedcode Analog Line Level 1 18th April 2013 08:19 AM
Preamp mod with a low cut shelf filter johanwahlberg Analog Line Level 11 11th December 2012 04:18 PM
high gain 2nd order high pass active filter topology sreten Solid State 21 23rd March 2006 01:26 AM
1st order low-pass filter, which cap ? Thmartin Parts 4 17th November 2005 08:37 AM
1st attempt of 2nd order active lowpass filter failed, please advise SimontY Parts 111 15th February 2005 10:32 PM


New To Site? Need Help?

All times are GMT. The time now is 11:07 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