Advice for Headphone Amp Ground Loop Hum

[thekrynn]

Hello all,

New user here coming from a youtube review video. Was looking for the best place to post a question about a ongoing nasty ground loop hum issue I'm having with a relatively new tube headphone amp. Without going into very long winded detail, the short version:

I have a headphone tube amp that is experiencing 60Hz and more so 120Hz hum which differs in volume and annoyance from day to day. I was able to talk to the owner of the amp company to troubleshoot through all the possible causes and narrowed it down to power. I've thrown a Furman SMP conditioner, a Furman UPS and several other devices at this thing. On recommendation of the amp maker, I added an EBTech humX which significantly helped but has struggled to keep the hum at bay during the winter months. The hum is louder when an electric heater is on in a room across the house. Changing outlets or rooms for the amp helps, but marginally. On some days, the hum is considerably better for no particular reason. The hum persists with no audio wires connected and regardless of amp volume. I used a ground tester to see if the house has a grounding problem, but the tester comes back fine. For reference, the house was built in the 1930s, and the neighborhood is known for having garbage power. I have been running Furmans and UPS' for decades, but have never had ground hum in any of my audio equipment until now, although this is my first piece of audio gear with a tube.

I was close to purchasing an isolation transformer, but the reviewer in the video actually mentioned that using one for this exact problem may not be the way to go and to first check the forums here. Any help would be greatly appreciated!

 

[johny_2000]

The hum persists with no audio wires connected and regardless of amp volume.

This is not ground loop noise. This is power supply noise that is poorly filtered in the device itself. All your external devices have little effect on the final result. I recommend getting a power supply schematic from the manufacturer and posting it here for analysis.

Read this article about Hum in Valve Heater Supplies

The Valve Wizard

 

[thekrynn]

This is not ground loop noise. This is power supply noise that is poorly filtered in the device itself. All your external devices have little effect on the final result. I recommend getting a power supply schematic from the manufacturer and posting it here for analysis.

Thanks for the reply, I can ask tomorrow. For reference, it's the Woo Audio WA7 Fireflies 3rd gen with the internal power supply.

 

[johny_2000]

For reference, it's the Woo Audio WA7 Fireflies 3rd gen with the internal power supply.

This device was reviewed here. Maybe a different generation, but similar.

Woo Audio WA7 & WA7tp DAC and Headphone Amp Review

This is a review and detailed measurements of the Woo Audio tube WA7 DAC and headphone amplifier together with its companion WA7tp tube power supply. It is on kind loan from a member. The cost of the pair is USD $1,599 so not cheap. As of this writing, shipping is free but I assume you would...

www.audiosciencereview.com

The FFT graph shows that the power supply noise (60Hz, 120Hz, 180Hz, 240Hz, 300Hz) is relatively high.

 

[Leeken]

I had a problem with the Wi-Fi interfering with my valve headphone amp,moving it further away (50cm) cures it,putting it back close brings it back straight away,maybe could be this?

 

[thekrynn]

including 3 FFTs of what I'm hearing tonight (feeding the headphone 1/4 out into our audio interface).

#1 (off_furman): the amp is off, no power, plugged into a furman 215A
#2 (off_furman_humX): the amp is off, no power, plugged into a HumX lifter which is plugged into the furman 215A
#3 (on_furman_humX): the amp is turned on and warms up for about 30s

No inputs connected, only power cable.

 

[thekrynn]

had a problem with the Wi-Fi interfering with my valve headphone amp,moving it further away (50cm) cures it,putting it back close brings it back straight away,maybe could be this?

Tried it in my bedroom with awful wifi coverage, no electronics except LED lightbulbs on the nightstand, still hums (does it in nearly every room of the house, putting it right next to electronic and wifi/bluetooth devices doesn't change it at all)

 

[johny_2000]

the amp is turned on and warms up for about 30s

Yes, it looks like noise from the built-in power supply with harmonics #2, #3... #6. Their amplitude seems to be small, but the relative scale of the amplitude does not allow me to estimate it.

The last thing that can happen is defective vacuum tubes. If there is a large leak between the heater filament and the screen/cathode, this will transmit power line noise to the amplification circuits.

 

[AnalogSteph]

Is the amplifier in question an IEC Class I device (a 3-prong power connector is a good indication) without input transformers, and the signal source not floating? Then that's an instant ground loop right there.

There are several factors why tube amps are commonly built like that:
1. Their designers tend to be more enthusiastic then great engineers, and often lack understanding of systemic issues like ground loops. Also, since nobody in the customer base really cares about their objective performance much (odds are you wouldn't bother with vacuum bulbs if you did), even basic construction flaws like bad grounding practices can go unfixed for decades in commercial products. The market is pretty much a fashion-driven wild west. Good tube designers exist but are about as common as hen's teeth, and I bet their demographic heavily skews towards retirement age.
2. Decent input transformers are Not Cheap™, much like any other signal transformers (they are a major contributor to tube gear traditionally being an expensive and hefty proposition, next to the shielding that the not-infrequent high-impedance nodes in the circuitry should ideally have). You can easily spend $30+ per channel. Also, you can't hope for much more than 10 kOhms of input impedance when using one. If you were aiming for 100k or even more, tough luck. The answer would be a transformer on the corresponding signal source output (as you traditionally see with coax S/P-DIF), but who does that. Adding an isolation transformer externally probably is as close as you're going to get.

In this case it appears that while there is a ground loop going, the majority of the hum really does seem to be internal power supply noise (and seemingly much worse than the device Amir reviewed). With it being variable, I may be tempted to hunt for some bad solder joints in the area (it might also be a filter cap that's faulty from manufacture, rare but it happens). Though honestly, given that the thing costs $1399, you shouldn't have to. It is an absurd amount of money to spend on a device that can't even get the basics right when perfectly fine DAC-amps are available for $1000 less.