Clicking from my AD converter

[2pulse]

My Lucid AD9624 converter is digitizing a quiet click in the left channel every couple of minutes. Some days it happens, other days it doesn't. I was wondering if anyone has had a similar problem in a converter before, and could shed any light on the issue.

Some observations:

- At 44.1 kHz, the clicks typically come through within about 1 dB of -26 dBFS. I just tested it at 96 kHz, and it's definitely louder, tending to be centered around -19 dBFS.

- It's always a downward/negative spike.

- It's always in the left channel.

- I popped the hood, and there are no visibly damanged components.

- This happens even with nothing plugged into the analog inputs or digital outputs - you can see it on the LED meter with no connections to any other source. Clock is set to internal.

Attached are screenshots of the click normalized in 44.1 and 96 kHz modes. You can see that at 96, there is a much more pronounced ramp on both sides of the click.

When I spoke with Lucid tech support a couple of years ago, they were much more interested in getting me to send it in and pay a flat $200 maintenance fee than troubleshooting over the phone. After a bit of pressing, the guy said, "it sounds like maybe a clocking issue, but we would need to take a look at it to tell you for sure."

In the interest of full disclosure, I'm trying to sell the thing and would prefer that the buyer not end up with a malfunctioning converter. Any ideas? Thanks for reading.

[dale116dot7]

Originally Posted by 2pulse

My Lucid AD9624 converter is digitizing a quiet click in the left channel every couple of minutes. Some days it happens, other days it doesn't. I was wondering if anyone has had a similar problem in a converter before, and could shed any light on the issue.

Some observations:

- At 44.1 kHz, the clicks typically come through within about 1 dB of -26 dBFS. I just tested it at 96 kHz, and it's definitely louder, tending to be centered around -19 dBFS.

- It's always a downward/negative spike.

- It's always in the left channel.

- I popped the hood, and there are no visibly damanged components.

- This happens even with nothing plugged into the analog inputs or digital outputs - you can see it on the LED meter with no connections to any other source. Clock is set to internal.

Attached are screenshots of the click normalized in 44.1 and 96 kHz modes. You can see that at 96, there is a much more pronounced ramp on both sides of the click.

When I spoke with Lucid tech support a couple of years ago, they were much more interested in getting me to send it in and pay a flat $200 maintenance fee than troubleshooting over the phone. After a bit of pressing, the guy said, "it sounds like maybe a clocking issue, but we would need to take a look at it to tell you for sure."

It does sound like a very particular clocking issue. The ringing indicates that it is probably happening between the delta-sigma stage and the digital filter within the converter IC itself, not with the data transfer once it is converted. The higher level at 96k also supports this theory. Excessive word clock (LRCLOCK) jitter can do this, but so can having an unstable MCLOCK or BITCLOCK. Most audio ADC (and DAC) IC's use three clock signals. LRCLOCK is at the sample rate. BITCLOCK is almost always 64 * sample rate. MCLOCK is used to run the ADC and the filtering, and is usually somewhere around 256*, 384*, or 512* the sample rate. This sounds like one of the clocks is moving around relative to the rest. Possibly an internal PLL stability issue? Especially if internal clock doesn't appear to work right. Perhaps unfortunately, unless you have a 60 MHz scope or faster, and know how to troubleshoot digital circuitry, your best option is probably to get somebody who knows those units to fix it.

I am assuming that your clocking arrangement is set up reasonably - that the ADC is the system wordclock master if you have its clock set to 'internal'. If it is not, then you need to straighten up your clocking properly first. Incompatible clocking modes can and will cause these glitches.

The shape of that trace is the impulse response of the ADC filter stage.

[2pulse]

Dale116dot7,

Thanks for sharing. That doesn't sound too encouraging. I was using the Lucid AD as the master clock, and slaving my MOTU Traveler via TOSLINK, but I don't think any of that even matters since the Lucid's meters showed transient activity without any digital or analog connections at all, running internal WC.

One thing I haven't been able to test is if clocking the Lucid with another source improves things. Am I correct in that I can't use TOSLINK or AES/EBU to send the Lucid a clock signal, since it doesn't have digital inputs? I guess I'll have to bring home a BNC cable and terminator before I can give that a try.