Hello there.. I had previously worked with the TDA2030A amplifier and made an 8 W sound-box which was nice. So I thought I should upgrade my plan and decided to make a 2 channel music system on my own. I have it all rigged up and mounted inside an old cassette player (with my own speakers) but it seems to have a problem. As soon as i turn it on, it starts to produce a near 3 Hz square wave kind of pounding in both the speakers. I have my overall design block diagram, The amplifier circuit schematic, and the datasheet attached for reference. The system is mono, but two channels for bass and mid-range. The spectrum is split by two second order op-amp butterworth filters, HPF and LPF both of cutoff around 150 Hz. Input is common to both filters and they are each followed by an amplifier then by respective speakers as in the image. The whole thing (both filters and both amplifiers) are parallel on a split supply of +12/-12 volts DC from two 12v lead acid batteries. So frequencies above 150 Hz goes to 10 W 8 ohm speaker (mid-range and high) , while below 150 Hz goes to 25 W 4 ohm speaker (bass and sub). Coming to the amplifiers, I have used the bipolar configuration as in the datasheet, except for the components marked in red below, Which I have not used. C8 is around 330 pF to give an upper bandwidth limit at 22 kHz. [Did I go wrong anywhere here, skipping the R5 which is supposed to be approx. = 3*R2] R1 for both the amplifiers is a pot to vary the closed loop gain. Now when I push the toggle switches of the supply, an intermittent 3-6 Hz square wave pounds both the speakers. Irrespective of the volume, input/no input. Plays nothing except this noise. The frequency varies with varying pot R1 value. I checked the amplifier IC's and heatsinks, they're fine and do not seem to heat up. I have four major questions.. Am I wrong in omitting any of those components marked? Are they crucial? What are their functions in fact? [R4, C7] Why are there two pairs of supply bypass capacitors? Isn't any one pair enough? [I felt C5,C6 larger caps would be enough] Anything else I have missed out in my overall setup? What could be the reason for the completely irrelevant output I am getting? I would be grateful for any help/insight/advice. Thank you.