Jolijt Headphone Amplifier
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+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+ +-+-+-+-+-+ +-+-+-+ +-+-+-+
Solderdude from 'DIY Audio Heaven' was so kind to send me the details of a passive filter to suppress the lower frequencies. This has to be placed between the headphone amplifier output and the headphones. See DIY Audio Heaven.
|soldered to the cable|
|showing the process of shoving the filter into the pipe|
|with a closeup|
|sealed with hot glue, but not air tight due to a ventilation hole|
|heat-shrink tubing applied, looks nicer|
|from left to right: PSU, filter, headphone amplifier|
How does it sound? Definitely more neutral, lineair. Bass is not exaggerated anymore as was the intention and voices are more pronounced than without. I will keep the filter inserted, I like it.
CircuitThe circuit is simple and the components are cheap: it consists of one dual opamp type OPA2134PA (see addendum for other opamps) and a power supply with two resistors and capacitors. These generate a positive and negative voltage for the opamp. This way the amp can be used with a single power supply (see addendum about other rail splitters).
BalanceBecause of component tolerances, not to forget off-balance hearing and a weird but interesting phenomenon described here and here it is possible one channel appears to be louder than the other. It made me think of the 'spinning dancer' effect where the brain tries to relate the direction to which the dancer turns with the visual clues it gets.
Since the stereo imaging of this headphone amplifier is better than before, I noticed the balance shifted a bit to the left, possibly because I try to relate the visual clues of what I look at with the auditory clues of the sound. Closing my eyes the balance shifted a bit back to the right.
Anyway, I wander from the subject but that is why I added a volume balance control at the back of the enclosure.
RFI payed special attention to block RF as much as possible at all 'paths', the input, output and power supply because I suspect the audio quality is negatively affected by this.
|the grey 2k resistors, 100k resistor and 100n capacitor are placed under the board|
WiringStart with wiring the power cables. Only power ground is connected to the aluminium case.
|balance control, fixed with epoxy glue|
|accesible from the outside with a screwdriver|
TestingConnect an old headphone at the first smoke test if possible and close the volume. Only then switch on the amplifier and open the volume slowly.
OscillationIf you hear unexpected weird sounds chances are the amplifier is oscillating.
I suspect the countermeasures to limit RFI play a major role in the
supporting fact is this: while building one version I forgot to add the ferrite
bead at the output (there is only one needed for both left and right). After the enclosure was closed I noticed it was
susceptible to hum when I touched it. When I added the ferrite bead the
hum was gone. The long headphone cable must have picked up RF creating a
loop when I touched the case. At least, that is my theory.
See the sources at the end of this article.
Other opamps?It is possible to use a different opamp but check some properties first.
Other possible ways to create a virtual ground?
|an implementation with an opamp rail splitter and a NJM4580|
Use metal film or carbon resistors?
Input capacitorsIn theory it is beneficial to get rid of the input capacitors as they influence the sound in a negative way. I tried that but I got DC offset at the output in return. The sound improvement is not worth the risk of damaging my headphones.
Class AIt is possible to set the opamp in class A, thereby lessening crossover distortion, resulting in a 'smoother' and more detailed sound.
Build your own switched power supply?
|the transformer is almost too big to fit in the enclosure|
|but sideways it just fits|
|to convert AC to DC I used several modules on a (very) old piece of phenolic paper|
Optimize the sound source
So, what is the best combination of components?I prefer the NJM4580 and NJM4556 opamp both set to class A, with the opamp or Sijosae rail splitter delivering +15 and -15 Volts. The NJM4580 and NJM4556 opamps have a nice warm (tube like?) sound that I (can) listen to for hours, pure magic. Try, you will be blown away by it, I am sure.
Sources:the original Cmoy article
These actions result in a click-free audio using an USB DAC and Linux. You may not need all of them, just try them, in this order:
1. Convert your >96 kHz flac files to 44 kHz. You probably won't hear the difference.
sox "inputfile_96k.flac" -G -b 16 "outputfile_44k.flac" rate -v -L 44100 dither
2. Use the MOC player (Music On Console)
3. Disable frequency scaling in the BIOS.
4. Improve Linux responsiveness by adding these lines to /etc/sysctl.d/99-sysctl.conf:
vm.swappiness = 1
vm.vfs_cache_pressure = 50
vm.dirty_background_ratio = 1
vm.dirty_ratio = 40
5. Add these kernel parameters :
intel_idle.max_cstate=0 processor.max_cstate=1 usbcore.autosuspend=-1 sysrq_always_enabled=1 isolcpus nohz_full=1-4 rcu_nocbs=1-4 threadirqs intel_pstate=disable
Underlined are most relevant.
In my experience this helps the best if dropouts occuring during playing a flac file.
6. If frequency scaling is not disabled in the BIOS, install 'cpupower', enable it and modify the following line in /etc/default/cpupower: governor='performance'
7. 'reflac' the flac files. Use compression '-2'.
This helps against dropouts when the flac file begins to play.
8. Install the package 'realtime-privileges' and add the user to the group 'realtime': usermod -a -G realtime <username>
9. disable 'Intel Virtualization Technology', 'C-state' and on-board sound in the BIOS.
10. change the contents of /etc/pulse/daemon.conf to:
high-priority = yes
nice-level = -11
realtime-scheduling = yes
realtime-priority = 9
rlimit-rtprio = 9
rlimit-rttime = -1
daemonize = no
11. Add the user to the 'audio' group: usermod -a -G audio $USER
12. Install ananicy.
13. Modify /etc/asound.conf to:
14. Modify /etc/security/limits.conf with the following lines:
@audio - rtprio 99
@audio - memlock unlimited
@audio - nice -10
15. Bypass PulseAudio when playing music with Deadbeef or Audacious.
In configuration, Sound, set the output plugin to 'ALSA output plugin'.
16. create ~/.asoundrc for near bit perfect audio via ALSA:
Beware your volume control is not PulseAudio anymore but Deadbeef or Audacious. If the volume in these are set to maximum, be sure to lower the volume first!
17. Convert your flac files to wav and back. In some cases this helps.
19. Add to /etc/pulse/default.pa:
load-module module-udev-detect tsched=0
Later I got an USB isolator to be placed between the USB DAC and the pc:
While it does not replace the actions above and the effect is almost too subtle to notice, perhaps the sound is a bit more 'spacious'. It does not hurt I guess.
Disclaimer: always beware of a high sound volume, it can damage your hearing! First close your volume control and slowly increase the volume.