How does a summing circuit work in terms of input/output impedances?
 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

 Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

 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
 16th May 2003, 03:08 PM #1 Saurav diyAudio Member   Join Date: Sep 2002 Location: Oregon, USA How does a summing circuit work in terms of input/output impedances? Hi, Everyone says to use Y splitters to divide one signal (output) to multiple inputs, but not the other way round. I've seen passive summing circuits using only resistors, but most circuits use an opamp, with all inputs to be summed going to the inverting input. Here's a typical circuit: Also, the reason Y adaptors are a bad idea is because each source device sees the parallel combination of the destination's input impedance (should be high), and the other source's output impedance (should be low). Thus, the combined impedance is too low, and the source gets distorted. My question - in the above circuit, won't each source still see the other sources in parallel with the opamp's input impedance (which is really high, so it doesn't matter). So is it R1 R2 and R3 that are used to make the effective (paralleled) impedance high enough? So in this diagram, I'd probably want to use 100K or so for each resistor... so then each source sees 100K || 50K? And the gain is of course set by the ratio of R1/R2/R3 to Rf. I have other ways to adjust gain (this is to use a single subwoofer for 2-channel as well as 5.1), so all I'm looking for here is the summing action. One more question - my linestage is a autoformer based passive, so the output impedance at DC is a near short (it's just a long wire). Does that complicate things in any way? If I wanted to sum the left and right channels of my linestage's output (it has 2 sets of outputs), would this cause the other (main) pair of outputs to get shorted and become mono, or something like that? Thanks in advance. Saurav
 16th May 2003, 04:27 PM #2 Fred Dieckmann   diyAudio Retiree   Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles summing time and the living is easy The summing juction is a very low impedance. The inverting input is following the non inverting input voltage by the mechanism of the negative feedback of the op amp. http://www-s.ti.com/sc/psheets/slod006b/slod006b.pdf
mrfeedback
diyAudio Member

Join Date: Feb 2002
Location: Perth, Australia.
Re: summing time and the living is easy

Quote:
 Originally posted by Fred Dieckmann The summing juction is a very low impedance. The inverting input is following the non inverting input voltage by the mechanism of the negative feedback of the op amp. http://www-s.ti.com/sc/psheets/slod006b/slod006b.pdf
IOW, the summing point is at 'virtual ground'.
The opamp keeps the summing point at 0 V wrt to local ground reference.
Inability to do this constitutes distortion at the output terminal.

Eric.
__________________
I believe not to believe in any fixed belief system.

Saurav
diyAudio Member

Join Date: Sep 2002
Location: Oregon, USA
Thanks, that's the point that I was missing. Here's a quote from that op-amp article:

Quote:
 The opposite end of the resistor connected to the inverting input is held at virtual ground by the feedback; therefore, adding new inputs does not affect the response of the existing inputs.
I used to know all of this back in college, but it's been a long time since I've had to use any of it

Thanks once again.

Saurav

 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 FastEddy Subwoofers 48 29th November 2015 05:50 PM cmckasty Digital Line Level 1 11th May 2005 04:03 AM breez Chip Amps 5 1st February 2005 02:17 PM duffy Chip Amps 4 2nd December 2004 07:08 AM Dominique Parts 5 5th August 2004 12:19 PM

 New To Site? Need Help?

All times are GMT. The time now is 02:28 PM.