Try this. Change the AC filament voltage going to the 12AX7 to filtered DC. Use like 2200 uF or more for the new design.
The 6L6 filament voltage potential should be elevated above the voltage of the cathode for minimum hum. 30V or more.
You may need to change that to DC as well. I know it's a real pain but sometimes hum can be a bitch!!! Especially with SE designs.
The 6L6 filament voltage potential should be elevated above the voltage of the cathode for minimum hum. 30V or more.
You may need to change that to DC as well. I know it's a real pain but sometimes hum can be a bitch!!! Especially with SE designs.
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Actually if I open the chassis as shown on the photos the hum is still the same - so I guess it is no coupling between output and power transformers.
I have checked the circuit ones again and it actually has CRC (CLC) filtering for output tube but CRCRC (CLCRC) for input tube. Do you think it is still not sufficient that it can produce audible 100 Hz hum?
I have checked the circuit ones again and it actually has CRC (CLC) filtering for output tube but CRCRC (CLCRC) for input tube. Do you think it is still not sufficient that it can produce audible 100 Hz hum?
Attachments
Yes, I have tried this. I have connected input to GND (inside the input CINCH connector and there was no difference.
One more thing I did - I disconneced AC heating voltage from one channel while having the AC heating of the second channel connected. The hum was only in the channel with AC heating connected. Of course the sound was not there in the channel without heating. Does this help in identifying the source of hum?
One more thing I did - I disconneced AC heating voltage from one channel while having the AC heating of the second channel connected. The hum was only in the channel with AC heating connected. Of course the sound was not there in the channel without heating. Does this help in identifying the source of hum?
I believe the GND is ok - please see attached picture. There is one single star GND point at power supply capacitors. Input GND gos here as well and input jacks are isolated from chassis. Earth (chassis) is near the power transformer and single wire from start GND goes here + power transformer shield + PE mains goes here as well.
I have tried different connection / disconnection of Earth GND, output transformer GND wires and input GN point but basically there was no improvement. Sometimes it was a bit worse but never beter. Therefore I thought it it not the ground problem. But I'm not sure. Maybe I'm doing something wrong.
I have tried different connection / disconnection of Earth GND, output transformer GND wires and input GN point but basically there was no improvement. Sometimes it was a bit worse but never beter. Therefore I thought it it not the ground problem. But I'm not sure. Maybe I'm doing something wrong.
Attachments
Well, the amp is done on PCB that I can't change and the supplier recommends to connect groundd to power supply capacitors (screw terminal close to theem on PCB).
What I need to connect to PCB are these grounds:
- input cable / connector GND - not connected to start GND now
- two wires of output transformers' GNDs - connected to star GND point now
- two GND wires from output terminal - connected to star GND point now
- Earth + shield of power transformer - connected to star GND point now
So where should I conect these GND wires if not to the star ground on power supply capacitors?
What I need to connect to PCB are these grounds:
- input cable / connector GND - not connected to start GND now
- two wires of output transformers' GNDs - connected to star GND point now
- two GND wires from output terminal - connected to star GND point now
- Earth + shield of power transformer - connected to star GND point now
So where should I conect these GND wires if not to the star ground on power supply capacitors?
Earth and shield of power transformer should connect to the chassis/case and incoming mains earth (if present).
The others can go to a star ground. The output ground from the PSU can also go to the star ground. My point is that the PSU caps should never be the star ground, as nine times out of ten the star ends up being at the beginning or in the middle of the PSU instead of the output.
The others can go to a star ground. The output ground from the PSU can also go to the star ground. My point is that the PSU caps should never be the star ground, as nine times out of ten the star ends up being at the beginning or in the middle of the PSU instead of the output.
Hi!
I wrote a small compendium about troubleshooting hum on my blog:
VinylSavor: Trouble Shooting Hum
Some of the things I suggest there you seem to already have tried, if not, try them first.
Then find out which stage causes the hum as described. What happens if you remove the driver tube? Still hum? Then it's caused in the output stage.
Have you contacted the supplier of the kit? He should have the most experience to fix hum in his design.
Best regards
Thomas
I wrote a small compendium about troubleshooting hum on my blog:
VinylSavor: Trouble Shooting Hum
Some of the things I suggest there you seem to already have tried, if not, try them first.
Then find out which stage causes the hum as described. What happens if you remove the driver tube? Still hum? Then it's caused in the output stage.
Have you contacted the supplier of the kit? He should have the most experience to fix hum in his design.
Best regards
Thomas
vubec ne
had this supply for headamp without 36w choke, it was bad (resistor instead)
check for floating filament windings, can also introduce hum
An externally hosted image should be here but it was not working when we last tested it.
i hope it had airgap
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Thanks, Vinylsavor for your excellent idea. I have removed input tube in one channel and there was no hum. So the conclusion is it is not the output tube part but somewhere between input jack and output tube.
What should I try now? Better filtering for input stage?
There is common heating AC voltage for each channel. If the output tube only doesn't hum does it mean that the heating voltage for input voltage is ok as well?
If the output tube only doesn't hum does it mean that there is no problem with grounding? Input tube is of course much more sensitive.
I have checked with manufacturer of this kit and he responded that they didn't hear about similar issue from their customers.
What should I try now? Better filtering for input stage?
There is common heating AC voltage for each channel. If the output tube only doesn't hum does it mean that the heating voltage for input voltage is ok as well?
If the output tube only doesn't hum does it mean that there is no problem with grounding? Input tube is of course much more sensitive.
I have checked with manufacturer of this kit and he responded that they didn't hear about similar issue from their customers.
Hi!
This narrows down the area of the problem at least!
next steps:
Try grounding the grid of the lower triode of the SRPP stack directly at the tube socket. If the hum disappears the problem is between input tube and RCA input. If not, it can be either of these:
inadequate filtering of driver stage: Try doubling the decoupling cap of the driver. Just clip lead in another cap. If the hum reduces, you need to improve filtering there. But since this seems to be a proven kit, this is unlikely.
Check the ground reference of the heater this is also a common cause of hum (has already been suggested).
A ground loop can still be the cause. Did you check everything related to that? When you did the test with the shorted inout, was the amp completely disconnected from the source?
That response from the manufacturer is very lame. They make profit with selling the kit, they should give you any help needed to get this amp hum free.
Best regards
Thomas
This narrows down the area of the problem at least!
next steps:
Try grounding the grid of the lower triode of the SRPP stack directly at the tube socket. If the hum disappears the problem is between input tube and RCA input. If not, it can be either of these:
inadequate filtering of driver stage: Try doubling the decoupling cap of the driver. Just clip lead in another cap. If the hum reduces, you need to improve filtering there. But since this seems to be a proven kit, this is unlikely.
Check the ground reference of the heater this is also a common cause of hum (has already been suggested).
A ground loop can still be the cause. Did you check everything related to that? When you did the test with the shorted inout, was the amp completely disconnected from the source?
That response from the manufacturer is very lame. They make profit with selling the kit, they should give you any help needed to get this amp hum free.
Best regards
Thomas
Thanks, Vinylsavor! I tried both:
- disconnect both inputs (L + R) and shorten them - no change. Actually I have also tried to shorten one input directly on the PCB but still the same. And the amp was completely disconnected from the source.
- shorten grid of lower tube to GND directly on the socket - no change. There still the same hum.
With this I think there is no issue between source and grid of input tube as well se there is no issue with output tube, transformer coupling etc. - silent when input tube unplugged
So what can be wrong with just the input tube?
I will try to increase filtering for input tube as you recommended.
Anything else to try?
- disconnect both inputs (L + R) and shorten them - no change. Actually I have also tried to shorten one input directly on the PCB but still the same. And the amp was completely disconnected from the source.
- shorten grid of lower tube to GND directly on the socket - no change. There still the same hum.
With this I think there is no issue between source and grid of input tube as well se there is no issue with output tube, transformer coupling etc. - silent when input tube unplugged
So what can be wrong with just the input tube?
I will try to increase filtering for input tube as you recommended.
Anything else to try?
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