Op Amp Audio Summing Mixer – Output Noise Issue - Page 1
Forum section: electronics / projects

author: Halban
author_rank: Contributor
timestamp: February 21, 2025, 12:20:37 pm
content: Hello everyone,I'm working on a simple summing mixer for an electronic musical instrument project. The circuit combines four audio inputs at Eurorack levels (-5V to +5V) and includes a potentiometer to control the output volume (ranging from -∞ to 0dB).I've already built this circuit on a PCB with proper ground planes and power supplies:The mixer functions well overall, but I’ve noticed a noise issue at the output. The noise is present even when no signals are connected to the inputs. Additionally, the noise level increases as I turn up the output potentiometer. When the pot is fully turned down (-∞), the noise disappears completely.What could be causing this, and how might I fix it?Thanks in advance for your help!

author: magic
author_rank: Super Contributor
timestamp: February 21, 2025, 03:49:58 pm
content: Probably Johnson noise. If so, the solution is to use lower value resistors. R83 in particular could be a short for a free 3dB improvement.

author: edavid
author_rank: Super Contributor
timestamp: February 21, 2025, 03:59:42 pm
content: Quote from: Halban on February 21, 2025, 12:20:37 pmWhat happens if you ground all the inputs?QuoteIt could be picking up EMI, or possibly oscillating.Quote from: magic on February 21, 2025, 03:49:58 pmI don't know the bandwidth of the circuit, but even with 1MHz bandwidth a 10K resistor only produces 12 microvolts RMS noise

author: AussieBruce
author_rank: Regular Contributor
timestamp: February 22, 2025, 03:56:20 am
content: Just a couple of observations that may or may not be related to noise.....You may be better off with a non-polarised capacitor on the output, the DC level at the opamp output will be very close to zero volts.The function of the series resistor on the output is to avoid possible instability due to load capacitance if the output cable is long. You should be able to reduce this to 100 ohms, a lower value will be better in terms of frequency response.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 22, 2025, 04:07:15 am
content: Is this by chance an SMT design with thick film resistors? Stick with metal films only. As above, make sure it's not oscillating.

author: Halban
author_rank: Contributor
timestamp: February 23, 2025, 07:02:35 pm
content: Hello, thanks for all you ideas! I will try them and get back to you.To sum up:* use lower resistors values* short R83* use metal film resistors (as I'm using SMD thick film resistors)* test: ground all inputs* other improvement: use 100R for the end resistor and non-polarized capacitorHow to make sure this circuit is not oscillating? I'm quite sure this is the sound of noise, I'm not hearing a single-frequency oscillation...Thanks!

author: magic
author_rank: Super Contributor
timestamp: February 23, 2025, 09:22:13 pm
content: Oscillation would likely be above audio band, so you need a scope on the output to see it.BTW, make sure you have some supply decoupling capacitors - none are shown on the schematic.

author: CaptDon
author_rank: Super Contributor
timestamp: February 25, 2025, 02:39:50 pm
content: If you are using line level inputs (keyboard, drum machine, MIDI voice module) this circuit should be relatively noise free (70 to 80 dB below 'average' signal level ONLY if connected to a LINE INPUT on your mixer board with the mixer board pre-gain control properly adjusted so your fader is running in the -3 / 0 /+3 range of its travel. A lot of noise reduction has to do with gain structure. If your gain is set up improperly such as running the fader on the mixer at -20 then yes you will have poor signal to noise ratio. If you are using your home made device to mix mic levels (-50 dB) at the inputs and then also feeding your device output to a mic level input on your mixer then your circuit design is improper and will produce very poor signal to noise ratios such as that which you appear to be seeing. A proper mic level 4-input mixing circuit is much more complicated to get an acceptable SNR. One of the very best beginner books on the art of audio in all of its studio and live venue applications is the Sound Reinforcement Handbook Written for Yamaha by Gary Davis & Ralph Jones. I own the second edition. It is a Hal Leonard book HL00500964. As a Broadcast Engineer and Sound Engineer since 1983 I still refer to this book when trying to explain things to new hires and band members. The ISBN is 0-88188-900-8. Best wishes, Cheers mate!

author: MathWizard
author_rank: Super Contributor
timestamp: February 25, 2025, 04:49:59 pm
content: What op-amp are you using? have you tried other ones ?I rebuilt 1 of my 5.1CH stereo's, it had lots of larger resistors than 10k, around the op-amps. And lots of cheap electrolytic caps, among others. And the way I soldered it up was a close packed nightmare. And a bunch of it is unshielded. But all in all, it sounded fine.Now I have a better 6-CH stereo, so I need to do away with my separate sub-woofer, and use the extra CH. I never crank up the volume anymore, so it will be perfect.

author: Halban
author_rank: Contributor
timestamp: February 25, 2025, 05:09:01 pm
content: @CaptDon : thanks a lot, that's super interesting, I'm already reading the "small signal audio design" by Douglas Self and I may read this one also@magic : there is no oscillation on the output, and I'm using decoupling capacitors (100nF on both rails).@MathWizard : I've indeed tried an other opamp and get the same result.I've lowered resistors values with thin film resistors, I've shorted R83 and it changed nothing.-------But I've finally found that the noise does not come from the mixer!- When I short INA, INB, INC and IND to ground, I get no noise at all.- If I take INA, INB, INC and IND and mix them in a pro mixer, I get the same noise as my DIY mixer.So it seems this noise is generated by my INA, INB, INC and IND inputs.I've made these sound outputs myself. I'm not using an audio DAC, but a industrial one (DAC8554, quad-channel) that is good at DC, but maybe poor at noise level. Its SNR = 95dB, THD = -85dB.When I listen to a single voice from the DAC, the noise is barely noticeable. However, when I add a second voice to the mixer, the noise appears to double. With all four voices mixed, the noise becomes significantly louder and very noticeable.If the noise increases when I mix multiple DAC outputs, does that mean the noise is internally generated by the DAC and correlated?Since uncorrelated noise should add in power rather than amplitude, it shouldn't increase as much—correct?So the noise should not come from my DAC opamp conditioning circuit?

author: CaptDon
author_rank: Super Contributor
timestamp: February 25, 2025, 06:38:59 pm
content: Since all of your sources have roughly the same amount of noise, the first connected source will set the lowest noise floor achievable and each additional source connected will raise the noise floor another 3dB. Depending on source impedance and summing methods each additional input could raise the noise floor by nearly 6dB as opposed to 3dB. It is good that you discovered the noise is outside of your mixer project.

author: edavid
author_rank: Super Contributor
timestamp: February 25, 2025, 07:05:33 pm
content: Quote from: Halban on February 25, 2025, 05:09:01 pmDid you take the next step and try setting your DAC inputs to all "0"?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 25, 2025, 11:21:09 pm
content: The DAC8554 requires an external reference, what did you use there? If the reference is noisy, you will get that noise on the output. If you just connect the reference input (pin 3) to Vdd (pin 4) and your 5V supply is noisy, you will get that noise on the outputs, proportional to the output voltage. You need either a very clean supply (the DAC8554 datasheet has some examples) or a separate reference voltage or at the very least an RC filter on the reference input.CheersAlex

author: Halban
author_rank: Contributor
timestamp: February 26, 2025, 11:33:08 am
content: @CaptDonIf I understand, even if the noise is uncorrelated (different noise on each output), I can hear it like it's twice as strong when I sum 2 outputs?@edavidYes, and it make no difference...@Alex NikitinI'm using a linear regulator for the 3.3V digital supply, an other linear regulator for the 3.3V analog supply, and a LM4040C25 for the 2.5V voltage reference (the datasheet says "low output noise: 35μVRMS" but I really know if it's good or not).I'm also using a second voltage reference (LM4040BIM3X) to offset the DAC output filtered signal (and get a signal centered on 0V, 10Vpp range, that I can use to output audio or output a "precise" DC signal thanks to the industrial DAC).My DAC circuit, and one voice of signal conditionner (filter + offset + amplifier):

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 26, 2025, 11:51:24 am
content: Quote from: Halban on February 26, 2025, 11:33:08 amHere is probably your culprit - the LM4040 is a VERY noisy reference - 180uV RMS 10Hz-10kHz noise! Replace it with, say, LT1021-10 (10uV or 18 times = 25dB less) . And the LM4040-2.5V for the DAC reference is also a bad choice for the same reason, use a lower noise reference there too.CheersAlex

author: CaptDon
author_rank: Super Contributor
timestamp: February 26, 2025, 01:58:51 pm
content: Halban, remember how power adds to the ear, a rough nearly accurate example would work this way (assuming each noise source is generating the same level of noise) Plug in one source, that is your noise floor reference, plug in the second source and the noise seems to double, you will now need to plug in both the third AND forth noise source to have the noise 'apparently' double again. The poster who mentioned the voltage reference device you chose as being very noisy is a good avenue to investigate!! In the digital recording world of -90dB noise floors or better it is hard to imagine we listened to cassettes and 8-track tapes with -48dB noise below average audio level and to vinyl where a sustained 1KHz tone would yield THD reaching 5%!!

author: Halban
author_rank: Contributor
timestamp: February 26, 2025, 04:03:10 pm
content: Thanks a lot!Indeed the 10V reference is more noisy than I imagined, the noise of these LM4040 vary with the voltage of these references.LM4040 2.5V = 35μVRMS noiseLM4040 10V = 180μVRMS noiseUnfortunately it seems that good noise voltage references are not in SOT-23-3 packages, so it will be harder to swap them with better voltage reference.I will let you know if I find a solution, thanks for your help!

author: edavid
author_rank: Super Contributor
timestamp: February 26, 2025, 05:40:12 pm
content: Before you get too excited, try putting RC filters on the references and see if it makes a difference.