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High frequency when IQaudIO DAC hat stops playing.
High frequency when IQaudIO DAC hat stops playing.
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Old 22nd June 2020, 03:21 PM   #1
edbarx is offline edbarx  Malta
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Join Date: May 2018
Default High frequency when IQaudIO DAC hat stops playing.

My IQaudIO DAC Pro emits a high frequency note when music stops. The DAC has been used without this issue since I installed Raspbian about 14 months ago.

Unfortunately, the Raspberry Pi 3B+ is very demanding on current, and I have to use an SMPS with all its unwelcome electrical filth.

The outputs from the DAC are taken from the RCA connectors which drive an LM3886 based stereo amplifier. The amplifier does NOT have automatic gain management.

I play music using a terminal using mpg123, mplayer and VLC or cvlc. I control volume using alsamixer. Pulseaudio and systemd were removed from the system. Alsamixer displays a long list of controls instead of single control.


Post Scriptum:
I will post an oscilloscope image of the irritating waveform as soon as the issue shows its ugly head.
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Last edited by edbarx; 22nd June 2020 at 03:47 PM.
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Old 22nd June 2020, 03:53 PM   #2
phofman is offline phofman  Czech Republic
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Quote:
Originally Posted by edbarx View Post
My IQaudIO DAC Pro emits a high frequency note when music stops. The DAC has been used without this issue since I installed Raspbian about 14 months ago.
Then at what point did it start?
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Old 22nd June 2020, 04:09 PM   #3
edbarx is offline edbarx  Malta
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This started to happen about two weeks ago and it does not regularly happen.
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Old 23rd June 2020, 06:40 AM   #4
edbarx is offline edbarx  Malta
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I forgot to write that in my neighbourhood there is a mains issue which is high voltage. I will investigate to determine whether the issue of this interference is caused when the mains voltage is too high. If this is the case, the Pi's switching power supply will need replacement.
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Old 23rd June 2020, 06:57 AM   #5
CallMeMike is offline CallMeMike  United Kingdom
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(as hinted by @phofman) Do you re-collect any changes (hardware or software) made around the time when the issue has started?

Try to:
- stream the music from the Headphone Out port of the RPi using the current set-up (bar the HAT DAC)
- use a loaned LPS/quality SMPS
- stream the music to a different amplifier (power amplifier or HP amplifier)

There may be some inspiration from this March thread:
Hum with iqaudio Dac+ when paused/stopped

EDIT: I've missed your last post... The solution may consist of a good Linear Power Supply for feeding the RPi + HAT DAC (or for better sound, separate power supplies for the RPi and the HAT DAC)

Last edited by CallMeMike; 23rd June 2020 at 07:11 AM.
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Old 23rd June 2020, 07:51 PM   #6
edbarx is offline edbarx  Malta
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If the cause proves to be the mains higher than normal voltage, I think, it is better to use double voltage conversion to power the Raspberry Pi mini-computer. The mains voltage will be first stepped down to about 20V, rectified and smoothed sufficiently for a step down DC-DC converter to produce the regulated 5V DC. The first step can be delegated to the amplifier's power supply, but I do not know whether a DC-DC converter will pollute the amplifier's power supply to noticeable levels.
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Old 24th June 2020, 06:45 AM   #7
edbarx is offline edbarx  Malta
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Further investigations showed it was not the mains higher than normal voltage that is causing the issue but an airconditioner. As usual, 'high tech' Chinese 'shortcut engineering' is the cause! Why would anyone bother to suppress interference from an electronic inverter powering an electric motor?!

The issue is predictable and repeatable. Simply written, if I turn on the airconditioning, the high pitched tone returns, if I turn it off, the high pitched tone vanishes.
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Old 25th June 2020, 06:03 AM   #8
CallMeMike is offline CallMeMike  United Kingdom
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Your current PSU feeding the RPi/DAC combo should not be so 'transparent' to 'noise'...
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Old 6th July 2020, 10:00 AM   #9
edbarx is offline edbarx  Malta
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Which implies supplying power to the Raspberry Pi and DAC from the same switching power supply is not a good choice. The DAC hat sites comfortably on the Raspberry Pi minicomputer: I have no idea how I can remove it from its current place to use another cleaner/linear power supply. The Raspberry Pi and DAC communicate through a multipin connector at one side of the Pi. I do not know how many pins of those are in use and what they do. The thought of having to modify something like that is daunting to me, especially at this age, when hand stability and eyesight are not so good.
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