hello, built and working, thank you
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I have lately been planning building some more hi-fi stuff. Basic DC power supplies aren’t good for dual supplying opamps so I’m trying with AC supply this time.
Layout is mostly based on Douglas Self’s schem, took just filtering from sound-au schem.
If this becomes a success I might go for LM317 and LM337 for even better results.
Layout should definitely be marked “unverified” (in the image itself).
I recommend a Schottky diode such as 1N5817 from the +12 V rail to ground. In “Figure 2” that would be D7 but use a Schottky, not a rectifier. Discussion:
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Yes Schottky diodes will be a better choice for that task. Updated the layout.
Overcurrent shouldn’t become a problem, there won’t be more big caps, just 100n bypass caps for opamps. And the opamps will draw max 300 mA 
The negative regulator doesn’t have the same startup problem as the positive, so doesn’t need a Schottky to ground. My understanding is It does need a diode to ground if there’s a large capacitance between the ground pin and ground, but that this isn’t the case here, so your D6 isn’t necessary. (But can’t hurt either.)
D5 and D6 in “Figure 2” are a good idea. They protect the regulators against damage from a downstream capacitance trying to discharge through them.
In a modular synth (if that’s what this is for), typically there will be ~10 µF bypass caps at the modules’ power headers. But that should not pose a problem. Current draw will depend on the number of modules as well as the particular design of each, but 300 mA will probably power a half dozen to a dozen or so modules.
Updated layout; 2200u caps are bigger than I expected. Also scaled resistors down to 4,7R just to make sure enough current can supplied. Added the regulator protection diodes as well. Also got some nice geometry going.
HAGIWO’s generative sequencer. Will build this week.
Confused about the LEDs, don’t they blink at the same time? Why have 2?
Edit - OK, one varies in brightness with cv range and one flashes with gate. That makes sense.
Taken from this schematic.
They are linked to 2 different outputs of the arduino so they wont blink “together”.
Yeah, I thought they would both just blink each time a new note was triggered making having a note and gate LED redundant, but the note led varies in brightness rather than blinking.
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I’m planning to create a basic toolbox for prototyping and testing modules.
One thing I had in mind was some kind of mixer+amp with a build-in speaker and headphones out. The idea was to drive a 4Ohm/5W speaker with a LM386N and the phones with NE5535, which are anyway lying around. I mixed together schematics parts of the Befaco outv3 (headphones part), the bass boost amp example from the LM386 datasheet (speaker part) and the basic kosmo mixer (inputs). It’s not meant to be used in production, just for developing other modules.
Not yet stripboard, but unverified schematics.
What do you think? Do I get away with that? Any parts I could leave out? I’m not an engineer, but rather trial and error myself into building stuff.
I don’t think you need 4 inputs for that use.
Or, in fact, yes, at least two, but with switches to switch instantly from the input to the output of the module under development.
The module may have more than one input and output, so yeah, maybe four inputs on the mixer… but with a rotary switch…
I’d leave the input attenuators for each input and put the switch at the common node between them and the opamps.
Or maybe the input to the left earphone “pad”, the output to the right…
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VR3 is very large compared to R9 and R10 (let alone the two in parallel). Normally the resistor on a voltage divider’s output should be at least about as large as the voltage divider itself, ideally larger, otherwise it seriously distorts the response curve. Besides, as you vary VR3, the cutoff of the C7/VR3+R9+R10 filter will change by about a factor of 10, though in any case it’s probably low enough not to matter much (it’ll remove DC without much affecting audio). I’d think about making VR3 something like 5k, equal to R9||R10. You could instead make R9–R12 200k but keeping them low reduces your noise floor. Or you could both lower VR3 and raise R9–R12 by lesser amounts. You can increase C7 to compensate but, as I said, even if you don’t the filter cutoff is probably not too high.
The 75Ω output resistors probably should be 1/2 watt (so give them more space than usual).
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Thanks for your explanations.
The original Befaco schematics put a 100k pot in front of a single 10k resistor. I derived these parts from the original without taking parallel R9||R10 into account. Thanks for pointing this out. I’ll go for a lower value for VR3 when testing on breadboard.
That’s a good point, making comparison much easier. What do you think about balance control to shift between two inputs?
Hm, I’m surprised. This is the behavior you’d get with the Befaco; with the pot turned halfway you’d be getting only 10% of full amplitude out, and you need to turn it about 90% of the way up to get half the full amplitude.
I suppose, though, that could be deliberate. It’s something like audio pot behavior with a linear pot. Yeah, maybe that’s it.
While I’m at it, 100k attenuator feeding two 10k resistors looks like this:
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If you put a big knob on it… 
I would be very careful doing anything “experimental” for prototyping that is used with headphones, especially using amplifiers that are not meant for headphones. A tiny mistake and you can permanently damage your hearing. I would suggest using old powered computer speakers to monitor circuits during the prototyping process.
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@K.ostas Very wise advice!
@Jos You can add me to that list of people who think about electronics in terms of schematics.
You’re on my list now …
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