# Amplifying an audio signal with an op amp

I'm trying to build a pretty basic color organ, but i ran into a problem and I can't seem to find the solution to my specific problem.

I built this circuit (using a 3.5mm jack connected to a phone or pc as audio input) and everything worked perfectly except for the fact that I had to crank up the volume to get the leds lit (far from being bright anyway), so I thought that I could try amplifying the audio input voltage to get a better result.

I got an op-amp and tried to build a differential amplifier with a gain of 2, but it's not working at all. Where have I gone wrong? Here is my design with the op-amp.

• You cannot reference a single-supply op amp to ground for AC signals. Mar 5, 2017 at 17:20
• Where or how have you supplied power to the opamp? Mar 5, 2017 at 18:33

Ugh! Sorry Joe but that is one plug ugly circuit and its wrong.

Layout basics: Circuits should normally read from left (inputs) to right, (outputs) supply top and bottom, components labelled with number and value.

The whole point of any circuit diagram is to make it easy to read. By labelling you make it easy to reference.

Electronics is made up from little building block circuits that feed into each other - so break down the problem into small steps.

(1) Convert the stereo signal (left and right channels) to a mono signal.

This is done by R1,C1 and R2,C2 which bring the two signals together at the inverting input of the op amp. The capacitors block any DC and also serve to limit the DC amplification of the op amp to 1. The inverting input is a virtual earth point so the signal can't go from one channel to the other.

(2) The op amp needs to do two things -

(a) Produce a DC offset to the output so that it sits at half the supply voltage. This is the job of R3 and R4 which form a potential divider.

(b) It also needs to amplify the signal coming in. The amount of amplification per channel is the ratio of R5 to R1 (or R2). The capacitor values are large enough so that we can ignore their impedance.

In this case I have chosen R5 = 10k and R2 = 2k2. That should give a value of 10/2.2 = 4.5 per channel, enough to amplify a small audio signal (say 0.5V) to a few volts. Too much or too little amplification? - change the ratio.

(3) Using AC to turn on a transistor.

If we just connected the output pin of the amp to the transistor the DC offset (about 4.5V) would be shorted out by the base-emitter and the transistor would be on all the time (not good). We first get rid of the standing DC voltage by using a capacitor (C3). We also limit the base current into Q1 using a resistor (R6). NPN transistors don't particularly like their bases being taken negative by AC signals so D1 is added to remove the negative part of the AC signal.

When the positive half of the audio is more than 0.6V Q1 will be turned on and current will flow through the LEDs and they light up.

(4) The LED output (driver).

With the values chosen (9V supply, 220R resistor) each LED will take about 32mA so 3 LEDs need about 100mA. The transistor (Q1) must be able to take this current (plus a bit more for a safety factor) so we select a suitable device capable of switching say 200mA - the 2N2222.

A tip 50 is far too meaty for this job and best saved for turning on a motor or something that requires a higher current.

Where to go next. Most light organs divide the signal into bass, mid and high frequencies. Then use these to drive separate banks of different coloured LEDs. These filters come between the output from the op amp (C3) and the driver circuit which needs to be repeated 3 times (one for each colour).

• Thank you very much! Just one thing: how does the op-amp work in this case? I don't get, for example, what's the purpose of the feedback input. Why does the input need to be connected to the output to be amplified? Mar 5, 2017 at 18:25
• It isn't the input must be connected to the output to amplify. It is that the output must be connected to the negative input to limit the amplification. An ideal opamp has infinite amplification. Negative feedback allows you to set the gain to (almost) anything you want between zero and infinity. Real opamps of course don't have infinite gain, but negative feedback is used to set the gain to something less than the maximum that the opamp can provide.
– JRE
Mar 5, 2017 at 21:06
• Without (negative) feedback (R5) the gain of the op amp would be so large that even a few uV at the input would cause the output to swing between +V and 0V. The (inverting) op amp circuit means the gain is controlled by the size of the external resistors and the output will be 180 degrees out of phase with the input. The reason we add a DC offset to the non-inverting input is because we are using a single supply (0, +V) and need the output to sit at half the supply so we can get both halves of the signal . In a dual supply (+V,0,-V) we would connect it to the 0V line). Google op amp circuits Mar 6, 2017 at 11:30

Your op amp selection is probably not suitable for this circuit. The audio input at the amp is never high enough to exceed the inverting input. That op amp would work better for you with +9V and -9V supplies, or a higher single supply like 15V. But with a single supply the Vin min would have to be higher than what you probably are getting from the audio input so you would need to offset it.