Greetings friends, I've had a productive couple days and completed work on a compact 6DJ8 Headphone Amp. While I became aware of the project thru @Suncalc I decided to use SS rectification and built the circuit as described in the DiyAiudioProjects website, with the Antek AS-05T160 (160v@150mA) planning to adjust the dropping resistors in the power supply to achieve the 135-150 voltage spec'd in the schematic. Heaters use AC, both coils paralleled and artificial center tap to Ground. It's built in a Hammond 1458VE3 enclosure:
The amp powered up just fine, I saw a little over 200v with no tubes installed. Drained the caps, installed a pair of tubes and fired it up with some headphones plugged in but no signal. The voltage leaving the power supply is about 166v, so I need to increase the dropping resistors. But I heard a savage hum coming from the headphones while they sat on the bench. Not loud enough to break em, but certainly too loud to use the thing. Sounds like 60Hz but I don't have a dummy load built so I can't say for sure. Hum was equal across both channels in the headphones.
As you can see, the open-frame OTs are rather close to the PT. Maybe I could put a steel cover around the PT.
And the leads from the OTs pass under the volume control pot and cross the input leads, but I'd expect any noise from that to be high-pitched...
any ideas, thoughts welcome. thanks for taking a look.
w
The amp powered up just fine, I saw a little over 200v with no tubes installed. Drained the caps, installed a pair of tubes and fired it up with some headphones plugged in but no signal. The voltage leaving the power supply is about 166v, so I need to increase the dropping resistors. But I heard a savage hum coming from the headphones while they sat on the bench. Not loud enough to break em, but certainly too loud to use the thing. Sounds like 60Hz but I don't have a dummy load built so I can't say for sure. Hum was equal across both channels in the headphones.
As you can see, the open-frame OTs are rather close to the PT. Maybe I could put a steel cover around the PT.
And the leads from the OTs pass under the volume control pot and cross the input leads, but I'd expect any noise from that to be high-pitched...
any ideas, thoughts welcome. thanks for taking a look.
w
A complete and accurate schematic is worth 1000 words, and can reduce the number of Posts required to get the correct answer by about 30 Posts.
Do yourself a favor, and draw one.
Push pull output transformers are less sensitive to power transformer's magnetic fields, and B+ choke's magnetic fields.
Single ended air gapped output transformers are more sensitive to power transformer and choke magnetic fields.
If the B+ capacitors can stand the unloaded voltage, then remove the 6DJ8 tubes, and see how much hum appears in the headphones with the power transformer powered.
Aluminum chassis?
For sensitive headphones, do Not use a magnetic steel chassis.
I had what seemed like a non-magnetic steel chassis once. It was non magnetic in the center of the chassis, but was slightly magnetic at all folds of the sides and all corners too.
Headphones are usually so sensitive, that even the lowest level ground loop will be heard.
My amplifiers for loudspeakers have less than 100uV hum (< 100 microvolts). Sorry, that is not good enough for headphones.
Good luck troubleshooting!
Do yourself a favor, and draw one.
Push pull output transformers are less sensitive to power transformer's magnetic fields, and B+ choke's magnetic fields.
Single ended air gapped output transformers are more sensitive to power transformer and choke magnetic fields.
If the B+ capacitors can stand the unloaded voltage, then remove the 6DJ8 tubes, and see how much hum appears in the headphones with the power transformer powered.
Aluminum chassis?
For sensitive headphones, do Not use a magnetic steel chassis.
I had what seemed like a non-magnetic steel chassis once. It was non magnetic in the center of the chassis, but was slightly magnetic at all folds of the sides and all corners too.
Headphones are usually so sensitive, that even the lowest level ground loop will be heard.
My amplifiers for loudspeakers have less than 100uV hum (< 100 microvolts). Sorry, that is not good enough for headphones.
Good luck troubleshooting!
I think I can relocate the power transformer to the top of the enclosure and then put a cap on that. If that's not it, I'll have to try a bigger enclosure.
Grounding - each tag strip has at least one ground point and they are all linked, like tree roots . There's no ground bus wire, too cramped for that. But no loops.
The top and bottom of the chassis are steel, that's prob no good.
Grounding - each tag strip has at least one ground point and they are all linked, like tree roots . There's no ground bus wire, too cramped for that. But no loops.
The top and bottom of the chassis are steel, that's prob no good.
If the hum was so loud that you could hear it with the headphones on the bench, don’t rule out a wiring error. As others have said, a schematic would help
Brian
Brian
If the hum were caused by inductive pickup in the OT transformers due to the proximity of the power transformer, then this would happen even if the tubes were removed. Is there any hum when it is turned on without the tubes plugged in?
There is very faint hum in the cans with no tubes installed. Like, less by a factor of 10 or more. I'm afraid it'll be a while before I can knock up a schematic but I do appreciate all the advice.
w
w
Can you see both heaters in the valve glowing orange? Just wondering about the chance for something being miswired. Since tube pinouts are as viewed from the bottom, it is fairly easy to get disoriented and wire everything up backwards.
Dear friends, when you assemble an amplifier... first install the power transformer in its place. And then while connecting the input of a headphone to the output of the audio transformer... put the power into the power transformer. In this case, find the best place to install the transformer. Rotating the audio transformer will help you find a position where there is no 60 Hz field induction. This is necessary at the beginning of montage.
Magnetic Steel Chassis do conduct magnetic fields from the power transformer and chokes to the output transformers.
A Toroid power transformer should have a fairly low external magnetic field, but probably not low enough for a headphone amplifier, unless other things are designed with care and knowledge.
The old rule of setting power transformers and chokes to have 90 degree orientation versus the output transformers, usually falls apart when . . .
1. magnetic steel chassis tend to change the magnetic path directions.
2. EI laminations and toroids magnetic fields follow different directions.
A circuit with this many parts, inputs, outputs etc. . . . Does have ground loops, no matter how small nor how short the loop length is.
The reduction or elimination of ground loop hum is an Art and a Science.
Parallel wires that carry lots of AC or Signal current, will induce a voltage into the other parallel wire(s).
A single central star ground does not eliminate ground loops, it merely makes them longer, and can increase the number of loops too.
Just my opinions and experience
A Toroid power transformer should have a fairly low external magnetic field, but probably not low enough for a headphone amplifier, unless other things are designed with care and knowledge.
The old rule of setting power transformers and chokes to have 90 degree orientation versus the output transformers, usually falls apart when . . .
1. magnetic steel chassis tend to change the magnetic path directions.
2. EI laminations and toroids magnetic fields follow different directions.
A circuit with this many parts, inputs, outputs etc. . . . Does have ground loops, no matter how small nor how short the loop length is.
The reduction or elimination of ground loop hum is an Art and a Science.
Parallel wires that carry lots of AC or Signal current, will induce a voltage into the other parallel wire(s).
A single central star ground does not eliminate ground loops, it merely makes them longer, and can increase the number of loops too.
Just my opinions and experience
Why do you need such a big toroid for such a small job....(200V @40ma and 8v @1.5V).That thing is radiating so much electromagnetic crud into your chassis your ecc88 are just drowning.
Remember that triode is an rf magnet as intended.
There is an internal photo of the project you linked.....copy it...why not.
"things should only be changed if they are wrong"
DC heaters as prescribed.
Poor Layout!
Remember that triode is an rf magnet as intended.
There is an internal photo of the project you linked.....copy it...why not.
"things should only be changed if they are wrong"
DC heaters as prescribed.
Poor Layout!
| The power supply provides direct current (DC) for the tube heaters and a well filtered B+. I believe that using alternating current (AC) on the tube heaters would also be just fine