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Headphone amplifier ground. Where to make the chassis connection?

Posted 2nd February 2016 at 08:11 AM by rjm
Updated 24th February 2016 at 02:02 AM by rjm

The discussion thread at the headphone forum is here, but I wanted to throw out the problem to the general blog-reading community here at diyaudio to see if anyone can nail this.

The earthed chassis (light blue) must connect to the circuit common i.e. "ground" (pale green). I do not know where the best place on the circuit ground is to tie that connection.

Suggestions please!

(COM and GND are completely equivalent pads on the circuit board, while IN- and OUT- also pads on the board but physically further away on the ground plane.)

****

Answer: as long as it connects at one point only, or the same point of both channels, it doesn't seem to matter at all. I have it connected at the ground tab of the headphone jack and that seems to be as good as anywhere.

****

The noise was in fact magnetic interference emanating from the transformers. Grounding layout changes / electrostatic shielding were ineffective, magnetic shielding was needed instead.

I bought a sheet of permalloy (mu metal) off ebay and wrapped a belt of the stuff around the transformers. This cut the interference drastically. RMAA-measured A-weighted S/N is now 97.7 dB.
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Total Comments 7

Comments

  1. Old Comment
    abraxalito's Avatar
    My choice would be at the input GND, so long as your trafo has an interwinding screen which also goes to chassis. Then CM noise doesn't need to flow through PCB 0V tracks. Headphone CM noise will only be RF pick up so that's probably less severe than noise from the source component, unless that's battery powered.
    permalink
    Posted 2nd February 2016 at 02:42 PM by abraxalito abraxalito is offline
  2. Old Comment
    rjm's Avatar
    Thanks. Actually what I think I'm looking at here is noise from the transformers coupling to the boards. It all happens within the chassis (the transformers and diodes are right next to the boards) which explains why I get noise regardless of where I connect the chassis to the circuit ground.

    My transformers are unshielded, with no screen. (Talema 62062)
    permalink
    Posted 3rd February 2016 at 01:25 AM by rjm rjm is offline
  3. Old Comment
    abraxalito's Avatar
    Having unshielded trafos does rather complicate the matter of getting the optimum grounding. Are you using a pair of full-wave rectified windings or is there a CT?

    Looking at your FFT I rather do suspect that the current loops when the diodes conduct are coupling magnetically to other circuit loops. As the harmonics don't go down very fast with frequency - tends to be indicative of haversine-shaped pulses getting in. Reducing the first res capacitor size and then having a series L normally works to increase the conduction angle and hopefully that would attenuate the higher harmonics somewhat.

    <edit> Nice pic which answers my question, thanks! I'd turn the top trafo through 60degrees (clockwise) so the lead-outs aren't facing towards your input wiring. Toroids have lowish leakage flux but its always highest where the wires come out, as that's a point where symmetry breaks down.
    permalink
    Posted 3rd February 2016 at 02:09 AM by abraxalito abraxalito is offline
    Updated 3rd February 2016 at 02:38 AM by abraxalito
  4. Old Comment
    rjm's Avatar
    One bridge rectifier for each of the dual secondary winding, so four in total. I've added a photo of the amp so you can see the details.

    I always try to keep the conductance angle under control by using moderate amounts of filter capacitance following the rectifiers. 2000 uF per rail in this instance, it sims out to about 10:1, or a quite modest 500 mA peak for 50 mA load.

    Magnetic coupling. I hadn't thought of that until you mentioned it, but, yes, it seems quite possible that the noise coupling mechanism is magnetic rather than electrostatic. That would explain why it is insensitive to the input wires and volume position. I'll try that trick rotating the transformer, but likely we'll have to rig up some flux containment. Pity there are no scraps of mu-metal about the place.
    permalink
    Posted 3rd February 2016 at 02:35 AM by rjm rjm is offline
    Updated 3rd February 2016 at 06:14 AM by rjm
  5. Old Comment
    For a tiny bit of help, rotate the bridges 45 degrees to get the secondary's AC parallel to the PWBs, and a just a bit further away from the PWBs.
    permalink
    Posted 5th February 2016 at 04:19 PM by disfunctionalshadow disfunctionalshadow is offline
  6. Old Comment
    abraxalito's Avatar
    I love the copper cummerbunds. Looking at your eye candy made me wonder something - your bridges are really looking quite 'heavy duty' for the application - have you considered snubbering them? Or swapping them for (discrete) soft recovery diodes?
    permalink
    Posted 8th February 2016 at 01:04 AM by abraxalito abraxalito is offline
  7. Old Comment
    rjm's Avatar
    I did consider snubbers, but caps in various places were to no avail. I realized later it isn't actually switching noise that is the problem here but instead The culprit is the 500 mA, 1 ms current pulses flowing in the secondary windings seen twice a cycle charging the filter capacitors. The diodes are just doing their job.
    permalink
    Posted 8th February 2016 at 09:35 AM by rjm rjm is offline
    Updated 8th February 2016 at 09:37 AM by rjm
 

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