Noise on subwoofer output - Onkyo TX SR876

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Hello there

I have a Onkyo TX SR876 which some times send out noise on the sub-woofer pre-out. I hope someone might give some suggestions or might have heard some similar noise in the past.

It is higtly in consistent: Some time the amplifier sounds fine for several hours, some times it is there from the start.
I have found the service diagram Onkyo-TXSR-876-Service-Manual.pdf - Google Drive

  • Normal subwoofer signal still play when noise is present
  • The issue is not present when using MultiCh input.
  • The noise is also present when perform channel-test
  • The noise occur some place in the signal chain before volume control.
  • If i turn off subwoofer in the speaker configuration, the noise also disappear
  • Noise is not present on any other channel (not even center-channel which share DAC)
  • The PCB and capasitors looks to be in fine shape

An overview diagram is seen here
2019-01-18_085513_onkyo_diagram_overview.jpg

or downloaded

I have used the Zone 2 of the amplifiers for passive subwoofers (see illustration), in the past and this might have caused some damage - however it is not this that trigger the issue today (i also get the noise when connecting an external amp to Subwoofer-preout)
2018-12-28_193202_Zone2Amping.jpg


An example of the noise can be
downloaded as wave here
or as mp3
An the spectrogram show it to be Pink/Brownian noise (-15 dB/dec)
2019-01-01_212421_noise_spectrum.jpg


It could be DAC and Onkyos special integrators (VLSC) as well as analog-switches
I suppose my best option is to start at the DAC output and see it the noise remain.
The receiver is extremely compact and takes approx 30 min to dis/assemble for each test run
 
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Hi there,

Was the amp working properly before this?

Have you tried a firmware update - see links

https://www.avsforum.com/forum/90-r...62-onkyo-tx-sr876-firmware-needed-please.html

Onkyo tx-sr876 firmware update - Google Docs

There is a very long owners thread - link below - you will probably find something about it there. I vaguely remember reading of an issue like this but it was a long time ago and it wasn't the model I have so I didn't pay too much attention.

Onkyo 876 owners thread | AVForums

Hope this helps and hello to Denmark - I worked there for 3 years.

Kind rgds,

Ray
 
Thanks for the inputs Ray
Was the amp working properly before this?

I am not a 100% certain. I bought it used and set it up at my parents. I did not hear the issue back then.

Have you tried a firmware update - see links
I have performed a firmware update after i noticed the sub-noise issue
However the noise did not change. I would however also expect the issue to be mostly hardware related (not saying you can't patch it in software)

There is a very long owners thread - link below - you will probably find something about it there. I vaguely remember reading of an issue like this but it was a long time ago and it wasn't the model I have so I didn't pay too much attention.
Onkyo 876 owners thread | AVForums
Thanks for the tips. I have come across this thread before. I did a search for 'noise', but i it seem no one posted a similar issue.

However i will try to post my issue in that thread too.
 
I suppose my best option is to start at the DAC output and see it the noise remain.
Agreed. I can't imagine it's an awful lot more than a bad solder joint on a resistor or opamp (or DAC) in the output stage on the sub channel, or a dodgy connector. Close inspection of that area of the board may be warranted. I like using an old wideangle lens in reverse, like a 28 mm f/2.8 - much better optical quality than your average loupe, and I have several of them floating around anyway.
 
Is the noise still there if you use that zone 2 illustrastion.
There is also noise if i couple through the Zone2 amp as illustrated- however i have also verified the Zone2 amps to not introduce any noise + the noise is present when sending Sub-Preout through different (external amplifier)

Is there lowpass on sub out?
Yes, and i am pretty sure it is handled in one of the DSP's since the x-over is adjustable.
I also believe this is one of the purposes of the ADC (to ensure a digital signal is created so a sub-signal can be made.

Cool idea if it works
The Zone2 amp idea worked just fine, there is just some noise influencing.
Actually i think it is stupid that such amplifier offer Bi-amping, but not offer a digital x-over to support it.
 
Agreed. I can't imagine it's an awful lot more than a bad solder joint on a resistor or opamp (or DAC) in the output stage on the sub channel, or a dodgy connector.
Have you tried listening to the noise? Does it sound like a bad connection to you?
To me it sounds a bit like a FM-radio with poor antenna.

Close inspection of that area of the board may be warranted. I like using an old wideangle lens in reverse, like a 28 mm f/2.8 - much better optical quality than your average loupe, and I have several of them floating around anyway.
I might be able to find a real DSLR cam at work. However the boards can be a bit overwhelming + most of them are crammed in so tight.
 
Ok i got a bit further.
The noise is also present right after the DAC section
I disconnected the capacitor C3877 and took the signal where the red cross illustrate.
2019-01-19_185053_schematic_dac.jpg


Hereby it could be:
  • Noise already present in the digital signal
  • The DAC chip it self (TI BB PCM1796)
  • Bad connections to the DAC (e.g. PSU noise at the
  • The I/V filter or other parts of the post filtering (Onkyos integrators)
 
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I am not certain how to proceed. I can measure resistors and verify connection, but do not know if i can measure the I-output (current) signal from the DAC
The best option would be to swap the signal with e.g. the front-channel DAC. However i do not know how sensitive the data pins are to load and delay of my patch-wires.

Do you have any suggestions?
Is the a very basic I/V circuit i could solder up of on a perfboard?

I have taken some pictures of the board, however not the best quality, a bit better images can be downloaded


2019-01-19_204622_DAC_SUB_up3_smaller.jpg

Upside1
Upside2
Upside3

2019-01-19_204540_DAC_SUB_Bottom_smaller.jpg

Bottom1
Bottom2
 
To determine if the noise is present within the digital signal i have ordered a simple I2S DAC from ebay, a PCM5102 which should be easy to adapt.
It should arrive in February.

In terms of verifying the I/V section i found a reasonably simple schematic for DAC output stage in the PCM1796 datasheet.
2019-01-27_063601_Texas_iv_section.jpg
 
A bit of progress.
I have made a spice simulation of the I/V section which is feed the current output of the DAC-chip.
It seem to have a high gain at LF (integrators tend to have ;-) ), 117 dB at 10 Hz and 100dB at 100 Hz. In other words -17 dB/decade, which is the same spectral characteristics as the noise i experience (the FFT i did earlier).
So if typical whitenoise is present at the DAC output the noise will be shaped to match.
2019-02-03_164055_Plot_onkyo_integrator.jpg
 
I also attached wires to the output of both negative and positive integrator (the DAC have balanced output).
2019-02-03_164651_dac_IV_diagram_wired.png

A fair amount of random white noise is seen on the oscilloscope, however the sporadic noise i described is ONLY seen on the negative integrator output.
The first picture shows the differential signal with sinus tone (yellow=negative, green=positive, pink is subtraction)
2019-02-03_164959_Ok_signals2.jpg


The next picture shows a point in time where the noise start
2019-02-03_165145_Noise_on_Balanced_minus1.jpg


The last picture is just contain a lot of noise on the negative signal
2019-02-03_165428_Noise_on_Balanced_minus3.jpg
 
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Please, try to check your power supply of the DAC low pass filters (OPamps) and also power supply of the digital circuit (DAC chip and other circuits).
I had long time ago some problems with too low voltage on the input of the linear regulator (not sufficient voltage on input of the regulator to have enough voltage difference for proper regulation) and strange noise present. Device was AV receiver NAD, 5V regulator for digital circuits was the issue.

After solving the problem with rectifier, noise was gone, regulator worked as it should. So my point is that something is wrong with power supply of mentioned circuits. If it is switching regulator maybe there are problems with ELCO's (bad ESR)...
 
Please, try to check your power supply of the DAC low pass filters (OPamps) and also power supply of the digital circuit (DAC chip and other circuits).
I had long time ago some problems with too low voltage on the input of the linear regulator (not sufficient voltage on input of the regulator to have enough voltage difference for proper regulation) and strange noise present. Device was AV receiver NAD, 5V regulator for digital circuits was the issue.

After solving the problem with rectifier, noise was gone, regulator worked as it should. So my point is that something is wrong with power supply of mentioned circuits. If it is switching regulator maybe there are problems with ELCO's (bad ESR)...

Normally i would also suspect power supply, but to me it does not make sense when only the negative side, put they use the same op-amp + DAC. There are multiple PSU pins for the DAC, but it seems to be divided into Left/Right.
(VCC1, VCC2L, VCC2R, AGND1, AGND2, AGND3L, AGND3R, DGND)

Could you explain ELCOs? (ESR sound like capacitors)
I might look the PSU tomorrow.
 
Initially i checked:
  • capacitors of the integrators - OK at around 2.88 nF vs. 2.7nF speced
  • connection from DAC to integrator input - OK at least 0.23 ohm
  • feedback resistor of negative integrator - OK at the 560 ohm

I decided to check the signal at the integrator input to investigate if it was DAC or OpAmp + ensure later part of the schematic did not influence.
2019-02-04_190452_dac_IV_diagram_wired2.png

I added wires to the other side of the capacitor of the integrators + disconnected the later parts of the signal chain by removing the output resistor (R3827) and connected a pull-down instead.

I took two pictures where there was sporadic noise + a sinus tone of ~100 Hz.
The blue signal is the signal before the integrator and the yellow signal is after the integrator.
It can clearly be seen that both signal drift away from being centered around 0 V. An integrator give a huge gain at LF, so it is much easier to see the tone playing after the integrator (yellow signal)
2019-02-04_191527_DSC_2242_before_intgrator_playing_tone.jpg



For comparison i also have a picture where the positive signal is present.
The pink signal (positive signal before integrator) is centered around 0V, whereas the blue (negative signal before integrator) is jumping around and therefore affect the yellow signal (negative integrator output)
2019-02-04_192306_DSC_2243_before_integrator_with_ok_signal.jpg
 
I tried to measure some of the supply-voltage while simultaniously observing the integrator output for noise.
2019-02-06_082958_dac_IV_diagram_measure_supplies.png

Both sides of:
  • L3722 (+5V_DAC and Vcc2R)
  • C3725 pins (VcompR), i think it was approx 2V
  • C3730 (decoupling of DAC and Opamp ground)
I did not spot the noise signature on any of these pins

I also measured the OpAmp supply voltages.
Here i clearly noticed that much of the noise on the OpAmp was correlated with the noise on supply voltage.
I measured the OpAmp supply voltage to +11,56 / -10,64,
This is quite far from the +/- 14V shown on the diagram, but still steady and there should be plenty of signal headroom anyway.

I will try replacing the NE5532 OpAmp with a TL072 later today. (the only soic8 within reach)
 
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