Bass preamp in a stompbox

[bluebunny]

Power supplies don't push current around a circuit.  Your circuit draws the current it needs.  Just because you have a power supply capable of delivering 100mA, that's not the current that flows.

To work out which resistors you need, you would have to work out the voltage dropped across each one and the current flowing through it.  From this you would calculate the required power dissipation for each resistor.  You'd find this a long and tedious exercise and the numbers you'd get out would not be useful.  Use 1/4W resistors throughout.

[Shlafenflärst]

OK, thanks !

I found clearer info to calculate the resistor for the LED : http://ledcalc.com/. So, for this LED I had in mind, that gives me a 735Ω resistor. Closest I could find was 750Ω. But since a blue LED might be very bright, I'm thinking about adding a trimpot to adjust the brightness.

[bluebunny]

These online calculators will tell you the smallest value resistor, to avoid burning it out.  The value you really want will typically be an order of magnitude higher, particularly with modern high-brightness LEDs.  750Ω will burn your retina.    Test beforehand on a breadboard to find a suitable value.

[Shlafenflärst]

I get it, but the test implies that I buy a bunch of resistors to find which one is good, so I'd need to at least have a clue what range of resistors I'd need before placing my order. It's not like there's a shop I can drive to if I don't have the right one (can't express how much I wish there was...). Hence my idea of the trimpot... but I realise it might be a bit pointless. So what kind of resistor is usually associated with a blue LED ?

[bluebunny]

Test beforehand on a breadboard to find a suitable value.

...with a trimmer.  Then measure the trimmer to decide what resistors to buy.

[Shlafenflärst]

I'd rather order everything at once. Since it's going to be delivered while I'm at work, I'll have to get the package from a post office, which are all closed when I get out of work, it's a pain in the ass to get one delivery, I don't want two.

Alternatively, I could just leave the tripot on, since it will do the job, and then replace it with a resistor when I have more things to order for a new project. Or don't replace it if I think it's convenient to be able to adjust the LED brightness. I have a 5K linear trimpot (actually the one currently on the circuit, I plan on replacing it with a regular sized pot), it would be good, right ?

[amptramp]

You could always use a photoresistive cell across the LED resistor so the LED is brighter in daylight than in the dark.

[Shlafenflärst]

Well that is definitely a clever idea ! I'll look into that !

[Shlafenflärst]

Back here with yet another question ! There's something wrong with my power plant, so I can't use the voltage doubler (well I can, but it still gives me 9V). So, in search for a solution, I realized I have 2 laptop power adapters with the exact same connector as pedal power cords. One gives 20V (actually 20.73V), 3A, 60W max. The other one gives 19V (actually 19,46V), 4.74A, no indication for the watts. What would be the risk in using one of those instead of the 18V I need ?

[PRR]

> What would be the risk

What polarity??

19V the wrong way can smoke parts.

[Shlafenflärst]

Well, it's always a good question to ask, especially in this case because the polarity happens to be the opposite of what pedal power supplies usually have, but I thought about this, and I plan to put a bridge rectifier on my project's power input to not have to worry about that.

[bluebunny]

Why not work out why the voltage doubler isn't working? I'd rather do this than kludge a rectifier onto a noisy laptop power supply. (And if you really want to use it, just rewire the plug to avoid the fire that Paul warns of.)

[Shlafenflärst]

I checked the wires on the voltage doubler, there is nothing wrong with it (at first I was worried they sent me a current doubler instead, but no). The problem is the power plant. It is said to have isolated sections, but the power outputs aren't isolated from each other. Don't know if it's faulty or it was supposed to be that way, I contacted the maker about this, since the warranty hasn't expired, but they haven't answered yet.

So I will, eventually, get a proper 18V supply. But since I need to order it online, I'd like to wait until I have other things to buy, to save shipping cost. Plus I have received everything I need this morning, I will probably build it in the week-end, I need to test it !

[bluebunny]

My misunderstanding. I thought you meant you'd built the doubler yourself. (It's an option, of course!)

[Shlafenflärst]

Well, I thought about it. And now that you mention it, I'm thinking about it again. Why not give my pedal a built in voltage doubler ? I have all the parts I need, including DC jacks with switches, so I can do something that would work with either one 18V or two 9V power cords. If I put just one diode between the two jacks, that would be enough to prevent a catastrophy if I accidently plug in one or two power supplies with the wrong polarity, right ?

[Shlafenflärst]

These online calculators will tell you the smallest value resistor, to avoid burning it out.  The value you really want will typically be an order of magnitude higher, particularly with modern high-brightness LEDs.  750Ω will burn your retina.    Test beforehand on a breadboard to find a suitable value.

I believed you when you said 750Ω wouldn't be enough... I just couldn't imagine how much more I would need ! I tried with 1K in the dark, and I could see enough to move around in the room ! I had ordered a 750Ω, they accidently sent me a 750K, and it was just a tiny bit too much. I made some tests with a 500K pot, and found out that 500K was good in the dark, and 150K good with the lights on. But even with the 750K I could see it with the lights on, so I think I'll stick with that one for the moment, maybe later I'll modify it with a 500K trim or a potoresistor.

[antonis]

the polarity happens to be the opposite of what pedal power supplies usually have, but I thought about this, and I plan to put a bridge rectifier on my project's power input to not have to worry about that.

Could you plz elaborate ..??
(in the mean of bridge rectifier reversing polarity..??)

[Shlafenflärst]

OK, I was using the wrong term. I looked it up, and actually it's supposed to be called a bridge rectifier when used to transform AC into DC. Since here that's not the case, I should just call it a diode bridge. English is not my native language, so I'm not yet familiar with all the technical terms, but I get how that can cause confusion, and I'm sorry for that.

Anyway, if I connect the DC plug on the AC side of the diode bridge, and the circuit on the DC side, no matter which polarity the power supply has, the circuit will always have it the right way. I thought of this as a safety feature in case I borrow a power cord and don't pay attention to the polarity (which is unlikely to happen, but I better be safe than sorry), and a way to use any power supply as long as it has the right voltage. But with my latest idea to have a built in voltage doubler, that would be a bit too complicated, so I'll stick to a simple diode to block the current if the polarity is reversed.

[DIY Bass]

I built a bass preamp from a kit.  (Growling Krizzly).  The schematic is available.  It has what you describe.  There is a voltage input that can be anywhere from 6-18V AC or DC any polarity.  It feeds a rectifier and then into a power supply module that outputs solid +15V/-15V DC.  That particular power module from memory is expensive though.  I am sure you can do something cheaper.   One thing though with a rectifier is that instead of the usual diode drop on the power supply you now have 2 drops, so a 9V input will end up being 7.6V pretty quickly if you use standard silicon diodes.