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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
gs5556
1,234 posts
 
Measure the voltage from circuit ground to earth. If it’s only a few tens of millivolts put a 10 ohm resistor in parallel with a 0.63 uF film cap between circuit ground and earth to lift ground a few volts. That may help.
atmasphere's avatar
atmasphere
11,807 posts
 
If you are using an AC filament supply, you might want to bias it at about 60-70 volts above ground so it can't cause secondary emission. If the filament supply is grounded and AC, hum seems pretty likely.

You could also be dealing with a ground loop. First, make sure that the grid and cathode resistors of the first stage tie together at the same point- and then ground that point to the power supply. Do the same for the power tube.


It would be a good idea to check your High Voltage supply to make sure its actually doing what its supposed to. Put an oscilloscope on the B+ line; it really should look like a straight line even if the scope is at high gain. 
steventrus OP
3 posts
 
Thanks for your feedback

The filament supply is DC for both tubes.  I use separate transformers and power supplies because the filaments use a combined 3A and my high voltage transformer only has 1a for filament.  

I have read about ground loops, but not exactly sure what that means.  The way i implemented this circuit is that i have a long wire that goes from part a to part b to part c to part d .... to part z and back to part a (in a circle) is that ok?

I am afraid that i dont have an ocilloscope but am pretty sure the high voltage supply is ok, for several reason.  First I used this same design on several other headphone amps I built and they don't have any hum (they are a slightly different design - white cathode follower with two 6922 tubes on the output stage instead of one 6AS7).  Also if i replace the heater supply with batteries the hum disappears.
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