Background Hum/Breeze/pink/white-noise

[Viktor]

Hi!

First I know this question have been asked a number of times before and I myself wrote about this back in 2011. However I never got around to fixing the problem and since I invested in a pair of HD800 headphones I really wan't everything to work 100%.

I have a low background hum present all the time when I have headphones connected. I wrote about it last week at head-fi and got a few tips. I rebraided my RCA connections with furutech cable, reflowed the solder-joints and I have tried three different power tubes and two different of the smaller tubes. I checked the grounding tutorial here and It checks out fine also checked the voltages again and everything looks good. It is really frustrating to have this background noise, I checked reference sounds at 60 and 120Hz and it is def. not any of those frequencies it sounds more like a very light breeze sound, mostly present when the music is quiet.

I tried the amp in different rooms to rule out any interference but the noise is still present. I lost at what I should do next.

Best Regards

Viktor


edit: It sounds exactly like the sound you here on some older jazz recordings when the microphone is picking up background noise.
Also, it does not change when the volume knob is turned.

 

[Paul Birkeland]

A breeze sound is not hum, so the recommendations from Headfi could be disregarded.

Does your Crack have a Speedball?  Is everything else stock? 

 

[Viktor]

Hi!
Yeah I guess you are right. I do not have speedball installed, but if I can manage to fix this I would be very interested in getting that upgrade!

Everything else is stock. The best way to describe the sound is, for example, the breeze noise present on Kind Of Blue and the songs there, for example So what. If you turn up the volume on that song there is a background present which has kind of a breezing sound to it.

Hope this helps,

Cheers

 

[Doc B]

The term for the sound you are hearing is rush, or more technically speaking either white or pink noise. It's most likely to be from a tube or sometimes a semiconductor can generate the noise, though that is not as common in our circuits.

 

[Viktor]

Hmm really weird problem then since I have tested two different 12AU7WA tubes and three different power tubes. The noise was somewhat lower using one of the 12AUWA7 tubes though.

 

[JamieMcC]

I don't suppose you are streaming? Have you tried using a different source cd dvd analog and seeing if its still there?

 

[Viktor]

I tried connecting the amp with no source at all and the sound was exactly the same. I have a Schiit Vali as well which is dead silent using the same source so I don't think that is the problem.

 

[Viktor]

No other idea or suggestion? I rechecked the solders once again but still the same sound.

 

[Doc B]

This is the kind of thing that is somewhat difficult to assist with without an idea of the relative level of the noise with respect to the music signal. If it is very low like the tape hiss you mentioned with respect to Kind of Blue, then it's probably a matter of trying different tubes until you find a level you are satisfied with. If it is more persistent than that, to where it intrudes in music playing, can you tell if it is in both channels equally? Sometimes a resistor can generate that kind of noise but that would typically be confined to the channel the noisy resistor is in.

 

[Viktor]

Yeah I understand that it is difficult but since some users state their Crack is dead silent it is kind of frustrating, but then again, I don't know what kind of tubes they use for their amps.
The noise is present in both channels, right now I'm just frustated about it and notice it all the time during quiet passages in music, but it could also be me being super sensitive!

As long as the voltage readings are ok it should be safe to use the headphones right?

Thanks for the input! Really appreciate it!


edit: Would the speedball upgrade lower the noise-floor? I'm confused about it since as I wrote about, some users state their Crack is dead silent, and it is stated that the speedball provides blacker background but to me it's not possible to improve dead silence?

 

[Paul Birkeland]

The Speedball upgrade might lower the noise floor, but then again it might not. 

There are some tests that we can do to eliminate potential sources of noise.  Do you happen to have a spare capacitor sitting around that's rated for at least 200V?  Maybe around 2uF-100uF?

We can use a capacitor to shunt all signal to ground, and potentially determine where the noise is coming in.

-PB

 

[Viktor]

Yepp I have such a resistor, how do I go about this shunting business? Soldering the capacitor to the part believed to be faulty thereby bypassing them?

 

[Paul Birkeland]

This is generally easier with a pair of clip leads.

Start by connecting the capacitor across the 22K resistor on terminals 1 and 2.  Listen to the amp with the cap connected here, observe changes.

Next, connect the cap to terminals 4 and 5.  Listen to the amp with the cap connected here, observe any changes.

(Power the amp off and be sure it's discharged before putting the cap in of course)

Let us know what you find.

-PB

 

[Grainger49]

If using an electrolytic capacitor be certain to observe the polarity.

 

[Viktor]

Hi!

I found two caps, one rated at 22uF @ 400V and the other one rated at 100uF at 400V. Which is best to start with or it doesn't matter?
Both are electrolytic, the negative lead is the shorter of the two legs, where should I put that one? Sorry for the n00b questions but this is new to me!

edit: What should I do if it is more silent with the capacitor present?

 

[Paul Birkeland]

The 22uF at 400V is OK.

Start by connecting the negative end to terminal 1, and the positive end to terminal 2, then run the amp. 

 

[Viktor]

I checked using the shunt capacitor. On terminal 1 and 2 I thought that some of the higher frequency "breezing" disappeared using the capacitor. There was still a very low hum which may have been a low 50hz tone. It was hard to determine.
Then I checked using the same cap at terminal 4 and 5 and the same thing was noticed but in the left channel.  What does this tell us? As I understand the capacitor removes the higher frequencies which makes sense but what new information has been learned here?

As a side note I also poked around the inside of the amp, the chop-stick test, and could not produce any noise by poking there.

 

[Paul Birkeland]

On terminal 1 and 2 I thought that some of the higher frequency "breezing" disappeared using the capacitor.

Is it gone, or still about the same?

 

[Viktor]

I would say that it almost disappeared. I talked to a friend who is experienced in DIY audio and he was surprised there weren't any capacitors placed there by default? He also suggested adding a grid-stopper before the first tube to reduce any small capacitances inside the tube.

 

[Paul Birkeland]

I talked to a friend who is experienced in DIY audio and he was surprised there weren't any capacitors placed there by default? He also suggested adding a grid-stopper before the first tube to reduce any small capacitances inside the tube.

I'd recommend listening to the amp with the capacitor installed, then giving your friend a hard time.  (There will be nearly no sound out of that channel)  Solid state equipment and some push-pull tube amps will have a very small cap to ground from the plates, but this is to prevent oscillations well above what we can hear. 

Grid stoppers don't reduce the capacitances inside the tube, but they do help with oscillations concomitant with high Gm tubes.  (The 12AU7 is low Gm)