DC Offset Question - 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 26th September 2003, 04:36 PM   #1
diyAudio Member
 
Join Date: Oct 2002
Location: usa
Default DC Offset Question

Could someone please explain, or point me to a tutorial on DC offset. Or tell me in simple terms "what it is". I've searched the forum and the net. Lot of articles. But none just come out and says it simple enough for me. I'm not exactly a beginner, I've built a lot of amps. I know and use ohms law but I only have a volt-ohm meter and just use the trial and error method. Is the DC offset the reason for putting a resistor directly to ground on the input? Is there a formula for this resistor? How do I measure DC offset - just put my volt meter on the amp input with no input device connected? Is this DC offset also a problem on the output? I've never blown a speaker, so surely I'm doing something right.

Have read this forum word for word for a year now, and literally having a blast. Thanks so much for all the info.
__________________
Where there is smoke....there is fire. (usually one of my circuits)
  Reply With Quote
Old 26th September 2003, 05:09 PM   #2
DrG is offline DrG  South Africa
diyAudio Member
 
Join Date: Aug 2003
Location: South Africa
DC offset is a net, no-signal DC voltage measured at the amplifier output, which should be zero. DC is unhealthy for speakers...

The input resistor is not strictly for determining DCO, although it does establish a zero reference for the input signal. It determines input impedance.

It's possible for a small input-offset (eg due to DCO of source) to become amplified at the output. This is commonly avoided by using a series capacitor at the amp input or between ground and the second feedback resistor. A separate servo for DC feedback can also be used.
  Reply With Quote
Old 27th September 2003, 06:38 AM   #3
diyAudio Member
 
Join Date: Oct 2002
Location: usa
Does this mean that my volt meter should not register any DC voltage on the output to the speakers? Is that not what the output capacitor is for - to block the DC?
__________________
Where there is smoke....there is fire. (usually one of my circuits)
  Reply With Quote
Old 27th September 2003, 11:01 AM   #4
weeghel is offline weeghel  Netherlands
diyAudio Member
 
Join Date: Aug 2001
Location: Netherlands
Send a message via ICQ to weeghel
Quote:
Originally posted by willy
Does this mean that my volt meter should not register any DC voltage on the output to the speakers? Is that not what the output capacitor is for - to block the DC?
Indeed, your voltmeter shouldn't register a DC voltage on the output.

And yes, the output capacitor is also for blocking DC. However, output capacitors are not normally used to block small (<100mV) DC offsets. The whole point of a direct coupled (no output cap) is that you don't have/need a output capacitor.
But some amps (with a single supply, say from 0 to 20V) have the output of the output transistor at half the supply voltage. Output caps are necessary for those amplifiers (else, you would have a 10V dc voltage at the output and no way to get rid of it)
  Reply With Quote
Old 28th September 2003, 01:21 AM   #5
diyAudio Member
 
dutch diy's Avatar
 
Join Date: May 2001
Location: NL, The Hague
Send a message via ICQ to dutch diy
Default DC-offset

To add to earlier posts. It's virtually impossible to achieve exactly 0V DC offset over the entire temp / power range of an output stage without capacitor.
A DC offset within a range of +/- 50 mV is acceptable, lesser values are preferred.

EG: My DIY JLH amp changes from +40mV (after an initial peak of 150mV) at startup to somewhat in the region of +/- 3mV over an hour warming up depending on ambiant temp.
  Reply With Quote
Old 30th September 2003, 05:42 AM   #6
diyAudio Member
 
Join Date: Oct 2002
Location: usa
Thanks so much members for those simple, easy to understand explanations. One of the things that was throwing me off was that all the amps I have built so far have single supplies. So I''ve always got about half the supply voltage on the output.
__________________
Where there is smoke....there is fire. (usually one of my circuits)
  Reply With Quote
Old 30th September 2003, 07:00 AM   #7
Shaun is offline Shaun  South Africa
diyAudio Member
 
Shaun's Avatar
 
Join Date: Aug 2001
Location: Cape Town, South Africa
Willy,

One drawback of using that topology is that the output cap has to be very large if you want your amp to work down to very low frequencies. Direct-coupled designs generally don't have this problem. It is also best to avoid having the signal pass through a capacitor if possible, to avoid introduction of capacitor-induced distortion.

Most designs nowadays are direct coupled.

Regards
__________________
Shaun Onverwacht
|||||||||| DON'T PANIC ||||||||||
  Reply With Quote
Old 30th September 2003, 07:33 AM   #8
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Shaun, you are a bit unclear. If you have an amp with fed only with single supply, you must have a capacitor at the output, (unless it is bridge connected) and if you have dual supply as most amps have you never use an output cap.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 30th September 2003, 08:09 AM   #9
Shaun is offline Shaun  South Africa
diyAudio Member
 
Shaun's Avatar
 
Join Date: Aug 2001
Location: Cape Town, South Africa
Yes, of course. Sorry about that. Some designs need the output cap.


Quote:
DrG:
The input resistor is not strictly for determining DCO, although it does establish a zero reference for the input signal. It determines input impedance.

I always believed that the main purpose of this resistor was to establish DC refence to ground, and that the input impedance was a consequence of the resistor value.
__________________
Shaun Onverwacht
|||||||||| DON'T PANIC ||||||||||
  Reply With Quote
Old 30th September 2003, 08:49 PM   #10
DrG is offline DrG  South Africa
diyAudio Member
 
Join Date: Aug 2003
Location: South Africa
chicken, egg... egg, chicken... whatever bends your boerewors, Shaun
  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
Question on DC Offset davodavo Solid State 17 19th July 2009 11:37 PM
DC offset question tschanrm Class D 9 23rd February 2006 02:32 AM
DC Offset Question dsavitsk Chip Amps 14 10th May 2005 11:52 AM
dc offset question? crippledchicken Solid State 4 7th April 2004 10:37 PM
DC Offset question wheezer Solid State 2 13th January 2003 10:30 PM


New To Site? Need Help?

All times are GMT. The time now is 03:39 PM.


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