How do I test an amp for Damping Factor?
 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
 Thread Tools Search this Thread
 13th August 2014, 07:27 PM #1 shredhead   diyAudio Member     Join Date: Aug 2011 Location: NC How do I test an amp for Damping Factor? Yeah, I get that damping factor is useless and it's really a matter of output impedance. If I understand correctly, output impedance of an amplifier varies with frequency? So that would mean that it would affect the frequency response of the amp right? I'm wondering how you would do an impedance sweep of an amplifier's output. Does anyone know the standard test that determines an amp's DF at a given frequency? Or do these companies just make up numbers?
 13th August 2014, 07:44 PM #2 Mooly   diyAudio Moderator     Join Date: Sep 2007 Well if you look it up you'll see damping factor is the ratio of the load impedance to the amplifiers output impedance. So a rough and ready way to estimate... if you set the amp to give (say) 10 volts rms output (with no speakers attached) as measured on a DVM (use something like 400Hz which any DVM should cope with), and then load the amp with a known resistance as a load (say an 8 ohm or 10 ohm resistor of suitable rating) you will see the 10 volts drop a little as measured at the speaker terminals. If we assume the amp internally is "perfect" as a voltage source then you can work out the damping factor. For example if you measured 10 volts and connecting a 10 ohm gave 9.8 volts then the internal resistance within the amp is dropping 0.2 volts. The perfect amp is delivering current I of 10/10 (I=V/R) which is 1 amp. The internal resistance is R=V/I which is 0.2/1 = 0.2 ohm. So the ratio 0.2 to 10 is 50 which would be the damping factor. That is a bit simplistic and there are other ways to measure damping factor more accurately and over a wider frequency range but if you only have a DVM it will get you in the right ballpark Try the same test at the end of the speaker leads too such that they are in circuit as well and their resistance is included. Now that amplifier with a claimed 500 damping factor isn't so much different to one of only 30 or 40.
 13th August 2014, 09:34 PM #3 jcx   diyAudio Member   Join Date: Feb 2003 Location: .. "tug-of-war" with a resistor load, another amp on the other end driven with your test signal such as a frequency sweep measure the V a your amp's output a soundcard is good down to its crosstalk limit for such a test
huseying
diyAudio Member

Join Date: Jan 2008
Location: İstanbul/Turkey
Quote:
 Originally Posted by Mooly Well if you look it up you'll see damping factor is the ratio of the load impedance to the amplifiers output impedance. So a rough and ready way to estimate... if you set the amp to give (say) 10 volts rms output (with no speakers attached) as measured on a DVM (use something like 400Hz which any DVM should cope with), and then load the amp with a known resistance as a load (say an 8 ohm or 10 ohm resistor of suitable rating) you will see the 10 volts drop a little as measured at the speaker terminals. If we assume the amp internally is "perfect" as a voltage source then you can work out the damping factor. For example if you measured 10 volts and connecting a 10 ohm gave 9.8 volts then the internal resistance within the amp is dropping 0.2 volts. The perfect amp is delivering current I of 10/10 (I=V/R) which is 1 amp. The internal resistance is R=V/I which is 0.2/1 = 0.2 ohm. So the ratio 0.2 to 10 is 50 which would be the damping factor. That is a bit simplistic and there are other ways to measure damping factor more accurately and over a wider frequency range but if you only have a DVM it will get you in the right ballpark Try the same test at the end of the speaker leads too such that they are in circuit as well and their resistance is included. Now that amplifier with a claimed 500 damping factor isn't so much different to one of only 30 or 40.
Thats a good way of testing amps' damping, i usually use this method but shredhead please keep in mind that the longer the cables wich you use to connect resistor to amp, the more wrong damping value you get , and also to decrease the cable's resistance use tick cable as posible as you can.
__________________
My ebay tube store
http://www.ebay.com/usr/hgungor10

Last edited by huseying; 15th August 2014 at 12:39 PM.

jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally Posted by Mooly For example if you measured 10 volts and connecting a 10 ohm gave 9.8 volts then the internal resistance within the amp is dropping 0.2 volts. The perfect amp is delivering current I of 10/10 (I=V/R) which is 1 amp. The internal resistance is R=V/I which is 0.2/1 = 0.2 ohm. So the ratio 0.2 to 10 is 50 which would be the damping factor.
Mooly, agree with most of your post but not on this. If the voltage drops to 9.8V with a 10 ohms load the Iout is not 1A but 0.98A...
Just to split some hairs

Jan
__________________
Advertising is the rattling of a stick inside a swill bucket - George Orwell
Get more Linear Audio for less! Check out my Autoranger and SilentSwitcher

 15th August 2014, 02:07 PM #6 Mooly   diyAudio Moderator     Join Date: Sep 2007 Mea culpa The train of thought wandered while typing and wielding Windows on screen calculator. Thanks
sgrossklass
diyAudio Member

Join Date: Nov 2004
Location: Germany
That's how you do it in theory and practice:
Quote:
 Originally Posted by jcx "tug-of-war" with a resistor load, another amp on the other end driven with your test signal such as a frequency sweep measure the V a your amp's output
Note that you can basically do this with two channels of one stereo amplifier as well, i.e. the resistor goes between R+ and L+, with playback on L and measurement on R. The resistor should be >10x the expected output impedance (current source approximation), but that shouldn't be an issue since you need something in the usual loudspeaker impedance range anyway. Sufficient load handling advised, shouldn't need to be anything too extreme though.

 16th August 2014, 06:38 PM #8 shredhead   diyAudio Member     Join Date: Aug 2011 Location: NC I want to test a new amp and an old one to see the differences in the 20-40Hz area but for the old one I'm scared that because it doesn't really have much in the area of protection circuits, it might release the magic smoke. Does anyone here have any experience in doing these 2 kinds of tests to an old quasi-complimentary dinosaur? Would one test be safer than the other?
 16th August 2014, 07:58 PM #9 Mooly   diyAudio Moderator     Join Date: Sep 2007 Just do the test at a couple of volts and then there is no danger of anything overheating. If you add a low value resistor (say 0.1 to 0.47) in series with the speakers then you automatically decrease damping factor down to at least 80 with a 0.1 ohm, and to just 17 with a 0.47 ohm. That assumes 8 ohm speakers and a perfect amplifier with a damping factor of zillions. Try it.
 16th August 2014, 11:43 PM #10 Kiriakos   Banned   Join Date: Jul 2014 Location: In from of my workbench There is no chance getting a good measurement with an multimeter in the range of 400Hz and up to 20k or 40kHz. This is an area only for modern Oscilloscopes

 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 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 chlorofille Solid State 4 3rd September 2010 09:37 PM jaya000 Solid State 25 25th September 2008 12:27 PM skrstic Class D 1 23rd April 2008 10:49 PM ben106 Tubes / Valves 9 26th September 2007 10:42 AM viatatto Tubes / Valves 0 27th August 2005 12:35 AM

 New To Site? Need Help?

All times are GMT. The time now is 08:18 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 © 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
Wiki