Primare A35.2 power amplifier Measurements

Sidebar 3: Measurements

I tested the Primare A35.2 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). As the A35.2 has a class-D output stage, I didn't perform my usual preconditioning by operating the amplifier at one-third the specified power into 8 ohms for an hour. With a class-AB output stage, one-third power results in the maximum dissipation in the output devices, but this is not relevant with a class-D design. However, because class-D amplifiers emit relatively high levels of ultrasonic noise that would drive my analyzer's input into slew-rate limiting, all measurements were taken with Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz. Without the filter, there was 433mV of ultrasonic noise present at the loudspeaker terminals.

The Primare's voltage gain into 8 ohms measured 25.8dB from the single-ended inputs and, unusually, 19.8dB from the balanced inputs. (A balanced input typically offers 6dB higher gain rather than 6dB lower.) The amplifier preserved absolute polarity (ie, was noninverting) with both balanced and unbalanced input signals. The balanced input impedance was 13k ohms from 20Hz to 20kHz. The unbalanced input impedance was 14.7k ohms at low and middle frequencies, dropping slightly and inconsequentially to 13.5k ohms at the top of the audioband.

The A35.2's output impedance was a very low 0.067 ohm at all audio frequencies. (This figure includes the series impedance of 6 feet of loudspeaker cable.) The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was therefore minuscule, at ±0.1dB (fig.1, gray trace). The response into an 8 ohm resistive load (fig.1, blue and red traces) was flat to 20kHz, with then a steep rolloff. The Primare's reproduction of a 10kHz squarewave (fig.2) featured a single damped cycle of ringing, this presumably due to the output stage's integral low-pass filter.


Fig.1 Primare A35.2, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (1dB/vertical div.).


Fig.2 Primare A35.2, small-signal 10kHz squarewave into 8 ohms.

Channel separation was excellent, at >90dB in both directions below 2kHz, reducing to a still-good 72dB at the top of the audioband. Measured with the unbalanced inputs shorted to ground and with the auxiliary low-pass filter in-circuit, the amplifier's unweighted, wideband signal/noise ratio was 52.6dB ref. 1W into 8 ohms in both channels. This ratio improved to 73.3dB when I restricted the measurement bandwidth to the audioband, and to 76dB when the reading was A-weighted. Low-level spuriae at the 60Hz power-supply frequency and its odd harmonics were vanishingly low in level in the Primare's low-frequency noise floor (fig.3). However, a pair of spurious tones of unknown origin were present around 500Hz, these low in level but a little higher in the right channel (red trace) than the left (blue).


Fig.3 Primare A35.2, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (left channel blue, right red, linear frequency scale).

Primare specifies the A35.2 as delivering 200W into 8 ohms and 400W into 4 ohms. Both powers are equivalent to 23dBW, which implies that the Primare amplifier behaves as a perfect voltage source. Using our definition of clipping, which is when the output's percentage of THD+noise reaches 1%, the Primare exceeded its specified powers. With both channels driven with a 1kHz signal, it clipped at 225Wpc into 8 ohms (23.5dBW, fig.4) and at 460Wpc into 4 ohms (23.6dBW, fig.5).


Fig.4 Primare A35.2, distortion (%) vs 1kHz continuous output power into 8 ohms.

The distortion at low powers in figs.4 and 5 was below the level of the noise floor, so I examined how the percentage of THD+noise changed with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The THD+N was very low in the midrange into 8, 4, and 2 ohms (fig.6), but rose at higher frequencies. The Primare amplifier's distortion at 50W into 8 ohms was predominantly the subjectively innocuous second and third harmonics (fig.7), these each lying at just –100dB (0.001%, fig.8). The second harmonic increased slightly at the same voltage into 4 ohms but was joined by the fifth and seventh harmonics (fig.9). When the amplifier drove an equal mix of 19 and 20kHz tones at 100W into 4 ohms (fig.10), the second-order difference product at 1kHz lay below –100dB (0.001%) and higher-order intermodulation products were not much higher in level.


Fig.5 Primare A35.2, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.6 Primare A35.2, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta) 2 ohms (left gray).


Fig.7 Primare A35.2, 1kHz waveform at 50W into 8 ohms, 0.0025% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.8 Primare A35.2, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (left channel blue, right red, linear frequency scale).


Fig.9 Primare A35.2, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 4 ohms (left channel blue, right red, linear frequency scale).


Fig.10 Primare A35.2, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 4 ohms (linear frequency scale).

The Primare A35.2 offers respectable measured performance, with superb matching between the channels.—John Atkinson

Primare AB
US distributor: MoFi Distribution
1811 W. Bryn Mawr Ave.
Chicago, IL 60660
(312) 738-5025

JRT's picture

Herb (Cc: JA2 & JA1), Within this review you mentioned your Magnepan 0.7 planar loudspeakers and linked to a partial review (no measurements) from 06 August 2015. You have had a long time with these, and I think a follow up review addendum to that article could be an interesting read, and very much more so if JA1 includes a set of measurements.

Bogolu Haranath's picture

If you deliver free pizza to JA1's home for next couple of months, JA1 may consider doing the measurements of Magnepan .7 speakers ....... Just kidding :-) .......

Bogolu Haranath's picture

Take a look at the measurements of Magnepan LRS ...... .7 measurements could be similar :-) ......

avanti1960's picture

with the 4-ohm terminals on the Harbeths I suggest you again try the 8-ohm terminals.
Doing so should greatly improve the dynamics and impact of the bottom six octaves.

georgehifi's picture

"All measurements were taken with Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz. Without the filter, there was 433mV of ultrasonic noise present at the loudspeaker terminals."

Of course they had to be other wise it would have looked like this.

When is Stereophile going to bench test a Class-D that get rid of this because of the use of 1.5mhz switching used with the appropriate output filter like on the Technics SE-R1 Class-D that won't need the Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz to be used to see it 1 or 10k square wave.

Cheers George

SNI's picture

There really is no reason to enclude a carrier wave into the measurements. If you could hear the switch frequency the yes, but i guess you can´t.
Typically the switch frequency is placed arround 5-600 KHz where there is a sweet spot, providing the best compromise between, losses, EMC and distortion in higher order modulated amps.
The only reason for Technics to include filtering is EMC approval.
Ripple in the output of an amp, cannot be accepted almost anywhere in the world. That is nogo.
Therefore you often se inductors applied to the output of class D amps, especially those from OEM manufacturers.
This because often OEM modules are used in active speakers, where no such EMC regulations apply.
No speaker in the world can replay those frequencies, and they are way to inductive, to cause any problem at all.
The same goes for passive speakers, but EMC rules goes for anything. Even your hairdryer has to comply with the same rules.
No ripple allowed.
Because of this you can see a variety of solutions to this problem in class D amps. Some use a ferrite core with output wires twisted arround. Some even uses transformer coupling and so on.
Anyways the best audiosolution is no filtering. No speaker reacts on HF ripple at all, and if doesn´t make your mobilephone unusable, one shouldn´t really care about it.

Bogolu Haranath's picture

May be HR could review the comparably priced ($3,995), Rogue audio DragoN tube/class-D hybrid amp, 300/500 WPC 8/4 Ohms ........ DragoN may provide that high frequency 'bloom' HR is looking for :-) ........

Briandrumzilla's picture

No thanks.

tonykaz's picture

Mr.HR is certainly describing & revealing High performance levels for the Reviled Class D.

The top Pro guys like Bob Katz have already embraced the enhanced capabilities of this new design, what increase in perfection is offered by Amp Companies presenting similar product at the $150,000 price level ?

Schiit make ( in California of all places ) far more complex products at far lower prices. Why do these Primare people feel compelled to have unknown sub-contractors building their gear ( our gear ) ?

I ask for fair priced, quality gear designed and built by Proud & loyal employees. Shopping out Asian Labor to make a fast-buck is Taboo.

Put a : Proudly made in Sweeden sticker on each unit and watch sales double.

Tony in Venice

ps. what is the point of three chassis feet?

David Harper's picture

instead of listening to your equipment with music why don't you guys try listening to music with your equipment?

zalimci's picture

Out of sheer wanton extravagance and curiosity I bought a slightly used A35.2 contrary to my gut feeling. The key lever was the reference to the way this review praised its ability to deal with the Magneplanars due to the possibly unprecedented high damping factor, I have both .7s and 20.7s. I have amps which quote damping factors as low as 10 and as high as 250, but I'm led to believe that the A35.2 value is greatly higher, inherent to class D technology.

Well it made a very good initial impression, sounding as clean and fast as expected. As with the .7s in the review the lower end of the sound picture was very quick and tight. Some familiar recordings sounded different, a couple possibly slightly better than previously. Some less pleasing...

Then back to ones that pleased, and they didn't please as well second or third time around. I had possibly been overkind to this amp in that for the week or so prior to arrival I had been using an elderly solid state amp by the name of Dawn Audio Command 2, a small Britsh enterprise based in Sussex. Plenty of power and grip etc, not overly refined and a gift from an older friend whose hearing has become impaired to the extent that listening is no longer a pleasure. So, this morning it was time to try a few of the discs that I keep for the performance not the recording quality, and they were the killers!

Class D may be the future, but for me that future is yet to happen. The A35.2 is definitely worlds ahead of my previous Class D experience, also Primare - their CD10 all in one system of a few years ago. The A35.2 is certainly a good amp, I would guess a huge leap forward for class D, but for all the grip it may have on the likes of my Maggies, it has little of the depth and body and presence of my valve (tube) amps. Looks like I shall continue to line the pockets of my electricity supplier for the forseeable....