Help tracing some noise in sound output circuit - Page 1
Forum section: electronics / projects

author: tony359
author_rank: Frequent Contributor
timestamp: July 17, 2025, 01:35:23 pm
content: Hi all,I have an old cinema sound player which has 8 sound outputs. The sound gets through DACs on a board which I am going to call the output board.The player is 10 channels, each pair going through a DAC (PCM1716E), an Op-Amp (OPA2134US), an e-pot (DS1867C1), two more op-amps and finally a switch on an IC (DG412DY).on the board there are some supply voltages (+5V, -5V), a digital section with a PIC, a 245 to feed the DAC I guess.I have some noise which reads 2.6KHz on an RTA on all channels but two. All boards I have show the same noise so I think it's a design flaw. The noise is VERY low, 10/13dB over noise floor but I'd love to tackle it and potentially remove it or mitigate it.See below.Tests I've done:- +5, -5 are generated on board via linear regulators. I tried feeding 5V from my bench PSU (-5V comes from a ribbon cable which likely comes from a PCI slot and eventually from the ATX PSU) and the noise remained. Those lines seem clean though.- If I change the channel gain via software (that would be the e-pot), the spike changes as well, suggesting the noise is injected before the DS1867C1.- I don't think the noise it's being injected from other parts of the machine, I ran the analysis while the board was physically removed from the chassis - but still wired - and while I moved it around and away from other components, the noise remained.- electrolytic caps were replaced.Because it's such a low level noise, it's difficult to probe it!Does anybody have any idea where this 2.6KHz could come from?There are at least two more revisions of this board, the latest is the one with the orange cable: it references the 2200uF cap on the -12V line coming from the ribbon cable to +5V line from the linear regulator instead of +5V coming from the ATX PSU (why not ground??)There is also a diode soldered on two SMD capacitors.Thanks!

author: Terry Bites
author_rank: Super Contributor
timestamp: July 17, 2025, 02:26:41 pm
content: Is it on your supply rails?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 17, 2025, 02:49:00 pm
content: It's a big guess, but if a small ceramic and a larger decoupling capacitor are used in parallel, and the small capacitor is defective, then the large one may start an oscillation on the power supply.Maybe it helps if you build a 2.6kHz notch filter with differential inputs and 40dB or so amplification.But the circuit does not seem very difficult. It's mostly a bunch of the same. Together with the datasheets it's probably not too difficult to reverse engineer a channel. If you've got a schematic that is close, you have a much better idea of what you are probing.

author: tony359
author_rank: Frequent Contributor
timestamp: July 17, 2025, 03:06:42 pm
content: Quote from: Terry Bites on July 17, 2025, 02:26:41 pmIt's hard to say as the noise is very faint.But when I fed 5V from my bench PSU the noise didn't disappear.This is one channel scoped AFTER the DAC, there are some test points. Using the FFT of my oscilloscope, I think I see the 2.6KHz I am looking for.This is the 5V from the 7805Quote from: Doctorandus_P on July 17, 2025, 02:49:00 pmIt would be great, I can try but reverse-engineering is not my best skill 🙂What would be the purpose if the notch filter? Not questioning, just genuinely asking 🙂Thanks!

author: radiolistener
author_rank: Super Contributor
timestamp: July 17, 2025, 04:04:42 pm
content: Quote from: tony359 on July 17, 2025, 01:35:23 pmTo better assess the issue, could you clarify your DAC's sampling frequency and whether any signals in your system operate at, or near, 2.6 kHz or at harmonics/multiples of that frequency (5.2 kHz, 7.8 kHz, ...)?It could be related to power supply ripple, ground coupling, digital leakage, or aliasing effects, especially if 2.6 kHz or its harmonics coincide with clock division products or PWM control signals within your system. Knowing the key frequencies present in your design would help narrow down possible coupling or modulation paths.For further analysis, you might find it helpful to monitor the interference using an SDR receiver with a CW demodulator. For 2.6 kHz you can use usual sound card withHDSDRsoftware. This can give you a clearer sense of the interference behavior - whether the frequency is stable or drifting.- If the tone is stable and clean, it likely indicates an intermodulation product or a direct leakage of one of your system’s clock signals.- If the tone is unstable, warbling, or frequency-drifting, it may suggest coupling from a PWM regulator or other load-dependent switching source or may-be some data-transfer interference.This approach can help distinguish between fixed-frequency clock-related noise and variable-frequency power-related interference.The 2.6 kHz tone could potentially correspond to a data transfer interval - such as the periodic transmission of update packets. You didn’t specify the data source feeding your DAC. If the data is delivered over Ethernet, the 2.6 kHz component could potentially correspond to the packet update interval, such as the periodic arrival of Ethernet frames carrying DAC updates.Ethernet can cause significant interference, especially due to the power supply layout requirements for the PHY. The external DC-DC stage associated with the PHY’s internal power domain has strict guidelines in the datasheets regarding trace length, placement of the inductor and capacitors, grounding, and filtering. Failure to follow these layout rules can result in substantial noise coupling into sensitive analog and mixed-signal sections of the design.

author: tony359
author_rank: Frequent Contributor
timestamp: July 17, 2025, 05:17:52 pm
content: Thanks@radiolistenerThis is not my design I'm afraid. So I am discovering things as I go.The board has a 12.288MHz crystal on it. The sample rate I see from the relevant clock going into the DAC is 44.1KHz. The DAC are in Hardware mode.I've tested HDSDR but I am not sure I understand it. I thought I'd identified the 1.6KHz peak but then I unplugged the device from the sound card and the peak was still there. Yet, when I play pink noise out of the unit, I do see noise reaching HDSDR. The peak does disappear on my RTA when I unplug the unit.So what is that peak at 2.6KHz I see on the graph that doesn't go away?I'm wondering whether I'm chasing ghosts but my RTA is quiet with another audio device when no sound is being played, no peaks at all.One main difference though: the device under test has UNBALANCED outputs. The "quiet" one is all balanced. The sound interface has stereo jacks, I'm bridging a "sound return" pin of the player with the "cold" pin of the sound interface.Basically I am doing this, from RANE interconnection guide.Sound is delivered via a ribbon cable from a board in the unit. The silkscreen says I2C. The whole box is an old Linux ATX PC.

author: alex_
author_rank: Contributor
timestamp: July 17, 2025, 05:57:06 pm
content: Some opamps tend to oscillate if their loading is not correct. You can check that by grounding the input (between the DAC and the first stage) and check the residuals at your output connector. It would be an easy fix to add some resistance at the opamp output.Your FFT shows spikes at 2.6kHz and 7.8kHz, both multiples of 1.3kHz, and a bunch more above that. Continue the series you reach 44.2kHz, suspiciously close to your DAC sampling frequency to not be a coincidence. I would check the 12.288MHz oscillator power supply, if you can probe it maybe you can spot some noise and improve its bypassing. Not all DACs reject well clock jitter (especially when they use some clock recovery that generates tons of jitter).

author: radiolistener
author_rank: Super Contributor
timestamp: July 17, 2025, 07:02:15 pm
content: It sounds like you may not have configured the input device in HDSDR correctly. Make sure to select your actual sound card as the input via the Options menu. After that, verify that you’re receiving the correct signal - apply a known test tone or noise source directly to your sound card input and check that it appears on the spectrum display.To simplify the visualization, I recommend switching the input mode to "mono" instead of "stereo IQ" (the same through options). The IQ mode is intended for complex IQ signal demodulation and will show both positive and negative frequencies, which can confuse the spectrum view when you're using a standard audio input - negative frequencies in this case are simply mirrored images of the positive ones. In mono mode, you’ll see a straightforward spectrum with only positive frequencies, making it easier to interpret.Once you’ve confirmed that you're monitoring the actual input from your sound card and not some other source, you can proceed. If you’re seeing a peak that remains even when your input is disconnected, it's likely caused by interference within your sound card itself - such as internal coupling or PSU noise, this is pretty usual behavior. Just ignore it.If you observed this peak using the same sound card, it’s quite possible that it’s an artifact of your sound card’s input stage rather than your DAC. I’d recommend testing with a different sound card, ideally on a separate computer and of a different type or brand, to minimize the chance of encountering similar input-related artifacts.Quoteyou need to set LO frequency to 0 in order to get proper frequency tuning with Tune regulator. It appears that actual frequency from your screenshot is 2600 HzWhen using LSB or USB demodulation, you'll hear signals that are offset either below or above the tuned frequency, depending on the mode. That’s why I recommended using CW mode instead - in CW mode, any carrier at your tuned frequency will produce a tone at 700 Hz. If the interfering frequency drifts, you'll hear the tone shift above or below 700 Hz accordingly.This makes CW mode convenient for accurately identifying small frequency shifts. In addition, you can monitor the audio spectrum display at the bottom of HDSDR to visually track the tone's exact frequency, and you can adjust the spectrum scaling using the slider to zoom in for more precise analysis.Highlighted area on the spectrum shows which frequency range you hearing.Select the CW demodulation mode by clicking the CW button, then set the Tune frequency to 2600 Hz to monitor the frequency of interest. Once configured, you can experiment by powering different sections of your system on and off to observe whether the tone changes or disappears. This process may help you identify the source of the interference.Based on the spectrum, the signal appears at stable frequency so it appears to be clearly synchronized to some clock source within your system.

author: tony359
author_rank: Frequent Contributor
timestamp: July 17, 2025, 07:23:20 pm
content: [ Specified attachment is not available ]Thanks so much for the suggestions.Ah yes, indeed. 2.594x 17 = 44100Hz. Interesting. Not all channels show the spikes and some only show the first two. (2.6 and 7.8KHz)The 44100 clock comes from the ribbon cable from another board. I think this unit can also do 96KHz so I'd imagine there are some components responsible for changing that when required. The 12.288MHz clock on the board should be for a nearby PIC, I'd imagine it's handling some logic to activate/deactivate the switches to enable or disable the outputs?The 44100 clock seems stable to me, I don't see anything moving on the scope. Is there a better way to measure it?I've traced what happens between the DAC and the first OpAmp. The outputs go through a 10uF capacitor (that's a high pass filter I think?) and then via some resistors into the Opamp.Are you suggesting I grounded the golden pads basically and see what I read at the outputs? This is a silly question but do I ground those pads with a resistor connected to ground?Thanks a lot!

author: radiolistener
author_rank: Super Contributor
timestamp: July 17, 2025, 07:41:11 pm
content: It’s also possible that what you’re seeing is a harmonic or a beat frequency resulting from interaction between 96 kHz and 44.1 kHz clocks. For example, a 96 kHz signal would alias to 7.8 kHz when sampled at 44.1 kHz, and the 2.6 kHz component could result from clock asymmetry, modulation sidebands, or mixing products due to improper isolation between clock domains.Clock leakage, jitter, or uneven phase relationship between different clock sources can easily produce such low-frequency artifacts. That’s why it’s generally advisable to clock all system components from a single master clock source. This helps prevent unwanted beat frequencies and intermodulation artifacts caused by clock domain interaction.I would recommend using 48 kHz instead of 44.1 kHz as your base sampling rate. If your system already operates at 96 kHz, 48 kHz is a more appropriate choice, as it is a direct sub-multiple. Using 44.1 kHz in such a configuration is generally undesirable, as it can lead to unwanted beat frequencies and intermodulation products between unrelated clock domains.

author: tony359
author_rank: Frequent Contributor
timestamp: July 17, 2025, 07:48:55 pm
content: @radiolistenerThis is a "closed" player, I didn't design it and I cannot change the sample rate I'm afraid. I'm investigating on whether the noise comes from the analogue section where I might be able to mitigate it.Thanks for the tips above - not sure what I was monitoring before but it was 100% the output as I could see pink noise when I was playing ping noise.Anyways, I think I've done what you said and I now have a tone matching my 2.6KHz peak.I think a video is worth many words so let me know if I've done my homework okhttps://youtu.be/ZI0OzAcaLHAThanks and apologies if I keep making a mess with attachments, I am struggling to understand the logic behind the attachment process!An update on my findings:The player has some sound test files I can run. Including a "digital silence" one. When I run that, I see the same noise.But if I run some actual sound test, when the sound fades out, before it loops from the beginning, the spikes are not there anymore. I see a slightly lifted noise floor - understandable - but the spike is 100% not there.So I am wondering whether I am chasing ghosts here, maybe this is a by-product of the codec or the main DSP.I think grounding the outputs of the DACs is a good idea: if the noise goes away I know it comes from the DACs.See attached: this is a "sweep" test, you can see that when the sweep is almost completed, there is no 2.6KHz spike but it comes back as soon as the file stops and restarts (ignore the lower frequencies spikes, that is the CPU fan which I can shield)

author: tony359
author_rank: Frequent Contributor
timestamp: July 17, 2025, 09:02:08 pm
content: I'm replying to myself to keep it tidier.I think I got an important clue: the spikes are there ONLY when there is NO DATA getting into the DACs on the DIN pin. As soon as there is data, even silence, then the spikes go away.The "digital silence" test sends no data to the DACsAnd remember when I said that there is no noise on the surround channels? Well, the Surrounds DAC gets data all the time even when idle. So weird!Does this tell anything on what the problem could be?Thanks!

author: radiolistener
author_rank: Super Contributor
timestamp: July 17, 2025, 09:33:18 pm
content: From what I can see, the 2.6 kHz tone is clearly a stable carrier, which strongly suggests it’s synchronized to one of the system’s internal clock sources, rather than being random noise or power supply ripple.Given that the tone disappears during playback but reappears during silence, it’s likely a by-product of the DSP or codec idle state - perhaps related to dithering, digital silence processing, or even an unintended leakage from the digital domain when no active audio is present. Many DSP systems enter different internal processing modes during silence, which could explain the change in noise behavior.Unfortunately, in closed systems like yours, identifying the exact source can be challenging, as interference can stem from internal clocks, codec processing modes, or PCB-level coupling. Since the tone is stable and clock-locked, my suspicion leans toward it being some idle state artifact of the DAC or DSP.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 17, 2025, 09:43:41 pm
content: Check De-emphasis control pins (HW mode).Code:De-emphasis control can be selected by DM1 (pin 27) and DM0 (pin 26).DM1 DM0 DE-EMPHASISL L OFFL H 48kHzH L 44.1kHzH H 32kHzBut it depends on the interface mode:Code:MODE (pin 24) = H Software ModeMODE (pin 24) = L Hardware Mode

author: iMo
author_rank: Super Contributor
timestamp: July 17, 2025, 10:14:21 pm
content: QuoteWhile looking into the DS such combination is not supported by the 1716 dac.. (Table 1)PS: also doublecheck the exact frequency of the crystal oscillator..

author: tony359
author_rank: Frequent Contributor
timestamp: July 18, 2025, 10:50:52 am
content: Quote from: radiolistener on July 17, 2025, 09:33:18 pmThanks.However, the noise only appears when there is NO DATA reaching the DAC. Wouldn't that rule out the DSP and anything before the DAC?Quote from: iMo on July 17, 2025, 10:14:21 pmOh you're right, I didn't realise the DAC needed a system clock as well.The system clock is 11.289MHz and comes from outside of the board via two lines entering a DS90C402, pins 3-4. The output, pin 2, is the System clock for the DSPs.https://www.ti.com/lit/ds/symlink/ds90c402.pdfThe on-board crystal is for the PIC and reads 12.288MHz.Quote from: DavidAlfa on July 17, 2025, 09:43:41 pmThanks. The DSP is in HW mode and both pins 26 and 27 are tied to ground so de-emphasis is OFF.I think I'd like to try grounding the DSP output as suggested by @_Alex - how can I do that safely?And, @_Alex when you mentioned the clock jitter, did you mean the DAC's system clock or the data clock?Thanks!UPDATEI've removed one of those 10uF capacitors in series with the signal. I've grounded the input of the op-amp via 10KOhm resistor.The output is 100% quiet. So the noise comes from the DAC. But only when it's "idle", that is, not getting digital data.Anything that comes to mind? It sounds like the DAC is getting into some sort of "disabled" state and makes a faint noise.

author: alex_
author_rank: Contributor
timestamp: July 18, 2025, 02:56:27 pm
content: Your grounding of the opamp is correct. A lower value would reduce thermal noise but you got the point.Your DAC generates some CM noise, and the differential output cancels it nicely.Not all DACs have good modulation methods. The good ones had differential current outputs, and you had to build your conversion to voltage output and filter with discrete parts.The industry standard is to measure them with a tone (1kHz, -xx dB) as they are never really quiet in zero output as they constantly toggle.I don't remember the quirks of this particular one, but I see a MUTE pin. See if it is wired and try to force it if it is not used.As others have said, try to set the DAC to 48/96kHz and see if the FFT spike moves.Sometimes to quiet a DAC we were adding a very low amplitude white noise to the signalClock jitter: I am talking of the DAC's system clock. Since you can't measure easily the jitter, check the noise on the power supply of the XO or the PLL attached to the Xtal. If you can see some noise on that DC line with your scope add caps or look for sth oscillating.

author: tony359
author_rank: Frequent Contributor
timestamp: July 18, 2025, 06:25:17 pm
content: Thanks.The fact that the DAC has its own zero-detection so that a mute circuit could be implemented, says a bit about the noise!Interesting to see that in HW mode pin 21 is just an output so a muting circuit could be implemented. In SW mode, a zero-mute system can be enabled, though with some potential clicks on the output.My DAC is in HW mode, would there be an easy way to use pin 21 to mute the output? There is a muting circuit on the board but it doesn't seem to be wired to pin 21. Nothing seems to be wired to pin 21. I'm wondering whether there was a good reason for that not to be used!

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 18, 2025, 08:24:06 pm
content: You might try replacing the dac with a more modern one like the 5102A.Not straight drop-in replacement but might worth the try.