"Millett Hybrid" redux: NuHybrid headphone amp, using the Korg Nutube

[silverfishla]

So if I connect a closed switched jack to the headphone output to make my Line Out work, do I have to wire all six connectors to the board? I only ask because I have the headphone jack floating above and off the board. And just to make sure that I have to replace the one that's already in there.

 

[pmillett]

So if I connect a closed switched jack to the headphone output to make my Line Out work, do I have to wire all six connectors to the board? I only ask because I have the headphone jack floating above and off the board. And just to make sure that I have to replace the one that's already in there.

You would need to get a jack that has switch contacts on tip and ring, and add two more wires to connect those contacts back down to the board.

If you don't care if the line out is active all the time (with or without headphones plugged in), you can also just connect jumper wires across the jack pins on the PCB.

 

[silverfishla]

You would need to get a jack that has switch contacts on tip and ring, and add two more wires to connect those contacts back down to the board.

If you don't care if the line out is active all the time (with or without headphones plugged in), you can also just connect jumper wires across the jack pins on the PCB.

Thanks Pete. Is there any downside to soldering in the jumpers and having the Line Out active all the time? Will it (or might it) create any unwanted side effects (like noise or something)?

 

[pmillett]

Thanks Pete. Is there any downside to soldering in the jumpers and having the Line Out active all the time? Will it (or might it) create any unwanted side effects (like noise or something)?

No, no side effects. Mostly I did this so that your speakers would mute when you plug in the headphones...

Pete

 

[HiGHFLYiN9]

Rocking a Switchcraft open frame TRS jack I see, good stuff Those are my favorite for guitar gear. They make switching mono jacks, but I'm not sure about stereo...

 

[silverfishla]

No, no side effects. Mostly I did this so that your speakers would mute when you plug in the headphones...

Pete

Thanks Pete, I just soldered the jumpers in about a half hour ago. Like you said, no noticeable side effects at all. Sounds great!

Rocking a Switchcraft open frame TRS jack I see, good stuff Those are my favorite for guitar gear. They make switching mono jacks, but I'm not sure about stereo...

My favorite jacks for guitar stuff too! That's why I went with it, tried and true.

 

[jacc1234]

I just built this amp and have a few questions. Overall it came together easily, I was able to set the output bias to 11v for both L and R and I does produce sound.

My main problem is a constant interference in the background with or without a source plugged in. I would say its on the mid range and the character of the noise changes if I mess with the volume but its always present.

Also, when I power the amp on there is a really high pitch harmonic sound which I believe is from the tube. If I don't disturb it, this seems to go away. I have seen a few other posts reporting this.

I stuck to the mouser project link but this ESD suppressor was backorderd:
https://www.mouser.com/ProductDetail/511-P6KE33A

So I subbed this "alternative packaging" version:
https://www.mouser.com/ProductDetai...P6KE33ARL?qs=sGAEpiMZZMv1cg40gLeF3moQwjhEPwXa

They seemed identical but I am very much an amateur at this so any feedback would be appreciated.

I am looking forward to posting a full build log once I hopefully get this all sorted out.

Thanks for putting this great project together!

 

[pmillett]

I just built this amp and have a few questions. Overall it came together easily, I was able to set the output bias to 11v for both L and R and I does produce sound.

My main problem is a constant interference in the background with or without a source plugged in. I would say its on the mid range and the character of the noise changes if I mess with the volume but its always present.

Also, when I power the amp on there is a really high pitch harmonic sound which I believe is from the tube. If I don't disturb it, this seems to go away. I have seen a few other posts reporting this.

I stuck to the mouser project link but this ESD suppressor was backorderd:
https://www.mouser.com/ProductDetail/511-P6KE33A

So I subbed this "alternative packaging" version:
https://www.mouser.com/ProductDetai...P6KE33ARL?qs=sGAEpiMZZMv1cg40gLeF3moQwjhEPwXa

They seemed identical but I am very much an amateur at this so any feedback would be appreciated.

I am looking forward to posting a full build log once I hopefully get this all sorted out.

Thanks for putting this great project together!

No issues with the TVS substitution, it is basically the same part.

The ringing you hear when you turn the amp on is indeed the tube, the filament singing like a tuning fork when excited.

Not sure about the noise, though. It could be some EMI coupling in... Have you tried moving the amp to a different location?

What kind of headphones are you using?

If they are very efficient (IEMs usually) you may hear the noise floor of the tubes. That is pretty mush a hissing or rushing sound...

Pete

 

[jacc1234]

No issues with the TVS substitution, it is basically the same part.

The ringing you hear when you turn the amp on is indeed the tube, the filament singing like a tuning fork when excited.

Not sure about the noise, though. It could be some EMI coupling in... Have you tried moving the amp to a different location?

What kind of headphones are you using?

If they are very efficient (IEMs usually) you may hear the noise floor of the tubes. That is pretty mush a hissing or rushing sound...

Pete

Thanks for your reply Pete! I really appreciate all the work you put into providing this DIY project to the community.

I did try moving the amp to a different location and was listening with my Grado SR225. I did not try any other headphones as a test but I will give that a try tonight and report back!

 

[jacc1234]

Ok I have run a few more tests and I am fairly sure the noise is EM interference. If I touch the bare metal on the pot I can faintly hear the radio. I have tested this with two pairs of headphones.

I have had EM issues with other equipment like my guitar so I am going to take the amp to work tomorrow and see if it behaves any better.

Below an image of my soldering job on the pot. Some of the rear pins are bridged with the empty holes that I think are used if you install the upgraded POT. Is that an issue?

I am going to run through each component and ensure every solder joint looks good and that it is in the right place. I took my time during the build so I am fairly confident that the right components are in the right locations but its possible I made a mistake somewhere.

 

[jacc1234]

Ok another update. I took the amp to my office to try and eliminate any possible environmental EMI and the issue is still present. If I touch the bare metal of the pot it gets worse. I am not sure if the following is helpful but, if I touch the tops of C13 and C14 the noise gets significantly louder but if I touch any of the tops of C3, C5 or C6 the noise is mostly gone.

 

[pmillett]

Ok another update. I took the amp to my office to try and eliminate any possible environmental EMI and the issue is still present. If I touch the bare metal of the pot it gets worse. I am not sure if the following is helpful but, if I touch the tops of C13 and C14 the noise gets significantly louder but if I touch any of the tops of C3, C5 or C6 the noise is mostly gone.

Yes, sounds like "normal" noise from the environment. Touching C13 or C14 basically connects a giant antenna - you - to the grid of the tube, and the noise you pick up gets sent into the amplifier input.

Likely the biggest noise source is coming from the power adapter. The power adapter is ungrounded, meaning it has no reference to a "real" ground. To make matters worse, they typically have an EMI filter that has "Y" capacitors from each side of the AC line to the negative output of the adapter. So if all is floating, the filter actually injects 60Hz onto the negative side of the output, which is ground in the amp.

When the amp is connected to a source that has a "real" ground (like a CD player or DAC that has a three-wire AC cord), often this noise goes away. In some cases, it can get worse, though, if there is excessive noise current flowing back through the ground. A computer source (soundcard or USB DAC) can be good, or bad. Unfortunately it is unpredictable.

If you can find a real ground, connecting a wire from there to the ground of the amp may help. It is possible to pick this up from the ground pin of an AC outlet or even the screw that holds the cover plate on. But you need to be very careful not to accidentally contact the line voltage!

Pete

 

[jacc1234]

Yes, sounds like "normal" noise from the environment. Touching C13 or C14 basically connects a giant antenna - you - to the grid of the tube, and the noise you pick up gets sent into the amplifier input.

Likely the biggest noise source is coming from the power adapter. The power adapter is ungrounded, meaning it has no reference to a "real" ground. To make matters worse, they typically have an EMI filter that has "Y" capacitors from each side of the AC line to the negative output of the adapter. So if all is floating, the filter actually injects 60Hz onto the negative side of the output, which is ground in the amp.

When the amp is connected to a source that has a "real" ground (like a CD player or DAC that has a three-wire AC cord), often this noise goes away. In some cases, it can get worse, though, if there is excessive noise current flowing back through the ground. A computer source (soundcard or USB DAC) can be good, or bad. Unfortunately it is unpredictable.

If you can find a real ground, connecting a wire from there to the ground of the amp may help. It is possible to pick this up from the ground pin of an AC outlet or even the screw that holds the cover plate on. But you need to be very careful not to accidentally contact the line voltage!

Pete

Thanks for all the details. I will say the the interference I am hearing is much louder than I have experienced with any other amps or dacs. I will see if I can connect it to a real ground and post results.

I was hoping to use this connected to a usb dac or phone so would a grounded power supply make a difference?

 

[hav2uo4]

I just built the headphone amp. Surprising headroom and power. My favorite build is still the Jonokuchi, but I digress. The nutube is definitely microphonic with a high frequency tone for the first few second after startup or if you touch the tube. Has anyone worked out a mod to subdue the microphonics?

I came up with an air-isolated shock-mount using an 18650 battery case. There are a couple of young kids above my place and when they stomp overhead it's enough get the nutube resonating. The only time I notice any ringing now is when I change the stepped attenuator setting with my most sensitive headphones.

 

[hav2uo4]

Thanks for all the details. I will say the the interference I am hearing is much louder than I have experienced with any other amps or dacs. I will see if I can connect it to a real ground and post results.

I was hoping to use this connected to a usb dac or phone so would a grounded power supply make a difference?

I've had luck attaching the circuit ground to my outlet's ground (don't try this unless you're confident you know what you're doing). Also changing the physical placement of the amp can make a big difference. An old girlfriend left one of those yoga foam block things at my place and it's useful for propping the amp to a sweet spot where there's very little noise. It looks like crap...but closing your eyes fixes that eyesore...and it makes the music sound better...haha.