My NAD Service request is 466557 is closed. Support by DALI GmbH and NAD was very helpful and friendly but could not provide a solution. Both DALI and NAD confirmed that there is a problem with the C 389 headphone amplifier.
The C 389 cannot drive any of my headphones correctly. The suggested “solutions” (see below) all do not work, neither listing at lower nor higher volumes (both was suggested) could fix the problem, connecting my low impedance headphone to the preout (output impedance 320 Ohm and 5V max output) is not applicable since the preout is required to drive a quiet power amplifier and sound quality would suffer significantly when going from high to low impedance.
NAD support had the interesting idea to record the headphone jacket output with a soundcard. I did so without any load, with 38 Ohm resistive load and my 2 real headphones loads. For comparison I did the same with an Apple iPhone 13 + cheap
Apple lighting dongle. Apple output is always fine, C 389 output with no or purely resistive load is also fine, but C 389 output with Grado or JBL real headphone load is distorted. I also tested Apple iPhone 16 + mid-price
Fosi Audio DS2 dongle, which can drive any load perfectly to very high volume levels. I have attached a short sample from the iPhone and C 389 both with Grado headphone load (C 389 Volume was set to -25dB, iPhone volume to max). The complete track from
Schiller “Liebe aus Asphalt” is available on the
Schiller official YouTube channel.
The impedance and EPDR measurements of my headphones show no problematic load:
NAD support mentioned that the C 389 is using a TI APO1654 chip. According to Texas Instruments spec
https://www.ti.com/lit/ds/symlink/opa1654.pdf (Section 8.2) that could be used to build headphone amplifiers with competent performance able to drive low impedance headphones and providing high output.
My conclusion on the C 389 is now, its noise floor is too high to drive > ~90db 2.83V/1m sensitivity loudspeakers quietly and its headphone output is faulty/unusable/distorted for the vast majority of headphones on the market, only the preout provides a clean signal to drive more quiet/competent power amplifiers.
Here are some snippets from the support replies from BERGMANN Electronic, Hidden Audio, DALI GmbH (all 3 translated from german) and my own NAD service request 466557) in order appearance:
10.02.2025-4.4.2025 Repair at BERGMANN Electronic Service GmbH, service report:
Device checked, device disassembled, the reported fault could not be determined. As a precaution, the entire mainboard, on which the headphone amplifier also works, was replaced.
7.04.2025 Hidden Audio dealer email:
I'm referring to the statement from the manufacturer-authorized repair shop. Quote: "Device checked, device disassembled, the stated fault could not be identified. As a precaution, the entire mainboard, which also houses the headphone amplifier, was replaced. Calibration and endurance test run performed." This doesn't allow for a second opinion. There is no defect.
10.04.2025 DALI GmbH email:
We have now been able to test your NAD C 389 extensively and reproduce the problem, thus confirming that there appears to be a problem with the headphone amplifier.
22.04.2025 DALI GmbH email:
We have now extensively tested your C 389 and also included a new one for direct comparison. Both devices exhibit the same behavior with our (25 ohm) DALI headphones. We contacted the manufacturer, NAD, and after further testing, they confirmed that low-impedance headphones can cause distortion in the high-frequency range. Replacing the device with a new one would produce the same behavior, so this is unfortunately not helpful. NAD recommends using the preamplifier output (pre-out) of the C 389 instead of the headphone output.
23.04.2025 NAD Service request 466557:
It is likely that you may need to consider increasing the volume level of the C389.... Therefore, I believe that increasing the volume remains the appropriate solution in this instance. There is no risk associated with using the C368 at higher volume levels, provided that any connected headphones can accommodate the output power. I trust this information will be beneficial. I will proceed to mark this ticket as resolved for the time being.
24.04.2025 DALI GmbH service report attached to send back C 389:
I reset the C 389 to factory settings and tested it with various headphones (DALI iO-4, iO-8, and iO-12, all 25 ohms). All headphones exhibited distortion in the high-frequency range at higher volumes. I retested with a new C 389 - the same behavior with all three headphones. I contacted the manufacturer, NAD. The behavior was reproduced. It is recommended to use the C 389's pre-out with low-impedance headphones.
25.04.2025 NAD Service request 466557:
...By chance, do you have a desktop computer with a line input? Would you be able to connect the C389 headphone output to the PC line input and record playback?...
28.04.2025 NAD Service request 466557:
...I can hear the noise in your C389 recording. The iPhone recording is clear....
29.04.2025 NAD Service request 466557:
...The C389 uses a TI APO1654 chip:
https://www.ti.com/product/OPA1654...
1.5.2025 NAD Service request 466557:
...I was able to test a C399 here in the office. During my testing, I was unable to get the headphone output to reproduce the issue I hear in your recording...My assumption now is that there is something unique about using 240V with the C389, as our tests here have been with 120V. The C389 uses a switch-mode power supply....
8.5.2025 NAD Service request 466557:
...That being said, we are also investigating the noticeable difference in headphone performance between the C389 and C399. For the time being, I don't believe we can resolve the issue with your specific C389. I feel there may be a hardware modification required for the C389 to correct the headphone performance. However, that's just my personal speculation. We'll need to wait for our NAD Engineering Team to complete their analysis. We will be working with DALI as well regarding the C389 and C399. For the foreseeable future, DALI will be your best point of contact for any updates to this situation. I'm sorry that I do not have a more immediate solution for you, but if you have any other questions or concerns, I would be happy to help!
