Electrical – Noisy BTL Audio Amplifier Circuit


I designed and breadboarded a simple audio amplifier circuit and when I play audio through the speaker, it is pretty noisy and very quiet.
The +5V is being supplied from a USB port.

I'm still probing and testing different spots, so I'll try to update with some scope shots and real measurements besides "it sounds quiet and noisy", but in the meantime, does anyone see any design flaws?

I'm thinking that I may not being supplying enough current and I've noticed that the inverted signal on the output of the second op amp seems to have a clipped voltage swing, about 1V less than the voltage swing on the output of the first op amp.

And as I've been typing, I'm wondering if I should tie the output of my push-pull outputs back to the op amps.

BTL push-pull output, had a typo on diode polarity

Best Answer

The first thing that struck me, starting looking from the left, that made no sense to me is your \$2.5\:\textrm{V}\$ reference. Why only \$10\:\textrm{pF}\$? Why \$1\:\textrm{M}\Omega\$ resistors?? It just doesn't make sense on its face.

Then I started looking at what the inputs of the opamps "see," looking backwards. And it's not even close to balanced, for either of them.

Then I saw the resistors and diodes as your "current source" for driving the output BJTs and knew that would be a problem. Then I wondered if the NE5532 could even hope to drive that thing, bad as it already was....

And then I saw the output diodes and nothing else mattered anymore.

Take a look at the polarity of the diodes tied across the resistor+emitter to collector of both \$Q_2\$ and \$Q_4\$.

Do you see any problem there?

EDIT: Okay. So that problem is really just a "typo" in your schematic.

Let's start at the beginning. You are going to all this trouble with a BTL topology and for all your trouble you intend on running this with only a \$5\:\textrm{V}\$ supply rail? Is that true? Or are you flexible there? What kind of output wattage do you expect to deliver into the \$4\:\Omega\$, \$3\:\textrm{W}\$ speaker? Explain your thoughts about your half-rail reference voltage design? (And yes, you will probably need feedback.)