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The One Watt Open Hardware Guitar Amplifier.

TOPIC: Replace LM386 with LM358?

Replace LM386 with LM358? 7 months 3 weeks ago #1713

Hi folks!

I have been pretty excited about the 1Wamp and decided I would try and assemble it myself to learn more about electronics. I have everything except for the required LM386 chip. Instead, I've got a few LM358s. I just can't figure out how to reconfigure the power amp stage!

Here's what I have so far

It seems the main difference between the two op-amps is that the 386 has pins dedicated to gain, while the 358 does not (I suspect that is because it's dual and restricts each op-amp to only 4 pins). So I designed an inverting configuration that depends on the feedback between the output and inverted input. I use a voltage splitter to bias my input voltage up to half my supply because this is a single power supply setup. I left out the aux input and power supply sections here, but they will exist in my final build.

I think my problem is the coupling capacitors - why is the output still biased after the capacitor?

Before this project I knew very little about electronics and how electricity actually flows, or even how capacitors worked. I'm looking to understand more so I appreciate any advice.

Thanks!
Matt
Last Edit: 7 months 3 weeks ago by stuffmattdoes.
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Replace LM386 with LM358? 7 months 3 weeks ago #1716

  • Ray
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Hi, I was not aware of the www.falstad.com/circuit/ simulator, it looks pretty cool.
So basically, you cannot replace the 386 with a 358

Why?
The LM358 is an op-amp, its maximum output current is around 40mA, not bad but not enough to drive speakers.
The lm386 is an integrated audio amplifier (not an op-amp), its maximum output current is around 300mA (e2e.ti.com/support/audio/f/6/p/360550/1267040#)

If you cannot find the LM386, you can try to use higher output current op-amps (like the JRC4556 that gives 70mA) and place them in parallel.
I never tried that myself, but there are some headphone amps like the O2 that use op-amps as a power amplifier (nwavguy.blogspot.com/2011/08/o2-details.html) or the A47 (www.head-fi.org/threads/i-need-an-update...p-parts-list.544829/)
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Replace LM386 with LM358? 7 months 5 days ago #1745

  • synesthesia
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There's no way. Do not just consider similar part numbers. LM358 is a BJT based op-amp as Ray mentioned, with mirror current drivers so it can run on double power supplies (+Vcc and -Vee) while LM386 is only single supply. By the way chip architects is not the same (LM358 contains two opamps through pins 1(out) 2(-in) 3(+in) and for 5,6,7 as posterior) so basically you need to change PCB and routes. That's a nightmare, believe me.
Desperate electronics enginner | Passionate audio hobbyist.
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