I've spent a bit more time on the linestage. Updated the power supply with 2 chokes and 1 resistor in a CLCLCRC. I've installed it in a case and am very happy with it so far. There is a little hum still after the unit is warmed up. Will DC heaters help this? Also, how will I use DC heaters with this design?
I made a similar amp recently, also had hum. Tried just about everything, could not get rid of it all.
Now waiting for a different transformer which I ordered, maybe that will silent it.
DC heaters made a difference for me, as well as the "groundloop breaker" scheme.
I used an old 12v switch mode powersupply for the heaters, grounded the negative (0v) to the star ground.
My version
And more here: Photos
Now waiting for a different transformer which I ordered, maybe that will silent it.
DC heaters made a difference for me, as well as the "groundloop breaker" scheme.
I used an old 12v switch mode powersupply for the heaters, grounded the negative (0v) to the star ground.
My version
And more here: Photos
Last edited:
For 12v you only use pins 4 and 5. No need for that cap on pin3 with the DC supply. Use it after the 12V reg. You only need a 20v cap, but higher won't matter.
I actually did not realise that the cap was on pin3, sometimes you have a cap and resistor in parallel on pin 1, which increases the gain (and any distortion.) Assumed it was the case here.
I decided to change a few things on the line stage last night. I tightened my ac heater wires. That made a little difference. I then rewired the signal wires with shielded. That made all the difference in the world. It is dead quiet now. I also threw in some different output caps. Just waiting for the system to settle back in, but as far as hum, it is completely gone. Even at twice as high a volume as I would ever listen to, it is quiet.
Thank you all for the help and direction.
Thank you all for the help and direction.
Here are a few pictures of line stage. I'm still trying to get a better way to label my sources on the face. I'm also not 100% settled on input and output caps. Those are Russian k75-10 3.3uf output and 1uf input. I previously had Mundorf silver oil caps of the same values in place.
Attachments
Good stuff 
What is the Denon, CD player?
I recently got a Denon DVD-2930 multiplayer, very nice unit.
My 12B4 still have a faint hum, and some hiss. I did get a better transformer which I still need to fit, unfortunately not a toroidal.
What speakers do you use and what is their sensitivity?
What is the Denon, CD player?
I recently got a Denon DVD-2930 multiplayer, very nice unit.
My 12B4 still have a faint hum, and some hiss. I did get a better transformer which I still need to fit, unfortunately not a toroidal.
What speakers do you use and what is their sensitivity?
Last edited:
It is not clear from the photos, but the supply wires to AC heaters should always be tightly twisted around each other and routed in the corners of the chassis, as far as possible from signal wires, that by itself makes a huge improvement in hum from heaters.
On my Mullard 4-20 mono blocks, I have no hum, by only doing this.
It never occurred to me to mention this earlier, should have asked for photos sooner.
On my Mullard 4-20 mono blocks, I have no hum, by only doing this.
It never occurred to me to mention this earlier, should have asked for photos sooner.
Attachments
) Sold it to a friend, should have kept it. After reading through this thread, I am astonished that no real scientific method has been applied to reducing the hum problem. I would say the first step is always an accurate description of a problem and we don't really have one, other than the unit has hum. How much hum? With the input shorted, what is the RMS value of the hum at the output with the design load? Would this be 50/60 cylce hum? What is the hum RMS value in the other unit?
So far, everyone just takes for granted that hum comes from tube heaters, and of course that is one possible source. Another source would be the plate supply, depending upon the type of tube, but most commonly if a triode is involved. That drawback of triodes is why the tetrode become so popular.
The fact that adding capacitance to the heater supply changes the hum does not prove that the heater wiring is at fault. Adding capacitance to one winding of a transformer affects what is going on in the other winding. Do you really need both 12v windings for one tube? The quickest way to prove your assumptions (should be theories) is to drive the heater with an external regulated d.c. power supply. I will believe it if your hum is completely gone if an external heater supply is used. However, my hunch is that you will still have hum.
Why doesn't this circuit use an input coupling capacitor? Your volume control actually changes the d.c. on the control grid? What kind of design is that? The d.c. operating parameters should be fixed. Its no wonder you are having trouble. Ground the input...hum?
Another possibility is actual magnetic coupling of hum into the amplifier's signal circuitry. It should be relatively easy to temporarily move the transformer out of the chassis area to check that possibility out. My point is that you could easily verify that you are working in the correct direction, rather than assuming you already know the solution. That's the way I see it anyway. If it means anything, you will not find an amplifier circuit like this anywhere else in the industrialized world. You might ask yourself why that would be so.
Good Luck and post the results of your tests.
RA
So far, everyone just takes for granted that hum comes from tube heaters, and of course that is one possible source. Another source would be the plate supply, depending upon the type of tube, but most commonly if a triode is involved. That drawback of triodes is why the tetrode become so popular.
The fact that adding capacitance to the heater supply changes the hum does not prove that the heater wiring is at fault. Adding capacitance to one winding of a transformer affects what is going on in the other winding. Do you really need both 12v windings for one tube? The quickest way to prove your assumptions (should be theories) is to drive the heater with an external regulated d.c. power supply. I will believe it if your hum is completely gone if an external heater supply is used. However, my hunch is that you will still have hum.
Why doesn't this circuit use an input coupling capacitor? Your volume control actually changes the d.c. on the control grid? What kind of design is that? The d.c. operating parameters should be fixed. Its no wonder you are having trouble. Ground the input...hum?
Another possibility is actual magnetic coupling of hum into the amplifier's signal circuitry. It should be relatively easy to temporarily move the transformer out of the chassis area to check that possibility out. My point is that you could easily verify that you are working in the correct direction, rather than assuming you already know the solution. That's the way I see it anyway. If it means anything, you will not find an amplifier circuit like this anywhere else in the industrialized world. You might ask yourself why that would be so.
Good Luck and post the results of your tests.
RA
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.