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Old 9th January 2011, 02:09 PM   #1
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Default Headwize 6N1P OTL headphone amp having hum

hi everyone

I bought the above amp ready built (schematic here HeadWize - Project: The 6N1P OTL Headphone Amplifier by Bruce Bender). it works fine but has a rather annoying hum that sounds like 50Hz. I'm asking here because I have zero experience with tubed gear and I know that they (tubes) have their specific problems.

Bruce mentions something about hum when the heater supply voltage.is not filtered but at a quick glance mine seems to have about 1800uF (4x470u) of capacitors on it.

any other places where I should investigate? thanks
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Old 9th January 2011, 02:51 PM   #2
Stixx is offline Stixx  Germany
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Hum as you have it most probably comes from a grounding problem... but also depends on how the amp was built. Does it have the same psu like the one in the schematic?
AC for the heaters should be no problem as long as the wires are tightly twisted and routed away from signal wiring... even with AC heaters the amp can be dead quiet. Can you post a picture of the internals?

you can also read about grounding here --> Star Grounding
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Old 9th January 2011, 04:16 PM   #3
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Quote:
Originally Posted by Stixx View Post
Hum as you have it most probably comes from a grounding problem... but also depends on how the amp was built. Does it have the same psu like the one in the schematic?
AC for the heaters should be no problem as long as the wires are tightly twisted and routed away from signal wiring... even with AC heaters the amp can be dead quiet. Can you post a picture of the internals?

you can also read about grounding here --> Star Grounding
hi Stixx

and thanks for the quick reply. I have maybe average knowledge of electronics but when it comes to tubes I'm a complete noob. I know about grounding schemes and I'll check that too when I dismantle the thing completely. meanwhile, this is how it looks with the cover off (if that can reveal anything at all). will get back soon with more details.
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Old 9th January 2011, 10:14 PM   #4
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took a look at the way it's made and it's a bit different from Bruce's schematic. the differences can be seen in the attached pic (drawn in red).

other differences not seen in the pic:
- the power supply doesn't use the cap-res-cap-res-cap scheme, instead all caps are just wired in parallel
- one rail of the heater supply is shorted to ground

and star grounding is not thoroughly obeyed...

where should I start?
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Old 9th January 2011, 11:37 PM   #5
tomchr is offline tomchr  United States
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Changing R5 will have a significant impact on the bias point for V2. What's the reason for the change?

500 kOhm input pot is rather high impedance. I suggest lowering that to 47~100 kOhm at the most. If this is indeed the problem, you should be able to make the hum go away by turning the volume all the way down.

Does the hum change with the setting of the volume control at all?

I would probably prefer to leave the 100 Ohm and 1 MOhm on the input in place. Then I'd AC couple with a 220 nF cap between the volume pot and the junction at the top of R1. That way any DC from the source won't interfere with the biasing of V1.

If you have hum with the wiper of the pot grounded, I'd start looking for ground loops and implement star ground. Also make sure you take the output from the power supply from the last filter cap and not earlier.

~Tom
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Old 10th January 2011, 02:58 AM   #6
singa is offline singa  Singapore
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Hi Mr.Push_Pull,
Fom what I can see the amp is OK with the addition
of a plate as EMI barrier.The only thing that is not implemented is the
tying of the common return (ground) of the amp and power supply.
Read the starground article given and understand how an electrical
circuit works.

The point is both boards don't have a "common" ground and therefore
both pcbs are referencing their respective "ground" so there may be a difference in ground potential or voltage and get into the amp through
lowest ground resistance as it is relative dedending on trace or copper track
size and length.

So first connect both pcb ground,choose a suitable takeoff point from amp pcb to power supply pcb,now where you choose to connect at the power
supply can also be critical as different location may have higher noise.
Try the thick common return of the filter caps just before it feeds the amp pcb.Then if there is no hum,then choose a suitable chasis starground preferably near the raw AC power entry wires and then to earth ground pin of the AC power receptor.This is important as if there is a electrical fault the excess voltage will have somewhere to go and not through the appendages of your body and then the ground you are standing on.
Always think SAFETY FIRST.Discharge the high voltage caps to ground with a resistor of about 5 watt or bigger first! Measure the cap voltage to ensure all voltage is discharge.If you see
1 volt or less it is safe to dive in.

If there is hum still disconnect from chasis ground and work your way around until minimum hum and then connect to chasis ground.

Edit: It is obvious there is ground connection between both pcbs ( 1 pair of wires ) or else the amp will not work and if this is a diy thing observe the pcb trace below and see how it runs and you will get the picture.Are the heaters running on AC?I don't see any diode bridge or is it behind
the heatsink. If that is so then there would be some hum but it can be minimised.

Regards.

Last edited by singa; 10th January 2011 at 03:19 AM.
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Old 10th January 2011, 06:22 AM   #7
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Quote:
Originally Posted by tomchr View Post
Changing R5 will have a significant impact on the bias point for V2. What's the reason for the change?
I have no idea. the guy who built it said it had some deviations from Bruce's schematic but couldn't tell exactly.

Quote:
Originally Posted by tomchr View Post
Does the hum change with the setting of the volume control at all?
nope. stays unchanged regardless.


Quote:
Originally Posted by singa View Post
Are the heaters running on AC?I don't see any diode bridge or is it behind
the heatsink. If that is so then there would be some hum but it can be minimised.
it is both rectified and filtered. you're saying that rectifying it would cause hum?


all other points taken. I'll start troubleshooting as soon as I get home.
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Old 10th January 2011, 06:41 AM   #8
singa is offline singa  Singapore
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Hi, If you're using DC for heaters there should not be hum at all so a
groundloop problem is more likely. But stange...since this is prebuilt
by seller he should QC before shipping?? Regards.
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Old 10th January 2011, 06:47 AM   #9
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Quote:
Originally Posted by singa View Post
by seller he should QC before shipping?? Regards.
it's DIY.
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Old 10th January 2011, 07:00 AM   #10
singa is offline singa  Singapore
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Quote:
Originally Posted by mr_push_pull View Post
it's DIY.
That explains it.Anyway this is a good way for you to learn and gain experience.
Regards.
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