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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
8 responses Add your response
gs5556
1,209 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
11,574 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.
atmasphere's avatar
atmasphere
11,574 posts
 
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?
Its not the best... Read what I wrote earlier about this. That bit about grounding the grid and cathode resistors at the same point is a big deal.


If using batteries sorts it that's a clue. Is your filament supply regulated?
steventrus OP
3 posts
 
I will check on your recommendation about the grid and cathode resistors and let you know.  Yes the filament supply is regulated with a LT1083.  It is based on the exact design of the similar headphone amp kit that works without any hum.
atmasphere's avatar
atmasphere
11,574 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
11,574 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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