Single Supply Audio Amplifier

I am currently designing an audio amplifier using the LM324. I am operating from a single supply of 9 V. Here is the schematic:

Here are the design steps I took:

1. I needed a gain of 100 so G = 1+(100/1) = 101.
2. The input capacitor C1 forms a high pass filter with R1||R2. The lowest frequency is 20 Hz so I chose f=15 Hz and C = $\frac{1}{2\pi\cdot 5000\cdot15}$ = 2.12 µF so I chose C1 = 4.7 µF as the closest value.
3. The non-inverting input is biased to VCC / 2 to allow maximum output swing.
4. C2 is just a normal decoupling capacitor.

After assembling it I am unable to get a decent output from it. Can anyone point out what could be the problem?

• Looks all good to me. What are you connecting at the output? – Nils Pipenbrinck Jul 30 '16 at 7:25
• What constitutes a "decent output", nobody here can read minds. Don't expect it to drive an 8 ohm speaker. – Andy aka Jul 30 '16 at 7:39
• This is where you want access to an oscilloscope, it's hard to debug otherwise. Nils and Andy has good points, I like to add that it would be interesting to know how you have assembled this. You may for example need bypass capacitors for the LM324 power supply. – pipe Jul 30 '16 at 7:58
• I'd rather power electrete mike (R3) with a cleener supply. A simple RC lowpass would do. PSSR thru that path is going to be very low. The same applies to R1/R2 bias network. Finally you need a capacitor in series with R4, otherwise op-amp is going to amplify DC bias as well and saturate. It would actually sound strange if circuit worked as is – carloc Jul 30 '16 at 8:07
• Use a power amplifier and not an opamp. – Andy aka Jul 30 '16 at 9:04

Your DC bias is wrong. R4 should be returned to virtual ground, not actual ground. As it is, the op-amp is doing its best to amplify the 4.5 V to 450 V.

You could fix the circuit by replacing R4 with a 2k to ground and a 2k to Vcc.

A good first step in debugging this kind of circuit is to measure the DC voltages with no signal applied.

Edit: The capacitor in series with R4 is a better solution because you don't have to worry about DC offsets or resistor tolerance.

• Hah. Amazing that everyone missed this. – pipe Jul 30 '16 at 8:13
• By virtual ground, you mean Vcc/2 around which my signal is centered? – hacker804 Jul 30 '16 at 8:32
• Or put a 100 uF capacitor in series with the 1 kohm (R4). – Andy aka Jul 30 '16 at 9:05
• Andy has it (though 10uF will do). As it's an LM324, see also electronics.stackexchange.com/questions/241561/… – Brian Drummond Jul 30 '16 at 11:00
• The bias voltage on some condensor mic's is as low as 1.5vdc. If this is the case here then this mic is saturated ON. – Sparky256 Jul 31 '16 at 2:24