So - a lot of you guys know your stuff, far better than someone like me, but I built a circuit today, I plan to breadboard it on my next day off - I figured I would post the schematic to see what everyone thinks.
It is pretty much the first circuit I have really built upon (since an LPG is a very basic circuit, i used that as the starting ground)
The LM386 is a low power amplifier fit to drive a loudspeaker. For this circuit you do not need such a ‘power house’. A simple op-amp should suffice.
have any suggestions for an ic?
Finally someone that uses the LM386 as the output amplifier it is designed to be 
EDIT: Or not, see below 
Looks fine to me (I mean, it’s a rather minimalist design, so not much to complain about 
), so if you’re happy with how it performs on breadboard go ahead and build a more permanent version.
C3 is perhaps a bit low but to calculate the right values for a boucherot cell you need to know exactly what speaker you’re connecting it to, which seems like overkill 
(or is it a bit high, the datasheet says 50 nF?)
C4 is larger than I’d expect, the datasheet has 250 uF there and looking at the usual suspects (ruby, little gem, noisy cricket, etc) 47-220 uF seems common, but a big one is probably ok too.
A TL072 should be fine.
Oh, I thought the point was to drive a speaker. Are you telling me that it’s the wrong chip here too? 
EDIT: Oh, right, the tone controls are on the output side. Oh well, maybe I should just stay away from LM386 circuitry.
Oh, I’m afraid I did not think a low pass gate would be used to drive a loudspeaker. If that is what is intended, then a simple op-amp will not do the trick. So @d42kn355 … what is it supposed to be connected to?
not a loud speaker lol.
@Jos -
Like this with a TL072?
Should I connect pin 2 to pin 1 or leave it at ground?
@fredrik TY for the value tips - I will dig into it a bit more as well
I am new to all of this designing stuff haha.
Now you have a comparator, not an amplifier. You need a bit of feedback. Connect pin 1 to pin 2 (i.e. output to negative input) and you have a voltage follower. Drop the network on the output side; for the LM386 it’s there to deal with a 4-8 ohm speaker and you don’t have that anymore.
Also, if you’re using a TL072, you cannot leave the other half hanging. Either use it as an output buffer (i.e. after the filter bits) or connect output to negative input there too, and positive input to ground. That gives it something to do, which is good since it keeps it from messing with the one that’s trying to do things (see here for more).
the network on the output being the 2 caps, switch and tone pot, correct?
No, get rid of C3/R3 and C4. The others did something with the tone, I think (if you have the circuit on breadboard, just go ahead and test various resistor/capacitor/pot combinations and see what happens).
(C3/R3 is a boucherot cell, also called zobel filter, which attempts to compensate for what the speaker coils are up to, and the big capacitor is there to filter out the DC component from the LM386 output so you don’t fry the speaker, but with no LM386 and no speaker they’re kind of pointless )
Gotcha!! Thank you
I like learning what specific sections like the zobel filter are known as, it would be lovely to have an dictionary of well known circuit names
How about this?
Sorry, had to go to sleep, so didn’t have the chance to read your questions. But luckily all got resolved. Looks fine to me now.
Now that you’ve removed R3, C3 and C4, the output is connected directly to the output of the opamp so C1, C2 and RV1 won’t have much effect at all (they now essentially just form another load in parallel with whatever you plus in the OUT jack).
The Eurorack connector is reversed, pin 1 should be -12V
That’s why I asked him to experiment on the breadboard (once the opamps are somewhat reasonably wired up, the rest can be figured out by random mutations. Or I guess reading a lot of literature and doing a proper analysis, but where’s the adventure in that )
(not sure a 1000 uF cap provides that much filtering, btw. You’d need like 5 ohms to get a filter cutoff in the audible range (maybe an argument for adding back the LM386? 
))
I noticed the 12v +/- swap right after i posted the schem lol. Ty 
Okay, so now that I have the op-amp established - that tone curve pot you said would have little effect… how do i slap something coming from the output of the op-amp before the out jack to adjust tone?
If I just moved that section up before the output jack, would it do anything? lol
Or should I remove the switch, the pot and the caps there all together and just use 1 cap and down to ground?
