Can’t I Just Leave the Input Unplugged?
If the input is left floating, meaning it’s not connected to anything, the voltage at that point can drift unpredictably due to electrical noise picked up from the environment. This can make the circuit much more susceptible to picking up hum, radio interference, static and all sorts of unwanted signals. There’s nothing more annoying than an amp blaring out random interference and static.
To prevent this, like most designers I usually design my circuits so that removal of the input jack short-circuits the input to ground. If there is nothing plugged into the input jack the circuit can’t pick up any of that stray interference.
A consequence of this is that I must have something plugged into the circuit to be able to listen and test for noise.
Why Test with a Dummy Load?
Imagine I’ve just finished building a pedal. Before I plug in my guitar, I want to check for unwanted noise, things like hum, hiss, or crackle, that might be present in the circuit. If I use my actual guitar for this, I’m introducing a lot of extra variables: the guitar’s own electronics, cable capacitance, and even electromagnetic interference from the environment. All of these can mask or mimic noise that’s actually coming from the circuit itself.
By using a resistor as a dummy input load, I eliminate these variables. The resistor acts as a simple, predictable load for the input stage, letting me focus solely on the circuit’s noise performance.
A properly chosen dummy load resistor “anchors” the input to a steady, known value, just like a real guitar would. This ensures the noise you hear is coming from the circuit under test, not from the input being left open to stray signals.
Choosing the Right Value for the Dummy Load
A guitar’s output impedance—the resistance it presents to whatever it’s plugged into—is typically in the range of a few kΩ to a few tens of kΩ, depending on the pickups, wiring, and control settings. For a dummy load, you can use any resistor in this range. If you want to be very precise, you can measure your guitar’s resistance at the output jack with your preferred settings and use that value for your dummy load.
Why Too High a Value Is a Bad Idea
You might wonder why not just use a much higher value resistor, like 100 kΩ or 1 MΩ, surely you’d be covered for any guitar? Here’s why that’s not recommended:
- Floating Input Problems: As the resistance increases, the input behaves more like it’s floating, with all the noise and instability that comes with it.
- Reduced Noise Immunity: High-value resistors don’t “anchor” the input very well. The higher the resistance, the easier it is for stray electromagnetic signals (like mains hum or radio waves) to induce voltage at the input, making your circuit seem noisier than it actually is.
- Unrealistic Loading: Your guitar never presents such a high resistance to the circuit, so using a very high value doesn’t accurately represent real-world conditions.
To sum up: using a resistor that’s too high in value can make your circuit appear much noisier than it would be in real use, and doesn’t give you as realistic a picture of how the circuit will perform with an actual guitar plugged in.
Should I Use a Potentiometer to Match the Guitar’s Resistance?
You might also wonder if using a potentiometer (variable resistor) and tuning it to match your guitar’s resistance would be even better. In theory, this could more closely replicate the load your guitar presents. However, there are some important considerations:
- Guitar Impedance Is Complex: A guitar’s output isn’t just a simple resistance; it’s a mix of resistance, inductance (from the pickups), and capacitance (from the cable). A plain resistor or potentiometer only simulates the resistive part, not the full frequency-dependent behaviour of a real guitar signal.
- Potentiometer Noise: Most potentiometers, especially carbon composition types, are noisier than fixed resistors. They can introduce their own contact noise, especially as they age or if the wiper isn’t making perfect contact. This could make your noise measurements less accurate.
- Practical Testing: For most noise testing, the main concern is to provide a load that prevents the input from floating. A fixed resistor matching your guitar’s typical output resistance—say, 10 kΩ or 20 kΩ—does the job well enough for practical purposes.
Advantages of Using a Dummy Load Resistor
- Consistent, Repeatable Testing: A fixed resistor provides a stable and known resistance, so every time I test, I know exactly what load the circuit is seeing. This consistency helps me compare noise levels before and after modifications or repairs.
- Eliminates External Noise Sources: Guitars, especially those with single-coil pickups, are susceptible to picking up electromagnetic interference. By using a resistor, I remove the guitar and cable as potential sources of hum or buzz, isolating noise that originates in the circuit itself.
- No Microphonic Effects: Guitars and cables can act like antennas or even microphones for mechanical vibrations. With a resistor, there’s no chance of picking up noise from physical movement or environmental vibrations.
- Safe for Circuit Debugging: If the circuit has a fault or is unstable, using a resistor avoids the risk of damaging a valuable instrument.
The Simplest Dummy Load: A Resistor in a Jack Plug
- Choose a resistor value that matches your guitar’s typical output impedance (more on this below).
- Solder one end of the resistor to the tip terminal and the other end to the sleeve terminal inside the jack plug.
- Close up the plug and label it with the resistor value for future reference.
When to Use More Complex Loads
If you really want to simulate the exact behaviour of a particular guitar and cable, you’d need a more complex test jig that includes inductance and capacitance as well as resistance. This is rarely necessary for basic noise testing, but can be useful if you’re designing circuits that interact closely with the guitar’s tone and loading characteristics.
Conclusion
In summary, using a resistor as a dummy input load when testing audio circuits offers clear advantages for isolating and diagnosing noise issues. The best approach is to use a resistor in the 4 kΩ to 40 kΩ range, matching your guitar’s real-world output impedance. This prevents the input from floating, which keeps out stray noise and gives you a true picture of your circuit’s performance. While a potentiometer offers flexibility, it can add its own noise and still doesn’t fully simulate the complex impedance of a guitar. For most troubleshooting and noise testing, a simple resistor is the best tool for the job.
If you want to get even more accurate, you could build a more elaborate dummy input that includes inductance and capacitance to better mimic a real guitar and cable—but for most practical purposes, a resistor is all you need.