My arch nemesis!
Coming into this a little less green but I still expect to make lots of mistakes.
I’ve done a few quiet practice OP amp circuits with success, but always simple isolated examples copied from art of electronics.
Something I’ve always wanted to learn is getting a stereo audio input from this 3.5 stereo TRS input and summing it into mono.
What I think I need is an inverting summing amplifier.
I feel uncertain because I don’t truly understand AC (with or without opamps).
Although I do feel like I understand audio wave forms and how that could be expressed by the push and pull of alternating current, I have zero intuition for what it means to have negative voltage.
Because I’m uncertain, I’ve made this schematic which I’ve blindly copied from here. Hopefully I got that right.
Best I can figure is that, since op-amps have inf input impedance, the current across R3 must be the sum of R1 and R2 simply because it has no-where else to go.
You can see I’ve chosen an LM358N but that’s just because I have used it the most when experimenting. I could believe there are superior choices for my application but I’m not sure what my search term should be? I’ve tried “Stereo to mono op amp schematic” and “Inverting summing amp best chip”. What comes out are some 240V circuits for pro-audio, nothing that makes sense for my 3.3v circuit. What should I search to learn more?
Please note that I don’t mind if the audio is inverted. The phase isn’t relevant to my application.
Yeah that was a great read. Thanks Jeff.
What did you google to get that link? “Summing Amp”?
One of my concerns in my post was working with AC.
I’ve noticed this circuit, which I’ve taken from the article you’ve linked, looks like my* schematic but with some caps in series with the audio. I think they are coupling capacitors like I’ve seen in PNP transistor small signal amplifier circuits.
Any thoughts as to why are they necessary in this circuit?
(coupling caps? What DC are they isolating?)
They say input filters but I don’t see what they would be filtering or why?
Hi Pix
Lets pull this apart.
Firstly there is an extra amp shown because the LM358 is a DUAL package. Last time I looked Dual meant 2, therefore 2 amps in the one package.
A quick look at the Data sheet would have confirmed this.
Why are you stuck with 3.3V. That is a hang up from the digital world. You are now getting into the analog scene where at 3.3V you are going to run out of head room, particularly as I don’t think that Op amp is rail to rail.
inverting is the only “summing” amplifier you can have I think. I don’t believe you can “sum” by inserting into the non inverting input.
Yes, that set up will do that but first a couple of points.
I just got side tracked for a while and just read the last few replies so will continue.
The caps are to get rid of any DC component on the AC audio signal.
If you follow the circuit you just posted you will need a positive AND negative supply or you will just get a rectified version of the audio which would sound very ordinary.
You will need a lot more than 3.3V. I would suggest plus and minus about 5V or even more.
Getting back to your original circuit the little triangle representing GROUND suggests a single supply and this is AUDIO GROUND (distinct from power ground. Caps should still be used on the input.
This “audio” ground is half Vcc or if the single supply is say 6V this point is 3V above power ground. This has the effect of shifting the AC signal 3V positive so all the "AC is now a varying positive DC. This will be the case right through the system until AC is restored by the inclusion of another series capacitor.
So you see if you only have a 3.3V single supply you will probably run out of puff and start clipping right at the first stage as most Op Amps will not swing the output the full supply voltage, you need quite a but of head room.
A bit rough but it is not exactly like that. You will need to swot up on the function of Op Amps. This could be a bit long winded to go into here so on with the reading glasses. While you are at it find out a bit about AC.
Of course if you already have a Mono amplifier the easiest way is to just connect the L and R channels together via a couple of about 5kΩ resistors and feed that into the Mono input. Works every time.
Cheers Bob
With a split (positive and negative) power supply Yes. With a single supply NO. You will get half of the input signal ie; half wave rectified. Unless you have a dummy “Ground” at half Vcc like I mention above.
Back to the reading glasses and research material.
By the way if you do research with Utube be a bit wary. Some of it is good and some is absolute garbage. You will need some understanding of all this so you have a chance of filtering out the rubbish and believe the good. So read solid material first.
Cheers Bob
To learn about stuff you need good reference material. Forums and YouTube videos often do not provide any useful information, sometimes they do but rarely in my experience.
Product data sheets by manufactures provide a wealth of good information.
The “Analog Engineers Circuit Cookbook: Op Amps” by Texas Instruments is an excellent example of quality reference material. There are many others too.
Courses provided by Tafe Colleges or Institutes of Technology are an excellent place to learn about Electronics or other subjects in a logical and quality manner.
If your looking for the quick path you won’t find it.
As @Robert93820 says read read read read.
The data sheet for the LM358 says it will work down to 3V.
But in my experience this is at the very limits of functionality. The data sheet does not mention audio. The Texas Instruments web site has a page for Audio Op Amps. Op Amps are not power amps, there is another whole range of devices that will provide quality audio amplification.
If you just want to combine stereo to mono, you only need two resistors as @Robert93820 mentioned.
Hi Jeff
Interesting.
I was always under the impression that the non inverting arrangement would output whichever was the highest input. Particularly in the unity gain configuration where the output is connected directly to the inverting input.
When I get a bit of time I will have a close look at that article and do the experiment.
I have not had a close look but as you say it seems to be pretty complete and should explain all.
I have mostly dealt with the inverting arrangement as used as the classic audio mixing role in commercial kit.
Cheers Bob
Great resources recommended here.
I found the cookbook here which looks like a fun read. Thanks @James46717
Totally, that’s why I added it to my schematic. I’m wondering if I could use it for a volume control. It was just a thought, it’s in the chip so I might as well use it.
Just because my power supply was 3.3v.
I can see the value of working with a higher voltage. I’ll switch to 5v
Yeah I think I understand. I need the coupling caps because the OPamp is DC because I’m powering it with a dc power supply.
Well that sounds interesting. I’ll do some research on that.
Well well well is it really that easy. Classic pix over complicating things.
Still there will be some situations an Op amp will be preferred. Good to have resistors in my back pocket as an option.
Hi Jeff
Had a closer look at the non inverting summing amp.
All very interesting but it also has the statement
“Note also that if the amplifier of the summing circuit is connected as a unity follower with RA equal to zero and RB equal to infinity, then with no voltage gain the output voltage VOUT will be exactly equal the average value of all the input voltages. That is VOUT = (V1 + V2)/2.”
Which is the way I have usually found this scenario. This holds true if the source impedance of the inputs is low and the 2 series input resistors can act as a voltage divider. If however ,as is often the case, there is a diode in the way and a high value resistor to ground as in the case of a half wave rectifier charging a cap with a high value R to discharge the cap this resistor is usually too high to allow the input Rs to act as a voltage divider and the output will follow the highest input voltage. Like sharing power supplies via diodes.
Assuming a low Z input and you want to have this as a summing amp you have got to provide some gain. That is if you study the text it says the gain has to be equal to the number of inputs so if you have 3 inputs the gain has to be 3. In effect you are outputting the average multiplied by the number of inputs which will indeed be the sum of the inputs.
Of course all this goes out the window if the inputs have series diodes as mentioned above.
As I see it the disadvantage here could be you have to know the number of inputs to ascertain the feedback network values before building.
On the other hand the inverting system has the capability of using and adding any number of inputs on the nun. The same thing is occurring but additional series resistors are effectively in parallel with existing ones so the overall value if the source Z is reduced so increasing the gain sort of automatically. Disadvantage here of course is the split supply requirement.
I presented a circuit recently to overcome this diode problem when the output from 2 sources need to be averaged. I used 2 Op Amps as unity gain buffers providing a low Z source impedance to drive a third OP Amp via 10k Resistors connected as a unity gain amp. This provides an overall output of the true average of the 2 inputs.
Don’t know the overall outcome yet but on test the OpAmp circuit worked wel.
The circuit is in this thread.
Yes. The non-inverting summing amplifier has a number of features that distinguish it from the inverting summing amplifier, as would be expected, but your comment implied that there was no such thing and I think that was worth remarking on. I don’t see much purpose in repeating the points of difference already identified in that article or in commentary relating to a different topic.
Jeff please note the words “I think”
I did not expressly say there was no such thing. Only that I have not seen a summing amp in that form in any equipment or usual applications I have had anything to do with.
If you are not interested in the averaging system in the other thread don’t click on the link.
Cheers Bob
Everything we discussed works.
Two parallel resistors brought together sums to mono fantastically and a pot in series acts as a reasonable volume control, this is certainly the simplest.
With that said for some of my applications I’m going to need a feeding to a low impedance input. I tried a few of the schematics and they worked great. A voltage follower after the potentiometer brings the audio signal into phase and is suitable for low Z out requirements.
I have specialist audio equipment including signal generators and professional DACs and ADCs. An osciloscope would be the simpler and superior method and I hope to own one some day.
No it doesn’t. The output of a voltage follower is in phase with the input.
Correct. That is where the voltage follower really comes into its own. Hi Z in and low Z out (usually about 75Ω). A good buffer where you might want to drive multiple inputs from the single output. Also will provide a good LOW Z half rail “Ground” where one is needed.
Also good for isolating things like a longer cable which usually behaves itself better when driven from a low Z source.
Cheers Bob
Hi Pix
Just had a look at that link.
For that to produce the result pictured (signal going from + 5V to -5V) the author does not indicate but he would have to be using a split (±V) power supply.
I bring this to your attention in case you connect such a circuit to a single supply and be disappointed with the result.
Classic pix over complicating things.
