Static-like Sound Coming from Left Channel?

[ThePhoenix]

Hello,

EDIT: the original "solution" proved to be a coincidence.

So, I was writing a big long post (saved below) and may have solved my own problem whilst doing so. I'm still posting just because, while the solution was one of those "d'oh" things, it might help someone else in the future.

TL;DR, I think my cheapo Amazon Basics power cable isn't grounded/shielded properly and was creating a static sound in the left channel. So, if someone from Google pops in with a similar issue, check a different cable before pulling your hair out like I have.

------Problem------

I recently purchased a used Mjolnir Pure BiPolar (S/N 020) and have an intermittent issue with static coming through the left channel.

I first thought "ground loop", but everyone describes that as a hum and this is more like white noise. I have tried several outlets/combinations of electronics powered on and off to no avail.

The sound only ever occurs in the left channel; its loudness is unaffected by the volume knob and comes and goes seemingly at random. The noise is usually relatively quiet, but can reach levels around what I'd describe as being a lower level listening volume. I would say that the sound usually doesn't appear for a few minutes after power up, but I wouldn't say that is a hard and fast rule.

Also, the static occurs regardless of whether or not something is connected to the inputs or if a source is powered on/transmitting signal.

I am disinclined to believe there is any issue with the headphone cable, as it is a new Sennheiser CH 650 S. I don't have a way of confirming my theory or not as this is my only balanced set at the moment.

-----Solution?-----

I believe I have solved the issue simply by trying the power cable I use on my DAC in place of the super cheap Amazon Basics cable I had in a drawer. I don't especially believe in high-end cables, but I imagine the Amazon cable must have a rubbish ground or something. So, I'm looking at a decent, non-insane power cord to replace it with. Here's hoping that's the end of it. [I will try and remember to report back to the positive or negative.]

EDIT: unfortunately, the static returned a few hours later, so it seems the "solution" was a coincidence. Back to the drawing board. 

-----Side Note-----

Spritzer, et. al, kudos to you all! The amp is fabulous and, in my opinion, money well spent. I don't have enough experience with different amps to review it, but it sounds great to my ears. I've wanted one for quite some time and just had to snag this one when I found it.

For those who care, the Mjolnir is replacing a Woo WA6-SE and is currently paired with a Bifrost Uber (I'm saving for an Yggdrasil or similar). At the moment, I'm using a set of Sennheiser HD 650s, and, once Drop finally sends my balanced cable for it, I'll switch to my HD 800s (non-S), which I'm really looking forward to.

Anyways, hope this helps some future person and is a reminder that one can think they've checked all of the obvious issues and realise the MOST obvious thing to change was the actual problem.

Edited January 14, 2023 by ThePhoenix
Adding new info in regards to the issue.

[ThePhoenix]

Just edited the original post as the static has returned; so, it appears the issue isn't only or isn't at all with the power cable. Damn.

Any ideas I haven't gone over? I find it hard to believe a cap is going out in the amp itself, but would that create an intermittent static noise?

[ThePhoenix]

Coming back to this for posterity; basically, despite thinking it was a poor cable, I was quickly proven incorrect (as stated previously). After more trail/error, I found that there seemed to be a correlation with my mini-split A/C & heat-pump running with the static (we had a few days of cool enough weather to shut it off completely; I didn't hear any static at all those two days - could've been luck, but I'm willing to bet there's correlation).

Anyways, I bought myself a Furman Elite15i power conditioner in hopes it can catch/resolve whatever is going on with the power and let me listen in peace. I just plugged it in, so I'll report back in a few days (or minutes, depending on whether or not I have any more issues) with whatever findings I have.

[ThePhoenix]

Yep; that was a bust. Static started a few minutes in; I thought it seemed quieter than usual (perhaps indicating the condition was doing SOMETHING), but that was shortly proven incorrect and no change seems to have been made. So, I guess that's back to the drawing board and, more likely than not, shipping this back to the seller so I can try something else.

Does anyone have any ideas on things to try? I still don't see how it could be an amp issue given it doesn't seem to be that old; that said, is there something I can look for to try and determine if there's an issue? I don't know how to solder myself, but I do have a multimeter I can use to test things if that would be of any help. I did think to ask the store I bought it from if the previous owner had mentioned any issues; they said, "no", but who knows if the previous owner even told them correctly.

@spritzer; sorry to tag you in one this, but I'm starting to lose my mind over this. Have you ever heard of a Pure BiPolar having intermittent static in just one channel? Does it seem more likely to you for it to be a power issue or an amp issue?

[dsavitsk]

This sounds like something is oscillating. It is almost certainly not due to a ground loop, bad outlet, cable, power cord, air conditioner, etc, and it likely going to require a scope and knowledge of the circuit to diagnose.

[ThePhoenix]

Thank you for your reply! I don't have an oscilloscope, but I'll look into getting one. I'm trying to avoid shipping it back to Iceland, but if that's the only solution, it is what it is.

[spritzer]

No, never had this issue pop up but first thing to check would be the offset voltage.  If something would fail, it could knock the servo out of kilter and damage the headphones. 

[ThePhoenix]

Okay, I would think I can do that. Though, dumb question, where should I be measuring that from and what values am I looking for? I know how to use a multimeter, but my knowledge of electronics is not very significant.

[spritzer]

You can measure at the output connector or just directly behind it on the main PCB inside the chassis.  Multimeter set to VDC and just a few millivolts should be present.  You measure with regards to ground so the black probe goes onto the chassis or the middle pin on the volume control (either side) marked GND. 

[ThePhoenix]

2 hours ago, spritzer said:

You can measure at the output connector or just directly behind it on the main PCB inside the chassis.  Multimeter set to VDC and just a few millivolts should be present.  You measure with regards to ground so the black probe goes onto the chassis or the middle pin on the volume control (either side) marked GND. 

Okay, so I removed my headphones from the output, took off the top panel, turned the unit on (with no input), and placed the black probe on the screw for the Ground by the volume pot and the red one on the output using the handy little L & R circles (I'm hoping/assuming that's what they're there for).

[For reference, the Multimeter is a Fluke 117; so, nothing super special, but I think it should be accurate enough for our needs.]

On R, which I've never noticed any static on, reads 1 mV pretty consistently and only hopped around between -1 and +2 mV for the thirty seconds or so I watched it.

On L, where the issue should lie, was much more jumpy; I saw everything between -7 and +9 mV.

I thought it might be more helpful if I knew what it was reading during a static-y period, so I plugged my headphones in again (after shutting the unit off, which I assume is recommended), and got the following values:

R: about the same, between -1 and +2 mV for the most part.

L: I didn't actually see as large of a swing when I got confirmed static; only around +-3 mV. I'm guessing I was getting more before, but am obviously not sure.

(naturally, I didn't get static for a long while when I was trying to measure for it. This after putting up with it for an hour earlier in the day.)

Thank you so much for your help; I really appreciate it!

[spritzer]

That's a bit high but not massively so, I doubt that is the issue here.  Intermittent issues are so hard to track down but do one thing, try to reseat the input BJT's (they are in the middle of the boards) and see if anything changes. 

[ThePhoenix]

10 hours ago, spritzer said:

That's a bit high but not massively so, I doubt that is the issue here.  Intermittent issues are so hard to track down but do one thing, try to reseat the input BJT's (they are in the middle of the boards) and see if anything changes. 

Intermittent problems are, certainly, the worst.

I just got done trying your suggestion; unfortunately, no change. I simply pulled them up and stuck them back in; should I have tried cleaning the contacts or anything? Also, since they're transistors, is there a way for me to test them for leakage or something (not sure if that's a thing with these)?

[spritzer]

You would need pretty fancy equipment to test them but this could be a THAT340 issue.  I'm not sure if you amp has THAT340 as the input (as I've used other parts in the past) but they have been known develop issues. 

[ThePhoenix]

1 hour ago, spritzer said:

You would need pretty fancy equipment to test them but this could be a THAT340 issue.  I'm not sure if you amp has THAT340 as the input (as I've used other parts in the past) but they have been known develop issues. 

Ah, okay, so no testing, then.

I looked up a THAT340 and I don't see anything like that as far as I can tell. Would a photograph of the boards in my particular amp be of any use to you?

[ThePhoenix]

Since I can't test them, would replacing the BJTs be a worthwhile investment so we can cross them off the list of potentials? If I'm reading them correctly, it looks like each board has a MAT02 and a MAT03.

[Also, it might just be me, but the issue seems to be getting worse/more consistent. I'm hoping that will help narrow stuff down.]

[spritzer]

Please send me some high res pictures as I might be able to spot something wrong. 

It is also worth it to try and swap the input devices between the channels and see if the problem follows the transistors. 

[chinsettawong]

Do you have other headphones to test and confirm that the issue is really from the amp?

[ThePhoenix]

3 hours ago, spritzer said:

Please send me some high res pictures as I might be able to spot something wrong. 

It is also worth it to try and swap the input devices between the channels and see if the problem follows the transistors. 

Okay; will do! I'll need to charge a battery or two, but I'll get those when I can.

Also, good idea - I'll try swapping them and report back.

2 hours ago, chinsettawong said:

Do you have other headphones to test and confirm that the issue is really from the amp?

Affirmative - I have tried two sets of headphones (my balanced cables for the HD800 finally came in) and the issue persists between the two of them. Both headphones are known good off of different amps.

[ThePhoenix]

All right, so I swapped the two MAT03 BJTs to no change (I heard static right away). I then swapped the two MAT02s; no change. So, it appears it isn't linked to them (unfortunately?)

However, two things I noticed trying to test to see if anything else seems unusual or possibly wrong:

1. I'm guessing this is normal, but there is an audible noise when you power the amp on only in the left channel. I don't know how to describe the sound, it's not really a pop, maybe a "plump"?
2. When adjusting the volume, there's a noticeable sound very similar to the intermittent static (perhaps not quite as rough-sounding, but still there), also confined to the left channel. I remember thinking I heard something with the volume control when I got the amp, but I haven't really noticed it while listening to music and I put it out of my mind (sorry). Also, it seems stronger now than it did; it's clearly audible over medium/low-volume music.

[ThePhoenix]

Doing some more listening; I'm more and more convinced there's a channel imbalance between the L/R channels (L being weaker). I'm not sure if I just simply didn't notice or if it's gotten worse over time, but I just swapped back to my previous amp to see if I was going crazy (or possibly damaged my headphones) and I'm confident it's there.

While I'd love to believe it's all due to the potentiometer, would it also be possible for whatever resistor/transistor (not sure how many transistors there are or if it's just the BJTs) causing the issue to have failed completely and is now not amplifying the channel fully?

[Also, I'm working on those photos. I'll have them up in a bit - sorry for the wait!]

[ThePhoenix]

6 hours ago, spritzer said:

Please send me some high res pictures as I might be able to spot something wrong. 

It is also worth it to try and swap the input devices between the channels and see if the problem follows the transistors. 

All right; I took a number of photos (yay, 30s exposures!) from different angles in an attempt to be as helpful as possible. I've posted them all to an Imgur folder; please let me know if a Google Drive or something similar would be easier for you to use.

I think everything is pretty logically organised, but let me know if you have any questions. I don't have a macro lens, so I did what I could with what I had readily available.

Imgur LINK (it's private, only those with the link should be able to view it).

I will reiterate that everything is exactly as it's been since I got it a couple of weeks ago (other than the swapping of the BJTs); I only mention it in case you see anything and go, WTF.

[simmconn]

Where are you located? We have forum members around the world, and someone close to you maybe able to help. Although the circuit is not complex, troubleshooting intermittent noise problems often require a lot of patience and decent test equipment.

[ThePhoenix]

10 minutes ago, simmconn said:

Where are you located? We have forum members around the world, and someone close to you maybe able to help. Although the circuit is not complex, troubleshooting intermittent noise problems often require a lot of patience and decent test equipment.

In the central bit of Missouri, USA (specifically, around the Lake of the Ozarks).

I knew asking for help with this particular issue and getting it resolved would be a long shot; I'm so thankful Spritzer and the rest of you have offered advice already.

[spritzer]

I've reviewed the pics and nothing stands out to me.  Couple of things, the input BJT's are sitting in a socket which is then sitting in another socket, did you try to separate the two?  That could be a factor. 

Now static sound from the volume control is always possible so one thing to try, swapping the input and output wires between the channels. 

[ThePhoenix]

12 hours ago, spritzer said:

I've reviewed the pics and nothing stands out to me.  Couple of things, the input BJT's are sitting in a socket which is then sitting in another socket, did you try to separate the two?  That could be a factor. 

Now static sound from the volume control is always possible so one thing to try, swapping the input and output wires between the channels. 

I did not attempt to separate the BJT sockets, but I'll fiddle with them to make sure they didn't separate.

For the volume control, are you saying I should switch the two L wires with the two R wires on the left of the pot? Or, that I should swap all of the wires from the left side (going to the circuit boards) to the right (from the XLR inputs)?