Cmoy resistor to ground
 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

 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 March 2011, 10:37 PM #1 yakideo   diyAudio Member   Join Date: May 2010 Cmoy resistor to ground Hello, I've been looking at and trying to understand the cmoy op-amp circuit. One thing that I couldn't figure out the use for was the R2 resistor connected from the input to ground. If C2 is used to block dc voltage what is the use of R2?
 26th March 2011, 10:41 PM #2 Atilla   diyAudio Member   Join Date: Jul 2008 C2 and R2 together form the high-pass filter that blocks the DC on the input.
yakideo
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by Atilla C2 and R2 together form the high-pass filter that blocks the DC on the input.
Would removing R2 still block the DC current? I mean why the high-pass filter, unless it would damage the headphones?

 27th March 2011, 06:59 AM #4 Mooly   diyAudio Moderator     Join Date: Sep 2007 R2 is vital and the circuit will not work and will quite possibly damage the headpnones without it. It defines the DC voltage on the + input of the opamp On a practical level, the value of 100k in combination with 0.1uf is not ideal for good low frequency response. I would suggest at least a 0.47uf cap. Filter Circuits with Capacitors [Also... from a theoretical point, the resistor modifies the "law" of the pot and adding a resistor of the appropriate value can be used with a linear pot to give a pseudo log response. You would need a much lower value than 100k though.] Edit... and why use C2 at all ? Firstly because you can not guarantee that the source feeding the amp will have no DC present. Second... any DC current that flows in the wiper of the pot tends to make it very crackly and noisy as it is turned. Thirdly... depending on the opamp, the DC offset will change as the pot is rotated. FET opamps will not exhibit that behaviour in this circuit. Last edited by Mooly; 27th March 2011 at 07:03 AM.
yakideo
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by Mooly It defines the DC voltage on the + input of the opamp
I though that C2 was there to block any DC?

Quote:
 Originally Posted by Mooly On a practical level, the value of 100k in combination with 0.1uf is not ideal for good low frequency response. I would suggest at least a 0.47uf cap.
Right, a 0.47uF cap would lower to cutoff frequency to 3.4Hz.

Quote:
 Originally Posted by Mooly Second... any DC current that flows in the wiper of the pot tends to make it very crackly and noisy as it is turned.
Wouldn't it be better then to have C2 (possibly R2) before the wiper?

 27th March 2011, 05:26 PM #6 Mooly   diyAudio Moderator     Join Date: Sep 2007 Not easy to explain quickly C2 is to block any DC from the source and to stop it affecting the opamp in any way. The two inputs of the opamp must have their DC conditions exactly defined... and that means tying or referencing the + input to ground. The - input has it's DC conditions set by the feedback network. I can understand your confusion. If it were AC coupled only the + input would just quickly drift to some ill defined level. The opamp inputs don't assume zero volts when not connected to anything. The golden rule of design for 99% of opamp applications is, "The output will do whatever is neccessary to keep the voltage difference between the inputs at zero Try and understand that, put some numbers in and have a go at calculating the output and feedback voltages and you'll see how it makes sense. Have a read at this, http://users.ece.gatech.edu/mleach/e...4/OpAmps01.pdf Location of C2 ? Good question. Where it is now stops any DC current in the wiper of the pot and that's essential to keep it quiet. Some opamps such as bjt types (NE5532 etc) have a significant current flowing from the inputs (it's in the data sheets) and that is enough to make the pot noisy. If you used a FET opamp then there is essentially zero current and so you could couple directly to the wiper and move the cap but that is not considered good practice... but yes you could

 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 chingyg Chip Amps 9 21st December 2009 03:48 PM TheSeekerr Chip Amps 2 23rd July 2008 09:20 PM sled108 Chip Amps 6 3rd June 2008 11:57 AM lanchile07 Chip Amps 11 23rd November 2007 02:16 PM markiemrboo Chip Amps 5 9th August 2007 03:50 PM

 New To Site? Need Help?

All times are GMT. The time now is 05:16 AM.