Recording - May 2018 - 65

a 60W solid state power amplifier. Most electrical tests on the
bench were conducted using an 8Ω 50 watt resistor as a dummy
load rather than a loudspeaker, though the Reverbrocket speaker
was used for listening and some bench tests.
WARNING: Don't try this with a tube amplifier. You could
overheat the output tubes or zap the output transformer.
Solid-state amplifiers only!
A potato battery?
My first hypothesis was that the potato could be acting like
a battery, biasing the speaker cone off its rest position, thereby
causing the speaker to be less linear. With the two steel nails,
my "potato battery" produced an open circuit voltage of 0.13V
and short circuit current of 72 µA, barely a mousefart. A proper
battery has dissimilar electrodes, so I tried replacing one of the
nails with a piece of 12-gauge copper wire. This gave me almost
1V, but with 180 µA maximum current-not even enough to get
a click from the speaker.

Electrical characteristics
With the dummy load connected and an oscillator driving
the amplifier, by measuring the voltage drop across the potato
and the current flowing through it, I was able to calculate the reactance of the potato. This varied from around 1900Ω at 100
Hz to around 1100Ω at 20 kHz, and was fairly consistent for
several potatoes. A full-range frequency sweep showed nearly
flat response between 20 Hz and 2 kHz, rising to +6 dB at 20
kHz. That's not a very useful filter, considering that there isn't
much energy above 3 kHz coming out of a guitar. (Figure 6)
The frequency response rise above 2 kHz suggests that the
potato is shunted by a capacitor. Lacking a capacitance meter, I
found an inductor from my junk box that, wired in parallel with the
potato, gave a pronounced resonant peak at around 1 kHz. From
this, I calculated the potato's capacitance to be around 6.3 nF.
Next came gain (well, loss) measurements. With the amplifier
(sans potato) delivering 30 clean watts into the dummy load,

RESOURCES
Circuit Specialists (for DMMs and more): www.circuitspecialists.com
A good on-line tone generator (creates WAV files for you to download):
www.wavtones.com/functiongenerator.php
An excellent stand-alone function generator for Windows:
vincent.burel.free.fr/download/generator_standalone2.zip

adding the potato in series doesn't change the amplifier's output
voltage, but since it's now loaded with about 1800Ω rather than
8Ω, the load is getting only about 130 milliwatts. This is Ohm's
Law at work. With the amplifier loafing at 1/8 of a watt, the power
loss of the potato is nearly 25 dB. No wonder it isn't very loud.
"Turn it up!" Well, you can't, at least not with any practical instrument amplifier. Mr. Ohm tells us that in order to pump 30 watts
into 1800Ω, we'd need 230 volts. An amplifier's maximum output voltage is its power supply voltage, which for this amp is 80V.
Compression? Speaker damping?
Compression is a commonly used guitar effect, so maybe the
potato acts like a compressor. It could, if its resistance varied
with voltage or current, but no dice. Its electrical characteristics
were uniform over the full output voltage range of the amplifier.
Proper loudspeaker damping requires that the speaker be
fed from a sufficiently low impedance, and the potato's high
series resistance would likely result in ringing in response to
transients. Adding the potato increased ringing from one cycle
to more than 10 cycles (about 100 milliseconds). Since a
guitar already has plenty of sustain, it's unlikely that this ringing
would be an audible effect. (Figure 7)
This is not an April Fool joke-but it could be
If you think you're hearing the sound of the potato, you've
been fooled-what you're hearing is mostly square waves from
an amplifier driven to its voltage limit. A 1500Ω 1-watt resistor
will work just as well and is less messy, though these days a
potato is probably easier to find.
If pushing the amp past its normal settings inspires the player,
that's a good thing. Perhaps this potato wackiness will challenge
him to come up with a new sound, or to play better. Sylvia's no
fool. She does nutty things in session to craft something different
and exciting, and, quirky as this trick appears, this is why people
hire her to engineer hit recordings. So fool around-but also
understand what's going on under the hood.
An oscilloscope plug-in to monitor for clipping when you're testing levels:
jaggedplanet.com/vst/jscope.asp
A precision digital level meter if your DAW meters don't have very
good resolution (Windows): robotplanet.dk/knef/vumeter/
Mini-TRS to 2x RCA cable: tinyurl.com/yc5czn3u
Banana Plugs: tinyurl.com/yaugod46
Online dBu-dBV voltage calculator: tinyurl.com/5jau56

RECORDING May 2018

http://www.jaggedplanet.com/vst/jscope.asp http://www.circuitspecialists.com http://www.robotplanet.dk/knef/vumeter/ http://www.wavtones.com/functiongenerator.php http://www.tinyurl.com/yc5czn3u http://www.tinyurl.com/yaugod46 http://vincent.burel.free.fr/download/generator_standalone2.zip http://www.tinyurl.com/5jau56

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