5CH Mode Power vs Frequency Response

The -3dB point of this amp is around 60kHz when driven to full unclipped continuous power 85wpc (0.1% THD + N) into 2 channels driven or 165wpc into 4 ohms with 2 channels driven. 85wpc was a bit shy (-1.5dB) of their 120wpc claim into 8 ohms but they did deliver rated power into 4 ohm loads. I was unable to sweep down to 10Hz at full power because amp kept switching into protection mode in 5CH Mode.

This did not occur in 2CH Bridged Mode however.

2CH Bridged Mode Power vs Frequency Response

In 2CH bridged mode, the PA350B was able to achieve a whopping 317wpc with two channels driven continuously above 50Hz (-3dB point around 53kHz) with less than 0.1% THD + N into 8 ohms. However, when driving 2 channels simultaneously, I was unable to exceed 265wpc x 2 without having the amp shut off if driving any greater power levels at low frequencies (below 50Hz) for sustained continuous conditions. This isn't surprising since in order to deliver 317wpc x 2 continuously, the power supply would have to be capable of delivering (317 x 2 / .5) = 1268 watts which the two 500VA transformers in this amplifier aren't capable of delivering. The amp would require at least 750VA dual transformers to accomplish such a feat. This would likely never be an issue in real world since we don't listen to continuous correlated test tones.

I was able to achieve unclipped 500wpc continuously at (0.1% THD + N) into 1 channel driven at 4 ohms. Unfortunately, the amplifier didn't have a large enough power supply to deliver its full power rating continuously into 2 channels. If I drove it any higher than 200wpc x 2 continuously at full bandwidth into 4 ohms, the amplifier would shut off. This is an unlikely real world scenario but something to be mindful of if you have an appetite for achieving LOUD SPL levels from 4 ohm, moderately sensitive ( < 89dB @ 1 watt/meter) into a large theater room (3000 ft^3 or more). Also keep in mind most review publications don't do continuous power measurements and this observation would have went unnoticed in their test procedures.

The PA350B is rated to deliver 350wpc x 2 into 8 ohms or 500wpc x 2 into 4 ohms but the manufacturer doesn't specify if this is a continuous rating with both channels driven, though its kinda obvious it isn't given the size of the power supply, and the rated max power consumption of 1000 watts, thus one cannot call the ACD power police on them for not meeting their power specification since I was able to validate it for at least 1 channel driven into 8 and 4 ohm loads on a continuous full bandwidth basis at 0.1% THD + N despite the manufacturer didn't even specify distortion limits of their ratings.

5CH Mode FFT Analysis

At 1 watt into 8 ohms, the PA350B exhibited impressively low distortion (8.879+104.055)dBv = 112.9dBv or 100*alog^-1(-112.9/20) = .00026% This is one of the lowest distortion measurements we've seen and a tribute to the fully balanced output design topology of this amplifier.

2CH Bridged Mode FFT Analysis

At 550wpc into 4 ohms, the PA350B didn't falter at providing excellently low distortion, unadulterated power. (33.492+47.677)dBv = 81.169dBv or 100*alog^-1(-81.169/20) = .0087% A rare achievement for amplifiers running at full rated power and again attributed to its fully complementary output topology.

In 5CH mode, the PA350B exhibited about average SNR performance from what we've come to expect of high performance amplifiers. 2CH bridged mode however exhibited a significant improvement in amplifier noise floor which is most apparent by the 6dB improvement at 1 watt. Using the balanced inputs further lowered the noise floor another 4dB or so to achieve a very respectable 80dB SNR @ 1 watt unweighted. The manufacturers SNR specification of 112 dBA seems about right for a full power measurement using A-weighted filtering.

The amplifier output impedance was superb in both 5CH Mode and 2CH Bridged Mode. It maintained well under 100 mohms (our benchmark #) for the entire audio frequency range. This amplifier should sound very consistent between different speaker types assuming it isn't driven beyond its power limits.

The amplifier output impedance was again under 100mohns in 5CH mode and about half that in bridged mode. The increased output impedance below 20Hz indicated perhaps where they chose the pole for high pass filtering the low end response to block DC. Incidentally this is the frequency area that caused the amp to shut off if swept at full power - again a non likely real world scenario. The accentuated output impedance at 40kHz into a 4 ohm load in Bridged mode was a bit peculiar but since it was well out of the audio range, I didn't give it much thought for further investigation.

The damping factor of the Thule amp in 2CH bridged mode was uniformly excellent at all power levels and load impedances as was expected based on the measured output impedance. I was a bit puzzled as to why the amps output impedance had such a sharp rise at 20Hz, causing poor damping, when driven to full power. I could only assume it was some intentional protective measure to NOT overdrive the amplifier. This did not cause any audible hiccups in my listening tests however.

Even in 5CH mode, the Thule amp exhibited excellent damping characteristics, though the puzzling 20Hz loss of damping at high power was also prevalent.