So i've noticed that on one of the protoboard iterations of the preamp ive been tinkering with, (one of the versions of it before i moved to a breadboard anyway, has big 250v 1uf input capacitors on it.) that if i waved my hand over the input cap on the left channel, that the left channel would hum while my hands was close to it. Thought it was the tube at first but just realized while messing with it, its that input cap.
The right channel did not do the same thing, but i DID notice that on the left channel, the input cap was wired in the opposite direction. Could tell by the printing on the cap. I flipped the cap around on the left channel, and now it doesnt hum when i get close anymore, only if i firmly grasp the cap directly or touch its leads.
I was under the impression that with on a non polarized film cap polarity doesn't matter, but i just experienced something to the contrary. It sounds the same but seems to create a path for noise to more easily enter the amplifier oriented one way over the other or something.
I reversed the right channel cap trying to learn something, and its doing the same hum thing now too, My cheap axial yellow chinese polypro caps do this too. None of my box style or safety caps do this though (all radial). Noise with box caps seems to be the same oriented either way, whether i touch them or not.
Why is this a thing thats happening if it doesn't matter which way you orient a non polarized cap? (obviously theres a reason, but i have no clue what it woud be))
The right channel did not do the same thing, but i DID notice that on the left channel, the input cap was wired in the opposite direction. Could tell by the printing on the cap. I flipped the cap around on the left channel, and now it doesnt hum when i get close anymore, only if i firmly grasp the cap directly or touch its leads.
I was under the impression that with on a non polarized film cap polarity doesn't matter, but i just experienced something to the contrary. It sounds the same but seems to create a path for noise to more easily enter the amplifier oriented one way over the other or something.
I reversed the right channel cap trying to learn something, and its doing the same hum thing now too, My cheap axial yellow chinese polypro caps do this too. None of my box style or safety caps do this though (all radial). Noise with box caps seems to be the same oriented either way, whether i touch them or not.
Why is this a thing thats happening if it doesn't matter which way you orient a non polarized cap? (obviously theres a reason, but i have no clue what it woud be))
Think of a film cap, and the way it is manufactured. Basically, metallised film is wound into a cylindrical form. Connections are then made to each end.
Anyhow, the outer turn should be at zero signal voltage if possible. If that is the case, the capacitor will not be sensitive to touch. Wire it the other way round, and the outer turn is at a sensitive position and will pick hum up.
Back in the day, tubular film capacitors had a red painted end - and that was the end connected to the outer film. So you knew which way to wire it to minimise hum or spurious signal pick up.
Anyhow, the outer turn should be at zero signal voltage if possible. If that is the case, the capacitor will not be sensitive to touch. Wire it the other way round, and the outer turn is at a sensitive position and will pick hum up.
Back in the day, tubular film capacitors had a red painted end - and that was the end connected to the outer film. So you knew which way to wire it to minimise hum or spurious signal pick up.
Input cap does not need to be specified for 250 V. That makes it very big, physically, prone to hum pickup.
1uF bipolar will be significantly smaller. Yes, you can put it in anyway you like.
1uF bipolar will be significantly smaller. Yes, you can put it in anyway you like.
oh i know, this is like 4 versions ago on this preamp, and the wasting of these protoboards is why i went to a breadboard. I havent bothered to pull the components off because its just a mess and i have more. The most recent design doesn't have input caps because source is an interface with known zero DC on its output. I was just playing with the wasted protoboards this morning lol.
Im just trying to learn why it hums more in one orientation then the other. it sounds exactly the same audio wise either way. At first thought it was the tube, determined was the cap, stopped when i flipped its orientation to match the other channel, and started in the other channel when i flipped that one around (But only if i got my hand close to it).
I've seen people mention "foil end" of axial film caps can affect things, but ive also seen people with very sound logic discounting that entirely. would that have something to do with this? Its not affecting performance of the circuit, but it is absolutely affecting its sensitivity to electromagnetic magnetic interference, or it at least seems that way from my perspective. lol im the textbook definition of novice here.
(i dont have a problem im trying to solve, i just wanna know why thats happening)
Im just trying to learn why it hums more in one orientation then the other. it sounds exactly the same audio wise either way. At first thought it was the tube, determined was the cap, stopped when i flipped its orientation to match the other channel, and started in the other channel when i flipped that one around (But only if i got my hand close to it).
I've seen people mention "foil end" of axial film caps can affect things, but ive also seen people with very sound logic discounting that entirely. would that have something to do with this? Its not affecting performance of the circuit, but it is absolutely affecting its sensitivity to electromagnetic magnetic interference, or it at least seems that way from my perspective. lol im the textbook definition of novice here.
(i dont have a problem im trying to solve, i just wanna know why thats happening)
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You can always shield the mp cap. Small piece of copper wrap grounded.
i know there's plenty of logical ways around that as a problem. In the most recent design i just ditched the input cap all together.
I was more fishing for some background theory, if anyone had such, as to why the cap is more sensitive to electromagnetic interference in one orientation over the other. (im also only assuming thats the kind of interference id be providing by getting close to it physically. If i touch something grounded first i cant get any noise out of the cap, my hand actually switches function to shield at that point) Its value, and its effect on energy flow in-circuit is the same in either orientation, but obviously its sensitivity to electromagnetic interference is not, or it somehow creates a path for said interference to enter a circuit with a bias toward one orientation over the other. Or maybe it has to do with manufacturing techniques some brands employ, idk lol.
With no caps on the input i have to get basically close enough to touch the signal path to get a similar noise. Same with caps in, i guess ill call it orientation 2, lol. But in "orientation 1", i get noise with my hand within a couple inches of anywhere on the surface of the cap.
I was more fishing for some background theory, if anyone had such, as to why the cap is more sensitive to electromagnetic interference in one orientation over the other. (im also only assuming thats the kind of interference id be providing by getting close to it physically. If i touch something grounded first i cant get any noise out of the cap, my hand actually switches function to shield at that point) Its value, and its effect on energy flow in-circuit is the same in either orientation, but obviously its sensitivity to electromagnetic interference is not, or it somehow creates a path for said interference to enter a circuit with a bias toward one orientation over the other. Or maybe it has to do with manufacturing techniques some brands employ, idk lol.
With no caps on the input i have to get basically close enough to touch the signal path to get a similar noise. Same with caps in, i guess ill call it orientation 2, lol. But in "orientation 1", i get noise with my hand within a couple inches of anywhere on the surface of the cap.
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I would have sworn that post wasn't there, and i have no idea how i missed it 😅
Ok so it does absolutely have something to do with the foil end. It did indeed seem like what was happening made sense somehow lol
Man, i even clicked like on that post and still glossed it over somehow. I apologize.
Ok so it does absolutely have something to do with the foil end. It did indeed seem like what was happening made sense somehow lol
Man, i even clicked like on that post and still glossed it over somehow. I apologize.
These noise symptoms you're describing for a proto circuit usually indicate the input grounding is non-existant and/or the input impedance is very high like there is no line to ground resistor or one that's a very large value.
Current design its 100k to ground, and doesnt have input caps. On this proto board that resistor to ground is 470k and had massive 250v input caps.. So, thats pretty likely i would guess.
Oh yes, especially if the first stage is high gain. An open input will pick up noise from anything between here and the galactic center.
Are these just DC protection caps? If both sides are at DC 0 volts, try shorting it. Sounds potentially like a faulty solder. A high impedance joint looking into 100 k ohms will have less effect on audio levels than noise susceptibility.
You probably have the input connected to a low-impedance source. When the outer foil is oriented towards the source, the hum is shunted to GND by the low impedance, but in the opposite situation the capacitance of your hand forms a capacitive divider with the 1µF, leaking some hum into the input.
Boxed capacitors often have a stacked construction, meaning there is no obvious outer foil.
If you want to determine the "polarity" easily and deterministically, you can use a device like this one:
https://www.diyaudio.com/community/...oil-tester-for-capacitors.347062/post-6019252
Boxed capacitors often have a stacked construction, meaning there is no obvious outer foil.
If you want to determine the "polarity" easily and deterministically, you can use a device like this one:
https://www.diyaudio.com/community/...oil-tester-for-capacitors.347062/post-6019252
viddi this. good stuff starts around 1:30. Need an oscilloscope? ya, you do. Don't forget the 9v power.
The caps are made with two foil or metallized sheets with insulation between them wound into a cylindrical shape. One sheet will end up as the outermost one and be just below the surface of the capacitor. The other sheet will be below the outer sheet and thus covered or shielded by it. The caps often couple a low impedance source, such as the plate of a tube amp stage to a higher impedance one, such as the grid of the following stage. If the outer foil of the cap is connected to the grid of a stage, then putting a hand near it may cause hum pickup. If the grid is connected to the inner foil, then it will be shielded by the outer foil which is connected to the first tube's plate. That why such caps have a band at one end, to show that that end is connected to the outer, shielding foil. So, it's a shielding, not a polar issue. The cap works fine in either direction otherwise.
I have a working tektronix scope i havent used much yet. And this was an old version of the preamp. Current one doesn't have any input caps because i didnt need them lol, they were indeed just dc protection, and im feeding this preamp from a focusrite interface. 0vdc on the output.. I was just playing with some of the old protoboard builds before i tried to pull the components off and i noticed this on one of them. But only on the one with the big 250v caps and i wanted to know why lol.
And yea, i figured it had something to do with the foil end. I (vaguely) knew that was a thing.
Thank you to everyone for all the answers, not one person replied i didnt learn something from. Finding this very interesting lol.
And yea, i figured it had something to do with the foil end. I (vaguely) knew that was a thing.
Thank you to everyone for all the answers, not one person replied i didnt learn something from. Finding this very interesting lol.
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Lol, thats the very video im referencing (i think) when i say i vaguely knew about the foil end.
I sit and watch Mr Carlson's lab videos in the evening to relax now. That guy was born to be an educator for sure. Just wish more information stuck, i dont have the memory i used to.
I sit and watch Mr Carlson's lab videos in the evening to relax now. That guy was born to be an educator for sure. Just wish more information stuck, i dont have the memory i used to.