Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Active highpass filter
Active highpass filter
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
Thread Tools Search this Thread
Old 28th June 2012, 05:33 PM   #1
dincz is offline dincz  Czech Republic
diyAudio Member
Join Date: Feb 2011
Default Active highpass filter

I'm trying to modify the high pass filter in a microphone preamp to drop its -3dB point from 75Hz to 40Hz. The original schematic is attached.

Should I simply increase the values of R24 and R25 keeping them equal, or is that likely to mess something up? Online filter calculators I've looked at seem to use a 2:1 ratio for these two resistors. What is the significance of this ratio and what are the effects of varying it?
Attached Images
File Type: jpg HPF.jpg (15.0 KB, 102 views)
  Reply With Quote
Old 28th June 2012, 11:47 PM   #2
Irakli is offline Irakli  United States
diyAudio Member
Irakli's Avatar
Join Date: Aug 2004
Location: washington, DC
Just google "unity gain sallen key filter". Even a Wikipedia page is very helpful.
My System
  Reply With Quote
Old 29th June 2012, 12:38 AM   #3
djoffe is offline djoffe  United States
diyAudio Member
Join Date: Oct 2008
assuming that everything about the first filter was as you wanted, except for the cutoff frequency, you could just increase the resistors by a factor of 75/40.

Alternatively, you could increase the capacitors by the same ratio...It's just usually easier to get closer with resistors.
  Reply With Quote
Old 29th June 2012, 08:39 AM   #4
dincz is offline dincz  Czech Republic
diyAudio Member
Join Date: Feb 2011
Thanks for the replies. I had already searched the web - including Wikipedia - but the answer seemed to be buried in layers of stuff way beyond me. Spent a few minutes with a design engineer this morning and problem solved.

Apparently changing both resistors (or capacitors) by the same amount just shifts the cutoff frequency, whereas changing their ratio alters the shape of the knee.
  Reply With Quote


Active highpass filterHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Highpass Filter Help Needed Greg M Multi-Way 5 19th May 2011 11:20 AM
Breadboard amp = echoes + highpass filter? dopeydavoid Solid State 2 12th April 2011 12:50 PM
Highpass filter via feedback cap scottw Chip Amps 12 24th October 2009 11:20 AM
Active highpass filter - ground skogs Analog Line Level 3 19th June 2007 06:25 PM
Highpass filter using sound card??? vsboxerboy Digital Source 0 18th July 2005 08:59 AM

New To Site? Need Help?

All times are GMT. The time now is 04:34 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio