Phantom Octave Troubleshooting

Started by MothGarden, October 17, 2023, 05:51:16 PM

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MothGarden

#20
Quote from: idy on October 19, 2023, 09:35:10 PMSorry Moth. I have spotted the difference before and should have known LM386 is power amp not opamp.

Also I was being stupid in not seeing why the LED was just an asymeteric clipper to ground, with audio going up and down from 0v. No mystery there.

Glad I didn't ruin your pedal or scare you away...

No problem at all. There is a lot of stuff to remember with all this pedal stuff, especially with not as common parts. I'm here to make mistakes. Would much rather do it on a pedal with around a dollar worth of parts vs something that has more expensive components. I will definitely take your advice on building a more "conventional" pedal. I'm excited to get a few projects under my belt so I can move on to messing around with some PT2933s. Love all the creative stuff people are doing with that chip.

EDIT: On the difference between power amps and op amps (in the case of pedal building), what is the main difference between them? What makes one more desirable than the other, maybe specifically in the case of this pedal. What is another situation where you would use this in a pedal?

idy

Opamps are basic building blocks; they can buffer (same voltage in and out, but can drive a lower impedance), amplify with easily controlled gain (and inverting or non inverting,) mix signals, make active filters including phase shifters and envelope filters, used in "precision rectifiers" that turn you audio into a DC control voltage..... They are everywhere. It is called "operational" because it can do analog computing; adding, subtracting, integrating and I forget the other.

The lm 386 is a power amp, its purpose is to amplify a signal until it can drive a little speaker. That is what you call a low impedance load, as low as 4ohms. That will draw a lot of current! The LM386 may produce up to 1 watt (probably not that much.)

An opamp, on the other hand, wants a load.... safe to say keep it above 1k ohms. So it should not be asked to put out loads of current.But that 1k load is pretty small in the audio signal world, so the opamp can drive the kind of loads we see inside circuits easily and match impedances. But they done't drive speakers directly!

The opamp must be biased when you are trying to amplify audio in a single side supply: you have to feed the non-inverting input a "mid-supply" voltage so that your signal will ride with crests and troughs within your supply. The LM386 has all that done for you inside. Just give it a signal and it comes out louder.

In your circuit it is being abused as a distortion engine. A lot of glitchy, out of control sounds come from abusing them. And they take even less parts than an opamp to do the trick. LM386 fuzz boxes are crude, noisy craziness.

MothGarden

Thanks for the extremely thorough explanation. So does that mean in most cases a power amplifier would be constructed with the building block of an opamp, plus a bunch of other stuff(I guess in this case the gain circuitry and bypass)? I always wonder if these circuits that abuse components like this were just stumbled upon by accident one day. I guess it pays off to experiment around and see what happens.

antonis

You can evaluate the cost/space of an audio power amp and an op-amp with a pair of power Darlingtons.. :icon_wink:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

idy

What Antonis said. You could take an opamp, add biasing, add a gain control (which you will want for the 386 too) and then use, as Antonis said, power transistors to give it... power. The data sheet for the 386 shows it has a "sziklai" pair, which is like a darlington (pair of transistors put together so there are only three leads and the gain is multiplied) but made of complementary transistors, one NPN, one PNP.

MothGarden

Quote from: idy on October 20, 2023, 11:19:04 AMWhat Antonis said. You could take an opamp, add biasing, add a gain control (which you will want for the 386 too) and then use, as Antonis said, power transistors to give it... power. The data sheet for the 386 shows it has a "sziklai" pair, which is like a darlington (pair of transistors put together so there are only three leads and the gain is multiplied) but made of complementary transistors, one NPN, one PNP.

Ohh this makes more sense. I was looking at the datasheet trying to figure out what was going on. There were so many nodes and transistors that it all started to become a blur! Someday soon I'm going to commit myself to practicing reading datasheets. Especially since they can get a lot more complex than this. Thanks for all the info everyone!

antonis

Quote from: MothGarden on October 20, 2023, 01:22:08 PMI was looking at the datasheet trying to figure out what was going on. There were so many nodes and transistors that it all started to become a blur!

This might help you..
(or not..) :icon_wink:

386 analysis
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

MothGarden

Quote from: antonis on October 20, 2023, 02:01:36 PMThis might help you..
(or not..) :icon_wink:

386 analysis

Couldn't have asked for a better resource about this chip