Naim Audio NAP 350 Mono Amplifier Measurements

Link: reviewed by Jonathan Gorse on SoundStage! Ultra on June 1, 2024

General information

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

The NAP 350 was conditioned for 1 hour at 1/8th full rated power (~25W 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 NAP 350 is a single-channel amplifier with one balanced (XLR) input and one set of speaker level outputs. An input of 330mVrms was required to achieve the reference 10W into 8 ohms.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Naim for the NAP 350 compared directly against our own. The published specifications are sourced from Naim’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 330mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the NAP 350 was able to sustain about 382W into 4 ohms (~2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (38.2W) 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 of the NAP 350 were warm 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 NAP 350 exhibits a near-flat frequency response across the audioband (0/-2dB at 20Hz/20kHz). At low frequencies, the NAP 350 is -0.3dB at 5Hz. The -3dB point is just past 100kHz.

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 NAP 350 does not invert polarity and exhibits, at worst, about -20 degrees of phase shift at 20kHz, and +5 degrees at 20Hz.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms 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 an 8-ohm load and no-load to be around 0.5dB. This is an indication of a low damping factor, or high output impedance, for a solid-sate amplifier. With a real speaker, the deviations are lower, at just over 0.3dB.

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 at 160W. The 1W data ranges from 0.002% at 20Hz, down to 0.0005% at 200-300Hz, then up to 0.01% at 20kHz. The 10W data ranges from 0.005% at 20Hz, down to 0.002% at 60-2kHz, then up to 0.01% at 20kHz. The 160W data ranges from 0.04% at 20Hz, down to 0.007% at 40-1kHz, then up to 0.1% at 20kHz.

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

The chart above shows THD ratios measured at the output of the NAP 350 as a function of output power for the analog line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.002% from 50mW to 20W, then up to 0.01% at the “knee,” at roughly 180W. The 4-ohm THD data were close, but 5-6dB higher, with a “knee” at just past 300W. The 1% THD thresholds were reached at 201W and 370W into 8 and 4 ohms.

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

The chart above shows THD+N ratios measured at the output of the NAP 350 as a function of output power for the analog line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.015% down to 0.0025% at 10-30W. The 4-ohm data ranged from about 0.02%% down to 0.004% at 10-30W.

THD ratio (unweighted) vs. frequency at 8 and 4 ohms

The chart above shows THD ratios measured at the output of the NAP 350 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields roughly 50W at the output into 8 ohms (blue), 100W into 4 ohms (purple), and 200W into 2 ohms (pink). The 8-ohm data ranged from 0.008% at 20Hz down to 0.003% from 30Hz to 2kHz, then a steady rise to 0.03% at 20kHz. The 4-ohm THD data were only about 5dB higher compared to the 8-ohm data from 20Hz to 2kHz, while above this frequency THD ratios were the same at 4 and 8 ohms. The 2-ohm data ranged from 0.02% at 20Hz down to 0.006% from 200Hz to 2kHz, then a steady rise to 0.04% at 20kHz.This shows that the NAP 350 is perfectly stable into 2 ohms, and it yields THD results that are comparable to those seen at 8 and 4 ohms.

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 NAP 350 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 freqencies, the two-way speaker yielded the highest THD ratios (0.4%). Generally, below 500Hz or so, THD ratios into the real speakers were higher than the resistive load, from 5 to 30dB. Above 1kHz, THD ratios into the real speakers were either lower (by up to 5dB) or close to the resistive-load values.

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 NAP 350 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). All three IMD plots are close, within about 5dB, most often within a couple dB throughout the frequency sweep. IMD values ranged from 0.01 to 0.03%, which is not a particularly strong result.

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 NAP 350 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 real speakers were lower than those measured into the resistive load, by about 5dB. IMD values ranged from 0.03 to 0.01%, again, not a particularly strong result for a modern solid-state amplifier.

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) and third (3kHz) harmonics dominate at around -95dBrA, or 0.002%. The subsequent signal harmonics range from the -110dBrA level, or 0.0003%, down to -140dBrA, or 0.00001%. The highest noise-related peaks are at the third (180Hz) and fifth (300Hz) harmonics at a low -125dBrA, or 0.00006%.

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 the signal’s second (100Hz) and third (150Hz) harmonics at close to -100dBrA, or 0.001%. Noise-related harmonics are much lower, at the -125dBrA, or 0.00006%, and below level.

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 relatively high -85dBrA, or 0.006%, level, while the third-order modulation products, at 17kHz and 20kHz, are lower at -95dBrA, or 0.002%.

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

Shown above is the FFT of the speaker-level output of the NAP 350 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 below the -120dBrA, or 0.0001%, 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 NAP 350’s slew-rate performance. Rather, it should be seen as a qualitative representation of the NAP 350’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. Here we find clean corners with only mild softening.

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

The final graph above is the damping factor as a function of frequency. We find quite low damping factor values, right around 35 across the audioband. This is a poor damping factor result for a modern solid-state amplifier.

Diego Estan
Electronics Measurement Specialist