Joechei Headphone Amplifier
I rarely listen to music over the speakers. Instead for a long time I used a headphone amplifier published by the Elektor of 1987: 'The Headphone Amp'. This headphone amplifier from Elektor is based around an old opamp, the OP-50, long out of production.
Tweaking the original circuit to dig up the sound qualities hidden behind this 'non-inverting opamp circuit', it eventually turned out to be an extended version of the original circuit that I am very happy with. Hence the name 'Joechei', an old Dutch word to express happiness.
It does not matter what kind of music you throw at it, but to judge its sound quality and 'musicality' while tweaking I mostly used classical music and Jazz.
Still using one opamp (which was my intention) it is fairly easy to build.
For me the result is proof that very good sound does not have to be overly expensive.
The 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).
Schematically I setup the circuit like this:
Because 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.
I 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.
RF is the reason why the 2.2 nF and 100 nF capacitors are ceramic ones.
If you hear unexpected weird sounds chances are the circuit is oscillating.
But unfortunately the value of the output resistor influences the sound. How much so depends on the type/brand and impendance of the headphones used. Preferably R5 and R8 are not used. Without R5 and R8 the lower frequencies of the amplifier sound more accurate, for lack of a better word and for lack of measuring equipment.
Luckily some opamps are less susceptible to oscillation than others. In my experience these resistors are not needed when the OPA2134, AD826 or NJM4580 are used, for example.
If you have to use output resistors, a R5/R8 of 38 ohms is suitable for headphones with an impendance of 32 ohm, for Beyerdynamic DT 770 Pro 80 headphones 10 ohm is sufficient.
Oscillation can also be caused by the high frequency range of a fast opamp. This can be stopped by placing a 100 pF film capacitor over the feedback resistor of 8.2 kohms.
To keep everything small and orderly I first made a drawing of the way the components are placed on the board:
|the grey 2k resistors, 100k resistor and 100n capacitor are placed under the board|
The Zobel network consisting of a resistor of 10 ohm and a capacitor of 100n in series at the output 'equalizes' the load to the opamp for higher frequencies.
I soldered it directly onto the headphone jack input.
The low pass filter to prevent RF from entering the input was also soldered directly onto the RCA input connectors.
The opamp is placed in a socket so it can be swapped later on if needed.
Start soldering the socket first, then the resistors, smaller capacitors and at last the bigger electrolytical capacitors. Watch out for the polarity regarding the bigger capacitors.
Another advantage from using a switched power supply is to avoid the stray magnetic field, resulting in hum.
Check the resistance between 'input/mains' and the 'output' of the switched power supply (when not connected), it should be too high to measure.
Start with wiring the power cables. Only power ground is connected to the aluminium case.
After that wire the signal ground. Connect signal ground to one point close to the signal input but do not connect it to metal enclosure. Therefore be sure to use isolated inputs and output if using a metal enclosure.
Here I used white cable for signal ground and red/black cables for power:
|balance control, fixed with epoxy glue|
|accesible from the outside with a screwdriver|
Try to keep the signal wires and power wires separate. In the photo above the power wires are (mostly) on the left and the signal wires are on the right.
Connect 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.
- mostly classical and Jazz music in flac format
- Audacious or Deadbeef music player bitperfect setting
- HRT Music Streamer II USB DAC with USB isolator
- Joechei headphone amplifier
- passive headphone correction filter
- Beyerdynamics DT770 Pro 80 headphones
Compared to the Musical Fidelity 'X CANS' headphone amplifier, Joechei sounds so much more detailed and less muffled that I am thinking of replacing the internals of these 'X CANS' headphone amplifier by Joechei.
The difference between good and bad recordings is clearly audible, yet it seems to extract the musicality even from bad recordings.
The soundscape is well defined, the tonal quality is well balanced. The amplifier does not sound harsh in the upper frequency range. If needed the lower frequencies are being called upon with quality and impact. Voices sound natural.
For example, listen to 'Ocean' of Gian, the dynamics of the recording and the instrument as it moves through the vast space.
Or in Marillion 'Brave' the smallest details can be heard without increasing the volume, a refinement my old headphone amplifier lacked.
Representing the sound of the piano is a challenge for amplifiers but in 'Irina in A - Schubert - Bach - Brahms' I am convinced Irina Mejoueva plays the real thing.
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.
It is possible to use a different opamp but check some properties first.
If you do not use a FET opamp this circuit has a higher chance of oscillating so beware. In my experience for example a LM4562 will not work but an AD826 or NJM4580 will.
If it oscillates experiment with the value of R5/R8 and/or use a 100pF feedback film capacitor. It seems placing a 10uF/35V capacitor over pin 4 and 8 also can help but I did not test that.
By now my preference is, high to low on a scale from 10 to 1:
NJM4580 in class A (9) - NJM4556 in class A(8) - NJM4580(8) - AD826(8) - TLE2082CP(7) - OPA2134(7) - prob fake OPA627(5) - MAX97220(4)
Other possible ways to create a virtual ground?
Joechei creates a virtual (signal) ground with two resistors and two capacitors, also called a rail splitter. However it is also possible to create the virtual ground another way.
I tried the Sijosae rail splitter:
and compared it with the original resistor rail splitter and this opamp rail splitter:
|an implementation with an opamp rail splitter and a NJM4580|
Use metal film or carbon resistors?
Carbon resistors are less stable, give more noise than metal film resistors, and the values are less accurate.
However I use metal film and carbon resistors interchangeably without adverse effect. I keep the values as close together as possible and keep both channels consistent e.g. two 8.2k metal film resistors, two 2k carbon resistors and so on, see the photo above.
In 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.
Then I swapped the 470nF capacitors for 1,5uF polypropylene capacitors. Any improvement in sound is probably my imagination playing tricks but it proves that a higher value can be used without problems.
It is possible to set the opamp in class A, thereby lessening crossover distortion, resulting in a 'smoother' and more detailed sound.
Just soldering two 10 kΩ resistors directly onto the opamp is necessary: one 10 kΩ resistor between pin 1 and 4 and the other between pin 7 and 4:
These resistors set the current at the output to a more or less constant 1 mA.
The result depends on the opamp, it seems especially older type opamps benefit from this hack. In my experience the NJM4580 or NJM4556 are suitable, these opamps handle the extra current with ease.
This hack is highly recommended.
|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|
So, what is the best combination of components?
I prefer the NJM4580 opamp set to class A, with the opamp or Sijosae rail splitter delivering +15 and -15 Volts. The NJM4580 opamp has a nice warm (tube like?) sound that I (can) listen to for hours. Try, you will be blown away by it, I am sure.
Disclaimer: always beware of high sound volume, it can damage your hearing! First close your volume control and slowly increase the volume.
the original Cmoy article
Cmoy amplifier and output test
Can an audio circuit be powered by a switched-mode power supply?
11 Myths About EMI/EMC
Eliminating RFI/EMI Interference from your Audio System