DC Offset Question
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 26th September 2003, 04:36 PM #1 willy diyAudio Member   Join Date: Oct 2002 Location: usa 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)
 26th September 2003, 05:09 PM #2 DrG   diyAudio Member   Join Date: Aug 2003 Location: Pretoria 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.
 27th September 2003, 06:38 AM #3 willy 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)
weeghel
diyAudio Member

Join Date: Aug 2001
Location: Netherlands
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)

 28th September 2003, 01:21 AM #5 dutch diy   diyAudio Member     Join Date: May 2001 Location: NL, The Hague 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.
 30th September 2003, 05:42 AM #6 willy 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)
 30th September 2003, 07:00 AM #7 Shaun   diyAudio Member     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 ||||||||||
 30th September 2003, 07:33 AM #8 peranders   Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden 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 PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid.
Shaun
diyAudio Member

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 ||||||||||

 30th September 2003, 08:49 PM #10 DrG   diyAudio Member   Join Date: Aug 2003 Location: Pretoria chicken, egg... egg, chicken... whatever bends your boerewors, Shaun

 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 Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Aussie Amplifiers         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post davodavo Solid State 17 19th July 2009 11:37 PM tschanrm Class D 9 23rd February 2006 02:32 AM dsavitsk Chip Amps 14 10th May 2005 11:52 AM crippledchicken Solid State 4 7th April 2004 10:37 PM wheezer Solid State 2 13th January 2003 10:30 PM

 New To Site? Need Help?

All times are GMT. The time now is 09:32 AM.

 Home - Contact Us - Advertise - Rules - diyAudio Store - Sponsors - Privacy Statement - Terms of Service - Top - Opt-out policy

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.