something to starve voltages for fuzzes (with psu)

[Narcosynthesis]

you can get some pretty cool sounds by using dieing batterys in fuzz pedals that give less voltage than the pedal 'wants'
so what i want is a similar effect but for a power supply - so you plug the box between the supply and pedal and it can reduce the voltage given

any ideas on a diy solution?

David

[bass_econo]

Couldn't you do this with a pot or a trimmer from the supply?

[R.G.]

The simple way is with a low dropout regulator that has a variable output voltage, plus maybe a series resistor.

The circuit's pretty easy.

[R.G.]

Do a google search on the National Semiconductor LP2951 regulator. Output is adjustable from 1.5V to 29V (if you have a 30V supply to feed it from...) or in your case, battery voltage minus 0.5V down to 1.5V with a pair of resistors. I believe that the resistors from my dying battery simulator at GEO will work just fine. It's an 8 pin DIP, puts out up to 100ma.

It could easily be integrated inside any pedal itself, with an external knob for voltage level, perhaps another knob for the series resistor.

[nelson]

wouldnt a simple pot on the 9v rail before it hits the rest of the circuit be sufficient enough for a starve control. Or a pot just before the output of a regulated PSU?


EDit just seen bass econo' post... :oops:

[bass_econo]

wouldnt a simple pot on the 9v rail before it hits the rest of the circuit be sufficient enough for a starve control. Or a pot just before the output of a regulated PSU?


EDit just seen bass econo' post... :oops:

Well I'm glad someone else had the same idea...

[Narcosynthesis]

i wasnt sure if adding resisters and crap in the supply will screw about with current draw or anything

i will have a look for the regulator (i guess you could use a switch to bypass it for full voltage) and the stuff on geo

David

[gaussmarkov]

wouldnt a simple pot on the 9v rail before it hits the rest of the circuit be sufficient enough for a starve control. Or a pot just before the output of a regulated PSU?


EDit just seen bass econo' post... :oops:

Well I'm glad someone else had the same idea...

+1 (don't pedals draw as much current as they require?)

[Narcosynthesis]

a pedal will only draw as much as it requires, but i think adding resisters in the power rails (therefore changing the resistance of the pedal as seen by the supply) will change the current required...

David

[gaussmarkov]

a pedal will only draw as much as it requires, but i think adding resisters in the power rails (therefore changing the resistance of the pedal as seen by the supply) will change the current required...

David

thanks, that makes sense.  but if that's the only consequence, isn't it still true that the trimmer would work as suggested above?  i guess the remaining issue is what happens to the current requirements.  

now here's where the weaknesses in my electronics self-education probably begin to show:   :oops:  because we are adding a resistor in series with the circuit, we can add up the resistance of the resistor and the original pedal to get the total resistance, right?  what's the current running through a fuzz?  say about 1mA?  i used a spice simulation of a basic fuzz face that i made a while back to get that number.  that means the resistance of the fuzz circuit is about 9k.

so to get a 2v drop we need a resistor (R) at the new current (I) such that RI=2.  for the circuit overall, we still have 9v, resistance is R+9000, and the new current (I).  solving, i get R=2.6k and I=0.8mA.  the current drops because total voltage is constant and the resistance is higher.  seems to work ok.

if the current required by the fuzz is 10mA, then the fuzz's resistance is 900 ohms and everything changes by a factor of 10.  the required resistor becomes 260 ohms and the current becomes 7.8mA.

so i'm still curious.  as long as the dc supply is already regulated, why another voltage regulator?

[R.G.]

thanks, that makes sense. but if that's the only consequence, isn't it still true that the trimmer would work as suggested above? i guess the remaining issue is what happens to the current requirements.

Correct - the circuit takes only as much current as it needs, and the current requirements then dictate the drop across any series resistor or trimmer pot.

now here's where the weaknesses in my electronics self-education probably begin to show:

No embarassment is necessary - you're actively trying to learn. That's GOOD.

because we are adding a resistor in series with the circuit, we can add up the resistance of the resistor and the original pedal to get the total resistance, right? what's the current running through a fuzz? say about 1mA? i used a spice simulation of a basic fuzz face that i made a while back to get that number. that means the resistance of the fuzz circuit is about 9k.

Here's the problem: some circuits act like resistors all right, where the current varies linearly with the voltage across them. Many do not. They change their current needs depending on what they're doing at a given instant. Averaging the currents with a filter cap helps, but different circuits use dramatically different currents, and may well draw quite different currents with different signals. It all depends on the circuit, and that can be, literally, anything.

This is the whole reason tweaking in a power supply with a resistor is a problem - the dirty so and sos won't stand still.

as long as the dc supply is already regulated, why another voltage regulator?

Which is why we want another regulator from a regulated power supply. The second regulator duplicates a worn out battery's voltage, which is lower than the 9.? volts it had when it was fresh, and the series resistor duplicates the increase in internal resistance that happens as the chemicals get used up inside.

[gaussmarkov]

thanks a ton R.G.!  i really appreciate your time and thoughtfulness.  

[Narcosynthesis]

so a pot in series with the supply (on the hot line) wired as a variable resister should work fine?

from what gauss said, something like a 5k pot should be fine and give me more than enough voltage drop for a fuzz

David

[gaussmarkov]

so a pot in series with the supply (on the hot line) wired as a variable resister should work fine?

from what gauss said, something like a 5k pot should be fine and give me more than enough voltage drop for a fuzz

David

no, unfortunately, a pot in series with the supply will not work fine.  the basic problem is that we cannot treat the overall fuzz circuit as a simple (as in constant) resistor.  so we have to simulate the characteristics of an "independent" supply like a battery.  this is where the voltage regulator comes in.  check out the power supply stuff on geofex.com.  i recommend this article as especially about what we've been talking about.

[petemoore]

OK...what about a voltage divider, just uneven R's per side?
 What'd happen if you seriesed a 10k and a 1k, 10k to ground 1k to V+ [9V], would that make for a 20% Voltage Drop?
  :oops: math  :oops:

[R.G.]

OK...what about a voltage divider, just uneven R's per side?
What'd happen if you seriesed a 10k and a 1k, 10k to ground 1k to V+ [9V], would that make for a 20% Voltage Drop?

No. In this one case, you have a simple resistance across a voltage. The current is 9V/11K = 0.818 ma by Ohms law. The voltage across any part of that resistance is just the current through it times the resistance. For the 10K part, it's 10K times 0.818 = 8.18V.

That's fine as far as it goes - but what happens if you connect an effect pedal to the junction of the 1K and 10K? The effect pedal sucks some current, right? That makes the voltage drop across the 1K larger. How much larger? How big is your pedal? Worse yet, your pedal may well *vary* in how much current pulls, so its drain from the 1K resistor changes. In fact, that destabilizes the circuit and many circuits will oscillate when you have a setup like this. Some people like that, some people don't.

In fact, what do you need the 10K for? Make the 1K a bit bigger and the voltage drops the same level. What are the number? Don't know. Varies with time of day, phase of moon, and so on.

You can't regulate voltage with resistors. You can drop voltage with resistors. You can dink with the resistances to get it to drop the right amount at the right times, but you can't start with the resistors and get it right.

[toneman]

Isn't the "STAB" control on the FuzzFactory just a 5K pot in series with the 9V batt??  
Seem's like U could do the same(?)
BTW, good explaination RG, & mucho patience....  
stayregulated
tone

[bass_econo]

This is the first thing I thought of when reading this post - http://aronnelson.com/gallery/album17/386_FUZZ

[analogmike]

One other thing to think about -  it's not just the voltage that gives an old fuzz the cool sound. The battery has other parameters when it dies that may have more to due with the sound than just the voltage. A-B test an old half dead battery and a voodoo labs PP2 with adjustable output set to the same setting and I don't think they will sound the same.

[R.G.]

No, it's not just the voltage. It's the voltage *and* the increased internal impedance, as I mentioned in my post above. The voltage decreases, the internal resistance rises. That is the essence of my "Dying Battery Simulator" project at GEO (which predated a patent application for the same thing by six months...).

I keep telling people to read GEO. Sometimes it works, sometimes not.