Yet another Lm4780 pcb layout

I had to revisit my gainclone story as I am in need for 8 channel amp, and I lost my previous eagle pcb design. So, here it is, yet another boring LM4780 pcb layout. Feel free to make suggestions for improvements, preferably before I order boards tomorrow.
 

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Forgot to mention - they use SMD 1206 feedback resistors, for the shortest possible feedback path and to avoid power plane piercing around pins, unlike in most of other boards I've seen. I also included smd footprint for optional ceramic stability capacitor (220p) between +- inputs. All connections use MKDS terminal blocks for easy PCB dismantling.
 
I spoke a bit too soon. Amp is stable with 10k pot, but with 100k pot turned half way, or without any pot at the input, it oscillates in lower audible spectrum. I am using values and topology from parallel schematics in LM4780 datasheet (page 7):

http://www.ti.com/lit/ds/symlink/lm4780.pdf

I tried with values from audiosector site, but still the same issue. Resistors at the output are Vishay LVR-3 0.12ohm.

So the board works fine when connected to low impedance output of some kind, but oscillates when nothing is connected to the input.
 
Amp is stable with 10k pot, but with 100k pot turned half way, or without any pot at the input, it oscillates in lower audible spectrum..

Just by any chance, do you have access to a couple of regulated power supplies? Preferrably standard lab power supplies. Your need two to create the split voltage. Try using this to power your amp and see if it makes any improvement.
 
Just by any chance, do you have access to a couple of regulated power supplies? Preferrably standard lab power supplies. Your need two to create the split voltage. Try using this to power your amp and see if it makes any improvement.

Thanks. I could probably try to set up something with two 20V laptop switching power supplies. I will also try to replace output resistors with some other types. They are supposed to be low inductance, but they are wire wound metal strip ones after all. If I short them, then the amp oscillates even with 100k pot wiper at 0 position (input shorted).

These are conditions the amp will never be in, but still would like to get rid of this.
 

Junm

Member
2008-12-23 5:52 am
Dau
Yesterday i have also encountered HF oscillations with my LM4780 amp...I integrate it with my preamp tone control..., bu removing the tone control the amp has some kind of hiss and too much high tone..to be sure...i put the zobel 2.7ohm + 0.1uf at the speaker terminal then upon turn on with at least 30 secs the zobel heats up that means i have oscillation and also heatsinks heats up rapidly....i had put another 0.1 ceramic caps at power supply but no good (my filter caps are two 15,000uf + 2 3300uf at the board)..Relocate other ground from star but no good...last remedy I had put a 33pf accros 22k feedback resistor and everything goes fine.
I had tested for several hours at moderate volume and no more burnt like smell and sound was clear and no hum, very punchy. I like the sound of this compared to my 2 previous lm3886 gainclone build.