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Old 1st January 2012, 04:51 AM   #11
gootee is offline gootee  United States
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We could probably offer better ideas if you would post some photos of the circuit, and a schematic.

Not sure about your scope results but you also say that you can HEAR the hum. So I have to believe that something is getting into the input.

Is this single-ended? How do you have the input's signal and corresponding ground conductors arranged/routed? And how is the grid-input's ground-voltage reference conductor (to star ground) arranged/routed?

The only two common "gotchas" that I can think of, to get hum into the input, are:

1) You could have "enclosed loop area", enclosed by the signal input and signal input ground conductors, and/or the conductor from the input ground voltage reference point to the star ground. Any conductive loop, in the presence of a time-varying magnetic field, will have a time-varying current induced in it (Faraday's Law). If the loop includes your signal input or its ground reference, that's definitely trouble. Your input jacks (both signal and signal ground) must be insulated from the chassis. The signal and signal ground wires must be twisted tightly together, all the way from each jack to the ends of the resistor betwen input and ground (the input ground reference point). If a shield is also used (which must NOT carry the signal ground), in addition to twisted pair, then the shield should only be connected to the chassis, and to nothing on the other end. In an extreme-gain case, I would probably also run/twist {the ground reference conductors that go from the ground ends of the input resistors to the star ground} with their respective "input signal and signal ground" pair, back to the input jack (but not connected there of course), and then take those ground-reference conductors (from the two jacks) and twist them together on their way from between the input jacks to the star ground. That should almost totally eliminate the enclosed loop area from the signal input, signal input ground, and signal reference ground conductors.

Of course, any loop with enclosed geometric area can (and will) be either a receiver or a transmitter or both. So you should also: run right next to each other, and tightly twist (even if shielded) if using wires, ALL "natural pairs" of conductors, including all AC pairs, DC power pairs, input pairs, output pairs, heater pairs, etc etc.

And of course you want to keep as much distance as possible between stuff with large and/or very dynamic currents and stuff with low-level or sensitive signals. And any conductors that must approach each other should do so orthogonally.

Do you get hum when the input jacks are not connected to anything? What about when you short the input jacks, or connect a small resistance across each one? If "no" for first question and "yes" for second question, or if there is a big difference in the hum for those two cases, then the hum is almost certainly due to enclosed loop area of the input signal/ground conductors.

Have you only tested it with the chassis open, and perhaps (but not necessarily) with fluorescent lighting nearby? With that much gain, I'd also try it with everything buttoned-up, to see if there's any change at all. If so, it's probably due to enclosed loop area in your input circuitry.

2) You could have the input ground voltage reference point's return to the star ground sharing some length of conductor with another ground return. Trace and wire resistance isn't the only problem, and may not be the main problem when the currents are time-varying. In addition to the voltages induced across the parasitic resistance, the conductor's parasitic inductance induces a voltage that's proportional to the time-rate-of-change of the current, regardless of the current's amplitude. Any such induced voltages will appear back at the non-ground end of all of the conductors that share any length on the way to the star ground. If one of them attaches to the "ground" end of a resistor that's between an amplification device's input pin and "ground", then those induced voltages will be arithmetically summed with the input signal voltage. And that's trouble.

If you have two probes and can use differential mode (one channel minus the other) you could put one scope probe at the star ground and one at the "ground" end of the resistor from grid to ground (assuming you have something like that in the circuit). Or, if they're close enough to each other, you might get away with attaching the ground wire from a single probe to the star ground, and probing the same input ground reference point. Disclaimer: I guess I can't remember actually trying that. It might not work with standard probe types.

Cheers,

Tom

Last edited by gootee; 1st January 2012 at 05:19 AM.
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Old 1st January 2012, 08:42 AM   #12
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put a .01 uf at 250V or higher on one side of your heater winding and DC ground. pm me to know why it worked.
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Old 1st January 2012, 09:18 AM   #13
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better yet, put the capacitor between the heater referencing resistors and DC ground. you need only to ac couple the heaters (through a cap) to dc ground. The hum balance range should be infinite now, and it doesn't matter if you reference it to one side of the heaters and without resistors too.

Last edited by DavesNotHere; 1st January 2012 at 09:23 AM.
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Old 13th April 2012, 02:28 AM   #14
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following up to an old post:

sorry, dave - that didn't work.

tom - just finding out how to draw schematics using programs. i'll get to it...

i went through extensive pains for a star ground, but, of course, that's not the issue.

i believe it's this anodized chassis (which is now butchered to shreds). i've had problems with grounding it simply by scratching off the anodized portions where the panel screws mount; even though on a DMM, I get continuity, I believe that not having full connection all around is causing a series of small ground loops throughout the chassis. even though the signal path is star grounded, there is an eddy of ground currents that radiates, in different amounts in different sections, the further away I probe from the AC chassis ground.

that's very interesting to me - how, with a complete signal star ground, and the only other ground point to chassis is the AC ground, there's still loops going down throughout various parts of the chassis...

i'm ordering a non-anodized chassis, will re-build, and see if that fixed it. i need to, anyways, for appearances.
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Old 13th April 2012, 10:52 AM   #15
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As mentioned don't rule out your power outlet.

I chased a hum around for weeks that went away the instant I plugged it into another outlet, away from the computer. Now it is silent, I can't really even hear anything with my ear against the speaker.
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Old 21st June 2012, 08:40 AM   #16
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reviving; had to move; life changes; now, back on the bench.

here's a picture of the o'scope looking at the chassis ground of the preamp. so, the ground noise is at about 5mV. this exact same waveform that is on the ground is hence on anything that is referencing to chassis ground.

does that make sense?

i've tried multiple grounding schemes and considerations, which only bring the noise of the preamp down to the noise seen on the ground, as seen here. currently, i have a star ground. the first filter cap and PT center tap are tied together at one chassis point; all other points are star grounded together (each filter stage; each tube stages' cathode; each stages' grid; each stages' vol pot; anything being considered has an individual heavy gauge lead to star ground); further, the mains ground is connected through a short, heavy gauge wire at the mains input. no hum improvement tying the two ground points (PT/first filter and 'the rest) together.

most specifically, this noise is seen on the (grounded) secondary side of the input transformer, so, it's being included with any input signal. ~5mv of noise then hitting an ~80db gain preamp adds up considerably. even BESIDES that, i'm still seeing the ~5mv noise at the output of the CF output (which is my last stage).

i've changed the chassis itself, which helped slightly.

further thoughts, everyone?

i'm always happy reading y'all solving problems - thanks for reading mine, yet again.

(any thoughts of connecting the star ground to chassis via a 10 ohm resistor, or similar method, to help this out?)
Attached Images
File Type: jpg ground noise 1.JPG (72.5 KB, 168 views)
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Old 21st June 2012, 09:36 AM   #17
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also:

the ground noise waveform is strongest at the power transformer mounting bolts.

should i put some rubber washers to isolate the power transformer from the chassis?
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Old 21st June 2012, 10:39 AM   #18
DF96 is offline DF96  England
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Remember that there is no such thing as 'a voltage'; there are only potential differences. So with respect to what are you seeing this signal? The oscilloscope ground? To see a few mV difference between two different 'grounds' is not at all surprising.
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Old 21st June 2012, 10:50 AM   #19
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Quote:
Originally Posted by themagicmanmdt View Post

further thoughts, everyone?
Have you seen this?

Audio Component Grounding and Interconnection
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Old 21st June 2012, 07:34 PM   #20
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Quote:
Originally Posted by DF96 View Post
Remember that there is no such thing as 'a voltage'; there are only potential differences. So with respect to what are you seeing this signal? The oscilloscope ground? To see a few mV difference between two different 'grounds' is not at all surprising.
most totally true - which is why i'm not just relying on my eyes/scope - but, rather, my ears (with a little help from my 'scope).





david - no, haven't seen that link! awesome. read the whole thing - rechecked and even did a sensical change to the signal reference -> power common flow. no change in noise, however, it 'feels' prettier.

i don't have a camera that takes a good close up picture, otherwise i'd take some.

it's a rather basic SE mic preamp; utc 0-8 -> 6sj7 -> ef86 -> 12au7 CF


mainly, the voltage ripple seen in the picture is amplified in each stage; especially the 6SJ7 which is set to have a voltage gain around 100. the noise on the plate of the 6sj7 is 40mv P-P. terminating the grid resistor in different values, and even grounding the grid, doesn't change the output noise. so, i'm ruling that the grid isn't where this noise is getting into the gain stage.

the noise looks to be 60hz, so it's not coming from the B+ (although I see the same small noise on the B+ that i see on the ground in the scope); i've tried 1uf filter cap bypass caps in the PSU, no change. currently removed.

going to try different grounding and bypass caps for the cathode. will report back.
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