Odd grounding problem

leftshoe

Member
2010-11-01 3:07 am
I've run into an interesting problem with my LM3886 amplifier. I get quite serious amounts of hum whenever the volume control on the amplifier is at anything other than 0 or full volume, when plugged into my diy DAC. This doesn't occur when plugged directly into a iPod, and interestingly it occurs even when the DAC is not plugged in! If the signal cable is not connected to anything it's fine, but just plugging it into the DAC without any power cable going into the DAC triggers it. Plugging in the DAC and turning it on doesn't affect the level of the hum.

I've also noticed that when turning the amp off, the hum stops immediately, while the music keeps playing for a few seconds. This seems weird to me, I would expect both to continue while the caps deplete.

I've tried floating the amp's ground by just not connecting it to main ground, but it didn't help. I can upload some pictures of its internals if need be.
 
I'm a relative novice, but here's a few observations:

I've also noticed that when turning the amp off, the hum stops immediately, while the music keeps playing for a few seconds. This seems weird to me, I would expect both to continue while the caps deplete.

That implies that the source of the hum is the transformer/rectifier in the LM3886 PSU.

...interestingly it occurs even when the DAC is not plugged in!

Hypothesis: a loop or something in the DAC output is picking this hum up. This antenna effect, being passive, would still happen with the DAC off.

Does the DAC work humlessly with other amplifiers when the LM3886 PSU is switched on in the vicinity?

Does the hum reduce when the LM3886 PSU is moved further away from the DAC?

...serious amounts of hum whenever the volume control on the amplifier is at anything other than 0 or full volume

When the volume is full off the 3886 input is shorted to input ground reference, so no hum.

Why the hum goes away when the volume is full up is less obvious to me. I'm gonna lurk around this thread 'til one of the smart guys explains it. :D

Good luck sorting it out. :)
 
Last edited:
I've run into an interesting problem with my LM3886 amplifier. I get quite serious amounts of hum whenever the volume control on the amplifier is at anything other than 0 or full volume, when plugged into my diy DAC. This doesn't occur when plugged directly into a iPod, and interestingly it occurs even when the DAC is not plugged in! If the signal cable is not connected to anything it's fine, but just plugging it into the DAC without any power cable going into the DAC triggers it. Plugging in the DAC and turning it on doesn't affect the level of the hum.

Presumably this is a stereo set up, so does the hum disappear or change noticeably when only one input is connected to the amp? It could be your DAC, by joining together two signal grounds, is causing some kind of loop. Another thing - what's the DAC connected to via its digital in? That could be the culprit - not all DACs have isolated digital inputs and many CD/DVD players don't either.

I've also noticed that when turning the amp off, the hum stops immediately, while the music keeps playing for a few seconds. This seems weird to me, I would expect both to continue while the caps deplete.

Its quite normal :) The source of the hum is going to be either the transformer's magnetic field (solve it by putting some more physical space between the amp and the trafo) or its the result of the high current charging pulses running into the main res caps cross-coupling into some sensitive place.

I've tried floating the amp's ground by just not connecting it to main ground, but it didn't help. I can upload some pictures of its internals if need be.

Certainly would be helpful to see a wiring diagram (with grounding clearly shown) and a schematic too.
 

leftshoe

Member
2010-11-01 3:07 am
I did some more experiments:

- Disconnecting signal ground resulted in no music.
- Hum occurs when only 1 channel is plugged in, only comming out of that channel.
- Not connecting a digital source doesn't help, niether does using a computer as source.
- Also, the hum is significantly louder than the music except at near full volume. It ramps up linearly at low volumes then stays fairly constant volume until near 100% volume.
- If I move the DAC about 6 feet away, the hum is the same as before with the amp off or on and the power cord plugged into the DAC, but when I unplug the power cord, the hum goes away. This sounds more like a normal grounding problem.

The DAC is a kit, the one in the recent Silicon Chip, metal cased, with a star grounding scheme. I think the DAC is properly grounded, so it must be something I did wrong with the amp. Any pointers on how the amp should be grounded? Its plastic cased.
 

leftshoe

Member
2010-11-01 3:07 am
I've attached photo. The photo doesn't show the volume control, which was a latter addition.
 

Attachments

  • amp_innards.jpg
    amp_innards.jpg
    79.2 KB · Views: 337
Look up "Faraday's Law".

Your main problem is that one or both pairs of your internal input signal and input signal ground wiring is forming open "loop area".

A time-varying (e.g. AC) magnetic or electromagnetic field induces a time-varying current in any conductive loop.

When an input is connected, the loop is closed, enabling induced AC current to induce voltages across any impedances in the loop, which would include the source impedance, and any resistance to ground at the amp input pin (as well as the resistance and inductance of the wires themselves).

The solution (mitigation, actually) is simple: Your input jacks should be isolated from chassis ground and each of your internal input signal and signal ground wire pairs should be inside of a shielded cable, the shield of which should be connected to chassis ground at the input end only, with the other end of the shield connected to nothing. If you can't use shielded cable, then tightly twist together the wires in each pair, for as MUCH of their length as possible. Only the shortest bare-minimum lengths of the wires should be unshielded (or un-twisted). And keep the wires as close to each other as possible, for those short unshielded or untwisted paths, even following against the amp input resistor to its ground end, with the ground wire, from where the signal wire connects to the resistor's other end.

Also make sure that you tightly twist together all OTHER conductor pairs, such as the AC input pair, transformer secondary pair(s), AC wire pairs to/from switch, rectifier output pair(s), DC power pairs, audio output pairs, etc, noting that Faraday's Law also works in reverse, i.e. a time-varying current in a loop will cause a time-varying field to be radiated. So tightly twist together ALL pairs, in order to minimize the effects for both "transmitters" and "receivers".

And ALSO, keep all small-signal and DC wiring as far away as possible from all AC and large-signal wiring and components. If such wires must approach each other, they should be perpendicular to each other.

Edit: Your amp is PLASTIC-cased?? Bummer. Then you might also have to worry about the interconnect cables from the DAC. But if you twist all of your internal AC wire pairs, you might not have to worry much about the interconnects.


Cheers,

Tom Gootee
 
Last edited:
You would not want to use coaxial cable that has one center conductor. You want a cable with two insulated conductors inside, plus an outer shield. If the shield is foil then there should also be an uninsulated "drain wire" for connecting the shield. (Tightly twisting should be "almost as good" as shielded, especially if you also twist all of the AC and other pairs. It's a bit of a pain but is something good to try if the shielded cable isn't immediately available.)

I would also twist all of the AC wire pairs before worrying about a metal shield. It might still be necessary but it's more difficult than you might imagine to shield magnetic fields. Better to first try to eliminate the fields radiating from the wiring, then see what's left. Toroids usually radiate mostly axially, anyway.

(Sorry, have to go to sleep now.)
 
Last edited:

leftshoe

Member
2010-11-01 3:07 am
I've rewired all internal wiring so it's now all twisted. It doesn't seem to have helped.
The original issue where the hum occured when the DAC was not plugged into power appears to have been caused by the DAC being in close proximity to the amp. Now that I've moved it futher away, it now only hums when the DAC is plugged in and connected to the amp. It doesn't matter if the DAC is on though.
 

leftshoe

Member
2010-11-01 3:07 am
The DAC schematic shows that the mains earth is connected to the 0v line going into the signal board, if I'm reading it correctly. For the amp it looks like I'm just running the 3 leads from the transformer to the board, no mains earth wire. I dont have a 'chassis' ground due to the (ill-advised) use of a plastic case. So that could be the problem.
 
The DAC schematic shows that the mains earth is connected to the 0v line going into the signal board, if I'm reading it correctly.

OK, its normally advisable to have some kind of ground-lifting network in place of a hard link. AndrewT should be able to advise on this, he's way more safety conscious than I:D

For the amp it looks like I'm just running the 3 leads from the transformer to the board, no mains earth wire. I dont have a 'chassis' ground due to the (ill-advised) use of a plastic case. So that could be the problem.

Well without a ground connection at all, there is a safety issue as you do have exposed metal parts. So to be totally safe you'd need to adopt double insulation standards so that a single point insulation fault doesn't turn your ins and outs into shocking hazards.

From the pov of your hum, the toroidal in your amp has appreciable capacitance to the mains (300-600pF is not uncommon). Once you connect your DAC there's a path back to ground, the amp 0V will cease floating and current (rather noisy with RF, not just mains frequency) will flow along your interconnect shields into the DAC. This is normally less than 1mA at mains frequency but will induce a hum voltage across your grounding wires. This might be part of your problem.
 
Abrax look after your fellow Members

I abhor the term "ground lift switch".
It implies that disconnecting the Safety Earth from the Protective Earth is an option that we users can decide to implement or not.

A ground lift switch MUST NEVER break the Safety Earth to PE connection.

A switch that bypasses, using a direct connection, the Disconnecting Network between the Safety Earth and Audio Ground does not introduce any safety issues if properly designed and implemented.
This switch which is user selectable is a completely different hazard from "Ground Lift"


Please, try never to confuse readers by leaving reference to "ground lift" without explanation of whether hazard or non hazard are being advocated.
 
I abhor the term "ground lift switch".

It seems from your header that you're referring to me :D But nowhere do I advocate a 'ground lift switch', and I would not recommend such a thing.

A ground lift switch MUST NEVER break the Safety Earth to PE connection.

Yeah, I agree.

Please, try never to confuse readers by leaving reference to "ground lift" without explanation of whether hazard or non hazard are being advocated.

I referred readers to you Andrew, as you have a well developed sense of electrical safety. But notice the idea of a 'switch' is entirely your own.