I am in desperate need of some help. I would be very appreciative of anyone who can help me solve a problem with hum.
First, some background. I built the following headphone amp and it works perfectly, no hum.
Nobsound 6N5P+6N11 Vacuum Tube Headphone Amplifier Board DIY KIT Single end Class A Amp|Amplifier| - AliExpress
Next I got more ambitious and built the following headphone amp:
A Single-Ended OTL Amplifier for Dynamic Headphones. – HeadWize Memorial
This 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.
First, some background. I built the following headphone amp and it works perfectly, no hum.
Nobsound 6N5P+6N11 Vacuum Tube Headphone Amplifier Board DIY KIT Single end Class A Amp|Amplifier| - AliExpress
Next I got more ambitious and built the following headphone amp:
A Single-Ended OTL Amplifier for Dynamic Headphones. – HeadWize Memorial
This 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.
'The negative of both transformers is connected to circuit ground.'
Hum or Buzz? Connect the 'negative' of the transformer to the first reservoir cap negative for that power supply. This localises the rectifier return current to stop it getting into your signal ground. So the HT return should go to C7, and the heater rectifier return to C10.
Hum or Buzz? Connect the 'negative' of the transformer to the first reservoir cap negative for that power supply. This localises the rectifier return current to stop it getting into your signal ground. So the HT return should go to C7, and the heater rectifier return to C10.
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Thanks for your response, but could you add some more detail as I am not quite understanding what you are suggesting that i do. Thanks
When i said the negative of both transformers is connected to circuit ground, what i meant was that the negative of the full wave rectifier after the transformer is connected to the circuit ground
Ok rectifier currents from the transformer should be kept local they circulate mainly around the transformer, rectifier and first reservoir cap. So the return to the transformer should be connected there. Then the 0V from this cap and others can be connected to you headphone amp board.
For this example see who the transformer return is connected, straight to C1 and C2.
My wiring is consistent with your first picture. For some reason I can't open the second picture. It get's stuck "loading" and never opens
Sorry had the same problem the pictures are the same. Do you have a photo of your wiring. It could be something else. Is it hum or buzz you are getting.
What I am hearing is a very loud hum, that doesn't change as I raise or lower the volume. The hum is there even if the volume is at 0.
The schematic is in 3 parts. The amp part is as described in the Headwaze link. The 6 volt power supply is as in the aliexpress kit - i have a picture but dont know how to attach it, but it is a simple supply based on an LT1083 regulator, full wave bridge, and caps on either side of the regulator.
The schematic is in 3 parts. The amp part is as described in the Headwaze link. The 6 volt power supply is as in the aliexpress kit - i have a picture but dont know how to attach it, but it is a simple supply based on an LT1083 regulator, full wave bridge, and caps on either side of the regulator.
There's a reference on the article "The shield between the two halves of the E88CC is grounded" - which is not shown on the schematic, this would correspond to pin 9. Did you implement this?
Another test to try: since the hum is not dependent on volume, short the input and remove the input tube. If the hum remains it is being injected by the output tube.
I did ground the shield between the two halves of the E88CC, and it did not help. I will try taking out the input tube and see what happens
I think removing the input tube will take the output tube out of its operating region and result in charred cathode resistors. Do you have a photo of the layout? I thought you said LT1083 not LT317. Before things get confusing could you post exactly what you have built please.
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So for your circuit you need a lot of heater current - more than a LT317 will provide. You may also need 20000uF of capacitance. The LT1083 is what you need and 5A rectifier diodes and transformer to match.
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the LT317 was a mistake in the diagram. I did use a LT1083 with high amp diodes. I am using a 10000uf cap and tried adding a second 10000uf cap, but it did not help
Good chance that the hum is induced from the filament transformer itself ... it should be put at some distance with longer wires to verify this .
When the filament was on batteries it was not powered ?
When the filament was on batteries it was not powered ?
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I will try that. When I ran it off batteries, the filament transformer was getting ac in, but was not drawing much current because the DC out was not connected to anything.
I was gonna say you must elevate the heater wiring to 20-80V so to eliminate hum leaking into the E88CC, but then I see you regulate the heater voltage.
As our Romanian friend says I also think this is magnetic. When you have such obnoxious amounts of cap you will have vulgar amounts of hard hitting currents every 2x50Hz cycle, and this can be bad if your loop is bad and or in close vicinity.
NO need to regulate the 2.5A heaters of the 6AS7. NONE!!! I would AC heat the 6AS7, regulate only the 330mA for the E88CC, get much less nasty currents pulsing thru the circuit.
If you scope the heater voltage and see little ripple, it is magnetic, because if the voltage is low in ripple, the current ripple is huge.
Then again, it can come from the B+. 6AS7 has such a low gain that all electrodes pass signal. Another way to look at it is the low rp of the 6AS7, the ripple settles across the much higher resistor value of the cathode resistor. This can be fixed by the good old cap mulitplier, greatly reducing the B+ ripple, or with some fancy injection of the ripple so to cancel it out.
What is the idle current? Say it's 50mA per channel, 100mA total...You have ca 425uF, the ripple on B+ will be approx dV= 100mA*10ms / 425uF = 2.35V
2.35V ripple aint that bad on a 150V rail, but for a 6AS7 it might be a bit too much, and with sensitive headphones...
As our Romanian friend says I also think this is magnetic. When you have such obnoxious amounts of cap you will have vulgar amounts of hard hitting currents every 2x50Hz cycle, and this can be bad if your loop is bad and or in close vicinity.
NO need to regulate the 2.5A heaters of the 6AS7. NONE!!! I would AC heat the 6AS7, regulate only the 330mA for the E88CC, get much less nasty currents pulsing thru the circuit.
If you scope the heater voltage and see little ripple, it is magnetic, because if the voltage is low in ripple, the current ripple is huge.
Then again, it can come from the B+. 6AS7 has such a low gain that all electrodes pass signal. Another way to look at it is the low rp of the 6AS7, the ripple settles across the much higher resistor value of the cathode resistor. This can be fixed by the good old cap mulitplier, greatly reducing the B+ ripple, or with some fancy injection of the ripple so to cancel it out.
What is the idle current? Say it's 50mA per channel, 100mA total...You have ca 425uF, the ripple on B+ will be approx dV= 100mA*10ms / 425uF = 2.35V
2.35V ripple aint that bad on a 150V rail, but for a 6AS7 it might be a bit too much, and with sensitive headphones...
