LM1875 Amp Distortion Problem

[PsychedelicFish]

I am getting a very mild, buzzy distortion of the bass end of the spectrum from my LM1875 chip amp. Many times I have checked the circuit and my wiring etc. over the past few days and have found no answers to my problem. I am using the circuit from here, Single Chip 25W Amplifier (Project 72), which doesn't seem to have any of the problems people have mentioned in other threads.

I am using a small piece of old, crappy veroboard, which I have checked multiple times for continuity, and everything seems alright. To be sure, I coated each of the tracks with solder, and rechecked the continuity, and nothing seems to have changed.

I am convinced it is the amp causing the distortion, although it is noticeably reduced when I connect it to two speakers in series, or use a transformer on the output to have a higher output impedance.

Thanks in advance for any help

[Redshift187]

Are you using a heatsink?

[PsychedelicFish]

yes, 1.1 c/w (Heatsink to suit SY4084/SY4086 - Jaycar Electronics), mounted with a silicone rubber insulator, no thermal paste (TO-220 Silicon Rubber Transistor Insulator - Pk.20 - Jaycar Electronics). I'm not using thermal paste, because the insulators supposedly do the same job. Even so, the chip is barely getting warm

[sofaspud]

Do you have an oscilloscope?
You checked for continuity, but did you check for discontinuity?
"Convinced" means it isn't the speaker itself?

[gfiandy]

Hi,

if the noise is present all the time (i.e it is there when no music is playing) it is almost certainly power supply noise.

If it is present only when music is playing then check the speaker on another amplifier first, if the speaker is OK you may have instability.

This can occur even if the circuit is wire up correctly. The most common causes are the decoupling capacitors are too far from the amplifier or if you have crossed the input and output signals over causing parasitic capacitive feedback. So look at your circuit topology to see if this is the case.

If you can borrow a scope this would identify this problem with a signal generator as you would be able to see the fuzz of instability on top of the signal wave form.

Regards,
Andrew

[Joseph1]

Your GND must connect to a metal case.Your GND is too small in the veroboard.

[PsychedelicFish]

I have the chip mounted on the heatsink, with the veroboard mounted on the chip. the Heatsink is then mounted on a large metal box which is connected to the mains earth and my circuit GND

[PsychedelicFish]

"Do you have an oscilloscope?"
No, because they're ridiculously expensive, and I don't really know anyone who would have one that I could borrow.

[PsychedelicFish]

And no, it certainly isn't power supply noise

[PsychedelicFish]

Ok, I think it is instability, because when I connected it to a preamp the other day, there was very loud oscillations and a lot of distortion. Any ideas why? My power supply filter caps are nowhere near the chip, could this be causing it? Anything else?