Gainclone PD Kit, slit hiss. Grounding issue?

farqwad

Member
2015-11-24 7:22 am
Hi All

Yep. Another gainclone grounding discussion. Or so I think. Just finished building my first gainclone. Peter Daniel's kit. Sounds astonishingly good. Taking inspiration from the example build, and Peter's beautiful Patek amp, this is what I've come up with. Apologies for the not-so-great soldering, and the long post.

Chip boards, and trafo+power supply board are all affixed to separate aluminum 200x200x4mm plates in their own wooden chassis. Aluminum custom legs made of sign post risers filled with epoxy, and two separate sets of feet I had lying around. Those thread against the aluminum plates. Wood to finish.

I thought this was a good idea for convenience of connecting the two boxes. The umbilical between power supply and amp boxes is a 6-pin PCI-E (minifit junior) connector I took out of a computer parts box and repinned. One wire each for V+, V- PG+, PG- and two for earth ground, which is of course connected to the aluminum plate, the aluminum legs, the power supply board, and AC ground. This AC ground is connected through the eyelet and screw you can see on one side of the the power-supply board, on the back of which is a brass standoff threaded into the aluminum plate.

In the amp box, V+- (the yellow wires) each get split and run to each amp board. Three of the other 6 wires (PG+, PG- and an Earth ground) are connected to the common ground wire (black solid wire running straight between the two amp boards) just as Peter suggests. The other ground is mechanically pinned to the aluminum heatsink plate and legs. Note: I plan to clean up that messy ground wire connection in the middle. I just had to see if it even worked!

The amp plays great! But, after shorting the inputs I still get a little hiss. One can only hear it within 10cm of my ~86 db sensitive speakers (CHR-70's) which is fine. But the perfectionist in me would love that to be gone.

To note, my toroid is actually kinda loud with mechanical buzz. I dunno if that would cause this. Probably not.

Maybe the buzz is picked up between the close proximity of the V+- wires which by poor design actually rest on the solid ground wire (or maybe because of the parallel runs inside the PCI cable, or ? I dunno.

Anyone have any suggestions?

Thanks for the help.
 

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Couple of observations that may or may not help:

The way you have connected the main safety earth in the PSU box seems a bit odd - why not connect it directly to the chassis using one of the nuts in the corner or a dedicated nut into the base? I think the latter is the safest way as strictly speaking I think safety ground should have its own, unshared and direct connection to chassis.

You mention the mess of the shared ground wire between the two channels - perhaps sorting this will fix the hiss. I notice that you differ slightly from Audiosector's approach in that Peter connects the speaker return to the PG on each board and not at what he calls the star ground point which is where your connections are made - will this make a difference? I can't see why (as it is essentially the same point) but you never know, and I guess he does things for a reason.

My traffo also makes a periodical mechanical hum that rises and falls every ten seconds or so - it does not produce hum or hiss at the speakers though (I have the same kit as you) so I'm guessing it aint that.

Perhaps the umbilical is the issue? This is a shot in the dark but given that the cases sit so close might they as well just be made into one box to neaten up the wiring? My thoughts were that in an amp so simple I should try and keep everything else to a minimum and I certainly found that in the test rig I built, which had messy and longish wires, the amp did produce hum and noise...this has gone now I've boxed it all up neat and tidy.

Anyway, a few things to try before you start adding caps and zobels etc..good luck :)
 
Sort the Safety Earthing.
You, the Builder, MUST ensure that any and every operator can safely use this equipment even while it is going faulty !
Don't kill the wife/kids.


That looks like it is copying the design philosophy of P.Daniel (Audio Sector).

I don't believe that separating the smoothing and the charging route from the transformer/rectifier group is good for audio.

I recommend that all the main smoothing is put beside the transformer.
Tauro agrees
I would add some filters at the rectifier side before bring it to the amplifier.
 
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farqwad

Member
2015-11-24 7:22 am
Thanks Tauro and Andrew. I will consider adding some filtering caps to the PS board.

AndrewT - What do you mean by sorting the safety earthing? Both boxes use mechanical mounting of the Earth ground directly to the chassis. So I am not sure what your specific concern is. Mind providing a little more detail?

Thanks
 
Hi Farq,

In UK safety earth needs to have its own connection (yours is dual purpose in as much as it also secures the PSU board to the chassis) directly to chassis as close to the power entry point as possible. The wire needs to be the correct colour (ie green and yellow), it needs to be secured with an appropriate nut, screw and star washer and the wire needs to be of such a length that it is impossible to disconnect it by pulling on the cable entry before the live and neutral wire connections are broken (I guess imagine a kid tugs so hard on the power cable that the IEC socket pops off - what happens if safety earth disconnects whilst mains contacts the chassis?). Perhaps these situations are unlikely and maybe you are the only person who plays with it but hey..better safe than sorry and maybe sued?

Regarding the hiss - Andrew and Tauro post valid solutions, however; there are hundreds of audiosector kits implemented without these additional components and I guess that is why people buy them..try sorting the wiring first if you'd like to keep it as close to PD's design..if that isn't a concern by all means add smoothing caps at PSU. To me it would seem sensible to untangle the wiring before resorting to this approach because your issue may not be due to the amount of smoothing you have (and certainly this design can be hiss free without those additional components).

Good luck :)
 
I think tonga is correct and that it is probably just a matter of getting a few details correct.

Have a look at Daniel's builds on the Audiosector site. Daniel places the amp boards close together so that joining ground wire can be very short. Also connect the speaker grounds at the top of the amp boards and not where they are now.