LM1875 Whooshing, Noise issue

[6Leinad7]

Fit the input filters and the Zobel.
They are on your sch.
The High Pass filter is C1 & R3 try 1uF and 47k
The Low Pass is missing. Add 1k to the input line. Add C of about 1nF in parallel to R3
The amplifier output Zobel is R7 & C5. Try 4r7 and 150nF
This is different from a speaker Zobel, which modifies the input impedance of the speaker.

Do not connect a speaker until after you complete your testing.
Leave the output open with just the Zobel attached.
Short the input with a zero ohms dummy plug.
Measure the output Hum+Noise using the 199.9mVac scale of your DMM.
measure the output offset using the 199.9mVdc scale of your DMM.

So I tried some what you suggested, and it helped tremendously. Specifically the HP filter- I already had a 1uF cap, but my resistor value was wrong. Correcting that resistor increased the gain by 2x, so I decreased the gain by reducing R6 to half of what it was. This alone made the noise become almost inaudible, only by putting my ear a couple inches away from my woofer in a quiet room could I hear it. I haven't fitted the Zobel because I don't have many capacitors on hand. I will order some and fix all the values that need fixing.

Secondly, I added a linear voltage regulator on the output of my switching PS. This also helped (I think) keep the noise at least more stable. It doesn't peak up randomly to slightly higher volumes as often now.

Measuring the dc output at the 200mV range, it seems to hover around 40-60mV consistently and jump up to 80-100 intermittently.

 

[AndrewT]

I am going to guess and suggest that the SMPS is injecting sufficient spikes/noise that it is causing the amplifier to become unstable.
Injecting very fast step signals into any amplifier that is not designed to handle signals of that speed makes the amplifier do unexpected results.
Check your low pass filter at the input.
Add more low pass filtering to the supply rails. Make sure the local supply rail decoupling is actually present and located where it should be.

 

[rsavas]

Secondly, I added a linear voltage regulator on the output of my switching PS.

Any inductance between the SMPS and lin reg? It may help too.

 

[6Leinad7]

I've got a .1uF cap right at the pins of the chip for the supply rail decoupling. I'll have to figure out the LP on the input, I don't have a ton of caps on hand.

I do have an inductor in between the + supply rail and the input, and a cap and resistor from that to ground, not sure how much it helps but I read up on reducing noise from SMPS' and implemented one of the techniques with inductance.