There is a high pitched ringing (~15khz) coming from my system which seems to fluctuate in volume slightly, almost like tinnitus , which I assumed it was after checking on scope and seeing absolutely nothing.
But after checking more thoroughly it is definitely the system, test tones of any frequency show up clear as day, rapidly changing waveforms like music or noise still show up clearly even if the scope cant track them.
How could it be that this noise is not being picked up at all on the scope?
Also I could still hear the noise through the headphones as the scope was attached.
But after checking more thoroughly it is definitely the system, test tones of any frequency show up clear as day, rapidly changing waveforms like music or noise still show up clearly even if the scope cant track them.
How could it be that this noise is not being picked up at all on the scope?
Also I could still hear the noise through the headphones as the scope was attached.
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Amp is SS. there is no volume control, volume is digital, but the noise is only present when an audio file is playing (even a file of pure silence), I think its coming from the I2S source ( an SD card player) but right now the focus is on why this noise cant be measured.
I assume it is that kind of noise I experienced with BT-receivers. How can you say it is 15kHz if you cannot measure it?
Btw, a scope is too unsensitive and has to much bandwidth to trace such noise of possibly a few mV audio. If you want to measure this a soundcard, a PC and REW/ARTA are your friend.
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ok I just discovered the source of the noise, its comes only from playback of 44.1khz files on the SD card player, I upgraded the 44.1khz rate XO a few months ago so the XO has to be faulty.
Anyway the scope is DSO188.
I am using a new DAC with low current output so I/V now has a lot more gain, it was probably inaudible before.
''~15khz'', meaning ''about 15kHz'' based on what my ears can hear.
I also further increased gain of the IV stage in the DAC, now it is easy to hear, still nothing on scope. the scope should pick up at least 10mV. i am measuring low frequency bass tones and super sonic tones just fine, how could this be a bandwidth issue?
Anyway the scope is DSO188.
I am using a new DAC with low current output so I/V now has a lot more gain, it was probably inaudible before.
''~15khz'', meaning ''about 15kHz'' based on what my ears can hear.
I also further increased gain of the IV stage in the DAC, now it is easy to hear, still nothing on scope. the scope should pick up at least 10mV. i am measuring low frequency bass tones and super sonic tones just fine, how could this be a bandwidth issue?
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Not anything. A white noise (hiss) of 100uVrms at amplifier output for instance may be well heard with a tweeter but is totally buried within the noise floor of the scope. And high pitched noise may be of similar low level and nonetheless audible. That is the point where the scope reaches its limits.
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You were right about this, when controlling the volume of a test tone there is a small margin where its still audible but the scope stops picking it up.
Not necessarily, it might just have a slightly different frequency than the old XO. If there is something in the system that generates intermodulation products between XO harmonics and some unrelated clock signal, it could be that such a product now happens to be audible, while it used to be above the audible range with the old XO. For example, crosstalk from an unrelated clock signal to the reference or clock of a DAC can cause effects like that.
Are there any clock frequencies around that are derived from a different crystal? If so, can n times one clock frequency minus m times the other clock frequency end up at 15 kHz, taking into account the crystal tolerances?
Do you have access to some of the crystals and if so, does the pitch of the tone change when you touch a crystal pin (assuming you can touch it without electrocuting yourself)?
I was able to fix it (or make so low that it became inaudible) by adding a 22r termination resistor, the old one was terminated with a resistor in fact.
Dont think it can do any of this, its one of the most basic scope you will find.
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