# Reducing noise in an audio amplifer using opamp

I am trying to make an audio amplfier using an opamp for a project. It works kind of okay but the only issue is there seems to be a lot of noise in the output of a 0.5 watt speaker. I have tried changing the various capacitor values but there still seems to be noise. Whts the best way to bring down this noise in the circuit? My circuit is identical to the one http://www.elexp.com/t_audio.htm. Could someone please help me out?

The input is from my laptop jack output . Could this cause any ground issues which cause noise?

• Are you driving the speaker directly with the op amp? Which op amp are you using? What component values? Draw the schematic with the Circuit Lab plug in. – Matt Young May 15 '13 at 17:39
• Yes. The output of the opamp is connected to the speaker which is then connected to ground. I am using LM741. The schematic is in the link I have posted above. Have edited the question to include it – Developer Android May 15 '13 at 17:41
• What are the values of R1 and R2? – Andy aka May 15 '13 at 17:48
• I have tried various combinations to adjust gain starting from 100k and 10k to 100k and 1k and even i00k and 33ohms – Developer Android May 15 '13 at 17:49
• My bet is what you're hearing is actually distortion. Your R1 and R2 values are all going to give you huge gains. If you're driving the amplifier with a laptop audio jack, I guarantee you're clipping it hard with even 100k and 10k$\Omega$. All you say about the speaker is it's a half Watt speaker, so it's probably low impedance, and that 741 doesn't have a prayer in driving it cleanly anyway. – Matt Young May 15 '13 at 17:51

With the exception of power op-amps, op-amps are not suitable for driving loudspeakers. The impedance of speakers is too low. You need a power amplifier chip for driving speakers, or to add a class B emitter-follower output stage made using two complementary power transistors.

Some op-amps are much more noisy than others. They produce noise and it cannot be eliminated; it is inherent to the op-amp. If you're doing audio work, and the circuitry doesn't have to be very low power, use an audiophile op-amp IC like LM4562 or the cheaper workhorse that is nearly as good: NE5532.

But neither of those will drive an 8 ohm speaker, or even a 32 ohm headphone speaker.

Also, do not expect to get anywhere near the 0V and 9V power rails with the signal swing. It's not realistic. Even if you get an op-amp that can do it, the speaker-driving output stage will not. I wouldn't plan on more than a +/- 2V displacement from the 4.5V middle bias, staying several volts clear of either power rail.

I sketched up the circuit below to illustrate the class B emitter follower output stage idea. The op-amp is basically used as a buffer with a modest voltage gain of 2 to interface the input to the output stage. You don't need a high gain because the available voltage swing is not so high. The output stage will only swing the voltage on the speaker to about +/- 2V before clipping. If the input signal is already +/- 1V, the gain headroom is only 2!

Note how everything is very similar to your original schematic, except that we have added the output stage, connected the feedback resistor R2 not from the op-amp output, but all the way from the output stage. This includes the output stage in the feedback loop, and so the feedback loop will cancel some of the distortion introduced by the output stage, such as its crossover distortion.

We still have serious problem here in that the output stage's input impedance is still too low for the op-amp! It's not nearly as bad as an 8 ohm speaker, but look at those 470 ohm resistors. Basically, they have to be considered in parallel, and so the input impedance is only about 230 ohms! Their values can be raised, but this cuts into our already limited voltage swing. We can fix that by using pairs of transistors in the output stage, to increase its current gain.

I can think of an op-amp off the top of my head which can handle this impedance as it is: the NJM4556. The op-amps on this chip can drive 150 ohm loads.

simulate this circuit – Schematic created using CircuitLab

• R5 shouldn't be necessary ;) – Bitrex May 15 '13 at 22:55
• Oops, you're right. We can open R5. – Kaz May 15 '13 at 23:17

You don't show the power rails. Clean power is key to clean analog circuits.

• Make sure you have enough decoupling capacitors between VCC and ground
• Use an analog LDO regulator to produce VCC

I faced a oversight problem which was causing huge noise — the feedback through the mic lowering gain using the desktop slider of the mic or muting it will stop this. Other is the RF using a RF bypass capacitor at the input of the amp say 330 like in this link http://www.markhennessy.co.uk/showpic.htm?preamp/main_audio.gif.

Bypass diodes in power supply with.o1mf capacitors.