Recently there has been a real resurgence of popularity in using old-style zinc-carbon batteries in stomp boxes. Zinc-carbon batteries are an older technology than that of the alkaline battery, even an older technology than the manganese dioxide “Heavy Duty” batteries at the dollar store.†
Old batteries sound better
A lot of folks claim older stomp boxes sound better with the old battery designs since they were designed to work with them. The problem is, they are hard to come by, and nowadays they’re only made in a couple of third-world plants. What we’d like to do, then, is to make an alkaline battery act like a zinc-carbon battery, so we could get the same sound while buying batteries at the corner market.
We can think of a battery as being an ideal current source, in series with an internal resistor. That internal resistor sets the maximum amount of current that the battery will produce. The “effective series resistance” in a car battery is very small, while that of a 9V battery is much higher, so the car battery can produce a lot more current.
Make alkaline behave like carbon batteries
Now, what we want to know is the effective series resistance of a carbon 9V battery, and that of an alkaline 9V battery. If we know these things (and we are sure that it’s a pure resistance, not a complex impedance that has both resistance and reactance in it), we can make an alkaline battery behave like a carbon battery.
We can figure this out by first measuring the unloaded voltage of the battery (V), which should be equivalent to the voltage of the ideal voltage source (since with negligible load the series resistance has negligible drop), and then putting a load resistor (R2) across the battery and measuring the loaded voltage (V2).
Check out the accompanying diagram. Ohm’s law tells us that the voltage across any resistor is the product of the resistance and the current going through it. That is, V=IR. So, in this circuit, we know these things:
1. We know the value of R2, since we put it there, we know the value of V2 since we measured it, and we know the value of V, since we measured that too.
2. The current going through each resistor is the same (since they are in series), and thanks to Ohm’s Law, I = V1/R1 = V2/R2.
3. The voltage from the ideal voltage source V is the sum of the voltage across the two resistors (V= V1+V2), so the unknown V1= V–V2.
What we don’t know is the value of R1.
But using (2) and (3) we know V1/R1= V2/R2 and V1=(V–V2), so we can substitute and get (V–V2)/R1 =V2/R2. With a little division, R1=R2(V-V2)/V2.
So, we can measure a few batteries, both new and old, with a 15 ohm and a 100 ohm load, then work out the derived value of R1.
Note that if a battery is producing a nominal 9V, a 15 ohm load draws 600 mA and a 100 ohm load draws 90 mA. I figure these are reasonable extremes, with most typical older pedals drawing near the low end of that.
The first thing that you’ll notice here is that the series resistance is not constant with load. This is because it is, in fact, not a simple resistance—there’s reactance in there too. But there’s no need to panic; for what we’re doing, it’s close enough that we can make some ballpark estimates, especially if we know how much current a device takes.
Looking at the chart, we can see that the series resistance of the sample carbon-zinc battery is around 20 ohms higher than the alkaline battery with a small load, and around 14 ohms higher with a greater load.
This means that if we add a 20 ohm resistor in series with an alkaline battery, we can get approximately the same behaviour that we got with a carbon-zinc battery. And that’s what we’ll do right now, making an adaptor with two 9V battery connector “snaps” and one resistor. (Editor Metlay nicknamed it the “Sagmaster” for the voltage sag it causes. Why not?) We’ll make two of them, one with a 10 ohm resistor and one with a 22 ohm resistor. See the parts list.
Wire the black wire from one snap to the red wire of the other. Put an inch of the smallest heatshrink tubing on the wire before soldering it, then put the heatshrink over the joint and use the side of the heating element on the soldering iron (which is less hot than the tip) to shrink it tightly over the wire without melting it or the wire’s insulation.
Clip the leads on one 10 ohm resistor down to about a quarter inch on each side. Get an inch of heatshrink tubing that is large enough to fit over the resistor, put it over the wire, and solder the red wire from one clip to one side of the resistor and the black wire from the other clip to the other side of the resistor. Slide the heatshrink over the resistor and shrink it down.
Now, do the same thing with two more of the clips and a 22 ohm resistor. Mark the two adaptors with a paint pen or a white marker, so you know which is the 10 ohm one and which is the 22 ohm one. (See the photo for the two I made.) Then try them.
They change the sounds of distortion pedals and wah-wah pedals noticeably. Other pedals? I don’t know, but it’s worth it to experiment on a variety of effects and see what you get. It’s another set of tones to have in your bag of tricks, for not very much money.
Scott Dorsey is a recording engineer and pro audio electronics enthusiast living in Virginia. He tends to sag when his batteries run down... but who doesn’t? He can be contacted via firstname.lastname@example.org