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DAC channel dropout

BillGrimm

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Hi everyone. I have a (very) inexpensive USB DAC that sometimes drops a channel, this last time it was the left channel. Before I spend money on something more substantial, I'd like to understand what is going on because I have had the same experience with another DAC I had. The solution seems to be to unplug the RCA plug from the DAC on the channel being affected and touch it to a ground, plug it back in and it (typically) works fine. Sometimes I have to fiddle with it more, like swapping channels, unplug it from the computer, etc. before it finally decides to work again. This only happens now and then, so it is not a big issue, but I would like to understand what is causing it to occur at all. Thanks for your help.
 

M00ndancer

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Have you tried a different cable?
 
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BillGrimm

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If you mean the RCA cable, yes, I did try that when I first experienced the issue with the previous DAC. That didn't make any difference. Getting another (admittedly cheap) DAC also had no impact on the problem -- I am still getting the dropouts. It seems to be an issue within the DAC itself since if I listen to the audio through the headphone jack on the DAC when a channel is dropped out on the RCA jacks, that same channel is also absent in the headphones. But why would this be a persistent problem over two different DACs?
 

Chrispy

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I'd suspect the source if two different dacs behaved the same way like that. You've ruled that out, the source doesn't matter?
 

M00ndancer

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Have you tested another USB port on your computer?
 

antcollinet

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And what is feeding the dac?
 
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BillGrimm

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The problem has persisted over two separate laptop computers (the source), the first using Windows 7 and my current (new) machine running Windows 10. Switching USB ports does not affect the issue -- I know because that was one of the first things I always try. Just to clarify: the issue only happens from time to time. It isn't something I am always having to deal with. I just wanted to get some understanding of it before I choose which DAC to purchase next. For instance, would a "balanced" DAC have any impact on the issue? That is, are balanced DACs constructed in such a way as to prevent this issue from occurring? These are the sorts of questions I'd like to find answers to.

As far as the rest of my "system" goes, the DAC via RCA cables feeds into a Topping TP60 which drives some old Polk Audio Monitor 10s.
 

antcollinet

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If you have had the same problem in two different dacs it is very unlikely to be the dac as root cause.

Is it possible that something downstream of the dac (interconnect or amp) is short-circuiting or overloading the RCA output? If the RCA output is also connected internally to the headphone amp, then this would also explain the headphone amp drops out at the same time.
 
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BillGrimm

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It is possible the amp may in some way be causing this, but I don't know how or what I could do to diagnose it. The only thing I have found to get the channel working is to fiddle with the RCA plug at the DAC on the affected side -- unplug it, handle it, touch it to metal on the amp or computer, that sort of thing. But it will eventually happen again. Next time, I can try messing with the plug at the amp I suppose, but I don't know what that will prove. It will likely also fix the issue for the short term once again.

I do leave the Topping amp powered on all the time. Could that be a problem? I thought Class D amps didn't care one way or the other.
 

antcollinet

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First change the interconnect, and leave it changed. Does it still happen? An interconnect fault is still most likely here.

Next time it does happen try listening with headphones, and just disconnect the interconnect from the DAC. Do the headphones start working again (i guess it is possible that if there is short circuit protection on the dac you may need to restart the DAC - but I don't think RCA outputs generally need this). If they do, then the problem is almost certainly downstream of the DAC. (interconnect or amp).

You could also try switching off the amp, and turning it back on - does this clear the fault? That would indicate a problem with the amp.

Do you have another amp (or can you borrow one) to try for a period longer than the usual time between failures? Alternatively can you just listen to headphones for this period, with the amp and interconnect disconnected?

Leaving the amp on will not cause this problem unless it has a fault.
 
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BillGrimm

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Those are all great suggestions -- I appreciate you taking the time to really consider it. Let give you some more detail about I did this last time to resolve the issue. When it happens, as it did in this case, the left channel just goes silent -- it doesn't just reduce volume in one channel like what my old HK795 did before it finally quit altogether a few years ago. This last time, the left channel did come back on a time or two briefly, as near as I could tell entirely at random.

Unless I am certain of the source, I typically make sure I've got both channels working at the computer first by unplugging the USB that goes to the DAC -- the audio then comes out the computer speakers and I can hear that it is working correctly. Plug the USB back in, the DAC gets identified and powered on, but the left channel is still out. You would expect that any short protection would be resolved with the powering on, but that doesn't appear to happen.

So I turn the amp off and back on. No difference. One thing I've not thought to do before is check if the amp is feeding both channels from the second source (the Topping amp has inputs for two sources). I did switch out the sources and it didn't effect the left channel issue, but I didn't power on the second source to see if both channels were working at the amp. I'll try that next time.

The next thing I did was plug some headphones into the headphone jack on the DAC, but all that did is reveal that the left channel was also out in the headphone circuit. Then I fiddled with the left channel RCA plug thinking it might be loose or something (for no reason since I never touch the DAC unless it is giving me a problem). The plug was well set, but I unplugged it anyway. I was handling the plug and touching it to various things (computer, amp, etc.) to see if I could hear any feedback of any kind. This may seem odd, but my old HK795 would give different feedback-like sounds depending on what an input plug was touched to. This was in the time before it finally quit -- it was sending a lot of noise into one channel for a while. There was no noise from the Topping amp, however. But when I did eventually plug the RCA cable back in, the left channel was working.

Here's the DAC: https://www.amazon.com/gp/product/B01M7QQQC7

Here's my other DAC: https://www.amazon.com/Dayton-Audio-DAC01-24-bit-Output/dp/B07JH8BKWN

Like I said, both have had the same issue. During this time, I have changed computers, but the amp has always been the Topping TP60. The cheapo DAC was actually my first one, but the channel dropout issue made me buy the Dayton Audio DAC to replace it. However, the Dayton Audio DAC did the same thing and it seemed to me, more frequently, so I switched back to the cheapo. There was another issue with the Dayton Audio DAC that I can't now recall, but it ended up in the garbage.

I wish I had access to another amp, but I don't. If this issue could be traced to the amp, I might consider getting something like the new Topping PA5 to replace it, but I don't yet see how the amp can be causing an issue like this.
 

antcollinet

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If the interconnect is broken (no signal to the amp) or short circuit (output of dac shorted) then you can switch the DAC on and off as much as you like and the problem will persist. The fact that it goes away when you disconnect the internconnect points strongly to a problem with the interconnect.

It could be a hardware problem with the amp - eg a bad solder joint on an input connector - that is moved slightly when you move the interconnect causing it to make contact.

Trying a second source into the second amp input is a good idea, since it will tell you if the power/output stages of the amp are working OK. But it won't tell you if the problem is the DAC, the interconnect or the first amp input.

Another thing worth trying is using a new interconnect - and connecting it into the second amp input. If the problem goes away you know it is either the interconnect or the amp input. Then plug back into the current input - if it stays away, you know it is the interconnect.
 
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BillGrimm

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Well, when I switched out the RCA cable that had been going to my CD player and connected it to the DAC, initially I got a high-pitched (but low volume) whine coming out of both channels. As I rotated the plugs in the jacks, that went away and it got quiet. Is it normal to get a low-volume hum from the stereo when you touch the conductors on an RCA plug?

Anyway, I will leave it as I have it now and see if the channel dropout happens again. Thanks for your help, Tony.
 

antcollinet

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Yes, normal. If they are not connected to a source at all, they will pick up noise like an aerial - especially if you touch the contact. Perhaps in this case you only had the gnd/screen connected until fully inserted which would pick up any DAC internal interference from the digital circuits. Better to connect the source end first.
 
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