using 12v on a 9v circuit?

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I have some 12v DC power supplies. Generally, is that pretty safe?

[ACS]

Generally speaking, yes.  BUT to be sure you'd need to check every component in the circuit to make sure it could hand;e that voltage...

Most effects will sound a bit different at 12V - some better, some not.  For example, my TS clone will run quite happily at 12V, but personally, I don't like the sound...

Aidan

[darron]

+1 on what acs said.

that would be excellent to make a 9v regulated supply! if your 12v supply was not designed for audio then it's possibly not regulated. all you'd need is a hand full of parts and any small plastic box.

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I actually checked one of the wallwarts marked 12v and it read nearly 18v on the multimeter...

So where can I find info on how to make a regulated power supply out of one of these?

[Transmogrifox]

http://www.national.com/mpf/LM/LM317.html

Check out the circuit on this page.  The LM317 makes this kind of thing pretty easy, and they regulate pretty clean power.

[.Mike]

Your 12V adapter reads 18V because you are measuring it without a load. When loaded, the voltage will drop to closer to 12V.

Even simpler than the LM317 that Transmogrifox suggested is the 7809 regulator. The difference between the two for your purposes is that the LM317 output is adjustable while the 7809 is fixed at 9V. Basically, you would need a small enclosure, a couple of 2.1mm DC jacks, a 7809 regulator, two capacitors, a small piece of perfboard, and some wire. Take a look at the 7809 datasheet.



Mike

[darron]

I actually checked one of the wallwarts marked 12v and it read nearly 18v on the multimeter...

So where can I find info on how to make a regulated power supply out of one of these?

the secondary on a transformer reads a bit higher until you put a load on it, so that sounds alright.

the lm317t is an adjustable regulator. you could use the LM340T9 with just 3 components though!: http://www.national.com/mpf/LM/LM340.html

make the capacitors on the input and output going to ground about 1,000uf each, and put a small 0.1uf cap going to ground on the output to add further filtering for RF.

like i said, very simple.


maybe add an LED on the output with a limiting resistor just to show power is on?

the only thing that you really need to not get wrong is the polarity on the wall-wart and the output of the regulator. most effects use a negative tip 2.1mm barrel with positive on the outside.

[petemoore]

 I have some 12v DC power supplies.
 Measured ?
 Generally, is that pretty safe?
 Not particularly dangerous, sometimes stinky.
 The power supplies that say 12v will measure different voltages, depending on the load/current draw. Stompboxes tend to be teeny current draw to these WW's, upwards of 16vdc is likely with one marked @ 12vdc.
 Calculate the voltage which will be applied to a component, add 10% is the general rule.
 The calculations for small voltage circuits [like most stompboxes] component voltage ratings is generally greatly simplified by making them all > than the power supply voltage.  
 

[Transmogrifox]

Your 12V adapter reads 18V because you are measuring it without a load. When loaded, the voltage will drop to closer to 12V.
Mike

As briefly mentioned by petemoore, even with a small stompbox load applied, the voltage can be in excess of 16V.  Usually a wallwart has something similar to this printed on it:
12VDC
350mA

That means that it will be at 12VDC when loaded at 350mA.  Anything less than 350mA, the voltage will be higher than nominal, anything more than 350mA, the voltage will be less.

If you overload a wallwart, your 12V wallwart could be giving you 6V. 

Just extra tidbits.

The biggest problem I have had using cheap thrift-shop 12V power supplies is they are incredibly noisy.  If you have a high gain distortion pedal, this can be really irritating.  As mentioned by darron, you will want to add a few thousand uF capacitance to the power supply.  I have a wallwart with a couple series inductors (RF chokes) and 470uF capacitance, which seems to be fine for low gain stuff like chorus & flange (etc), but still isn't enough for a high gain pedal.  1000uF would be the bare minimum, I think.

[zombiwoof]

I've wondered about this question, I measured the 9 volt transformers that come with the BBE pedals (unloaded) and they measure over 13 volts.  I measured a Dano 9V transformer the same way, and it measured exactly 9.6 volts.  If the voltage is lowered when connected to a pedal, does that mean the Dano one will be putting out less then 9.6 volts?  I couldn't believe the BBE ones measured that high.

Al

[ACS]

Nope, it most likely means that the Dano one is regulated - so no matter the current load (up to the stated max) it should always be putting out about the same voltage...

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So if I just want to filter the power supply (not regulate it), do I just put a cap across the two rails?

What size cap?

[darron]

DC is AC that has been filtered. if you want to filter it further then putting caps across is what you do. generally a massive electro and a small  0.1uf film or ceramic cap. the electro can be as big as you need. single pedals often have 100uf in them. you could use 1,000uf or 2,2000 uf.. whatever.... if you want to give further regulation add a small value resistor in series with the positive line after the filter caps and add more filter caps again. the resistor can be anything from 100ohm to 1k, depending how much current you want to load it with.

[Transmogrifox]

... add a small value resistor in series with the positive line after the filter caps and add more filter caps again. the resistor can be anything from 100ohm to 1k, depending how much current you want to load it with.

Best to add the resistor first in the sequence.  There are two reasons for this:
1)  The resistor creates a voltage divider for AC voltages with the cap.  By placing it first in the line, you provide more attenuation to the AC voltage.
2)  When the power supply is first turned on, it behaves as though the output is shorted until the capacitors are charged.  If the capacitance is much larger than the design rating for transformer and diodes allow, the charging current will be large for a long period of time and you may either smoke your transformer, rectifier diodes, or both.  The series resistor limits the inrush current, so you can add large capacitance without much danger of damaging the wall wart adapter.  Use a resistor with a power rating equal to the charging current requirement.  This is less critical in low power wall warts because the transformer windings and the diodes can handle short circuit currents indefinitely.

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What does the small value cap do? I thought that capacitance just adds up when in parallel? So wouldn't it go from 1000uF to 1000.1uF, for example?

Or is there something with it being a non-polarized cap that does something different?

[Mark F]

What does the small value cap do? I thought that capacitance just adds up when in parallel?

The small value cap filters high(er) frequencies, RF. The larger caps, 1000uF etc. filter the lower frequencies such as the 50 or 60 Hz. from your transformer (wallwart) giving you a smooth, constant, steady DC voltage.

[Transmogrifox]

What does the small value cap do? I thought that capacitance just adds up when in parallel?

The small value cap filters high(er) frequencies, RF. The larger caps, 1000uF etc. filter the lower frequencies such as the 50 or 60 Hz. from your transformer (wallwart) giving you a smooth, constant, steady DC voltage.

Just to add some technical background about where this comes from:
Thru-hole capacitors are not the ideal components we think they are.  Every capacitor has parasitic series inductance and resistance. 

An inductor in series with a capacitor (such is the case of the leads in series with the capacitor), there is a frequency at which there is resonance, then the impedance increases due to the inductor.  Simply, any capacitor is actually an inductor at really high frequencies (usually several hundred MHz).

A larger value capacitor will resonate at a lower frequency, and the impedance starts to climb from there upward.

When you get into the microwave frequency range, filters are designed by defining the length of circuit board traces and putting bends in certain places.  Two circuit board traces running parallel for a certain distance may actually be a component.

The key to mitigating this is to put multiple capacitors in parallel.  Some large to extend attenuation to low AC frequencies, and some to attenuate radio frequency noise in regions where the inductive impedance on the large capacitor leads is relatively high.

In short, putting small caps in parallel will do a better job of filtering noise from your neighborhoood WiFi, cell phones and cordless phones.

A good pattern:
1000uF
10uF
.1uF
10nF
100pF
1pF

I usually stop at 10nF because I haven't actually observed much added noise from stuff above the frequency where a 10nF film cap stops behaving like a capacitor.

"But those frequencies aren't audible".  The perverse nature of things is that this high frequency garbage gets demodulated by the semiconductors in your audio circuit due to parasitic effects that aren't even listed in the datasheets...and it becomes audible.

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Just as a test, I put three 1000uF electrolytics in parallel with my DC input from my cheap thriftstore 9v supply (800mA). I have this all connected to a multi FX guitar synth box I am making (PWM, gargletron, passive ring mod w/boosters, and a mini mixer at the end - soon to have a couple more circuits).

I could hear that with each one I added (started with one, then two, then three), that the 60Hz AC hum got quieter... but I can still hear it - it isn't terrible, but not like running from a battery. It is 60Hz plus maybe a few harmonics. It is not overbearing, but it is audible. I haven't had a chance to drop in some lower value caps yet.

I have a question though - don't lower value caps filter out the low frequencies more? I am just speaking from my experience with input caps on fuzz pedals - higher value caps seem to make for bass-y, thick fuzz, and lower makes for treble-y thin fuzz.

The PWM is basically unaffected by any of this, BTW, it is the gargletron and to a lesser extent the boosters for the passive ring mod where I can hear the hum most (hard to tell on the ring mod as everything is ring modulated anyway). I can't recall if I could hear any hum off of the mixer.

[darron]

I have a question though - don't lower value caps filter out the low frequencies more? I am just speaking from my experience with input caps on fuzz pedals - higher value caps seem to make for bass-y, thick fuzz, and lower makes for treble-y thin fuzz.

You are right that lower value caps pass more bass on the inputs of fuzz circuits etc., but that's when they are acting a decoupling capacitors. They pass a lot of bass from your guitar input onto the transistor for amplification. In power filtering though they are connected to ground, so you'd want all of those lower frequencies to 'pass' to ground rather than continuing along into your circuit and causing noise. Similarly you'd use a tiny cap if you only wanted to send the high treble and radio frequencies to ground while letting the rest pass to the transistor.




here's the Huminator by Beavis Audio Reaserach:

http://www.beavisaudio.com/projects/Huminator/index.htm][url]http://www.beavisaudio.com/projects/Huminator/index.htm[/url]

It's the same thing that everyone has been talking about here, but with protection diodes. The 100uF isn't very much though. The 100ohm resistor is a really good idea and acts as a sort of regulator.

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That's great, and makes a lot of sense, thanks!