LM3886 - Noisy signal unless inputs shorted

[Hogwild]

No, and yes. I used a pair of white wires. But I kept track of which was + and which was return.

 

[scottjoplin]

Well, if they are tightly twisted they will probably be ok, I've no idea why the coax you used was noisier, it shouldn't be

 

[Hogwild]

Hmmm....the shipping seems to be awfully slow. It looks like the POTs won't be here until tomorrow, or even Monday or Tuesday.

 

[flexus]

scottjoplin: thx a lot for explaining dc offset! i think i got the same problem here. after my case build i will try out the 10k pot on the input.

if it's ok i post my 3886 problem here.
atm i'm building my case. progress is good but i got shortage of space. the psu doesn't fit on the floor so i wanted to mount it "freeair" with 1mm steelband.
my question: is this ok regarding safety of operation? i'm afraid after stronger movement the psu moves around with this kind of mounting. hope you understand what i mean.

 

[scottjoplin]

Nice heatsink, it wants to be mounted on the outside of the case, won't that give you room for the power supply?

 

[flexus]

right, the heatsink is looking gorgeous, but i got 6 of these puppies (amps) and wanted to maintain a steady look because in the end they will sit next to each other. my collection of heatsinks all look different. "only" a visual appearance problem, but i will take a look, maybe i find more of those heatsinks secondhand.
..or i build a bridge with the steel band. some riveting on the outside won't look too bad.
i'm excited to fire them all up without dc offset problems *rubbing hands*

 

[Hogwild]

scottjoplin:

Got the POTS a couple of days ago. Wait, I'm confused, if I'm not putting it in a screened case (I don't even know what that is) you're saying I shouldn't bother with aluminum foil? Then what I should I do for next test? Just connect RCA outputs directly to POT and POT to amp boards? Does everything need to be soldered to reduce noise, or can wires just be twisted together with caps or ??

Big newbie here.

 

[scottjoplin]

Hi, a screened case just means one made out of metal usually, yes, wire the sockets to the pot and the pot to the amp, and solder all the connections. Mount the sockets and the potentiometer in the case so they are held securely in place

 

[Hogwild]

scottjoplin:

Thanks, will try to do that soon. No case yet. Still in testing stage.

 

[Hogwild]

I assume it will suffice to use cheap, really thin RCA cables? I don't have any high-end interconnects.

 

[scottjoplin]

Yes, they'll be easier to work with too

 

[arnd13]

I see no problem. You've only a few mV noise with shorted input, that's OK.

If you measure with open input the input acts like an antenna for all kind of noise,
thats not the right way to measure. IMO everything is fine.

 

[Hogwild]

Okay, well I've finally go time to solder. Any particular wire colours used traditionally for POT in/out/ground? I might as well make things as simple as I can.

 

[Hogwild]

I'm confused.

I've soldered:
CD player RCA Out--->POT Input
POT Output-->Amp signal Input

Do I solder the CD player output shield and POT ground to signal ground on the Amp board? The amp GND pads are small, so I don't have room to solder both wires together easily. Should I find a common point somewhere else and wire both to that?

 

[scottjoplin]

Yes, take the shield to the pot then from the pot to the amp (ground). Do you know how to wire the pot signal in and out?

 

[Hogwild]

By the below, you mean

Source output RCA shield --> POT ground
and
POT ground ---> Amp ground

POT output-->Amp input
Music Source RCA output to POT input

right?

Yes, take the shield to the pot then from the pot to the amp (ground). Do you know how to wire the pot signal in and out?

 

[scottjoplin]

Yes, hopefully this image will help clarity what I'm saying https://qph.ec.quoracdn.net/main-qimg-0034648e79a35431ff0cb9e6cdbbd063-c

 

[Hogwild]

Damn! Something else must've got screwed up. Since I attached the POT, no music plays, but I do hear a faint buzz if I turn up the POT.

Here are a couple of photos, albeit dark, showing what I've done. Any idea what is wrong?

 

[scottjoplin]

What terminal on the amp is the dark blue wire going to? The screen from the source should be going to "earth" on the pot together with the white wire it's connected to.
Ah, blue wire is going to "chassis ground" on board, disconnect that.

 

[Hogwild]

scottjoplin:

Thanks for sticking with me on this. Actually, I forgot to mention-the wire you mention is actually green, but the colour is off -it's a dark green. To make things worse, I took that photo at night and the camera made it look even more like blue. I soldered the green wire from chassis ground on the amp to ground on the POT (but not to any actual ground or earth). That okay?