DC blocking capacitors - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 25th October 2005, 12:32 AM   #1
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
Default DC blocking capacitors

I have found that I need to add some DC blocking capacitors to my LM3875 amplifier and LM833 Preamp.

I get large amounts of DC amplified in the preamp due to the source equipment and it causes my sub to pull in about 3/4" when on max volume. At half volume the DC causes it to pull in about 1/4"

First, where do I need the capacitors, and second, what value/type do I need?

Thanks,
Mike
  Reply With Quote
Old 25th October 2005, 12:42 AM   #2
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
If the DC is coming from your preamp, then all you'd need is a cap at the input of your amp, or output of your preamp. The value depends on frequency response, the -3dB point would be given by 1/2piRC, R being the input impedence of your power amplifier and C being your cap. So, bigger cap yeilds lower response, but you wouldn't need to large a value at all. And try and use a polyprop and stay away from the electrolytics.
  Reply With Quote
Old 25th October 2005, 12:51 AM   #3
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
The amount varies depending on the source. For example if I use my plain CD player it's not that bad, but if I use my active sub crossover, its terrible.

So I'm assuming it's caused by the input source, and I need one on the preamp input. But could one on the amp input hurt as well, in case some is caused by the preamp?

The input impedence of both the preamp and power amp is 500ohm/channel.

That was a mistake, and I've been planning on changing them to the 10Kohm they should be.

In that equation where does the frequency at which it rolls off come into play?

I don't want it to start rolling off until at least 20hz so sound isn't messed up at all. Using the input impedence of 500ohm, how would I set this up? I think it's wrong, but with 20=1/2pi*500*c I got C to equal .025uf. Does this sound right?
  Reply With Quote
Old 25th October 2005, 01:08 AM   #4
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
I'm at work and don't have a calculator but, you'd need a larger value of C than 0.025uF, the value of C is in F and the value of R is in ohms. With a 500R input impedance, you'll need an input cap of at least a few uF to get down under 20Hz. But, you really should change the input impedance from 500R to something reasonable, at least 10k.

Also, it would depend on your source, but it probably isn't good to drive a 500R load.

Adding a cap to the input of your amp and preamp is never bad practice, with a higher input impedance, you'll need very low values so good quality caps are still cheap at those low values.

But the first thing I'd do is change input impedence, for sure...
  Reply With Quote
Old 25th October 2005, 01:36 AM   #5
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
Thanks for the reply.

In your equation is the resistance measured in ohms or Kohms? So am I right using 500, or should I use .5? And for the 10k resistor, should I use 10000 or 10?

Thanks again,
Mike
  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
DC Blocking Capacitors dome406 Chip Amps 18 4th May 2011 05:07 PM
DC Blocking on Pre-Amp Puffin Chip Amps 8 22nd January 2009 08:08 PM
DC Blocking fdeck Digital Line Level 5 12th November 2008 03:55 PM
Favorite film capacitors for DC blocking? The Peasant Parts 18 1st December 2006 04:37 PM
BZLS (aka, BoSoZ?) Blocking Capacitors...what am I missing??? NV&H Pass Labs 7 5th February 2005 08:03 PM


New To Site? Need Help?

All times are GMT. The time now is 08:43 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