Hum problem

user68 · 21st July 2010, 01:57 PM

I'm trying to fix hum coming from both channels of my diyhifisupply 300B Joplin. The multimeter gives readings of 2.3mV each side and hum is audible through the speakers at 14 feet. Hum should be around 0.9mV

The hum level doesn't change if I turn the volume control nor does it stop if I remove the 6072 and 5687 pre-amp signal tubes. The hum is present with any one of the 300Bs inserted.

I'm using short 25g pure silver teflon coated hook up wire for the audio grounds but not twisted or shielded. Have checked all connections and they're good. The ground from the volume control runs parallel with the HT PCB but about 2 inches clear. Rerouting it just makes the hum worse.

Can anyone please suggest what I could do to reduce the hum? Would twisting or covering the ground wires with tubular braiding be a good idea?

mario13 · 21st July 2010, 03:25 PM

Hello,
The heater supply can cause hum. How is your wiring? Does the PT give you a CT for the heaters? This ct should be grounded or a vertual ct can be made with two 100 ohm resistors to earth from the heater supply. This is if the heater supply is at fault.... anyway its something to check out. Other than that check your earth connections.
Best of luck
Mario.

cbj591 · 22nd July 2010, 03:11 AM

Good evening,

I found in my amp, the power supply was pretty dirty causing a large ripple in my B+ going to the output tranny. There was a consistant level hum until I cleaned it up with a choke and additional filter stage.

Good Luck
Brian

jazzy939 · 22nd July 2010, 05:02 AM

I was also having humming issues with a 5687 based tube preamp. I managed to minimised the hum considerably by using DC heater voltage. There is however some humming left (not affected by volume control) which I suspect, is caused by B+ ripple.
Will add another stage of choke and/filter stage to see if it helps.

levelfive · 22nd July 2010, 10:03 AM

Quote:

Originally Posted by user68

Can anyone please suggest what I could do to reduce the hum? Would twisting or covering the ground wires with tubular braiding be a good idea?

Hi, User68

What frequency is your hum: 50Hz(60Hz) or 100Hz(120Hz)?
Can you show schematic of output stage (better full amp schematic)?

Do you use linear pots for filament hum level eliminating?

Give us some more information and we will certainly find a solution!
Kind Regards, Konstantin.

Arnulf · 22nd July 2010, 05:05 PM

Short the 300B grid to ground. If hum presists, post the schematic that shows your filament supply arrangement for the output tubes.

kmtang · 22nd July 2010, 05:10 PM

2.3mV of hum at the speaker from 300B is very normal. Is the 300B with DC filament supply? If it is with DC supply, the hum probably come from the B+ ripple. You can insert an RC B+ filter before the CLC filter to clean up the ripple at the B+.

Johnny

user68 · 22nd July 2010, 06:00 PM

Thanks for all your suggestions. I have attached schematics.

@ levelfive: it sounds more like a constant 100hz hum with an intermittent higher pitch crackling buzz. Have a 47R hum balance pot for each power tube and 22K signal balance pot between each pair.

triode_al · 22nd July 2010, 06:52 PM

Hi User,
Menno van der Veen describes a procedure on his site to adjust an output stage. see: Menno van der Veen, audio electronic research & consultancy, section 'Modify the UL40-S2 into a Super-Triode amplifier.'

he describes an interesting find:

Quote:

" The supply voltage Vo always has a ripple voltage on top of the DC voltage Vo. This ripple voltage only balances to zero in the balanced output transformer when both EL34 tubes have exactly the same effective plate resistance. "

Thus from that we could see that if both side of your amplifier do not have equal plate resistance (a slight imbalance between the tubes) then this might lead to some hum.!

Well just a thought.

In my own 300BC project I am working on, I get a hum from the rectifier block - while when I use a tube rectifier (5R4), there is no hum. Annoying. I can trace the hum all the way on the gound plane ( a solid wire) from the first capacitor till the point where it is grounded on the chassis (at the entry point) and there the hum is lowest, or absent. I can hear it on the speaker (very faint) so it is no artifact from my equipment ground wires. It's the commutation spike.

albert

user68 · 22nd July 2010, 06:56 PM

Thank Triode_Al. I'll read up on this.

21st July 2010, 01:57 PM	#1
user68 diyAudio Member Join Date: Jul 2010	Hum problem I'm trying to fix hum coming from both channels of my diyhifisupply 300B Joplin. The multimeter gives readings of 2.3mV each side and hum is audible through the speakers at 14 feet. Hum should be around 0.9mV The hum level doesn't change if I turn the volume control nor does it stop if I remove the 6072 and 5687 pre-amp signal tubes. The hum is present with any one of the 300Bs inserted. I'm using short 25g pure silver teflon coated hook up wire for the audio grounds but not twisted or shielded. Have checked all connections and they're good. The ground from the volume control runs parallel with the HT PCB but about 2 inches clear. Rerouting it just makes the hum worse. Can anyone please suggest what I could do to reduce the hum? Would twisting or covering the ground wires with tubular braiding be a good idea?

22nd July 2010, 05:05 PM	#6
Arnulf diyAudio Member Join Date: Feb 2009	Short the 300B grid to ground. If hum presists, post the schematic that shows your filament supply arrangement for the output tubes. __________________ mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."

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21st July 2010, 03:25 PM	#2
mario13 diyAudio Member Join Date: Jul 2010	Hello, The heater supply can cause hum. How is your wiring? Does the PT give you a CT for the heaters? This ct should be grounded or a vertual ct can be made with two 100 ohm resistors to earth from the heater supply. This is if the heater supply is at fault.... anyway its something to check out. Other than that check your earth connections. Best of luck Mario.

22nd July 2010, 03:11 AM	#3
cbj591 diyAudio Member Join Date: May 2010	Good evening, I found in my amp, the power supply was pretty dirty causing a large ripple in my B+ going to the output tranny. There was a consistant level hum until I cleaned it up with a choke and additional filter stage. Good Luck Brian

22nd July 2010, 05:02 AM	#4
jazzy939 diyAudio Member Join Date: Nov 2007	I was also having humming issues with a 5687 based tube preamp. I managed to minimised the hum considerably by using DC heater voltage. There is however some humming left (not affected by volume control) which I suspect, is caused by B+ ripple. Will add another stage of choke and/filter stage to see if it helps.

22nd July 2010, 05:10 PM	#7
kmtang diyAudio Member Join Date: Nov 2004 Location: Vancouver	2.3mV of hum at the speaker from 300B is very normal. Is the 300B with DC filament supply? If it is with DC supply, the hum probably come from the B+ ripple. You can insert an RC B+ filter before the CLC filter to clean up the ripple at the B+. Johnny

22nd July 2010, 06:56 PM	#10
user68 diyAudio Member Join Date: Jul 2010	Thank Triode_Al. I'll read up on this.

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