Noise with Computers, USB DACs, and Powerline Ethernet Kits

[Paul Birkeland]

One thing I have noticed after doing the diode mod is that my tubes, especially the power tube, doesn't get as brightly lit as before or as compared to when I use the same power tube in another tube amp. As you can tell from my posts, I severely lack technical knowledge in this area, but is this a general side effect of this mod?

No, not at all.  That would tend to indicate a loose connection in the amp that may have been bumped when you installed the mods.  I would go back in and do a DC voltage check and start a new thread if anything is off.  The twisted pair of wires that goes to B7/B8 makes the 6080 glow, and if one of those wires is loose on either end, it will not glow nearly as brightly. 

[jivex5k]

For the purpose of this mod, you can think of a diode as erecting a barrier that is approx. 700 mV tall: @700 mV and below, no current flows, but everything above 700 mV passes over the barrier. (The "height" of the barrier, called the diode's "forward voltage" -- the minimum voltage required to turn the diode "on" and permit voltage and current to pass, varies a bit by type of diode). This applies to a "forward biased" diode - i.e., one where the anode is more positively charged than the cathode (banded end).  Because the mod uses two "reverse paralleled" diodes, there is a forward-biased diode in each direction: to earth ground and from earth ground -- so it blocks sub-700 mV currents in both directions.

In reality, the "barrier" is sloped and some small amount of current makes it through when the applied voltage is less than 700 mV.  See attached graph, and specifically the region to the right of the X,Y origin.

cheers, Derek

Oh wow, thanks! That graph helped me understand it better. This implies small voltages over ground can cause noise, I don't really understand why but I'm going to try and find out.

[Paul Birkeland]

This implies small voltages over ground can cause noise, I don't really understand why but I'm going to try and find out.

The maximum peak signal voltage you could send into a Crack with the volume control all the way up is 1.5V.  If you have 500mV peak of noise current because of a polluted ground, that's not a small voltage at all!

[jivex5k]

The maximum peak signal voltage you could send into a Crack with the volume control all the way up is 1.5V.  If you have 500mV peak of noise current because of a polluted ground, that's not a small voltage at all!

Hmm, interesting. I was thinking it had to be related to 120 AC volts somehow due to the connection to the main power input. I think I need to take a step back and figure out how the audio signal differs from the 120 AC in, and how the ground differs and can affect the audio. I believe the simple answer is the noise is coming from the audio input rather than the power input, therefore the voltage is significantly lower.

I've been doing some experimenting to try and understand the way this noise is introduced. First I've realized there are two noises:

1. Can be heard with nothing plugged in but the headphone and power, but I have to turn the volume way up. It's a constant buzz that is audible after about halfway, more present in the left side. I'm going to guess this is either a ground loop buzz, or there are some weak solder joints. I think the ground loop occurs when nearby stuff is plugged into a different outlet or something? This amp is on top of a PC and next to a monitor, which are all plugged into the same power conditioner. I don't really know enough about this to determine much about it.

2. Needs a signal into RCA.

2a. If I connect them and touch my fingers to the other side I get a buzz, I'm guessing this is a ground loop caused by my body.

2b. When plugged into my DAC output I get intermittent little buzzing, similar to the cell phone interference sound, but lacking a distinct pattern. The DAC is powered via laptop currently, which runs on battery and is charged by the same outlet, but not on a power conditioner. The sound is present regardless of the laptop being plugged into an outlet, guessing this is something related to the battery of the laptop sending something over the 3.3V USB powering the dac.

2c. When I plug the interface into my main PC USB, the noise is drastically reduced in volume. This PC is plugged into the power conditioner. I'm guessing here, it's the same 3.3V sending noise, but due to a better PSU installed in the system it's more stable.

I've been reading up on ground loop hum, and will continue to try and learn more about this. It's one thing to know how to install a fix for noise, it's another to understand why it fixes it. I'm very interested in the why of it all. Thanks for the info on this.

I just got the pair of rectifiers today for this mod, going to install them after work.

BTW: The longer I use this amp the more I am enjoying it. Sometimes the bass notes hit so nicely, I'm extremely happy with this amp and have greatly enjoyed building it myself. The active community on these forums is a very nice bonus to it all.

[Paul Birkeland]

1. Can be heard with nothing plugged in but the headphone and power, but I have to turn the volume way up. It's a constant buzz that is audible after about halfway, more present in the left side. I'm going to guess this is either a ground loop buzz, or there are some weak solder joints. I think the ground loop occurs when nearby stuff is plugged into a different outlet or something? This amp is on top of a PC and next to a monitor, which are all plugged into the same power conditioner. I don't really know enough about this to determine much about it.

This is covered in the sticky FAQ on this forum.  With open inputs and high input impedance, turning the volume control up with nothing plugged in is just amplifying ambient grunge.

I've been reading up on ground loop hum, and will continue to try and learn more about this. It's one thing to know how to install a fix for noise, it's another to understand why it fixes it. I'm very interested in the why of it all. Thanks for the info on this.

It's more about when two grounds aren't in perfect agreement with each other, and in this case when you have the ground in the Crack that is really solidly earthed and another ground via USB that has a few hundred mV of noise riding on it.  If those two meet, that few hundred mV of noise becomes signal current as it seeks earth in the Crack, or it doesn't if it can't follow that loop (with the anti-phase diodes).  When we put out our own DAC, we were very careful to only use the USB power as an indicator that something was plugged into the USB socket and nothing else!

BTW: The longer I use this amp the more I am enjoying it. Sometimes the bass notes hit so nicely, I'm extremely happy with this amp and have greatly enjoyed building it myself. The active community on these forums is a very nice bonus to it all.

We are certainly here to help.

[jivex5k]

Just installed the mod, works like a charm. Zero noise picked up from DAC, via laptop or PC connection. I even turned the knob all the way up and still nothing.
Great stuff, thanks Paul!

[Sektor7G]

I don't know what kind of voodoo magic was involved in this modification idea, but all I can say is that it works 100%. I just wish I had done it sooner.

[Deluk]

As I posted after I had added the diodes to my Crack with great success, IMO they should be provided as part of the original build. If nothing else it would cut out a lot of posts from unhappy owners. Costing just a few cents, it's a no brainer.

[Paul Joppa]

I should point out that it does not always work for every situation. The ways of hum and noise are many and varied!

But yes, it is included in the new Moreplay preamp.

[Laudanum]

RL207 or UF4007?  I have both on hand.     The RL207 isn't a fast recovery.  Does it matter?

[Paul Birkeland]

Either one is OK, the RL207 would be a worse rectifier than the UF4007, but would be slightly better in this position.

[Laudanum]

Either one is OK, the RL207 would be a worse rectifier than the UF4007, but would be slightly better in this position.

What characteristic makes the RL207 "better" than the UF4007 for this duty?   The higher current and peak current ratings? 

[Paul Birkeland]

You can use either, the RL207 has a very high peak current rating, and that's not necessarily a bad thing.

[Laudanum]

Thanks Paul.  I have a surprisingly very low amount of "hash" present in both the Crack and a Hybrid tube headphone amp when hooked up to my DAC.   It's only present in the Hybrid when the DAC is run through the Foreplay before the Hybrid.  DAC direct to hybrid and the hybrid is silent.   It's present in the Crack either direct from the DAC or through the Foreplay first.  Only slightly worse when the Foreplay is in the mix.  But really,  it's low level either way.   Plan on installing the diodes in both the Foreplay and Crack to see if I can get rid of it.   But I'm considering myself lucky because it's really low level as it is. 

[pofofo]

Ive got some DAC noise on my SEX with some of my headphones. And I was wondering if I need to include both ground leads connected to the ground of the inlet plug for it to have the desired effect?

NVM, theres only one going to the inlet ground..