Basic Amplifier Measurement Techniques

There has been much discussion on many of the audio forums lately regarding mid priced A/V receivers power capabilities as well as their abilities to drive low impedance loads. Much of the controversy steams around the infamous " All Channels Driven Test " which simulates a best case test load in a worst case environment and by all intents and purposes an unrealistic real world scenario. For more information on this, the reader is encouraged to read the following articles: The All Channels Driven Test Controversy and Product Managing Receiver Platforms

With that, lets all first remember that receivers in the $1k price range from the leading companies today are an absolute bargain regarding their processing power, features, and performance. I like to think of them as an excellent Preamp/Processor (usually rivaling many costlier Pre/Pros) with a free amp thrown in. Whether the amp section within the said receiver is " good enough " depends on the following variables:

• Loudspeaker Impedance
• Loudspeaker Efficiency
• Room Size
• Room Acoustics
• Listening Preferences
• Whether or not you apply bass management to your Loudspeakers and utilize a dedicated powered subwoofer(s).

As you can see, there are a number of variables and the answer is simply not that straight forward. Let's continue on and establish some of the critical test criteria for gauging amplifier performance.

THD and IMD Distortion - This is a measure of how well an amplifier can faithfully reproduce a signal without introducing any artifacts or harmonics. While this is an important metric to consider, most solid state amps these days boast pretty low figures here (usually well below audibility) and are rarely a real world problem for well designed amplifiers. In addition, distortion measurements into a resistive load are generally good on amplifiers. Reactive load distortion testing is rarely done, but of interest if a quality load could be constructed and not act as the distortion mechanism. This will be revisited at a later date and incorporated into this test procedure when appropriate.

Signal to Noise Ratio - This is a measurement of importance to consider as it is a ratio of the magnitude of the signal and background noise. Many amplifier manufacturers publish their SNR figures at full power, which can be misleading and hard to compare between different amplifiers at differently rated power levels. Many also apply weighing filters to yield better results such as dBa or dBc. This can make over a 10dB difference compared to unweighted measurements and should be noted when comparing various amplifiers. It is best to look at 1 watt levels (where the amp spends most of its time) and interpolate the SNR at the amplifiers full rated power. We will leave this topic for another article in the near future.

Frequency Response - This is a measure of how well an amplifier will reproduce a signal without attenuation or boosting with ideal being flat within the range of human hearing 20Hz to 20kHz +/- 0dB. This again is rarely a problem with well designed amps and there should be little reason why an amplifier cannot maintain at least +-.5dB from 20Hz to 20kHz. Some amps do result in excessive power fall off at high frequency due to slew rate induced distortion and this is something to watch out for. Thus why we check frequency response at 1 watt and full power for our amplifier testing.

Amplifier Output Impedance - This again is an important metric as it determines the amount of interaction or frequency response variation an amplifier may produce when driven by a reactive loudspeaker load. Usually lower is better, but some do prefer high output impedance tube amps for example since they can excessively roll off high frequency response and result in a warm sound. Note, sonically preferring something doesn't necessarily mean its more accurate or precise.

Damping Factor - This specification is somewhat overstated by most manufacturers and the reader is encouraged to reference our article: Damping Factor - Effects on System Response for more information. I still like to check for this as it is a good indication of amplifier output impedance and power supply capability. However, in real world scenarios, cable impedance usually nullifies the benefits of a very high damping factor and thus beyond a certain value as stipulated in the aforementioned article it becomes mostly academic.

Amplifier Measurement Techniques

Required Test Equipment

• Non inductive Resistors (8 ohms, 4 ohms)
• Oscilloscope
• Short 10AWG Speaker Cables
• Function Generator

Measuring Output Impedance and Power of Amplifiers

Step 1: Adjust the function generator until you read 2.82Vrms on the u nloaded amplifier as depicted in the left figure above. Record these results in Table 1. under "Vopen" over the entire frequency range.

Step 2: Attach a non inductive Load to the amplifier and re-measure the output voltage over frequency and record the data in the appropriate "Vload" column specific to the load impedance used in this test in the three Tables below. Do NOT vary amplitude of the function generator, only Frequency.

Step 3: Calculate the amplifier output impedance (Zo) and Damping Factor with the following relationship:

VLoad = (RL / RL+Zo)*Vopen

Solve for Zo = > Zo = RL*Vopen/VLoad - RL

Damping Factor = Zload / Zo

Step 4: Plot the data from the Tables into Figures in excel.

Step 5: Find the maximum unclipped voltage the amplifier can provide into the specified test load

Step 6: Record the voltage over the specified frequency range.

Step 7: Detach the test load and record the voltage "Vopen" vs frequency repeating step 3 to calculate output impedance.

Note: The following variables limit the absolute accuracy of this test method:

• Cable impedance and stray inductance in resistor
• Variance in line voltage (since all tests are done without a line stabilizer)

However, this is more of a real world than typical ideal test condition, thus for all intents and purposes serves well for this application.

Test Notes

• Record the 8ohm test load + test cable measurements: (ie. Rdc = 8.04 ohms ; Ls = 5uH (1kHz) )
• Record the 4 ohm test load + test cable measurements: (ie. Rdc = 4.03 ohms ; Ls = 3.8uH (1kHz) )
• All measurements should be done in "Pure Direct Mode" or analog bypass
• If Line Voltage is NOT held constant, variance (if any) should be noted in the measurements
• All tests should be conducted using EXT Multi-CH Inputs and Outputs when appropriate