Fixing this "Hibou" chipamp, anybody seen this PCB?

[pjanda1]

I took this troubled little chip amp (LM3875) in a partial trade. It sort of works, but it has typical chip amp problems: hum and noise. The grounding scheme appears to be one big loop, so that would explain the former, and maybe even the later. I'm going to try a couple of things, but I figured I'd see if anybody has seen these PCB's before. The gain is pretty high, but I'm assuming that those resistors under the chip pins might be feedback and gain setting. I'm certainly not going to pull the chips off, as I don't have much for desoldering tools.

FWIW, this amp originally came from Ebay, and certainly shows why buyers should be careful when getting someone else's DIY project. Grounding and noise problems aside, the rails are +/- 37V, so this one isn't going to work well with 4 ohm speakers! I may ultimately throw some regulators in there.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[infinia]

No I havent seen those
Your grounding scheme is wonky tho, look at optimum grounding thread here

Your star ground should be at a single lug the junction of the 2 biggest caps.
Bypass those big caps with some 10-25uF smaller caps using either good newer electrolytics or film caps. Especially if tying the speaker returns there.

[pjanda1]

That isn't my grounding scheme. I didn't build this thing! It seems like they attempted to construct two stars: one at the input and output jacks and one at the junction of the caps! That is the loop I see anyway. My plan was to separate the grounds at the jacks, and as you said, make the junction at the caps the main star. What I don't know is if I need any additional connections to all those unused terminals on the PCB's.

Those big caps are silly too, aren't they? They aren't as big electrically as they are physically: only 5600uf each. This thing will probably become a sub amp (not used above 60hz), so I don't know if bypassing them will be necessary. I do have some little, new-ish 'lytics I could toss in if need be.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[infinia]

Sorry I misunderstood, I thought you had determined the hum was due to PCBs not realizing the whole shootin match was Ebay'd LOL/

Those big caps are silly too, aren't they?

Something else to ponder
Could be be a hum problem if the total off connected caps is getting to be over 5-10% of the common bulk storage capacitance. You want the big 120Hz charging pulses away from the amps! You can add some series resistance or change the ratio of capacitance.

[hybridfiat]

Id question the need for even having 5600uf caps. 1800uf is ussually more than adequate.
If you have hum then you have earthing probs

[infinia]

Unless yer poppin rectifiers or dimmin lights there is no such thing as too much caps in audio PAs.