Aloha 'goners! I'm going to do my best to get the details down here as I am a bit stumped on this one.
I have a Wadia 121 DAC that was recently repaired. The issue was the RCA/XLR outputs produced mostly noise and little music. The headphone side was fine.
As the unit wasn't functioning properly I hadn't had the chance to hook it up and use it. When I got it back and hooked it up there was a nasty ground loop(?) that led to a blown speaker (Thiel 1.6). The buzz produced was the angry bee buzz followed by a loud electrical sounding crack and the woofer was gone. The amp (Creek Classic A53) went into protect mode and I shut it off. I then disconnected the Wadia and hooked up a Mytek Brooklyn+ and there was no buzz. Same cables same outlets same everything except the DAC. I tested the amp and it seems to be ok through another pair of speakers.
At this point I wasn't sure what was going on. I tested 3 different amps with the Wadia and 2 of the 3 buzzed. With the Mytek none of them buzzed. Of the 2 amps that buzzed one of them had a 2 pin connection and the other a 3 pin. The only amp that didn't buzz was a 2 pin Adcom 535 MkII. The other amps were a 3 pin Creek and a 2 pin Carver.
I'm not an expert of any kind, but what leaps out at me is the Wadia has a bad problem that is passing a very destructive signal down the chain. I think the volume must have been turned up a great deal as it was loud enough to blow a speaker (I'm surprised it was a woofer and not a tweeter).
I'm relatively sure what you're hearing is not a ground loop, but a sound I've heard unhappy digital equipment make when something is very wrong in their circuitry. I would throw away the Wadia, or at the very least, have it repaired properly. Sometimes, though, that can happen when something is not connected properly. So, check everything you can. Good luck, Dan
It’s not clear to me what you mean when you refer to a "2 pin connection" and a "3 pin connection." Initially I thought you might have meant RCA vs. XLR, but several rear panel photos I found as well as the manual for the Creek amp (which you described as "3 pin") show it to only have RCA inputs. And the cable you indicated was being used between the DAC and the amps is an RCA cable. Can you clarify that?
Also, by any chance were you using XLR-to-RCA adapters between the DAC and any of the amps?
In any event, I agree with Dan that it does not sound like the problem was caused by a ground loop.
P.S: Now that I think of it I presume that "2-pin" and "3-pin" were references to the number of prongs on the power plugs of the amps. And since the problem appeared when you used the Carver which has a two-prong plug that would seem to further confirm that the problem was unrelated to a ground loop.
The Adcom 535 Mk II is capacitor coupled at the input and the Creek and Carver are not. What you have is DC at the output which the Creek passed through to the speaker before going into protection mode. The Adcom's input cap blocked the DC and didn't pass the buzz-causing DC to the speaker. DC is enough to blow out a woofer.
The problem in the DAC can be anywhere in the active circuit. I would present a repair bill for the Thiel to whoever worked on the Wadia.
Looking at a photo of the particular meter shown here I see that there are two settings of the mode switch marked "200m." One is for DC current, which it seems safe to assume is not the one the OP used, and the other is the most sensitive of the DC volts settings. So it sounds like the worst case of any of the readings the OP reported was 1 mv (millivolt) DC, which won’t hurt anything.
I second your suggestion of trying an AC volts measurement, although the most sensitive AC volts setting provided by this meter has a full scale of 200 volts.
I tested using meter set to the V with the 3 dashes below the solid line.
That's correct, and that is what I assumed you were doing.
I suppose it is possible for a large DC output to occur intermittently, but it seems unlikely. Especially if the problem has occurred when no signal was being provided to the DAC (i.e., when a CD was not being played).
• Multiple stages of regulation were designed into the main circuit board to ensure quiet,
stable DC power is available for all sensitive circuits.
• A dedicated voltage regulator for the master oscillators is used to provide clean pure
DC power for maximum clock stability.
I wouldn't know where to start testing those pieces and do not want to dig around inside of the unit.
On the repair bill they stated that the internal power supply was replaced as well as an IC.
My aim again with all of this is to get some measurement of the issue prior to sending it back to them.
Not sure what to suggest at this point, other than measuring the DAC’s output on the 200 volt AC scale as was suggested (while no music is playing).
It would be nice if it were possible to also measure the output voltage(s) of the DAC’s external switching power supply, but that would seem to be impracticable due to the nature of the connector and the lack of definition as to how it is wired. I suppose it's possible that it has been intermittently misbehaving, and caused the failure of the internal supply.
The only way to get a handle on what's going on is to play a 50 hz 0 dBFS test signal through the Wadia and measure the output voltage. It should be 2.0 volts RMS (the 50 hz tone will be more accurately measured since most DMM's are optimized for 50/60 hz). The DC offset can vary as a signal is passed through so the 1.0 VDC reading with no signal won't give you the extent of the problem.
So the answer is the no signal condition will be there most of the time but the actual offset may not.
The Adcom 535 Mk II is capacitor coupled at the input and the Creek and
Carver are not. What you have is DC at the output which the Creek passed
through to the speaker before going into protection mode. The Adcom's
input cap blocked the DC and didn't pass the buzz-causing DC to the
speaker. DC is enough to blow out a woofer.
Can you put a
output and input of the
Carver to confirm that?
Update: I went to test the Creek and it wouldn't power on. Opening it up I found the 2 fuses on the board were blown. Unfortunately they didn't blow before the speaker was damaged. The unit even worked a short period after the initial incident. Checked the fuses and they were the correct rating.
I'm trying to think of a way that a simple ground loop, DC or ultrasonic noise would destroy a woofer and leave the amp intact. None of those seem to be the issue; a ground loop can't do that, DC is blocked in any sane design by coupling capacitors and ultrasonics take out tweeters, not woofers. So what could cause a really loud angry buzz before outright failure of the woofer? I'm guessing the 'snap' was the woofer voice coil bottoming out. AC line voltage is one possibility- but this happened without the Creek being damaged.
I tested the amp and it seems to be ok through another pair of speakers.
I'm wondering if the DAC is grounded correctly. If the green wire and white wire were exchanged on the IEC it might be able to do something like this. A DVM should tell the story- measure the resistance from the center pin of the IEC connector to the chassis and see what you get. It should read 0.something ohms- shorted. What is the resistance to ground of the other two pins? If either is shorted to ground its a problem!
I went to test the Creek and it wouldn’t power on. Opening it up I found the 2 fuses on the board were blown. Unfortunately they didn’t blow before the speaker was damaged. The unit even worked a short period after the initial incident. Checked the fuses and they were the correct rating.
I did see that. But what we know is that the amp didn't fail- those fuses provide power to the amp, but the amp worked because filter caps in the power supply still held some voltage. That's my assumption that checks all the boxes anyway. We'll know more after the fuses are replaced and the amp tested.