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Very nice performance! You use two different MCLK frequencies. I assume that you get optimum results for THD+N with 49 MHz and best results for THD with 24 MHz. Do you know how much you gain by changing the MCLK frequency? Another question regarding the ES9039Q2M: on the ES9822PRO you found...

Were the inputs 2 - 4 unconnected? If so, I would expect a lower crosstalk level by connecting them to a low impedance output/termination.

Two of the OPA1612 are for I/V. And yes, they are fed directly from the DAC chips. It does look like they only use one channel of each DAC. This means that they have opted to keep it simple and not try to reduce the noise as much as possible. The dual DAC design gives a good channel separation...

If the volume control is digital the internal converter part of the DAC will then see a level lower than 0 dBFS. With some DACs this will have an influence on the distortion.

Thanks for the AES paper and the video. Good to get some additional information on exactly how the measurements are done and how the numbers are calculated. One minor detail has slipped though your review process: on page 4 you mention "DACs that mute at 0 dBFS". Some DACs mute when the input is...

I am sure you are aware of it, but someone less experienced may forget it. As you correctly point out, designing an input stage with very low noise and protection against high voltages is not trivial.

It might be useful to add a resistor on each input to ground to allow use with an AC-coupled source.

I just checked the voltage from my L30 transformer. With no load I measured 19.2 V AC, so 18.9 V sounds fine. My guess is that the OPA1612 of the L30 has been destroyed, most likely from an ESD pulse. For the E30, I have no idea.

That is correct. A replacement is of course the best solution. Repair is the next best. And yes, the DC protection of the old version did not work very well. If the ESD issue is solved, the risk of getting DC on the output should be relatively small, provided the repair is done correctly (mainly...

@GD83 If you want to attempt to repair it, I think there is a good chance that you can do so by changing the OPA1612 op-amp. And if that solves the problem, you should add some wires as I did, shown in the second picture in this post...

Yes, I guess I overlooked that. I was looking for an FFT spectrum similar to the ones for the RCA and XLR outputs. To me that reveals a lot of additional information.

I see some mention the headphone amplifier, but I don't see any measurements. @amirm Did you make measurements on the headphone amplifier output?

Is it even legal to sell this thing? From the picture it looks like the protective earth terminal has no connection. And the safety distances on the PCB look questionable. A further investigation could determine that. People are asking for a listening test. Perhaps Amir did listen to it and...

It is actually fairly easy to see. Look at the white dielectric around the center conductor of the BNC's. On 75 ohm BNC's there is no solid dielectric around the center conductor, only air.

75 ohm BNC connectors are available, but judging from pictures of AP equipment, it looks like they use 50 ohm BNC connectors. Here is a picture, which shows the difference between the two types: 50 and 75 ohm BNC For SPDIF it probably isn't terribly important what connector it is.

In the frequency response figure you write "100 pF and 10 uF load settings". I assume you mean "100 pF and 10 nF load settings". Regarding inductive load: Can an inductive load be expected to trigger (test for) instability or is it only the capacitive load, which is of concern here?

Thanks. Very nice result:)

What numbers do you get, if you only measure THD, without the noise?

On the higher resolution picture it is clear that the anodes of D1 and D2 are connected to the ground plane. The cathodes of D3 and D4 are connected to the filter capacitors C31 (electrolytic) and C32 (ceramic) as well as to the input of the 12 V regulator (U2). The diodes D5 and D6 protect the...

No. D1 - D4 are just a regular bridge rectifier. But the transformer may output around 11 - 12 V ( x 1.41) at light loads, so perhaps there will be some voltage for regulation. But it looks a bit marginal. With no detailed information abot the transformer it is difficult to know for sure of course.