power supply filter series regulator resistor question.... (:

[darron]

i was thinking about building a simple dc ripple/hum filter as it would be something easy. something like this:

http://www.beavisaudio.com/Projects/Huminator/index.htm



the idea is very simple. polarity protection with diodes either series or parallel. ripple + RF filter caps of course, and also a series small value resistor. the idea of the resistor is that it will reduce a bit of the ripple by regulating the incoming voltage to a non-fixed output voltage, at the cost of a bit of voltage drop of course. this is the popular way or powering pretty much any tube amp.

the value of the resistor used here was 100ohm, quite small. this made me curious of the voltage drop that it would create....

please pardon any ignorance i demonstrate here, but this is why i am asking. there is a great wealth of information from forum members here and it comes as second nature to people who haver used it for many decades, longer than i have been alive.

my assumption is that the voltage drop will varied based on current consumption of the following effect / load. my next assumption (where i may be wrong) is that the voltage drop will be calculated as a shared voltage division between the 100ohm resistor and the resistance of the following load (stomp box).

so i will assume a voltage of 9V (even though it could be more), and a pretty common pedal current draw of 30mA. then i will calculate the load of the pedal:

R=V/I
R=9/0.03
R=300

hope we're still going good?

so from here the total load on the power supply will be the 100ohm filter in series with the 300ohm pedal, 400ohm.
now my assumption will be that the filter resistor voltage draw will be: (incoming voltage) / (total load) * (resistor load) = 9/400*100 = 2.25V

So the power that the pedal will end up receiving will only be 9V-2.25V = 6.75V, and with a series protection diode = 6.55V

that poor starving pedal...

these numbers made me that maybe my logic is flawed, or the circuit was designed for a different load?


shine some light on me please someone?


thanks for reading all of that!!

[GibsonGM]

It will work great with a 12V supply, drawing 30mA.  It will get higher with lower draw, and lower with higher draw, of course!   This might not be the best way to get "clean power", I think.    I would opt for using an LM7809 and come up with a regulated voltage.  This configuration depends on you having a steady load, and adding/removing a pedal from a chain would change your V out.

I simmed this in LT Spice to see just how lossy it is,  and you are absolutely correct, there is no way to get anywhere near 9V out if you are using 9V in!  I'm sure it is a good filter, it just needs a higher Vin and a dedicated load. 

[darron]

a simulation is just what i needed. i should have breadboarded it i guess. thanks for doing the work (:

i actually have a DIY toroidal regulated supply, which is absolutely perfect. the main idea of this is to try putting it in front of a single pedal. you could easily make these in a very tiny package. i'd probably put in bigger caps (1000, 2200uF?) and maybe a cap before the filter resistor too, where the LED is.

the biggest reason that i asked this is because i'm interested in putting in this kind of power filter in future builds and would like to calculate an appropriate value of resistor. maybe the 100ohm would be alright for a fuzz drawing 2mA or something, but now i know that i'm thinking on the right lines i might use a 10ohm or something, depending on what it's going into.


thanks heaps for the prompt and satisfying reply!

[earthtonesaudio]

I had similar thoughts about a year ago.  My conclusions which I've kept to this day are:

-Regulators are great.  Easy to use, better than passives, and nearly as cheap.  LDO's are better still, and still pretty cheap.

-Even better than regulators is a circuit which still performs well if the power supply is changed (linear wall wart, switching wall wart, battery)

-But even with such a circuit, regulators are nice.

[PRR]

> a pretty common pedal current draw of 30mA.
> R=V/I
> R=9/0.03
> R=300
> these numbers made me that maybe my logic is flawed

The logic is fine.

But many pedals pull MUCH less than 30mA.

If anybody still used 9V batteries (which eliminates your buzz concern!), battery life is just a few hours with cheap batts at 30mA.

Look at 1-transistor pedal with 10K collector load. The most it could pull is 9V/10K or 0.9mA; in fact it must pull less to have room to swing. So 0.5mA is nominal for some pedals.

Look at TL074 pedals.... datasheet says (I think) like 3mA for the whole chip, maybe 9V/200K for the V/2 divider, so about 3mA. This will increase for large signals under load, but 2V signal into 50K load is around 0.04mA dynamic current and we can overlook it.

Now you could figure pedal "equivalent resistance", then do the resistance-divider thing.

But if you are TOLD the pedal draws say 3mA, then you can cut to an answer quicker. 3mA flowing in 100 ohms is 0.3V. Your 9.0V becomes 8.7V. Any "9V" pedal should work fine at 8.7V.... if anybody still used 9V batts, they'd ride them down to 7V or 6V before replacement.

This does avoid the question: is it 3mA at 9V and something less at 8.7V? In this case the voltage change is so small that we may simply ignore the issue. And in most "add-in power filter" schemes we will hope for "small" voltage drop, so small that whatever the current was at 9V it will be about the same at 8.7V or whatever.

If you truly have a 30mA pedal which wants a large part of 9V, then this 100 ohm plan seems dubious.

You "can" scale everything to lower impedance. 30mA in 10 ohms is only 0.3V DC drop. However with the same 100uFd cap it is less ripple-drop too. So you up-size the bulk cap to 1,000uFd. Now DC drop is good and ripple-loss is good.

> i might use a 10ohm or something, depending on what it's going into.

See? You knew all that, you just had to work it out.

But going to say 1,000mA (1A) gets silly: 0.3 ohms and 30,000uFd! You must wonder if there is an easier/smaller/cheaper way. (There usually is.)

I must point out, in a DIY Pedal forum, that you can just build the equivalent filtering INto your pedals. Adapted for the current and filtering THAT pedal needs and the voltage-loss it will tolerate.

You can also use "good" power packs.

[darron]

PRD, i suppose the 30mA that i was thinking of was more of an average for the more problematic pedals such as delays, digital effects, and multi effects. or if you wanted to share a couple/few off the same filter. definitely not a problem for out little fuzz boxes (:    most people around here too also add a pretty bright led with a low limiter resistor, but like you mentioned, if you were building something like this into the effect itself then it wouldn't make sense to have the LED circuit as a part of what is being filtered, if you've got a protection diode in the circuit too of course.

i'm glad that i wasn't thinking stupidly. i don't have a formal education for these things (specifically anyway) and sometimes need the reassurance for what would be half a thought for others.

i do have a good power supply and agree that that is what is usually needed to fix problems rather than work-arounds. but good filtering in everything i think is a good practice. i often work with TL07x chips and the like and was thinking down the lines of what you have just covered, so i'm also glad that i can settle on a 10ohm or so resistor with probably 220uF/+ or so filtering.

[PRR]

> i don't have a formal education for these things

Bah. An EE degree gives concepts but no practical insight.

Smoke $100 of resistors for a few decades and *in your field* you can become an "expert", wise beyond most EEs.

> good filtering in everything i think is a good practice.

Yes, but not wanton over-kill. After a point, there's usually a better place to put a buck. "Balance in all things."

> i often work with TL07x chips

They have very good power-pin rejection, at least in the low audio band. Stunningly sloppy power often does not get into the audio.

Of course nasty power-wires wandering all over the board near sensitive audio points will make trouble.

Your signal ground needs to be clean. In simple systems we often use a "Vref" jacked up to half the supply voltage. This needs to be very clean. However this is usually a low- or "no"-current point, so hi-R and small-C is sufficient.

[darron]

> good filtering in everything i think is a good practice.

Yes, but not wanton over-kill. After a point, there's usually a better place to put a buck. "Balance in all things."

Well everything that I design comes with a 220uF low esr electro in parallel with a 100nF mkt and nothing more.... but i was thinking of doing some things hi-fi style, with a small series resistor between a few to a few hundred ohms.

Of course nasty power-wires wandering all over the board near sensitive audio points will make trouble.

Your signal ground needs to be clean. In simple systems we often use a "Vref" jacked up to half the supply voltage. This needs to be very clean. However this is usually a low- or "no"-current point, so hi-R and small-C is sufficient.

in most cases my half supply is two metal films tied with a 22uf electro to ground. then i try to keep lfos and high gains away from each other. where practical i lay extra earth blobs around the place if they can easilly connect back. i haven't had to use any shielded wire or anything, it would be completely unnecessary.


in some ways im happy not having an EE or anything.... it makes me feel like i've come a long way (: