Audio Research D-80 Stereo Amplifier Measurements

Link: reviewed by Jason Thorpe on SoundStage! Ultra on January 15, 2026

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The D‑80 was conditioned for 1 hour at 1/8th full rated power (~10W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The D‑80 is a two-channel amplifier with two balanced (XLR) inputs and three pairs of speaker level outputs: 4-, 8-, and 16-ohm taps. Unless otherwise stated, the 8-ohm taps were used for these measurements. An input of 520mVrms was required to achieve the reference 10W into 8 ohms.

Our typical input bandwidth filter setting of 10Hz-22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz-90kHz was used. Frequency response measurements utilize a DC-to-1 MHz input bandwidth.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Audio Research for the D‑80 compared directly against our own. The published specifications are sourced from Audio Research’s website, either directly or from the manual available for download, or a combination thereof. Assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz. Our primary measurements revealed the following (unless specified, assume a 1kHz sinewave at 520mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the balanced line-level analog input (unless specified, assume a 1kHz sinewave at 520mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the D‑80 was able to sustain about 81W into 4 ohms (~3% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (8.1W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the sides and top of the D‑80 were very hot to the touch.

Frequency response (8-ohm loading)

In our frequency-response (relative to 1kHz) plot above, measured across the speaker outputs at 10W into 8 ohms, the D‑80 exhibits a near-flat frequency response across the audioband (0/-0.25dB at 20Hz/20kHz). At 5Hz the D‑80 is at roughly -3dB, and the high frequency -3dB point is at 70kHz.

Phase response (8-ohm loading)

Above is the phase-response plot from 20Hz to 20kHz for the balanced line-level input, measured across the speaker outputs at 10W into 8 ohms. The D‑80 does not invert polarity and exhibits, at worst, only -20 degrees of phase shift at 20kHz.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 10Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find the maximum deviation between a 4-ohm load and no load to be around 2.2dB through most of the audioband. At 20kHz, the deviation is about 2.5dB. This is an indication of a very low damping factor, or high output impedance, endemic of most tube amplifiers. With a real speaker, the deviations from 20Hz to 20kHz were lower, but still within the potentially audible range, at roughly 1.6dB.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input. The blue plot is at 1W output into 8 ohms, purple at 10W, and pink is at the maximum achievable power (67W). THD ratios increase as power is increased. At 1W, THD ranges from 0.02% at lower frequencies to 0.1% at 20kHz. At 10W, we find THD ratios between 0.15% and 0.7% at 20kHz. And at 67W, THD ratios exceed 1%, ranging from about 1.5% to 5%.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (8-ohm taps)

The chart above shows THD ratios measured at the output of the D‑80 as a function of output power for the analog line-level input for an 8-ohm load (blue/red) and a 4-ohm load (purple/green), for the 8-ohm taps. The 8-ohm data ranged from 0.005% at 50mW, steadily increasing to 1% at the maximum power output of 67W. Increasing the input voltage beyond this point only serves to increase THD while maintaining the same 67W. The 4-ohm data ranged from 0.015% at 50mW, steadily increasing to 2% at the maximum power output of 84W. Increasing the input voltage beyond this point only serves to increase THD while maintaining the same 84W.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (8-ohm taps)

The chart above shows THD+N ratios measured at the output of the D‑80 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red) and a 4-ohm load (purple/green), for the 8-ohm taps. The 8-ohm data ranged from 0.02% at 50mW, steadily increasing to 1% at the maximum power output of 67W. Increasing the input voltage beyond this point only serves to increase THD+N while maintaining the same 67W. The 4-ohm data ranged from 0.015% at 50mW, steadily increasing to 2% at the maximum power output of 84W. Increasing the input voltage beyond this point only serves to increase THD+N while maintaining the same 84W.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (4-ohm taps)

The chart above shows THD ratios measured at the output of the D‑80 as a function of output power for the analog line-level input for an 8-ohm load (blue/red) and a 4-ohm load (purple/green), for the 4-ohm taps. The 8-ohm data ranged from 0.005% at 50mW, steadily increasing to roughly 1.5% at the maximum power output of just past 40W. Increasing the input voltage beyond this point only serves to increase THD while maintaining the same power. The 4-ohm data ranged from 0.02% at 50mW, steadily increasing to 1.5% at the maximum power output of just past 60W. Increasing the input voltage beyond this point only serves to increase THD while maintaining the same power.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (4-ohm taps)

The chart above shows THD+N ratios measured at the output of the D‑80 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red) and a 4-ohm load (purple/green), for the 4-ohm taps. The 8-ohm data ranged from 0.02% at 50mW, steadily increasing to roughly 1.5% at the maximum power output of just past 40W. Increasing the input voltage beyond this point only serves to increase THD+N while maintaining the same power. The 4-ohm data ranged from 0.03% at 50mW, steadily increasing to 1.5% at the maximum power output of just past 60W. Increasing the input voltage beyond this point only serves to increase THD+N while maintaining the same power.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the D‑80 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields roughly 10W at the output into 8 ohms (blue), 20W into 4 ohms (purple), and 40W into 2 ohms (pink). The 8-ohm data ranged from 0.1-0.2% between 20Hz and 4kHz, then up to 0.7% at 20kHz. The 4-ohm THD data ranged from 0.5% from 20Hz to 200Hz, then steadily up to 2.5% at 20kHz. The 2-ohm data yielded THD ratios from 2% at 20Hz up to 8% at 20kHz. This shows that the D‑80 is stable into 2-ohms, but will exhibit very high THD ratios.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the D‑80 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At very low frequencies, the two-way speaker yielded the highest THD ratios (2%). In the all-important 300Hz to 5kHz range, THD ratios into all three loads were close, with the data into real speakers hovering above and below (+/-10dB) the 0.02-0.03% values seen for the resistive load. At the highest frequencies, the three-way speaker yielded the highest THD ratios (0.2% at 20kHz).

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the D‑80 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The IMD results into the resistive load are fairly consistent, from 0.03 to 0.05% across the sweep. The results were lower and higher for the real speakers, ranging from 0.02% to 0.07%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the D‑80 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The IMD results into the resistive load remained constant at 0.1% across the weep. The results were lower and higher for the real speakers, ranging from 0.05% to nearly 0.2%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. We see that the signal’s second (2kHz), third (3kHz), and fifth (5kHz) harmonics dominate at -60dBrA (2/3kHz) and -70dBrA (5kHz), or 0.1% and 0.03%. Other signal harmonics can be seen from -90dBrA (0.003%) to below -120dBrA (0.0001%). There are power-supply noise-related harmonics throughout the FFT, at -100dBrA, or 0.001%, and below.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most dominant (non-signal) peaks are again the signal’s second (100Hz), third (150Hz), and fifth (250Hz) harmonics at -60dBrA (100/150kHz) and -70dBrA (250Hz), or 0.1% and 0.03%. There are power-supply noise-related harmonics throughout the FFT, at -100dBrA, or 0.001%, and below.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. The input RMS values were set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at the -65dBrA (0.2%) level, while the third-order modulation products, at 17kHz and 20kHz, are at the same level.

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the speaker-level output of the D‑80 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are at the -90dBrA, or 0.003%, level.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the balanced analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the D‑80’s slew-rate performance. Rather, it should be seen as a qualitative representation of the D‑80’s relatively wide bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. In this case, we see a reasonably clean result, with mild ringing in the plateaus.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. We find very low damping-factor values, from 6-7 from 20Hz to 2kHz. This is a very poor damping factor result, but typical for tube amps.

Diego Estan
Electronics Measurement Specialist