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power supply design question.


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5/6/1998 5:25 PM
nic
power supply design question.
Hi I am looking at many differnt power supplies for stomp boxes. I looked at Anderton's bipolar design. But I think I might need more current than that, as I am useing quite a few Boss boxes. I heard they consume up to 70mA a piece. For pedals like these will I need a bipolar supply? What can I do about pedals like the fuzz face that needs a negative supply? I also need an AC primary outlet with three a prongs so I can power my MXR distortionII. Is it safe grounding the third plug with the secondary common ground? As you might be able to tell I need a lot of advise on this. Can someone lead me through a good design???
 
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5/6/1998 7:39 PM
R.G.
The standard for pedalboard supplies is an AC power transformer, rectifier/filter, and a three terminal regulator. Jack Orman will doubtless tell you about the schematic on his site, there was/will be one on mine when the schematics go back into operation. This is not very high tech as electronics go.  
 
For Boss pedals, no, I'm not aware of any that need a bipolar supply. For the very few pedals that use positive ground, the germanium versions of the FF being the only popular/common one, yes, you need a negative supply, so you'll need bipolar for Boss plus FF, if you have a germanium PNP FF. Some later FF's were NPN silicon, and are negative ground. Gotta know your equipment...  
 
You can also just power most of it from the power supply, and battery power the FF.  
 
Grounding the third wire safety ground to the secondary common ground is not only safe, it's the recommended practice.
 
5/7/1998 12:17 AM
nic

RG Thanks!!!  
 
I am pretty sure that all of my pedals are negative ground but I want to build a power supply that will be flexable to expansion. Plus I want something that I can use on my work bench.  
 
Say I read a lot about Fulltone's power supply. It is really too much money for me to buy, considering what it is. I have one question that is more for reference than for me to put into practice. Is the setting that mimicks a dying battery a gimmick or is it good for anything? How do they go about having that option?  
 
 
nic
 
5/7/1998 7:24 AM
R.G.
Let me preface this with - I have not seen the inside of the Fulltone power supply.  
 
A dying battery has two differences from a new one. The most obvious is that it has a lower voltage, but more importantly to the sound of a pedal, it has a higher internal resistance. I suspect that if Mike did his homework, the dying battery setting lowers the output voltage to 7.5 or 8V and inserts a resistor in series with the output to mimic the internal resistance increase. The extra resistor might be enough by itself, depending on the effect. Some experimentation might be necessary to find the right resistance. Shoot, maybe you could take a carbon-zinc battery drain it into resistors and measure the internal resistance (using injected AC, not with a meter), and find out all you need to know.
 
5/7/1998 1:09 PM
Doc

nic-  
 
You can build a bipolar supply, that will be good for your bench or powering effects, using building blocks like the one R.G. suggested at Jack's DMZ site. By using a transformer with a center tapped secondary (or two separate transformers), two bridge rectifiers, and two regulator circuits, it's easy. The LM317T is a 1 amp adjustable positive regulator in a TO-220 tab case. There's a similar one (can't remember the device number) that's set up for negative voltage polarity. The center tap becomes the common ground rail between the positive output and the negative output.  
 
Doc
 
5/8/1998 9:09 AM
Jack Orman
Doc,  
 
The LM337T is the negative supply regulator. One thing to watch for, is that as I recall, it has a different pinout than the 317.  
 
regards, Jack
 
5/8/1998 9:15 AM
R.G.
I got the schematics page back up last night, and the pedalboard PCB layout is there, but I notice that the schematic is not available. I'll see if I can get that back up tonight.  
 
This supply includes the spot for the "senile battery" resistor.
 

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