Cheap opamp no good ? - Page 1
Forum section: electronics / projects

author: Jan Audio
author_rank: Frequent Contributor
timestamp: April 28, 2019, 02:39:54 pm
content: Hi, how bad can a opamp be ?I,m buying some opamps and i see some cheap ones :Texas instruments : LM258 for €0,15 each per 100Diodes incorporated : AS358P-E1 for €0,13 each per 100I could buy some to fill my cart, only would it be bad to use for audio ?Is there a big difference between these opamps ?Why are these ones cheaper then others ?For what would you use this ?It seems the diodes incorporated has rail to rail output, only the datasheet dont find the word rail.I,m not buying, just curious, i have enough NE5532 already.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 28, 2019, 02:57:37 pm
content: There are quite different OPs for different tasks. It is not only about lower grades and higher grades, but also about different properties, like supply current, noise and voltage range. The LM258 can be a good choice for some tasks, but it is a poor choice for audio. I don't know the AS358, but it's likely very similar.The price depends on the process needed, the required parameter tests , possible needed trimming and the numbers sold. The LM358 and related are still about the most sold ones and at lowest cost end. They can be a good choice for something like slow tasks (e.g. voltage / current regulation, temperature control), but have quite some cross over distortion and are thus not really good for audio. One can work around it, but it's usually not worth it.Low cost audio suitable OPs are more like TL072 and RC4558 and similar.

author: Audioguru
author_rank: Super Contributor
timestamp: April 28, 2019, 09:04:20 pm
content: An LM158, LM258 and LM358 were invented for general purpose and use a very low power supply current. The low current causes pretty bad crossover distortion, poor high frequency slew rate and noise (hiss) which are horrible for audio.But hey, if you want cheap sound then use cheap opamps.

author: DaJMasta
author_rank: Super Contributor
timestamp: April 28, 2019, 09:33:11 pm
content: If you're looking for cheap audio opamps, look towards datasheets that specify THD and then look for lower noise figures. There can be some crossover distortion issues and stuff with some architectures, but generally if it's got a THD spec, it's going to be at least passable for basic audio stuff.On the cheaper end, I find LM833s to be a pretty high performer for the price. For a bit more there's something like an NE5532 which can sound really good, but which I think requires wider voltage rails. If you're trying to run from a 5V single supply or something, you've got a bet a bit more careful with your opamp choice or you will get very little range for the signal.

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2019, 12:32:29 pm
content: Quote from: Jan Audio on April 28, 2019, 02:39:54 pmThe only real difference, between the LM258 and AS358, according to the datasheets, is the AS358 is rated to 36V, where as the LM258 is only rated up to 32V. Neither op-amps are rail-to-rail. Their inputs need to be 1.5V, less than the positive rail to work properly and their outputs don't go to the positive rail either.https://www.diodes.com/assets/Datasheets/AS358-358A-358B.pdfhttp://www.ti.com/lit/ds/symlink/lm158-n.pdfThe TL072 is another good and cheap audio amplifier IC. It has J-FET inputs, which means lower bias currents and lower current noise, which makes it more suitable for higher source impedances, than the NE5532.Quote from: Audioguru on April 28, 2019, 09:04:20 pmThere are other applications for op-amps than audio. For example, the LM358 is quite useful in single supply, DC amplifier applications, since it works with its inputs at the op-amp's negative rail.