Summing mixer  - Page 1
Forum section: electronics / beginners

author: Jbliss
author_rank: Regular Contributor
timestamp: June 14, 2016, 12:08:52 pm
content: HiIm looking a building a Inverting Summing Amplifier like thishttp://masteringelectronicsdesign.com/wp-content/uploads/2010/01/3_input-summing-amplifier.pngPretty Simple. However just wondering if it is possible to sum 20 inputs using this design or would it be better to say make 5 of the summing circuits and then sum the outputs of them together. The signals are Line level Audio signalsThanks you for any help

author: dmills
author_rank: Super Contributor
timestamp: June 14, 2016, 01:39:38 pm
content: What is on the back of that link is not an inverting stage, but assuming you are in fact building a standard virtual earth stage, you can indeed do 20 inputs, but the noise tends to come up because the noise gain increases with each input you add.This may or may not be a serious problem in your application, and there is some good discussion of the subtle points in "Small Signal Audio Design" by Douglas Self.Regards, Dan.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 14, 2016, 03:09:55 pm
content: Hi Danthanks for the reply. is there a way I can combat the noise issueThanks

author: Jbliss
author_rank: Regular Contributor
timestamp: June 15, 2016, 06:34:42 am
content: Would it be better to do it in stagesQuote from: dmills on June 14, 2016, 01:39:38 pm

author: danadak
author_rank: Super Contributor
timestamp: June 15, 2016, 10:31:02 am
content: Short answer is depends on G thru signal path.Take a look at page 47 -http://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Motchenbacher_Connelly/Low-noise_Electronic_Design.pdfRegards, Dana.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 15, 2016, 11:19:34 am
content: Quote from: danadak on June 15, 2016, 10:31:02 amThanks for the link Interesting.would isolating each input reduce noise. Eg using a bufferThanks

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2016, 07:48:21 am
content: Quote from: Jbliss on June 14, 2016, 12:08:52 pmWhat's the intended gain and bandwidth?The op-amp sees all the input resistors in parallel so with 20 of them, negative feedback will be 1/20th of what it would be with one input. For example, if the design gain is unity, then expect to see the same, bandwidth, transient response, output impedance etc. as you would if the same op-amp were configured as an inverting amplifier with a gain of 20.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 16, 2016, 01:34:30 pm
content: Quote from: Hero999 on June 16, 2016, 07:48:21 am

author: Jbliss
author_rank: Regular Contributor
timestamp: June 16, 2016, 01:38:12 pm
content: Would it be better to do it passively.http://wiki.diyrecordingequipment.com/wp-content/uploads/2011/12/schematic.pnghowever i would lose gain due to the resistors ?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2016, 10:24:29 pm
content: Quote from: Jbliss on June 16, 2016, 01:38:12 pmThe passive option would work and yes you're right, there will be some attenuation, by a factor of 20 minimum.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 17, 2016, 01:03:23 pm
content: Ok thanks every onewhat would be the best option for summing 20 Channels of audio each with there on volume pot and no attenuation due to summing resistors (use of preamp) also the least Noisy??thanks you all very much.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 18, 2016, 07:42:25 am
content: Quote from: Hero999 on June 16, 2016, 07:48:21 amI assume this would this Cause distortion as well ??

author: ruairi
author_rank: Frequent Contributor
timestamp: June 18, 2016, 07:55:09 am
content: Hey J,Mastering engineer here, 22 years into the game. A question - why are you building a summing mixer? (I know there are several possible answers, curious to hear yours specifically).Cheers,Ruairi

author: Jbliss
author_rank: Regular Contributor
timestamp: June 18, 2016, 10:55:09 am
content: Quote from: ruairi on June 18, 2016, 07:55:09 amWell its actually for monitoring a rack of radio Mics for a music theatre show. So the quality does not have to be amazing as it not or studio work.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 19, 2016, 11:32:20 am
content: Quote from: ruairi on June 18, 2016, 07:55:09 amWhat would you recommend thanks.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2016, 12:43:50 pm
content: Why not just use an ordinary, inverting summing amplifer?http://www.electronics-tutorials.ws/opamp/opamp_4.html

author: Jbliss
author_rank: Regular Contributor
timestamp: June 19, 2016, 12:59:00 pm
content: Quote from: Hero999 on June 19, 2016, 12:43:50 pmHi Thanks your reply. Thats a great idea just wondering how much noise there would be.Thanks

author: Jbliss
author_rank: Regular Contributor
timestamp: June 19, 2016, 01:03:28 pm
content: Quote from: Hero999 on June 19, 2016, 12:43:50 pmAlso what is the benefit of using a Inverting summing amplifier instead of a non invertingThanks

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2016, 01:09:19 pm
content: The amount of noise depends on the type of op-amp used and the source impedance.What sort of microphones are they?

author: Jbliss
author_rank: Regular Contributor
timestamp: June 19, 2016, 01:15:50 pm
content: Quote from: Hero999 on June 19, 2016, 01:09:19 pmI would be a line level output from a RF Mic receiver.

author: Hideki
author_rank: Frequent Contributor
timestamp: June 19, 2016, 01:46:20 pm
content: The typical way to mix audio signals (in an audio mixer) is the inverting circuit shown above, and NOT the non-inverting one from the first post. It will sum the currents at the virtual earth node, and you avoid each input interfering with the others.Noise from the mixer itself isn't going to be a problem for your application as long as you keep the resistor values reasonably low. Less than 10k would be good.

author: ciccio
author_rank: Frequent Contributor
timestamp: June 19, 2016, 02:39:23 pm
content: When I read the first post I though the OP wanted to buid a "standard mixer".Now I understand that his needs are simpler.He has 20 line level, low impedance outputs.If he does not need individual volume controls, a passive mixer (it does not needs to be balanced) will be enough.Using 10K resistors everywhere, it will have an attenuation of 20 times, eg - 26 dB.I think he will need some monitoring amplifier, that can easily supply the gain needed to compensate the attenuation.The amplifier will have a volume control, to adjust monitoring level.If he wants to go into experimenting, all this can be built with a "chip amplifier" (LM3886 or equivalent, boards can be found on ebay) summing the channels' resistor on the volume pot.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2016, 10:05:48 pm
content: Quote from: Jbliss on June 19, 2016, 01:15:50 pmI'd just use the circuit linked to in my previous post with 1k resistors all round and an NE5532 op-amp.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: June 20, 2016, 04:17:37 am
content: Might want to stick a small cap on each input too to block any DC.Also perhaps a pot for each input (along with a smaller fixed resistor) to set relative levels for each source?

author: Jbliss
author_rank: Regular Contributor
timestamp: June 20, 2016, 01:21:41 pm
content: Quote from: Hideki on June 19, 2016, 01:46:20 pmThanks everyone !!! great help all of you. So would say a 10 k Pot then in to a 1uf cap then a fixed 1k resistor??thanks again

author: Jbliss
author_rank: Regular Contributor
timestamp: June 27, 2016, 01:54:25 pm
content: Quote from: Hero999 on June 19, 2016, 10:05:48 pmSo you would say 10k potentiometer then a 1k fixed

author: Audioguru
author_rank: Super Contributor
timestamp: June 27, 2016, 03:00:40 pm
content: An inverting opamp with 1k input resistors cannot use 10k volume controls because the 1k will load down the 10k control so much that it will be difficult to adjust the level. Maybe use 20k input resistors fed from 10k volume controls.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 28, 2016, 01:56:13 pm
content: Quote from: Audioguru on June 27, 2016, 03:00:40 pmThanks for your input will definitely implement.So would you Have the pot first then the fixed or other way.thanks

author: Audioguru
author_rank: Super Contributor
timestamp: June 28, 2016, 03:38:50 pm
content: Quote from: Jbliss on June 28, 2016, 01:56:13 pmIf the input goes to the fixed series resistor which feeds the pot to ground then when the pot is turned down the low frequency gain of the opamp will be a few hundred thousand and its output noise will be very very high. Actually the output of the opamp will be as high as it can go and it will not amplify because it will be amplifying its DC bias voltage or its input offset voltage instead.

author: Jbliss
author_rank: Regular Contributor
timestamp: June 29, 2016, 01:29:01 pm
content: Quote from: Audioguru on June 28, 2016, 03:38:50 pmso pot first then fixed. thats what most of the circuits I have seen implementthanks

author: Jbliss
author_rank: Regular Contributor
timestamp: June 30, 2016, 02:06:19 pm
content: Quote from: FlyingHacker on June 20, 2016, 04:17:37 amso say a 10k Pot then a 20k Fixed resistor.Thanks