Help with designing a power supply for a bunch of tube pedals (UK voltages)

[frequencycentral]

I have built a lot of tube pedals over the last year! All are 12 volts DC, with a range of ma requirements from 50ma up to 300ma. I have a few more tube pedal designs that will require 300ma and 600ma. I have a few wall warts I use to power my pedals at the moment, but I think it's time to make a purpose built power supply.

The supply needs to have 12 volts output and put out a lot of milliamps. I'm thinking 2 amps or maybe 3 amp, to 'future proof' it, as I have more pedals in mind. Another idea is to have a transformer capable of providing say 5 amps. and have a number of daughter board with rectifier/7812/caps each providing 1 amp. I could add more daughter boards as I need them.

I do need some help with this project if anyone cares to give it - I'm happy enough to put it all together, but need help identifying the right transformer and designing the circuit. I've been thinking of a toroidal transformer because they are pretty neat and low noise. This need to be a really clean supply - hiss and hum free.

So, where do I start? Transformer I guess. Which one?
http://www.rapidonline.com/Electronic-Components/Transformers/Toroidal/Toroidal-transformers/35506/kw/Toroidal%20transformers#techspec

Then bridge rectifier - I have plenty of 1n4002's and 1n4007's.

Then 7812's and filtering.

Any help, pointers, guidance and /or schematic would be great.

Thanks in advance!

Rick

[slideman82]

Pretty simple! just Use a 7812, and from 12V (a 1000uF cap to ground), many 100 ohm metalfilm resistor plus 4700uF filter cap with polyester 100n caps in parallel, you won't get hum with 2 outputs per cap... try it! Pretty simple! just space for large caps!

[punkin]

I'm gonna give this a bump. I'm in the same boat. The ideal transformer seems to be the ticket and every where I go it seems to get avoided in the conversation. The rest of the circuit is straight forward stuff IMHO but some of the transfomer specs aren't quite clear to me.

By the way, I've got a couple tube pedals as well. I'm finding that 13.6 for the 12u7's works nicely...a little extra juice but I understand that the 12aux type won't tolerate the voltage bump for the filaments. ...so far all I can say is I think we need to know the total load demand then back up into the circuit to know what will be the draw on the transformer. That's the limit of my suggestions but I'll be watching...hoping to learn a little something here.

[yeeshkul]

I recently bulit an adapter 9V/320mA and collected some precious experience making mistakes, burning my fingers and spending necessary money, haha:

1. 78XX gets hot like hell - use TO220 and a metal cooler (+ conductive paste so the cooler sticks on the TO case fine - the bolt itsef cannot guarantee that the cooler sits flat on the chip's surface). The bigger the cooler is the better for you.
2. use 100nF caps(preferably film) at the input and output of the 78XX as close as possible to the chip's legs
3. use filter cap that works up to 105°C, they are big, but considering that you should use 1000uF for each 1A of the current draw, you should be fine with 470uF
4. do not place the filter cap any close to the 78XX or the trafo
5. Some transformers claim for a fuse at the secondary side - i used a fuse at the primary side because i measured the primary current ...
6. If you gonna get 12V stabilised output, pick a transformer with more than 12V secondary voltage, you will need at least 3 volts across the 78XX to get the chip working properly. Some sources say 2-2.5V but count 3V at least to be sure.
    This applies for other voltages too. You can count the voltage at the 78XX input ... Vi = (Vtrafo sec * 1.4) - the drop on the diodes. This must be more than output voltage +3V. Whatever result you get, try to pick the trafo the way i described.

How to pick a primary fuse:
1. buy a load resistor the can take the expected power(!) P > output voltage*current draw ... R=output voltage/current draw
2. count on the fact, that the load resistors can be made in China and can start burning at lower wattage because they may have air bubbles in the isolation - so stay around
3. stick the ampermeter in between the steel fuse holder sides and measure the primary current .. XmA
4. mercilessly short the output for a second and measure the critical primary current ... YmA
5. pick a fuse in between X and Y. I used a medium slow fuse, not the fast one.

EDIT:
How to pick the transformer:
every transformer is defined by the primary power VA (voltamper)
VA value = mains voltage * primary current flow
1. Let's say you need 12V/ 300mA draw stabilised - > go for 15V secondary.
2. That means 15*1.414 - diode drop 1.2 ~ 20V at the 78XX input and that's fine (20-3>12)
3. Count on Psec ~ 0.8* Pprim(small transformers have bad effectivity - that's why that rough 0.8 ) => Pprim = Psec/1.25 => Pprim = 15*0.3/1.25 = 3.6VA
4. you should be fine with 15V sec 4.5VA trafo

If you go for a not stabilised power supply, thinks will get a bit easier, but this one is more challenging, haha

After you built the power supply give it a check. Let it go fo at least 3 hours at the maximum current draw, to be sure. And be around all the time unless you have a spare house.

[frequencycentral]

Found this:

http://www.quasarelectronics.com/cfe013.htm

http://img.icnea.net/Forum/E6001/ftp/Fe-13(English).pdf

What do you guys think?

[yeeshkul]

Rick read first what i wrote. If you need a normal supply you may get away with that one, if you wanna stabilise the output you will need more (see the edit).
Look for HAHN transformers. They are plastic-cased, tested and reliable german stuff. The smaller of them (up to 3.5VA) are short-proof.

EDIT: Rick i am sorry, i looked somewhere else  - the module looks fine if you willing to pay 60 pounds for it. It actually gives much more than what you may need.

[yeeshkul]

If you decide to build your power supply check out these
http://www.elfaelektroonika.ee/cgi-bin/web_store.cgi?vw=28a1b&pcode=373&lng=eng

[Andi]

6. If you gonna get 12V stabilised output, pick a transformer with more than 12V secondary voltage, you will need at least 3 volts across the 78XX to get the chip working properly. Some sources say 2-2.5V but count 3V at least to be sure.

Doesn't the AC rectify to a higher DC voltage anyway? I'm sure I read that somewhere - 1.4 times the AC voltage IIRC.

[yeeshkul]

That is right, but (it is the capacitor that charges up to the peak voltage) also subtract the voltage that is lost on the rectifier and 3 more volts to be sure that 78XX is working. If you get result that is still let's say 1-2 volts above the XX output - go for it  .
The thing is, that if you balance on the edge with the voltage at the 78XX input you can never tell what is gonna happen. Better to be safe than sorry. I had to go for another transformer ... so i know what i am talking about haha.

EDIT: i guess i will post the build report to another thread, so people can avoid the troubles i ran into . Power supplies look easy as hell and that's why they cause so many troubles.

[punkin]

I recently bulit an adapter 9V/320mA and collected some precious experience making mistakes, burning my fingers and spending necessary money, haha:

1. 78XX gets hot like hell - use TO220 and a metal cooler (+ conductive paste so the cooler sticks on the TO case fine - the bolt itsef cannot guarantee that the cooler sits flat on the chip's surface). The bigger the cooler is the better for you.
2. use 100nF caps(preferably film) at the input and output of the 78XX as close as possible to the chip's legs
3. use filter cap that works up to 105°C, they are big, but considering that you should use 1000uF for each 1A of the current draw, you should be fine with 470uF
4. do not place the filter cap any close to the 78XX or the trafo
5. Some transformers claim for a fuse at the secondary side - i used a fuse at the primary side because i measured the primary current ...
6. If you gonna get 12V stabilised output, pick a transformer with more than 12V secondary voltage, you will need at least 3 volts across the 78XX to get the chip working properly. Some sources say 2-2.5V but count 3V at least to be sure.
    This applies for other voltages too. You can count the voltage at the 78XX input ... Vi = (Vtrafo sec * 1.4) - the drop on the diodes. This must be more than output voltage +3V. Whatever result you get, try to pick the trafo the way i described.

How to pick a primary fuse:
1. buy a load resistor the can take the expected power(!) P > output voltage*current draw ... R=output voltage/current draw
2. count on the fact, that the load resistors can be made in China and can start burning at lower wattage because they may have air bubbles in the isolation - so stay around
3. stick the ampermeter in between the steel fuse holder sides and measure the primary current .. XmA
4. mercilessly short the output for a second and measure the critical primary current ... YmA
5. pick a fuse in between X and Y. I used a medium slow fuse, not the fast one.

EDIT:
How to pick the transformer:
every transformer is defined by the primary power VA (voltamper)
VA value = mains voltage * primary current flow
1. Let's say you need 12V/ 300mA draw stabilised - > go for 15V secondary.
2. That means 15*1.414 - diode drop 1.2 ~ 20V at the 78XX input and that's fine (20-3>12)
3. Count on Psec ~ 0.8* Pprim(small transformers have bad effectivity - that's why that rough 0.8 ) => Pprim = Psec/1.25 => Pprim = 15*0.3/1.25 = 3.6VA
4. you should be fine with 15V sec 4.5VA trafo

If you go for a not stabilised power supply, thinks will get a bit easier, but this one is more challenging, haha

After you built the power supply give it a check. Let it go fo at least 3 hours at the maximum current draw, to be sure. And be around all the time unless you have a spare house.

NICE...thanks!

[Andi]

That is right, but (it is the capacitor that charges up to the peak voltage) also subtract the voltage that is lost on the rectifier and 3 more volts to be sure that 78XX is working. If you get result that is still let's say 1-2 volts above the XX output - go for it  .
The thing is, that if you balance on the edge with the voltage at the 78XX input you can never tell what is gonna happen. Better to be safe than sorry. I had to go for another transformer ... so i know what i am talking about haha.

Ah - that makes sense. Most helpful - thanks!

[punkin]

wow...those transformers are a bit costly...oh...wait...thats listed in a foreign currency...duh.

[yeeshkul]

I got mine for aboutr 4 bucks.

... another thing that makes you think about a bigger trafo is the 10% mains fluctuation.

[Ripthorn]

If you purchase a transformer with a higher current rating than you will use, the secondary voltage will actually be higher than the listed secondary, so that can buy you some extra volts for regulation.  I am designing a power supply for a low voltage tube amp and the current draw in my circuit fluctuates some, so it makes it hard to pin down an exact load current.  But I think that being a little cautious (or a lot cautious) is not a bad thing.

[frequencycentral]

Wow thanks guys, that's great!

I thought I'd add these words from my brother - he's an EE and emailed me this today:


6V+6V 50VA TOROIDAL TRAN (12020) (RC) 88-2614

Connect the primary windings in series (being careful of polarity) to giveyou 230V input. According to the datasheet you connect grey and violettogether. Blue and brown to the mains.

Connect the secondary windings in series (being careful of polarity) to giveyou 12v AC at approx 4.2A (50 VA / 12 V = 4.166A). Red and orange togetherwith thick wire. Black and yellow to the bridge (~ and ~).

The 12V is RMS so it will give you about 12 * root(2) V peak = 17V.

Bridge rectifier, 8A

http://www.rapidonline.com/Electronic-Components/Discrete-Semiconductors/Bridge-Rectifier-Diodes/4A-8A-In-line-bridge-rectifiers/29673

A big smoothing capacitor, 4700uF @ 25V.

And then 4 x 7812 with common inputs and earths (but not outputs) and each with its own small decoupling caps (100nF) close to the device (in->gnd andout->gnd), and an extra 100uF @ 16V on the outputs (out->gnd).

Each 7812 could have three connections to the outside world (parallel)giving you 12 x 12v @ 333mA (average).

The 7812s will need heatsinks of course. Luckily, the casing of the 7812 isGND, so you can use your metal enclosure as the heatsink and you won't evenneed to insulate the devices. A squidge of thermal paste is probably a good idea.

Earth the enclosure. Use thick wire throughout. Shall I draw and scan a detailed schematic or will the generic one (enclosed) be enough?

So it's:

Toroid
Bridge
1 x 4700uF @ 25V
8 x 100n
4 x 100u @ 16V
4 x 7812
12 x output jacks
Case, fuse?, switch, led. (thermal paste?, bolts for mounting the 7812s)

EDIT: Forget the "Red and orange together with thick wire". Obviously it's atoroid and has wires not tabs, so just shorten orange and red, twist,solder, and robustly insulate. Same for grey and violet.

[yeeshkul]

I completely agree with everything your bro said but the output voltage: 17V - 1.1(to 1.4 - see the catalogue for the particular bridge) -3 = 12.9 ... i would call it a bit risky but i don't want to put you off mate, just give it a try. It is sure big enough to feed 12 outputs, the only trouble can be caused by the non-linear behaviour of 7812 when it has not enough voltage across the legs. Also mind the 10% mains drift. After all the best way to learn is to put your foot into it, no pussyfooting and learn from your own possible mistakes. If you find out the strange things are going on with the output voltage when you draw something close to 300mA - you can always make a non-stabilised power supply out of it.
I think that stabilisation is a bit exaggerated feature anyway.

[frequencycentral]

Thanks again Jan, I'm just digesting all the info right now, power supplies are new to me. I'll decide which way to go over the weekend.

That kit I linked to - they do a 2 amp version too which is much cheaper: http://www.quasarelectronics.com/cfe011.htm

[yeeshkul]

Rick one more thing - your bro mentioned 4700uF cap, which is indeed a clever idea - the more current you draw the bigger cap you need to get rid of the ripples.
The problem is the temperature inside the box. You will most likely need a cap that works at 105°C and those are big like a house - make space for 4x1000uF/35V/105°C caps instead. 25V may be ok, although the rule says 2x as much as the voltage on the cap is - at least.

[yeeshkul]

The smaller module looks good. What total current draw you expect you may need? I am not experienced in tubes at all 

[frequencycentral]

The smaller module looks good. What total current draw you expect you may need? I am not experienced in tubes at all 

I have three tube pedals that draw 300ma each. I have four more that draw up to 150ma each. My total draw with what I have right now is 1.5 amps. I'm planning more tube pedals such as phaser, envelope filter, wah, and maybe a sample rate reducer. The phaser and filter could both end up being four tubes each and drawing 600ma each. I guess I need to build one with enough ma not to run out of steam in the near future!