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I have been having issues with light switches and household appliances causing ticks, pops and in some cases, causing my DAC to protectively mute.One recommendation was to check my system grounding. The process involved removing RCAs from my amp (DIY 6C6 - 2A3 SE) and listening for hum level to set a reference then make comparisons with various components attached.
Well, removing RCAs from my amp creates one hell of a hum! Is this normal or does it suggest an underlying issue? Other details: with inputs connected, my amp has low noise (for AC filaments); the amp has continuity between the chassis (various points tested), RCA grounds and safety earth; there is 120k resistance between RCA signal and ground, as expected; in all other ways, the amp has been reliable.
Shorting plugs would force the input to 0V (I think)... but why would there be voltage on the signal input without?
Thanks for any assistance.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 11/02/15Follow Ups:
Thank you all for taking the time respond.Before building this amp, I spent a good deal of time researching grounding approaches and selected a multiple-star scheme, but it seems it can still be tweaked a little.
I learned a lot from the build, but it did include some compromises that future builds will avoid.
Strategy (for starters):
1/ Add 10 to 100R between 0v signal grnd and chassis.
2/ The wire to the control grid cap of the 6C6 is unshielded and runs near the output transformer; it was an unplanned change. The wire could be picking up noise. I will use some shielded wire to the control grid.
3/ Recheck the wiring to ensure connections are correct and nothing is touching the chassis.
4/ Consider returning the 6C6 cathode circuit to the PS cap that supplies the stage (rather than to the screen bypass cap).The 0v signal grnd connects to the chassis in place only: close to the safety earth connection. In future builds, it might be better to make this connection at the RCA return.
Thanks again all.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 11/07/15
I once had a noisy light switch in the bathroom, every time it was switched off it drove my parasound (solid state) amps protection circuity crazy. Worn out /pitted or loose I guess. JH
On my St-35 clone I had an identical problem. Both channels. 20mVAC on the outputs. Went away with shorted inputs. I was sure it was a grounding problem.
Nope. One channel's ultra-linear wire was sticking out through the ckt bd far enough to contact the chassis. Pulled it back a bit and presto. Hum gone.
Check the boards for protruding wires that might contact something.
The amp has an issue and it does sound like you need some education on it.
For example it should be pretty quiet if nothing is hooked up to the inputs. You asked about that so I assume you didn't know that. So that's one example of 'both'.
the amp has continuity between the chassis (various points tested), RCA grounds and safety earth
This is an example of incorrect grounding technique. The chassis should be at ground through the ground pin of the power cord, but the circuit should not. Instead, there should be a low resistance (10-100 ohms) between the circuit ground and chassis. This practice allows the chassis to shield the unit but prevents ground loops should some other component in the system be grounded to the AC ground connection. So that's a second example of 'both'.
Now solving the issue of pops when other electrical items are used is a bit trickier. One thing that can cause that is improper grounding of the audio circuit, such that noise from the power supply can be fed into the audio input via the ground. IOW, each ground point in the amp should have its own connection to ground (this is called 'star grounding'). Now one exception to this rule is when you have a grid circuit and a cathode circuit of the same tube. Those two points should be tied together and use a common ground to the power supply. If any noise is present on such a ground it cannot get amplified as it is common to both inputs (grid and cathode).
Secondary emission from filaments can cause troubles too- especially if the power transformer has poor electrostatic shielding. For this reason it is a good practice to 'elevate' the filaments. Of course you can't do this so easily with DHTs, but if you are using 6SN7s or the like for driver or voltage amplifier applications its something to be considered.
I think this is enough for now. Take a look at these things in your amp and see if you can correct them. Then let's see what sort of progress you made.
"Well, removing RCAs from my amp creates one hell of a hum!"
When the input wires are open-ended (no source impedance across them), they act like antennas. The amount of hum they pick up will depend on a number of things, including how they're routed inside the chassis. Proximity to a power transformer or choke can create excess hum. A wooden (unshielded) chassis can make this problem worse. It's helpful if the "hot" wires are twisted with a ground wire, or better, made of shielded cable. Tube circuitry is more susceptible than solid-state to the pops and clicks you've described. This is due to the higher operating impedances, which allow a higher voltage to be induced by external influences. Again, a shielded chassis is beneficial.
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Buy Chinese. Bury freedom.
Each ground point should have one path back to earth. Try shorting the RCA jacks to see if the hum goes away. If it does, you have a problem with a system ground, how your amp handles signal ground sending it back to the source component on more than one path.
If the hum does not go away, the hum is contained in the amp itself. There are three primary sources: Ground Loop, Inductive Coupling of wiring and/or transformers, and ghosts.
Layout is important, twising AC wires, keeping signal wiring and stages away from AC, orienting and spacing the transformers to minimize inductance, etc.
The good news is that hum issues can usually be solved. The bad news is that it can be hard to find the solution. But when you do, the sense of Joy is overwhelming. If this was easy, everyone would have a nice stereo.
"the amp has continuity between the chassis (various points tested), RCA grounds and safety earth; there is 120k resistance between RCA signal and ground, as expected"
Which is it continuity or 120K? Chassis should be earth. RCA should be power supply and signal ground. Please clarify.
Dan Santoni
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