Question Can DC offset cause intermittent left channel?

lovetravelsfast

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My setup: Technics turntable, rega phono pre-amp, NAD integrated amp (or Harmon Kardon integrated amp), speakers.

Issue: At seemingly random times, the left channel drops to like 10% volume and is super distorted. Fiddling around with the stylus, the inputs, the connections, etc. seem to restore to normal for a while, until it happens again.

Reason for this question: I identified the NAD amp as the culprit, swapped it out with my Harmon Kardon amp instead, and lo and behold, the issue persists. However on the HK I noticed some fairly loud hum from the transistor (not speakers), pointing to DC offset issues, which doesn't surprise me in our old house with old appliances, tons of dimmer switches etc. Now, my question: Can this DC offset issue in my house really cause this weird left-channel issue on multiple amps?

Whenever Emotiva gets the CMX-2 back in stock, I'm picking that up to see if that eliminates the hum and/or the left channel issue, but in the meantime I'm very curious if anyone here has any thoughts?


For those curious about my troubleshooting naming the amp the culprit:

While the issue was happening I swapped the speaker cables around on the back of the amp, and confirmed that the issue moved to the other speaker. Which confirmed to me that the speakers and the speaker cables work fine and that the distorted sound is being output of the left speaker channel on the amp.

I then tried a bunch of source changes. I plugged in my phone on both the same channel as the turntable runs on (AUX) and on at least two different channels. This source had the exact same issue, so this ruled out my turntable or cart or anything like that.

I also tried swapping the left and right RCA cables going from the turntable into the amp, but the issue still was showing in the left speaker. So in other words, no matter what signal is input on the left channel on any of the inputs, the left speaker output will have this issue, intermittently.

So, those are the steps that made me confident the amp was at fault. So I was very surprised to hear the exact same issue on my other amp. Where I went through all the same troubleshooting again, and still confirmed it indeed looks like an amp issue.
 

insider9

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Here's how. Please make sure you know what you're doing as I don't want you to blame me for electrocuting yourself

 
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lovetravelsfast

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Here's how. Please make sure you know what you're doing as I don't want you to blame me for electrocuting yourself


I read this article which led me to believe I have DC offset issues, and based on a few random comments I saw here and there about measuring it, I assumed I didn't have the equipment necessary to measure it. For some reason I got the impression a multi-meter wasn't enough. I'll take a look at that how-to and promise I won't blame you for any zaps.
 
I'd urge caution in performing any tests like this unless you happen to be a competent electrician.
Can you source another, more modern amp to put into your system to see if the same effects happen?
Personally I would be working backwards and eliminating cartridge then photo preamp.
Can I assume your Nad and the HK integrates don't have an inbuilt phono stage?
Apologies I have just read further into your description. Please ignore this.
 
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lovetravelsfast

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I'd urge caution in performing any tests like this unless you happen to be a competent electrician.
Can you source another, more modern amp to put into your system to see if the same effects happen?
Personally I would be working backwards and eliminating cartridge then photo preamp.
Can I assume your Nad and the HK integrates don't have an inbuilt phono stage?
Apologies I have just read further into your description. Please ignore this.
Your advice on caution is still valid, but I feel competent enough to do this since its just measuring voltage in the speaker outputs it looks like. But I appreciate the answer!
 

lovetravelsfast

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Here's how. Please make sure you know what you're doing as I don't want you to blame me for electrocuting yourself

Just got a chance to measure it. Worth noting is that the transistor hum also comes and goes. Lets the amp on for 10-ish minutes and then measured. The hum was not present, so I'll do another measurement when the hum shows up again. But for this first measurement I got something weird/interesting:
The left channel (the one with the issue) is around 10mV, which should be fine (right?) but the right channel measured a staggering 130mV. I'll post again when the hum returns.
 

lovetravelsfast

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Transistor hum is there now, still measures the same ~10-15mV on left channel, ~130mV on the right. These are on the HK amp. What's even weirder is that the NAD amp shows ~5-10mV on the left channel and ~15-20mV on the right. So right channel worse than left on both amps, but on NAD both numbers fairly healthy.

And still, both amps exhibit this behavior, and my troubleshooting has ruled out any of the other components. One thing I didn't mention in my troubleshooting is that I have another turntable, another set of speaker wires, another set of speakers that I have also tried, and still the same. Like no matter what combo of anything I put together, I get this behavior. I'm about to pull out my hair.
 

lovetravelsfast

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Sounds like a partial short circuit on the speaker cable to me. You eliminated the amps. DC offset has nowt to do with it.
Indeed I did rule out the amp, but the speakers and speaker cables are also ruled out. I'll reiterate what I tried to say above in my troubleshooting with this little diagram:

First setup:
Amp Speaker Channel L -> Cable L -> Speaker L (issue somewhere on this path, bad sound/dropout in speaker L)
Amp Speaker Channel R -> Cable R -> Speaker R (No issue here, so all these three should be good, right?)
Result so far: Known goods: Amp Speaker Channel R, Cable R, Speaker R

Swapping Cables on the back of amp:
Amp Speaker Channel L -> Cable R -> Speaker R (Issue now in speaker R)
Amp Speaker Channel R -> Cable L -> Speaker L (No issue)
Result so far: Known goods: Amp Speaker Channel R, Cable R+L, Speaker R+L

So, only thing left to blame is Amp Speaker Channel L. Which made me confident it was the amp. So when I swapped out the amp, and the problem persisted, (and I'm 100% sure its not upstream of the amp), the only thing left to blame is power, right?
 

Gray

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I think you're conflating two DC offset types.
You started off talking about the possibility of mains DC offset issues.
But you've been measuring the DC offset at your speaker terminals - which will not tell you about any DC on the mains.
As it happens though, by checking the amp's DC offsets, you've discovered that excessive 130mV. Whatever else, you should get that reduced, which may or may not be something that you can do yourself.
Back to your original question. My answer would be no. DC in with the AC mains has reportedly resulted in transformer hum for some people - but I'm sure something else is causing your left channel issue.
Your elimination process needs to be refined (and very specific, to stop you going in circles).
 

lovetravelsfast

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I think you're conflating two DC offset types.
You started off talking about the possibility of mains DC offset issues.
But you've been measuring the DC offset at your speaker terminals - which will not tell you about any DC on the mains.
As it happens though, by checking the amp's DC offsets, you've discovered that excessive 130mV. Whatever else, you should get that reduced, which may or may not be something that you can do yourself.
Back to your original question. My answer would be no. DC in with the AC mains has reportedly resulted in transformer hum for some people - but I'm sure something else is causing your left channel issue.
Your elimination process needs to be refined (and very specific, to stop you going in circles).

Gary, thank you! That is very helpful. I got an AVA Humdinger on the way to see if that takes care of the transformer hum. So that's one issue potentially taken care of. And it makes sense to me that the dropout in sound is another issue. But -- I don't get why you think my elimination process is unrefined or circular? Maybe I'm just bad at explaining the steps I've taken.

I'll try to reiterate my steps. These are all things I've done while the dropout issue was present.

Suspect #1: Audio source (starting with my turntable)
* I tried replacing the cart. No change.
* I tried swapping the L/R RCAs down by the amp. If the turntable was emitting the dropout, this should move the issue from one speaker to another. But no change.
* I tried whole other sources (a phone, an ipod, a sonos thingy). All had the same issue. Dropout in speaker L. I tried plugging all those sources into multiple inputs. Tuner, Aux, Video, etc. And tried swapping L/R for all of them. Literally no matter what I do, the issue is dropout in Speaker L. No change.

Result
: Nothing wrong with the audio sources, so it must be either amp, speaker cables or speakers, right?

Suspect #2: Left speaker
* I tried swapping the speaker cables at the amp like I explained above. Now the issue moved to speaker R. So in other words, speaker L and speaker cable L are both capable of NOT having the issue. So in other words. Both speakers and both speaker cables have exhibited healthy behavior at the same time as the other set exhibiting dropout. Ergo, the speakers and speaker cables should be fine.

Result: The only remaining culprit is the amps LEFT speaker channel. No matter what else I swap around, the dropout persists through the amps LEFT speaker channel.

Suspect #3: The amp
Now that I've narrowed it down, I wanted to verify by swapping the NAD amp with the harmon kardon amp. Everything else is the same. And I've ruled out everything else. EXCEPT THE ISSUE PERSISTS ON THIS WHOLE OTHER AMP. What the heck. I'm going crazy. So, I start from the top with suspect #1, suspect #2, and reach the same conclusion with this whole other amp. The issue is with the left amp speaker channel.

What have I done wrong in my troubleshooting?

And like I mentioned above, I've tried an ENTIRELY DIFFERENT SETUP. Different cables, different turntable, different speakers, different amp. No matter what combination of anything I do, the amp has issues on its left speaker channel. Which is why I'm losing my mind, and why I'm looking at the power being an issue.

Next opportunity I get, I'm bringing my whole setup to someone elses house to verify this hypothesis.
 

Gray

Well-known member
If there's DC on your mains, there's every chance the Humdinger will kill (or at least reduce) the transformer buzz.
As listed above, your elimination process is as logical and thorough as it could be.
You've done nothing wrong in your troubleshooting.
The only reason I mentioned double-checking was - and you don't need reminding - it's what's driving you crazy: The chances of two different amps having that same, strange L channel symptom.....well it would be a massive coincidence wouldn't it?
That suggests an external influence affecting both amps. But, again, the chances of two different brands of amp reacting in an identical (specific to the L channel) way, seems just as unlikely to me.
Anyway, it certainly won't hurt to get that buzz killed off - I'd be surprised, but pleased if it cured your L channel fault.......keep us posted.
 
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lovetravelsfast

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If there's DC on your mains, there's every chance the Humdinger will kill (or at least reduce) the transformer buzz.
As listed above, your elimination process is as logical and thorough as it could be.
You've done nothing wrong in your troubleshooting.
The only reason I mentioned double-checking was - and you don't need reminding - it's what's driving you crazy: The chances of two different amps having that same, strange L channel symptom.....well it would be a massive coincidence wouldn't it?
That suggests an external influence affecting both amps. But, again, the chances of two different brands of amp reacting in an identical (specific to the L channel) way, seems just as unlikely to me.
Anyway, it certainly won't hurt to get that buzz killed off - I'd be surprised, but pleased if it cured your L channel fault.......keep us posted.
Thank you Gary! Yeah you hit the nail on the head. Like what are the chances? Seems so unlikely. I'll continue swapping components around, using different circuits in the house, and trying it all at someone else's house too, like I mentioned. Plus the humdinger. Something has to start making sense at some point. I'll keep you posted!
 
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