Identifying and Testing a Type A (Audio Taper) Potentiometer
A Type A potentiometer features an audio (logarithmic) taper, meaning its resistance changes non-linearly as you rotate the shaft. This taper mimics the human ear's logarithmic perception of volume, making it the standard choice for audio amplifier volume controls. By contrast, a Type B potentiometer uses a linear taper, where resistance changes at a constant rate.
Before testing, you must understand a critical regional naming convention that catches many DIYers off guard. In North America and Asia, Type A denotes an Audio (Logarithmic) taper, and Type B denotes Linear. However, in many European standards, the letters are reversed: 'A' means Linear and 'B' means Audio. Always verify the manufacturer's datasheet—such as those from Bourns or ALPS—rather than relying solely on the casing stamp.
Testing a Type A pot serves two purposes: verifying the logarithmic curve to ensure it is not a mislabeled linear pot, and checking for carbon track degradation (dead spots or 'scratchiness') that causes audio popping.
Never measure resistance on a live circuit. Disconnect the potentiometer from power and discharge any parallel capacitors before probing. While typical audio bench work involves low voltage, any multimeter used in an environment where mains-adjacent circuits (like tube amplifiers or active subwoofers) are present must carry a minimum CAT II safety rating to protect against transient spikes. For high-voltage tube amp bias pots, use a CAT III rated meter and high-voltage probe leads.
Multimeter Setup and Probe Placement for Pot Testing
Accurate taper verification requires a stable baseline. Set up your digital multimeter (DMM) and probe the lugs using the following parameters.
Meter Setup Block
- Dial Position: Resistance (Ω). Ensure the meter is not set to Continuity or Diode Test, as these apply different test currents that will skew resistance readings.
- Lead Jacks: Black lead into COM (Common). Red lead into V/Ω/mA (Voltage/Ohms).
- Range: If using a manual-ranging meter, select a range at least one step above the pot's nominal value (e.g., use the 20kΩ range for a 10kΩ pot, or the 200kΩ range for a 100kΩ pot). If using an auto-ranging meter, allow 2-3 seconds for the range to lock before recording the value.
Probe Placement Sequence
A standard potentiometer has three lugs: Lug 1 (Counter-Clockwise / CCW), Lug 2 (Wiper), and Lug 3 (Clockwise / CW). Orient the pot with the shaft facing away from you and the lugs pointing toward you.
- Total Resistance Check: Place the red probe on Lug 1 and the black probe on Lug 3. Rotate the shaft fully back and forth. The reading should remain stable at the nominal value (e.g., 10kΩ ±20%).
- CCW Baseline: Rotate the shaft fully counter-clockwise. Place the red probe on Lug 1 and the black probe on Lug 2 (Wiper). The reading should drop to near zero (typically 0.5Ω to 2Ω due to wiper contact resistance).
- The 50% Taper Test: Rotate the shaft to the exact mechanical midpoint (50% rotation). Keep the black probe on Lug 2 and move the red probe to Lug 1. Record this value. Then, move the red probe to Lug 3 and record that value.
- The Sweep Test (Noise Check): Place probes on Lug 1 and Lug 2. Slowly rotate the shaft from 0% to 100% while watching the display. The numbers should climb smoothly without dropping to zero or spiking to 'OL' (Open Loop).
Expected Readings: Good vs. Bad Type A Potentiometer Values
To determine if your component is functioning correctly, compare your measurements against the expected logarithmic curve. The industry standard for a Type A audio taper (per All About Circuits reference standards) dictates that at 50% mechanical rotation, the resistance between the CCW terminal and the wiper should be approximately 15% of the total resistance, while the wiper to CW terminal should be 85%.
Below is the expected reading table for a standard 10kΩ Type A Potentiometer at the 50% rotation midpoint.
| Test Point | Good Reading (Nominal) | Acceptable Tolerance | Bad Reading (Defective) |
|---|---|---|---|
| Lug 1 to Lug 3 (Total) | 10.00 kΩ | 8.0 kΩ - 12.0 kΩ (±20%) | OL (Open) or < 5 kΩ (Shorted track) |
| Lug 1 to Wiper (at 50%) | 1.50 kΩ (15%) | 1.0 kΩ - 2.5 kΩ | ~5.0 kΩ (Indicates a linear Type B taper) |
| Lug 3 to Wiper (at 50%) | 8.50 kΩ (85%) | 7.5 kΩ - 9.0 kΩ | ~5.0 kΩ (Indicates a linear Type B taper) |
| Wiper Sweep (Dynamic) | Smooth numeric increase | Minor jitter (±5 Ω) | Sudden drops to 0Ω or spikes to OL |
Numeric Verification: If your 10kΩ pot reads exactly 5.0 kΩ on both sides at the 50% mark, you do not have a Type A audio taper; you have a Type B linear taper (or a European-marked Type A linear). If the reading jumps from 2.1 kΩ to 8.9 kΩ with a slight physical nudge of the shaft, the carbon track is worn and the potentiometer must be replaced or cleaned with DeoxIT.
Common Measurement Mistakes That Give Misleading Readings
Even with a high-quality Fluke or Brymen meter, operator error can lead to false diagnostics. Avoid these common bench mistakes:
- Measuring In-Circuit: If you test a potentiometer while it is still soldered to the PCB, parallel components (like pull-down resistors or coupling capacitors) will create alternative current paths. A 10kΩ pot might read as 4.7kΩ due to a parallel resistor network. Always desolder at least two of the three lugs to isolate the component.
- The 'Finger Resistance' Error: When testing high-value Type A pots (e.g., 500kΩ or 1MΩ volume controls found in tube guitar amplifiers), holding the metal tips of both probes with your bare fingers places your body's skin resistance (typically 50kΩ to 200kΩ) in parallel with the pot. This will severely skew the 50% midpoint reading. Use alligator clips or probe hooks for high-impedance measurements.
- Misinterpreting DMM Refresh Rates: A standard digital multimeter updates its display 2 to 4 times per second. If you sweep the wiper quickly, a momentary 'dead spot' (micro-interruption where resistance spikes to infinity) might occur between display refreshes and go unnoticed. To properly test for scratchiness, sweep the pot very slowly, or connect the wiper to an oscilloscope with a pull-up resistor to visualize voltage dropouts in real-time.
- Ignoring the Wiper Contact Resistance: If your total resistance (Lug 1 to 3) reads 10.0kΩ, but your minimum resistance (Lug 1 to Wiper at 0%) reads 45Ω instead of <2Ω, the wiper spring tension is failing or the contact pad is heavily oxidized. This will cause a 'dead zone' at the bottom of your volume sweep.
Frequently Asked Questions About Type A Potentiometers
What is the exact difference between a Type A and Type B potentiometer?
The difference lies in the resistive track's geometry. A Type A (Audio/Logarithmic) pot has a track that changes resistivity gradient along its length, yielding roughly 15% of total resistance at the mechanical midpoint. This matches human hearing, which perceives volume logarithmically. A Type B (Linear) pot uses a uniform resistive track, yielding exactly 50% of total resistance at the mechanical midpoint, making it ideal for tone controls, panning, and lighting dimmers, but poor for audio volume.
Can I use a Type B linear potentiometer instead of a Type A audio taper for volume?
Technically yes, but the user experience will be poor. With a linear pot in a volume circuit, the perceived volume will remain very quiet for the first 70% of the knob rotation, and then suddenly jump to maximum volume in the final 30% of the sweep. If you are stuck with a Type B pot in a pinch, you can approximate an audio taper by soldering a fixed resistor (typically 10% to 20% of the pot's total value) between the CCW lug and the wiper, though this alters the total input impedance of the circuit.
Why does my Type A potentiometer read 'OL' (infinite resistance) on my multimeter?
An 'OL' reading between Lug 1 and Lug 3 indicates an open circuit. This is usually caused by a cracked carbon track, a broken solder joint at the internal lug-to-track crimp, or severe overheating from a previous soldering attempt. If you read 'OL' between the Wiper and an outer lug, but Lug 1 to Lug 3 reads correctly, the wiper has lost physical contact with the track due to bent spring fingers or heavy grease contamination.
What safety category (CAT rating) multimeter do I need to test potentiometers?
Potentiometers are passive, low-voltage components. When tested completely isolated from power, any basic multimeter is safe to use. However, if you are probing pots inside live equipment (such as adjusting a bias trim pot on a guitar amplifier or a live mixer board), your meter and probes must carry a minimum CAT II 600V rating to protect against internal power supply transients. Never use unrated, cheap promotional multimeters for live-circuit adjustments, as the internal shunt resistors can arc and cause severe injury.
