What a difference a feedback resistor makes!?
 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

 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
 28th October 2003, 07:31 PM #2 MarcelvdG   diyAudio Member   Join Date: Mar 2003 Location: Haarlem, the Netherlands This doesn't sound abnormal to me. It seems like a normal result of the internal frequency compensation of the chip amplifier. Even with perfect resistors, if you use a conventional operational amplifier or a chip amp at a closed-loop gain considerably higher than the minimum for which its stability is guaranteed, it is normal that the circuit has approximately a first-order behaviour, with a small-signal step response with approximately a (1-exp(-t/tau)) shape. Theoretically, the time constant should be equal to tau=(R1/R2+1)/(2*pi*GBP), where R1 is the feedback resistance from the output to the negative input, R2 is the resistance from the negative input to ground or to the signal source, GBP is the gain-bandwidth product (in Hz) and pi is 3.14159265358979... If the amplifier is well-designed, the step response will gradually change into a well-damped second order response when you reduce the closed-loop gain to the minimum recommended value.
 28th October 2003, 07:33 PM #3 MarcelvdG   diyAudio Member   Join Date: Mar 2003 Location: Haarlem, the Netherlands I forgot to mention: for frequencies well below 1/(2*pi*tau), one would also expect the output sine wave to be delayed by a delay time approximately equal to tau.
 28th October 2003, 07:53 PM #4 kropf diyAudio Member   Join Date: Dec 2002 Location: Illinois, USA Thanks for the details about the delay. That makes good sense. It surprised me because I hadn't seen such an obvious shift from input to output before. Of course, I have worked mostly with DC in the past. Assuming the National Semiconductor literature to be accurate, the numbers work out to be very close to the measured 1.1 microsecond delay observed. That leaves the question of the poor sound... I'll have to go back and change one thing at a time and see what the effects are. Jeremy
 2nd November 2003, 09:36 PM #5 Pedja   diyAudio Member   Join Date: Jun 2002 Location: Belgrade Hi Jeremy, Something like 1us delay is exactly what I see running the LM1875 (similar GBW) model. This delay goes down together with the amps gain. However, I am not sure why there will be such a difference between small and large signal pulse response that you found. That is interesting. Joe Rasmussen found strange behavior above 320kHz and that started to happen rapidly at 330kHz. http://members.ozemail.com.au/~lisaras/design.htm (scroll down to the bottom half of the page) Note that he uses a higher gain (1M/(18k+4.7k+Zout) feedback). If this freq would be shifted with lower gain, I’d rather expect to see it shifted up. If you repeat that measurement with 100kHz (not bad to go even higher) using the cheaper/other resistors, please, keep us informed about it. It will be interesting to know if anything of this is related to the used (type or brand of the) resistor. Pedja
 4th November 2003, 06:48 AM #6 ppl   diyAudio Member     Join Date: Apr 2001 Location: Seattle, WA U.S.A. One thing to keep in mind is that Bulk Metal Foil Resistors are highly inductive while carbon composition and Metal Oxide are relatively low inductance. Bulk Metal Foil resistors while great at noise specs are not the best choice for feedback networks. This can cause time delay in the larger values because the self inductance of the resistor is larger at higher values and can get to the point of reacting with the parasitic capacitance both on Board and on the Chip to create a L/c resonant circuit.
 4th November 2003, 10:44 AM #7 pooge   diyAudio Member   Join Date: May 2003 Location: Northern Va. Bulk metal foil resistors are NOT highly inductive. They are particularly designed with a pattern to have very low inductance.
 4th November 2003, 11:29 AM #8 jan.didden   diyAudio Member     Join Date: May 2002 Location: The great city of Turnhout, BE I think, reading all the answers, that kropf really must do a comparison with a "normal" resitor. The thread is called "what a difference ....", but I cannot find what this difference is, if there is any. Without that, any cause attributed to the feedback resistor remains pure speculation, if not fantasy. Jan Didden
 4th November 2003, 03:31 PM #9 ppl   diyAudio Member     Join Date: Apr 2001 Location: Seattle, WA U.S.A. While Bulk metal foil resistors have alot lower inductance than wirewound thay are not as low as carbon compisistion and Metal oxide.
 4th November 2003, 04:29 PM #10 ppl   diyAudio Member     Join Date: Apr 2001 Location: Seattle, WA U.S.A. it appears that the caddock Bulk metal foil resistors are rather low in inductence the 10 nH inductence is real Low and on par with the Wima Polopropylene Box capacitors often used for bypassing thay also have about 10 nH of inductence the caddock resistors can be found at http://www.caddock.com/Online_catalo...000_Series.pdf

 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         Holton Precision Audio         CSS         Dx Classic Amplifiers         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 Ouroboros Solid State 810 21st May 2013 09:25 AM dantwomey Digital Line Level 10 16th June 2009 05:45 AM blindsjc Tubes / Valves 4 18th June 2008 10:40 PM Alexnick Multi-Way 12 27th January 2006 11:21 PM

 New To Site? Need Help?

All times are GMT. The time now is 01:33 AM.