I've been prototyping a headphone amp for the past several months and have gone through close to a dozen iterations at this point. I think I have settled on a design now, but am having some problems with buzzing. The power supply is just a bridge rectifier with a basic Pi-Filter arrangement. Two 100uF caps separated by an inductor or resistor. Earth ground tied to DC ground. If my scope ground is plugged in to the amp power supply ground then it's all good. Nice and quiet. As soon as I pull that scope ground off of the breadboard though I get a very loud hum/buzz. Where the scope ground is plugged in doesn't matter, and plugging in a direct link to earth ground does nothing. Any ideas what I'm doing wrong here? I'm at a bit of an impasse.
You have a ground loop in the amplifier, or some other problem.
Is the hum present in your headphones when:
The input is shorted?
The input is open?
A signal source is connected, i.e. CD player?
Each of the above with and without the scope connected?
What headphones?
How many Ohms?
What sensitivity?
Headphones are so sensitive. They are 1 inch off of the ear, essentially no sound attenuation.
Speakers 3 feet away (36 inches) are attenuated by 36 x 36 = 1/1296 (-62dB).
Getting hum low on a headphone amp is not trivial (as you have found out).
A complete schematic is always helpful to doing troubleshooting, please post it.
Troubleshooting without a schematic is like being blindfolded and trying to shoot an arrow at the target.
If your amplifier will not become a commercial product, you have nothing to protect,
and you can post the schematic. That will be a good start.
Thanks!
Is the hum present in your headphones when:
The input is shorted?
The input is open?
A signal source is connected, i.e. CD player?
Each of the above with and without the scope connected?
What headphones?
How many Ohms?
What sensitivity?
Headphones are so sensitive. They are 1 inch off of the ear, essentially no sound attenuation.
Speakers 3 feet away (36 inches) are attenuated by 36 x 36 = 1/1296 (-62dB).
Getting hum low on a headphone amp is not trivial (as you have found out).
A complete schematic is always helpful to doing troubleshooting, please post it.
Troubleshooting without a schematic is like being blindfolded and trying to shoot an arrow at the target.
If your amplifier will not become a commercial product, you have nothing to protect,
and you can post the schematic. That will be a good start.
Thanks!
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My current headphones are nothing special. JVC Mr-60. I don't know any other specs on them. The hum stops when I short the input to ground. I can just barely hear a bit of rectifier hum but that's okay. The buzz I'm talking about is very similar to when you unplug the source from some amps. Loud and extremely grating sound. When a source is connected on this one it actually gets slightly louder. When I plug in that scope ground though then all is well. It kills the buzz no matter where on the breadboard I plug it in, so long as I plugged it into ground of course. I'm not sure how to lift the scope ground. I'm still pretty much a novice at this stuff. I hope you can read this schematic.
Attachments
First off if you have a scope you should be able to tell us what frequency the hum is, either by counting 1/ (hor divs times by timebase setting) or reading off the value if you have a fancy digi scope.
Second your scopes ground lead is/should be connected to mains earth, therefore connecting your BB ground to earth stops hum, so, that could mean you may have some capacitive voodoo going off between the BB and bench or between wires or air of something. That or some part of the amp is acting like an antenna, picking up stray mains 50/60hz, or you have a cathode htr short. could be one or many things.
Am guessing your scope ground is acting as a LF, low Z short to mains earth, effectively killing them. Guessing here, you could have some other ground issue too.
Andy.
Second your scopes ground lead is/should be connected to mains earth, therefore connecting your BB ground to earth stops hum, so, that could mean you may have some capacitive voodoo going off between the BB and bench or between wires or air of something. That or some part of the amp is acting like an antenna, picking up stray mains 50/60hz, or you have a cathode htr short. could be one or many things.
Am guessing your scope ground is acting as a LF, low Z short to mains earth, effectively killing them. Guessing here, you could have some other ground issue too.
Andy.
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By "lift ground" i was thinking you might have a way to plug in the scope in a way that the ground pin wasn't connected like when using a 2 prong extension cord or the like. If the scope uses an IEC cable, and you have several like I do, you can just break the ground pin off of the cord. Once again, this can potentially cause a dangerous situation for you (unlikely) or sensitive electronics (more likely) and shouldn't be left like that.
From the schematic, I can say the 2k2 resistor (grid stopper) is on the wrong side of the 100k grid leak. It's probably not causing your problem though. I can say the input stage values are way off for a 6FQ7 IMHO. I mean they will work, per se but starved for current and running in a crappy part of the curves. Usually it's more like 47k plate resistor and 470R cathode, and even then, that's usually with 250V+, not only 170V.. Those values look more like something you'd see for a 12AX7 in an instrument amp to me but again the low voltage
Especially if it's driving a tone control from the plate!It sound to me like your input isn't grounded. The noise you describe sound like when you plug in an RCA cable. Positive connects first, followed by ground which is why you get that noise. If you had a bad ground from input to the amp that would also happen.
Yesterday I found out I needed to pinch a pair of RCA cables with pliers because they were too loose and one channel didn't have a local ground instead using the electrical system. No hum, but full blast and or switching noise from everything in the neighbourhood on the channel with missing ground.
It could also be a grounding issue with the tone control but unlikely since without a ground it wouldn't work.
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You were right about the ground. Turns out I made a stupid mistake.. The earth ground wasn't connected in the power supply section. Connected it there and the problem stopped. As for the B+ and 6fq7, I've just been working with what I have laying around. I have other triodes that may be more suitable, but that's just what was on the top of the pile. Sounds as good or better than anything I've ever heard. I changed the resistors to your recommendations. I can't tell much of a difference but then again I can't exactly listen side-by-side. I only included it to offset the attenuation of the tone control. (It's a cheap ebay special thing) This is using an edcor 170v transformer. I have a vintage 130v transformer that I salvaged out of something. I may try running a doubler off of that to get closer to the in-spec voltages. What I am particularly excited about though is the OPT. It isn't really an OPT. It's a line-matching transformer for PA systems and they cost $3 US each. I was expecting zero bass but this thing can really rattle my brain and it still sounds nice and clean. Then again I'm no audiophile. This project has morphed from a "How cheap could I make one" to a "how much better can I make it without buying expensive mainstream stuff." It's been fun.
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