{"id":5342,"date":"2011-07-28T00:39:10","date_gmt":"2011-07-28T07:39:10","guid":{"rendered":"http:\/\/www.nearfuturelaboratory.com\/?p=5342"},"modified":"2017-08-18T17:58:48","modified_gmt":"2017-08-18T17:58:48","slug":"lm358-op-amp-issues","status":"publish","type":"post","link":"https:\/\/blog.nearfuturelaboratory.com\/2011\/07\/28\/lm358-op-amp-issues\/","title":{"rendered":"LM358 Op-Amp Issues"},"content":{"rendered":"
\n\"Audio<\/a><\/p>\n
\nShould be simple…here’s the test circuit. It’s derived from the multi-channel mixer below. I’m just testing the circuit. It should basically pass through the one input signal perhaps with some attenuation. Instead, it distorts it all to hell-and-gone. Any ideas?\n<\/div>\n<\/div>\n

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So \u2014\u00a0back to hammering and less yammering. I’m trying to build a small, portable, DIY audio mixer. Thus far \u2014 I’ve found nothing off-the-shelf that meets my, you know..specifications. There are lots of inexpensive and rather expensive field mixers but those are honking things with gigantic XLR inputs for proper microphones. I want about two 1\/8″ stereo inputs, and maybe two or three 1\/8″ mono inputs. I want to mix a few things and I want the whole thing about the size of deck of cards or something small like that. The only thing I can think to do is to build it.<\/p>\n

Plus..we like to build things.<\/p>\n

It should be simple but I’m about a decade out of analog electronics. I used to be able to debug op-amp circuits but not any more. And back when I could they didn’t have the internet the way they have now, so I’m sorta putting it out there with as much evidence of the problem as I can muster.<\/p>\n

I built the circuit you see above using an LM358 op-amp. Seems like a reasonable go-to unit to build around and many op-amps, as I recall, are generic enough that I feel like it should work in this circuit. (But, I’m prepared to be wrong.)<\/p>\n

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The problem is that the signal gets super distorted right from the get go.<\/p>\n

I’m using my MBP to generate a signal using Max\/MSP. It’s just a 1kHz pure tone \u2014\u00a0a sine wave. The output comes out of the computer, right? Max\/MSP generates the signal to go out on one channel and that’s what I feed into my circuit.<\/p>\n

Basically \u2014 it gets distorted, even at the input. Check out the signal at pin 2 (the + on the left-most op-amp) \u2014\u00a0it’s what comes after a resistor and capacitor from the input signal. Why’s it so clipped even before it gets processed except for a couple of discrete components? I think that’s the heart of the issue cause the signal doesn’t even touch the op-amp?<\/p>\n

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\nSo \u2014\u00a0obviously screen shots from my scope. The top cyan trace is the audio signal I injected into the circuit. It’s a 1kHz sine wave generated from Max\/MSP and output over my MacBook’s audio output. It’s about 280mV P-P.<\/p>\n

Pin 2 – Basically the input signal after a resistor and capacitor. It’s like..totally clipped there. Why it’s clipped between the raw signal and the input on pin 2 is baffling me. I’ve been so far from analog circuits for so long..I’m just not even sure why putting it on pin 2 alone clips it this way.\n<\/p><\/div>\n<\/div>\n

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\n\"\"<\/a>\n<\/div>\n
\nThe output from the first Op-Amp in the mixer circuit..looks kinda hinky..worse than the input on pin 2.\n<\/div>\n

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Things only get worse from there. The output of the first op-amp looks like worse crap. This is basically the op-amp processing one signal and mucking it all up.<\/p>\n

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\nNow we’re at pin 6 which is the input to the second op-amp which comes after pin 6 with a 10k resistor in between.\n<\/div>\n<\/div>\n

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\nPin 7 is the final output. This should be a clean signal basically like the input for this test.\n<\/div>\n<\/div>\n

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\nFinal-final output after a couple of resistors and a capacitor. Distorted crap.\n<\/div>\n<\/div>\n

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Why do I blog this?<\/strong> Well..I can’t figure out what might be wrong and because the circuit is so simple, I figure the problem is something as simple as the circuit. But, I may be a bit out of my depth and I’m baffled especially because the signal as measured at pin 2 \u2014\u00a0effectively the input \u2014 is distorted already.<\/p>\n

Any ideas?<\/p>\n

<\/p>\n","protected":false},"excerpt":{"rendered":"

Should be simple…here’s the test circuit. It’s derived from the multi-channel mixer below. I’m just testing the circuit. It should basically pass through the one input signal perhaps with some attenuation. Instead, it distorts it all to hell-and-gone. Any ideas? So \u2014\u00a0back to hammering and less yammering. I’m trying to build a small, portable, DIY … Continue reading LM358 Op-Amp Issues<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[23,24,78],"tags":[1181,323,470,492],"yoast_head":"\nLM358 Op-Amp Issues - Near Future Laboratory<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blog.nearfuturelaboratory.com\/2011\/07\/28\/lm358-op-amp-issues\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"LM358 Op-Amp Issues - Near Future Laboratory\" \/>\n<meta property=\"og:description\" content=\"Should be simple…here’s the test circuit. It’s derived from the multi-channel mixer below. I’m just testing the circuit. It should basically pass through the one input signal perhaps with some attenuation. Instead, it distorts it all to hell-and-gone. Any ideas? So \u2014\u00a0back to hammering and less yammering. 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