audio amplifier circuit design (how to choose LOW Noise op-amp) - Page 1
Forum section: electronics / projects

author: menushadesanayaka255
author_rank: Newbie
timestamp: January 08, 2014, 11:02:22 am
content: Helloi am studenti 'm designing it using STA540 ic and i need to control the volume.. so i was searching for op-amp.i want a low noise one and it should be famous (easy to buy from any place).Also if somebody can say what are the important things take into consideration when selecting op-amp for audio application?thanks alot for any help.

author: VK5RC
author_rank: Unknown
timestamp: January 08, 2014, 11:30:55 am
content: I have very little knowledge myself but I believe Douglas Self and Peter Bandaxall are worth a read. In Elektor April 2012 an ultra-low noise audio pre-amp design was published using multiple NE5532s, paralleled to (in my simplistic terms) 'average out' the noise ( a design and article by Self). He deliberately uses a low impedance design to reduce noise, but it added some problems in getting double ganged 1K pots for the volume and tone controls though (I recall they were $20each).Hope this is of some help.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 08, 2014, 11:40:25 am
content: NE5532, if not available, go somewhere else, if nowhere else is available, try for OPA2134, NJM2068There may not be perfect but will do the job just fine and have done so for years. Pick the one that is available, comes in the package you want, has the power supply requirements that match your widget ect.Things important; INTENDED APPLICATION - people who make the op-amp will probably know what they're good at, if it says "audio", it will probably be good at audio, Low noise, Low THD, Unity Gain Stable (you're controlling volume, so you want it to work well at low volumes too, right?), other things are more situationally dependant, if you're driving low impedance loads, output current is important, if you don't want to implement decent power supply decoupling and input filtering, CMRR and PSRR is important, if you're driving whatever directly, DC offset maybe important, but you're probably AC coupling so it doesn't matter.

author: Rerouter
author_rank: Super Contributor
timestamp: January 08, 2014, 12:20:52 pm
content: i'll second that CMRR and PSRR are very important if the gain is high (which it normally would be with volume control)

author: menushadesanayaka255
author_rank: Newbie
timestamp: January 08, 2014, 12:34:26 pm
content: Thanks for the quick reply...from the 2 audio branded op-amps (OPA2134 and MC33078) both are very similar.i am thinking OPA2134 because i can get it easily from eBay and it also cheap.i was just wondering whether; it's important for the op-amp to be high speed for audio application?

author: Rerouter
author_rank: Super Contributor
timestamp: January 08, 2014, 12:37:10 pm
content: generally not being the highest harmonics you would be dealing with would be only a few hundred kilohertz,

author: menushadesanayaka255
author_rank: Newbie
timestamp: January 08, 2014, 12:46:05 pm
content: thanks for all the help.....

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: January 08, 2014, 01:45:26 pm
content: With modern devices and approaches, noise in power amplifiers and line-level preamplifiers have more to do with resistor value selection than the silicon.What is your source? A standard CD player should give 2V RMS for 0dBFSHow much input do you need to apply to the chip? The chip has 20dB gain in single-ended mode, and 26dB in bridged mode, so knowing the output voltage of your source, and the clipping point of the IC, you'll be able to work out the required gain. Include a bit more for quieter tracks and/or source equipment.Newsflash: In high quality audio terms, the STA540 is not a great performer. It'll sound absolutely fine, I'm sure, but it's pointless to worry about using high-end op-amps for a preamp. Even a 741 could be made to work better than the power amp itself! The LM833 is a good cheap choice - the NE5532 is excellent, but overkill. The OPA2134 is also overkill, frankly. The dominant noise sources are the resistors built into the IC for gain setting, and the dominant source of distortion will be the STA540. Well actually, your implementation might be the dominant source - screw up the earth layout and you'll easily increase distortion by 10 or 100 times.Volume controls are usually implemented as potential dividers - if you use them to alter the gain of an op-amp, you'll have problems with DC on the track-wiper interface, causing noisy operation after a short period of use. The usual way is to place the volume control at the input, and follow it with an op-amp, then feed that into a power amplifier. I'll attach an example. If you use a different IC, you might want to proportionally increase the value of the gain-setting resistors, as most op-amps exhibit an increase in THD when driving low impedance loads. Go no higher than 10k, and you'll be fine for noise.The op-amp bandwidth is at unity gain. Build a circuit with a gain of 10, and the bandwidth will be a tenth. And so on. If making microphone preamps (with a gain of 60dB), that becomes significant.Wide bandwidth means more NFB is available at higher audio frequencies, meaning more effective cancellation of distortion.

author: GamerAndds
author_rank: Regular Contributor
timestamp: January 08, 2014, 03:21:39 pm
content: Mark H is right, you should have just hand build a little 741 op amp circuit. Personally I always hand build my "front end " (the op amp) using transistors (set as differential pairs) instead of ic's, that's what I have in "The Beast", my 1000watt per channel (2channel) amp.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 08, 2014, 03:47:25 pm
content: actually.... what are you driving the ST540 with... if it is with say... a PC soundcard, into 2x bridged loads, you could get away with NO opamp...

author: menushadesanayaka255
author_rank: Newbie
timestamp: January 09, 2014, 03:15:04 am
content: I'm currently planning to use it for a laptop and Tv audio out....yes, i agree it's an overkill, i went through about 15 op-amp data sheets and tried to compare them but different vendors datasheet are different, an even the same vendor datasheets are different; as i lack in experience in electronics, no idea on audio stuff, i went with lowest price and audio branded op-amp.After all you information i found 10xNE5532 for $3... so i thought of getting them...... because as you mention it's can produces good quality, it might be overkill but very cheap when compared with OPA2134.About hand building 741 op-amp... i just didn't know which transistors to choose (difficult to compare transistor good for audio stuff because i don't know which parameter to take into consideration) so i thought of buying pre- built op-amp.#quality of audio need to be good but not professional.... since it is use as laptop and TV and their audio will not very good.#i will be using 2 x 8W 4ohm High quality speakers for output left and right and sub-woofer for bass, input for sub-woofer will be stereo signal.Questions1) since it is not professional use, should i built a op-amp or buy a NE5532?2) (input is stereo signal for sub-woofer) should the op-amp and power-amplifier side have any special filters if it is connected to a woofer (that mean should i give low frequency stuff only for woofer) ?3) input signal for for sub woofer, should it be stereo or mono?ONCE AGAIN THANKS FOR ALL THE HELP!!!....... any of your thoughts and ideas to improve the design will be highly valued, ST540 is fixed because i have them.

author: Lightages
author_rank: Unknown
timestamp: January 09, 2014, 03:29:20 am
content: If you want to learn and spend time troubleshooting instead of getting a working circuit as fast as you can, then build a discrete op amp. If you want a working circuit fast then use the 5532. You won't be disappointed as it is a very good opamp yet to be bettered enough by more modern designs for most people to measure, never mind hear.