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Author Topic: (Resolved) Low hum/buzz even when DAC is not connected.  (Read 3218 times)

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Offline elgringo81

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The biggest improvement was made once I grounded the choke (You might be rolling your eyes by now:))

Instead of the single shielded microphone cable I was using, there is now a pair of them. 1 for each channel. I believe this brought the buzz/hum even further down.

I made myself some RCA shorting plug. Took a cheap RCA and cut two ends off it.
Then I connected the wire to shield/ground on both of them. This I should have done a long time ago. There is much less noise when I have them installed compared to when they have nothing installed.

After these improvements and discoveries, the noise is lower but it has also changed.

Hizzing:

Now I can hear a low hizz that responds to different volume settings. (Note: this hizz is louder with the Clear tops than other 12au7 tubes.

These tests are done with the Clear Top installed. Tested with shorting plugs installed.
*0% Volume = Very low hizz. I can hear the difference when I plug in an out my HD650s
10% Volume = hizz is going up with volume
20% Volume = the hizz has now stopped going up in volume
80% Volume = The hizz is going down in volume.
100% Volume = Now the hizz is the same as at *0% volume.

50-60 cycle buzzing:
Every time my hand is on the volume knob or closer the the 12au7 tubes, there is a 50-60Hz buzz.
The buzz is volume dependent in exactly the same way as the hizz is. At 80% volume, the buzz starts to fade out. I am pretty sure its 50Hz because I have been listening to different guitar buzzing videos and it sounds more like 50Hz then 60Hz.
It might have something to do with that I am in Denmark and the mains here are running on 230V 50Hz, correct me if I am wrong.

I have measured resistance on ground connection accordingly to this excellent threat below.
http://bottlehead.com/smf/index.php?topic=4812.0
Prefect ground was 0.6 Ohm on my el cheapo meter.


I have attached few pictures that show the amplifier in current state. Not state of the art, but only small sonic issues left.








 




« Last Edit: May 23, 2017, 09:11:15 AM by elgringo81 »
Guðmundur Ásgeirsson
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Offline elgringo81

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..One strange thing a hear now is that. If I turn off the amp, disconnect the 12au7 and keep the headphones on, I can hear a very low 50Hz buzz and it goes away when I touch the 12au7 socket.
I hope that can be evidence on what is going on here.
When I keep the amp turned off and plug in the 12au7, the 50Hz buzz goes away.

I forgot to note that I have found a contact cleaner that I am using on my tubes.
the name is Kontakt IPA/Isopropanol from Germany. It has not solved my new hizz/interference 50Hz noise problem but its got to have. I will get my hands on DeoxIt or similar soon.

I feel like giving up on the Clear Tops I have and focus on stock 12au7. I dont need more components to worry about when I am trying to figure this out.

But the problem remains.
1. Low volume depended hizz.
2. 50Hz buzz when my hand gets to the volume pot or closer to the 12au7.

I am sorry for how long this is, and thank you very much for reading so far, but I cannot get this out of my mind :)
« Last Edit: May 23, 2017, 10:09:57 AM by elgringo81 »
Guðmundur Ásgeirsson
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Offline attmci

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Dr. B answered one of your questions in the other thread which is very true:

Rule 3 - if the noise is coming only from the tube, it can't go up and down when you turn the volume control. The volume control is ahead of the tube. Noise that goes up and down with the volume control is coming from ahead of the volume control, i.e., cables or source component. Different tubes might expose more of this noise if they have more high frequency emphasis.

You crack looks very nice. But the noise.........

« Last Edit: May 23, 2017, 03:12:51 PM by attmci »

Offline elgringo81

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Dr. B answered one of your questions in the other thread which is very true:

Rule 3 - if the noise is coming only from the tube, it can't go up and down when you turn the volume control. The volume control is ahead of the tube. Noise that goes up and down with the volume control is coming from ahead of the volume control, i.e., cables or source component. Different tubes might expose more of this noise if they have more high frequency emphasis.

You crack looks very nice. But the noise.........
Thank you.

With rule 3 in mind, could it be that this is a ground related issue.
As I said before, If I turn off the amp and remove 12au7 and then put on my 250 Ohm DT-1770-Pro headphone, I hear a faint buzz.
again at this state, the power switch is Off.. the buzz goes away if I touch the 12au7 socket.

I live in Denmark and its pretty usual that main sockets down have ground connected to it.
I dont have a meter to test it so I guess I need to open op the socket and look inside.
Its just that my landlord is a pain in the *** and I dont want to fittle with it if I dont have to,


I have also though about the choke placement. I am using 40mm nylon standoffs and I have not experimented with different locations with it.
I am wondering if I am in a zone that picks up interference coming from the transformer. I would like to test going back to stock PS configuration
and see if it gets resolved. If that has an positive impact, I would like to test the choke with a shorted standoffs, 20mm.


I have tried different interconnects and different DACs (Chord 2Qute and MF M1DAC) and the hizz and 12au7 tube interference is the same.
I have also tested it with an RCA shortening plug and it yields the same results as if I have the DACs connected.
If I have nothing plugged in. the hizz gets louder from 80-100% volume.

This is what comes to mind that I need to test. I am open for all suggestions and correction.

1. Disconnect the choke and PS output film capacitor and go back to stock PS setup. If that lowers the hizz/buzz, I will try with shorter standoffs.
2. Remove the new TKD pot and install ALPS blue.
3. Replace the shielded microphone cables with 3 wire braided solid core cables from RCA socket to pot.

Guðmundur Ásgeirsson
DIY Beginner.

Offline 2wo

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Ok first off, testing for noise with out shorting plugs or DAC plugged in, or the tube removed, is not really going help moving forward. However if you still have noise with the amp turned off that means that the amp is not generating the noise. If you suspect your mains, take the amp and shorting plugs somewhere else, to work or something and have a listen

The next thing I can think of is to remove the choke and replace the resistor...John
John Scanlon

Offline elgringo81

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Ok first off, testing for noise with out shorting plugs or DAC plugged in, or the tube removed, is not really going help moving forward. However if you still have noise with the amp turned off that means that the amp is not generating the noise. If you suspect your mains, take the amp and shorting plugs somewhere else, to work or something and have a listen

The next thing I can think of is to remove the choke and replace the resistor...John
Once I discovered the shorting plugs, I have been using them to test this hizz issue I have ever since.
You can see it earlier in this threat that I made some and did the test with them plugged in.

I am basically not interested at this point how the amp sounds with empty RCA inputs.


I will need to wait for a good time to roll back the choke, I will most likely test that here over the weekend.

Here are my thoughts at the current moment.

I would say that if I dont use the Clear tops, both my HD800S and HD650 are silent at all volume levels.
If there is hiss, its even hard for me to notice it. But I can notice a small hizz like I have descripted earlier with my DT-1770-Pro closed headphones (Impedance: 250 Ohm, SPL: 102 dB) They seem to be more sensitive or that its more noticeable because of the closed back design.

I am starting to wonder if this hizz is basically normal because HD800S and HD650 don't seem to be picking it up anymore.

If that (Tired) conclusion is correct. Then I only have one small issue remaining.

That is the 50Hz buzz interference when my hand gets close to the Pre tubes.
Could it be because if ground issues in the apartment I am living in?
I have tried different sockets in the kitchen but its does not seem to matter.

I will report back with results from (rolling back) the choke upgrade.





Guðmundur Ásgeirsson
DIY Beginner.

Offline elgringo81

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I have ordered 1x Mullars 12au7 and 2x Clear tops.
I should be getting these tubes next week.

it will be interesting to see how that goes.
Guðmundur Ásgeirsson
DIY Beginner.

Offline elgringo81

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Just wanted to update this threat.

I could not wait so I have now rolled back the choke upgrade. (So far I cannot hear much difference in SQ, I will spend some more time with it like it is)

the Hizz remains the same and maybe as expected,
the 50Hz buzz is still there when I come close to the 12au7 tubes.
I have re-flowed the pre socket 2x times and re-flowed the TRS socket and pot.

I am starting to give this new TKD pot a bad eye..

Guðmundur Ásgeirsson
DIY Beginner.

Online Paul Birkeland

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If you have hum that appears when your hand gets close to the 12AU7, then the Crack is still not properly grounded.  I would recommend checking to see if the electrical outlets where you are using your Crack are grounded.  This isn't all that uncommon in older structures, and it is a nice upgrade to add a grounding rod and provide a solid ground path for your electronics.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline elgringo81

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Re: (Resolved) Low hum/buzz even when DAC is not connected.
« Reply #24 on: May 27, 2017, 09:45:40 AM »
@Paul.

Thanks Paul.

You were right. The amplifier was not grounded, but that has been sorted now.

I have now connected it to a second outlet which I have opened and verified it has a ground wire in place.
The 50Hz interference is now gone for good. I will check the old socket for faults later.

I am going to stop worrying about the low hizz when I use the DT 1770 Pro headphones. I believe its just because they are more sensitive than the HD800S and HD650.

I bought them because they got great reviews and they are 250 Ohm for my Crack. I really like them,
but it seems that they would need lower gain then the Sennheisers, the crack was build around.

I have no complains about the TKD pot now, it works fine and the Crack sounds great.

Thank you for all the information and help.


« Last Edit: May 28, 2017, 05:15:16 AM by elgringo81 »
Guðmundur Ásgeirsson
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Online Paul Birkeland

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Re: (Resolved) Low hum/buzz even when DAC is not connected.
« Reply #25 on: June 01, 2017, 10:48:32 AM »
I'm glad you got it sorted.  It's certainly worth checking all your outlets to be sure they are grounded.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man