Bipolar PS plus AC coupled output from opamp? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 27th July 2009, 04:21 PM   #1
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Default Bipolar PS plus AC coupled output from opamp?

Just wondering, if implementing a power opamp which uses BOTH a bipolar power supply and a big electrolytic to AC couple the output, are any special tricks required to guarantee a bias across the cap? Something like diodes from each rail to the opamp output? Sorry if this seems like a trivial question, but I had trouble searching for this info.
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 27th July 2009, 04:54 PM   #2
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Do you speak English?
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers. -- Plato, 447-367 BCE
  Reply With Quote
Old 27th July 2009, 05:07 PM   #3
diyAudio Member
 
RocketScientist's Avatar
 
Join Date: Mar 2008
Location: Northwest
If you need much voltage swing, don't use a polarized cap. If the required output voltage swing (peak) is much smaller than your supply rails, you could create an intentional DC offset.

But a much better solution is to use audio-grade bipolar electrolytics (search on Mouser.com for "audio electrolytic" or "muse") or a film capacitor depending on the value needed.

If you explain more about your application it would help? What lower cutoff frequency do you need? What impedance would the cap see in whatever it's driving?
  Reply With Quote
Old 27th July 2009, 05:21 PM   #4
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Thanks for your reply. As for application, it is a DIY guitar practice amp with an LM3875 for the output. Rails are roughly +/-25VDC The reason for the bipolar PS is that it keeps parts count down as I don't have to bias and AC couple the front end stages. The reason for the big electrolytic is to preclude any thump or DC of any kind on the speaker. A big electrolytic is cheap, and the negatives don't really come into play for a guitar amp, so that's what I want to use.
Attached Images
File Type: jpg accoupledoutput.jpg (7.8 KB, 95 views)
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 27th July 2009, 07:02 PM   #5
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
Go for it,no biasing or anything is needed. Just make sure the input to the next (power) amp stage isn't floating,and you're good.
  Reply With Quote
Old 27th July 2009, 07:14 PM   #6
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
Surely it would make sense to use two small cheap ordinary electrolytic capacitors to AC couple the input and a capacitor in the feedback path than one large more expensive capacitor in the output?

The LM3875 has pretty low offset when done this way, and no thump. I use an LM3886 in a similar fashion and get no poweron/off thumps. I do not use the Mute pin on that chip (just 22K to -V, no capacitor)
  Reply With Quote
Old 28th July 2009, 11:12 AM   #7
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
the big cap on the output will have to charge on start up. That will thump.
Use a delayed relay to connect the speaker to the output.

You can also use this same relay for disconnect when the amp or the source goes faulty, or on loss of mains supply.
__________________
regards Andrew T.
  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
Opamp?? FET vs Bipolar evo9 Chip Amps 9 4th August 2013 11:25 PM
Cancelling DC Offset in a bipolar Opamp ygg-it Chip Amps 16 5th May 2008 04:47 PM
Opamp that can be dc coupled ? beppe61 Solid State 37 20th January 2008 08:21 AM
What preamp should i build for my 3886 GC??Tube?MOSFET?OPAMP?Bipolar?? thomgun_lc Chip Amps 45 14th August 2005 04:27 PM


New To Site? Need Help?

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