Power supply

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Power supply

Postby stuntman mike » 21 Mar 2011, 09:11

Hello. I am new to stompbox building and can't wait to finish my first pedal. I have been reading as much as I can and am pretty sure I could build a from schematic with the component values already there. My only problem is about power supply... Is the way I've been reading about powering (by dividing the 9v and with the + at the schematic top and the - at the bottom/ground), is this a standard method - because sometimes it seems left out of some schematics? Do some stompboxes have other battery inputs and why? As I say I'm new to this so it's probably a dumb question. Thanks! :) :)
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Re: Power supply

Postby DrNomis » 21 Mar 2011, 10:12

stuntman mike wrote:Hello. I am new to stompbox building and can't wait to finish my first pedal. I have been reading as much as I can and am pretty sure I could build a from schematic with the component values already there. My only problem is about power supply... Is the way I've been reading about powering (by dividing the 9v and with the + at the schematic top and the - at the bottom/ground), is this a standard method - because sometimes it seems left out of some schematics? Do some stompboxes have other battery inputs and why? As I say I'm new to this so it's probably a dumb question. Thanks! :) :)




Yeah, I agree, sometimes the power supply is left off a stompbox schematic, usually the convention is to have the + of the power supply at the top of the schematic, this is the case for schematics using a Negative grounding system, occasionally, you'll come across schematics which employ a Positive ground system, usually for old fuzz stompboxes which use PNP Germanium transistors.... :)


Hope that helps.... :)
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Re: Power supply

Postby stuntman mike » 21 Mar 2011, 12:18

yeah that's a great help thanks :) i think i've seen some small round batteries in boxes too, do you know what the additional power from them for? Also... the end caps that prevent the DC at the circuit start/end... can they be used anywhere in the circuit if there is any possible wave issue? This site has been great help so far by the way, so thanks :D :)
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Re: Power supply

Postby DrNomis » 21 Mar 2011, 12:36

stuntman mike wrote:yeah that's a great help thanks :) i think i've seen some small round batteries in boxes too, do you know what the additional power from them for? Also... the end caps that prevent the DC at the circuit start/end... can they be used anywhere in the circuit if there is any possible wave issue? This site has been great help so far by the way, so thanks :D :)



No worres,


Those small batteries that you've seen in those boxes, are commonly used to supply power to CMOS memory chips, a good example is the Bios battery in desktop PC motherboards.... :)

My Yamaha DX7 Digital FM Synthesizer has one of those batteries soldered onto the PC board... :)

Those caps at the start and end of the circuit are called coupling caps, their job is to couple the output of one part of the circuit, to the input of another while blocking any DC components of the signal being fed through the circuit, so yes they can be used anywhere in the circuit where they're needed, sometimes they may have an effect on the shape of the waveform, a good example is a treble-cut tone control circuit.... :)
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Re: Power supply

Postby stuntman mike » 21 Mar 2011, 12:51

cheers. this is great help. I have one last question, least for today :D when you see a schematic which is labelled 9v at the top, has this power source already been dividing with a circuit that is not on the schematic or is everything as shown? Because I read you need half the voltage for the AC? thanks again :o :)
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Re: Power supply

Postby DrNomis » 21 Mar 2011, 13:12

stuntman mike wrote:cheers. this is great help. I have one last question, least for today :D when you see a schematic which is labelled 9v at the top, has this power source already been dividing with a circuit that is not on the schematic or is everything as shown? Because I read you need half the voltage for the AC? thanks again :o :)



Yep, the 9V that you see at the top, means that the circuit runs on 9 volts, if the circuit is something like a clean booster made up of a single transistor and four resistors, the "C" terminal of the transistor needs to have at least 4.5V (half of 9V) on it for the transistor to "amplify" the input signal without distorting it, in the one transistor circuit, there are two resistors, one is connected from +9V to the "B" terminal of the transistor, another resistor is connected to this terminal with the other end connected to circuit ground, what these two resistors are doing is turning the transistor "on" enough to have 4.5V on the "C" terminal, this is where the half-voltage for AC that you read comes from... :)


Circuits using Op Amp ICs will commonly have two resistors of the same value conected in series from +9V to ground, this is called a Voltage Divider since the supply voltage is divided between them, the two resistors are equal in value so each one has it's half of the supply voltage dropped across it, the half-supply voltage generated by the two resistors in series is used to set the DC condition of the Op Amp so that as for the transistor amp, it can amplify signals without distortion too, so in answer to your question, usually using voltage dividers, the circuit is designed to supply these half- voltages for AC automatically.... :)
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Re: Power supply

Postby stuntman mike » 21 Mar 2011, 13:35

ah got it. so the the 9v volts gets divided with these resitors anyway so one 4.5 v can be fed into an op amp to condition it for the input signal. :) and simular with the transistor :D think I understand how the circuit works more now thanks. :D
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Re: Power supply

Postby DrNomis » 21 Mar 2011, 13:50

stuntman mike wrote:ah got it. so the the 9v volts gets divided with these resitors anyway so one 4.5 v can be fed into an op amp to condition it for the input signal. :) and simular with the transistor :D think I understand how the circuit works more now thanks. :D


No worries.... :)

It's a little bit diffferent for a transistor, since you're trying to turn it on enough to get 4.5V on the C terminal, but the principle of the voltage dividers is the same... :)


Yes, that 4.5V can, and is fed to the opamp, and sometimes more than one at a time, check out the freestompboxes.org library if you're looking for more books to read on electronics theory, you might be interested in downloading a copy of The Stompbox Cookbook by Nick Boscorelli, it's a good read with lots of interesting stuff in it.... :)
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Re: Power supply

Postby bigmufffuzzwizz » 27 Mar 2011, 18:54

DrNomis wrote:It's a little bit diffferent for a transistor, since you're trying to turn it on enough to get 4.5V on the C terminal, but the principle of the voltage dividers is the same... :)


Whats the real purpose of biasing to 4.5V? Is that when a transistor would create the "smoothest" most music fuzz/distortion?
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