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Need help with a 6AS7 Headphone Amp

I am in desperate need of some help. I would be very appreciative of anyone who can help me solve a problem with vacuum tube headphone amp hum.

First, some background. I built the following headphone amp and it works perfectly, no hum.

https://www.aliexpress.com/item/32898304278.html?spm=a2g0s.9042311.0.0.4c484c4dfhenE9

Next I got more ambitious and built the following headphone amp:

https://headwizememorial.wordpress.com/2018/03/20/a-single-ended-otl-amplifier-for-dynamic-headphone...

This second headphone is almost identical to the first. The amp part I built is exactly as in the Headwize link. The power supply for the filament is identical to the one from aliexpress. The high voltage power supply is a custom based on a bridge rectifier, followed by 47uf cap, followed by 50 ohm voltage dropping resistor, followed by 330uf cap, followed by 50 ohm voltage dropping resistor followed by 47uf cap. I have separate transformers for both filament and high voltage. The negative of both transformers is connected to circuit ground. The high voltage supply is on a separate board. The filament supply is on the same board, but on the other side of the board. the filament transformer is about 2 inches from the 6AS7.

The problem is that the Headwize amp has a severe hum. The hum is the same no matter what the volume is set to. The hum starts about 30 seconds after power on. If i turn the power off, the music continues to play for a few seconds without any hum. Also, I tried using 4 D batteries for the filament supply and the hum went away.

I have tried everything I can think of to solve this problem with no success, so as I said in the beginning I would greatly appreciate any help I can get. I used high end parts in the Headwize amp, hoping to have a better sound than the aliexpress one.
steventrus
atmasphere's avatar
atmasphere
11,760 posts
 
@steventrusDoing this sort of thing without an oscilloscope is really difficult!

But you can use a DVM to help you. Put it on the AC scale (not DC) and see what you measure when you connect its probes to the plus and minus outputs of the filament supply. You should see very nearly 0 volts. If its anything significant (more than 0.1V) than your regulator might not be working right.
steventrus OP
3 posts
 
PROBLEM SOLVED !!!!!!!!!!!!
Thanks for all the suggestions.  Based on your input this is what I did.  I took the complete filament circuit and separated it from the amplifier circuit so that it is only connected to the amplifier ground in one place.  Before I had the filament circuit connected to the amplifier ground at several places.  For some reason the hum completely disappeared.  This is beyond my knowledge of electronics to understand why that fixed the problem, but I am happy that it is fixed and I have learned an important lesson.
atmasphere's avatar
atmasphere
11,760 posts
 
For some reason the hum completely disappeared.
You had a ground loop. They can seem quite tricky and mysterious. Probably what was happening is the way it was grounded had something to do with the gain stages in the preamp- that they were obtaining their grounds the same way the filament supply was, or that they both get their grounds through the same circuit. That way the current of one circuit becomes conflated with the current(s) of another- allowing for that current to become a noise vector somewhere in the circuit.


At any rate I'm glad you got it sorted!
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