@mzero, in response to my posting in the minimal mixers thread you said it would be easy to convert the mixer i shared to take DC.
so…i’m curious if you or another knowledgeable individual would be able to explain the changes required to convert my mixer from taking AC to DC based on the pictures below.
ABOUT THIS THREAD
i created this thread as a place for folks to ask for help on diy projects that don’t already have their own dedicated thread.
It seems the mixer is running on a dual rail off +5V and -5V. I honesty think it’s much easier to find a AC adapter than to modify this mixer in this small case. It would even be easier to modify and old transformer based DC adapter to output AC.
Where I live there are lots of second hand stores that usually have a crate of old adapters. Anything outputting AC from 7V to 12V will probably work fine.
EDIT: sorry on closer look it’s seems to running on +15V and -15V so you’ll need an AC adapter above 15V
thank you for the advice. i have the required ac adapter already but it is relatively bulky and @mzero mentioned it wastes more electricity than a dc adapter would.
A possible mod could be getting the big caps and 7815 and 7915 out and replace it with something like this.
https://www.digikey.nl/nl/products/detail/recom-power/RKZ-0515D/2314577
It’s not drop in so you would have to bypass all the pcb connections before the 7815 an 7915.
And maybe you would need some caps after that. And you might end up with extra noise.
I feel like I can’t give you real guidance from a photo. But, here’s what I’d do if I had it in front of me:
There are four diodes just below the two big caps in the picture. I’d guess that these were the rectifier circuit. Some probing with a continuity tester or multimeter would confirm the connections as a diamond. And then, and oscilloscope would quickly confirm which two points in the diamond are the AC, and which two are the DC (and polarity of those).
I’d try to confirm that the only thing between the AC input jack and that rectifier diamond are those two big caps.
If all that were true - THEN I’d test what happens when I apply 9V DC to the AC inputs of the diode rectifier. It should all just work: The DC points on the rectifier should show the same voltage and polarity.
If that works - then I’d need to figure out how to rewire the jack: If there are really just those two caps between the AC and the rectifier - I’d just cut out the caps and bridge the points. Or even just solder wire across the cap legs, making it reversible.
The hardest part here is reverse engineering the schematic from the AC plug to the rectifier.
I don’t understand how you will get your negative voltage rail from a dc input?
hrm… i’m thinking that if the voltage spread between the - and + points of the output of the rectifier diamond is the same as it gets when using AC… then it doesn’t matter that the power adapter thinks it is (say) GND and +12V, and the device thinks they are -6V and +6V.
I’ll be the first to admit I could be talking out of my arse here… 
Missed this post, but I’ll throw in my bit since I’ve been playing with a small mixer module.
Many of these small mixers simply convert whatever AC supply into 9v and then create a virtual ground at 4.5v ( effectively creating a +/-4.5v supply. These mixers are simple to convert to DC supply.
Modern minimal mixers can do this due to modern low noise low voltage op amps such as the NE5532
However, what you have here has an lm7815 and lm7915 pair of voltage regulators to create a proper +/-15v split rail supply and will require a 15v or greater AC supply. Note, there’s nothing inherently wrong with this and it’s much closer to fully featured mixer design.
Curious about those regulators: the picture of the back off the unit in the original post studies the case explicitly says it takes 9VAC.
Ahh of course, that’s why we where expecting a 9vdc internal regulator. And I have the answer for that too, it will have a voltage doubler circuit on the AC input.
I was thrown off by the 4 diodes (typical full wave rectifier), but tracing the circuit out in circuitjs I see that it is two half wave rectifiers, more typical of voltage doubling. I suspect the simulation is doing me dirty as I would expect to see higher output voltages than the simulation is showing.
I have a pair of 20hp eurorack rails that I need to shorten by 3.5 mm. I have no experience working with metal. What is the cheapest way for me to do this, if it is feasible?
Yes, totally doable.
Use a hacksaw. Should take 5 minutes. If you don’t have one lying around, buy one at a big box hardware store and return it after you’re done, if you like. In exchange for decimating mom and pop stores, they offer extremely lenient return policies.
Might be more complicated depending on the type of rails and how they attach to a case. Can you post a screenshot and/or a description of how they attach?
This is one of the rails:
I want to mount them inside a Pelican 1050 case. I’m honestly hoping to just cut them to the right length such that I can force them in and they will be held by the tension of the case, although that is probably unrealistic and I will have to drill holes on the sides of the case.
Yeah, you can cut those down using a hacksaw without any other issues. Remember: “Measure twice (or thrice!), cut once.”
Using a square to mark your cut line on all the edges will help keep your cut straight. I think that’s aluminum, which is soft…it can also help to scribe the guide lines (lightly cut them into the metal) using a straight edge and a sharp knife.
Be sure to secure the rail before starting your cut. Most mistakes truly are fixable…for example, an uneven cut can be filed so it’s square. There are always ways to compensate, if that helps things feel lower stakes. But cross those bridges when/if you come to them.
Given how expensive modules are, I’d recommend using more than tension to attach the rails. The possibility of modules bouncing around in my case would make me nervous.
I recommend just focusing on the cut for now, and thinking about mounting options when that’s done. There are a bunch of ways you could mount the rails, but we can talk about that after.
Hope someone can help 
I’m building a diy kit and accidentally touch one of these styrene capacitors with my soldering iron, which seems to have melted some of the plastic outer casing. I’m not too familiar with the technical make-up of these ones.
Is it still ok or should I seek a replacement?
I could be ok. You could measure if there is no short between the 2 poles of the cap.
When there is no short you can safely test it with it.
Looks like it’s part of a filter. Is that so?
IIRC those poly caps can be particularly sensitive to heat so it might be toast but also maybe not!
As @Shiftr said check for a short and there isn’t one you’re probably ok to test it. I think those types of caps are really only used in the audio path (as opposed to power) so little to worry about there.
Looks like you still have a bunch of other components to solder in- I’m guessing you don’t have a spare poly cap on hand just in case? Placing an order somewhere for just a cap is obviously painful.
It’s a 258 oscillator for which I’ve just finished the wet build. Have just bought a multi meter to test the capacitor with (and also for calibration later on). Fingers crossed it’s ok!
Update 7/11/22:
Got a multi meter and not getting much of a capacitance reading from it, but also measuring its neighbouring styrene capacitor with reads ~24pF, when both should be 4.7pF…?
Not sure what’s going on there but may as well order a few more to be on the safe side.
Working on something at the moment (pic below) and I’m thinking of adding a status LED along the underside, but given that it’s running off a 9v battery, I’d like to not use very much juice at all.
What is the absolutely smallest amount of power I can draw with an LED? And does 3mm/5mm/color matter for this? Ideally this would be white since it would add some illumination to the underside, but not married to that if that’s an expensive color.
