just curious what's on the back side of 1et400a amp board, could you please post a detailed photo of the 400a back side?This is a review and detailed measurements of the Purifi 1ET400A Class-D amplifier module. Company was kind enough to send me a complete, assembled kit consisting of two modules, power supply and case, ready to go. They have not disclosed the cost of the unit. Modules will be available to DIY channel in Q4 of 2019.
As cases go, the prototype unit came in an attractive one:
A touch of color on the lid goes a long way to break the monotony of DIY aluminum cases. Someone should take this case, put two giant VU meters in the front and make me happy!
Here is a quick teardown:
A single Hypex SMPS1200 is used to power both channels. That feeds two 1ET400A amplifier modules.
A single "Amp Connector Board" feeds both channels using balanced input through an optional gain stage of 13 dB. Here is more detail from documentation on the features of this EVM board:
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I was pleased to see high-quality Rubycon capacitors used in the amplifier modules:
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The power supply seems like the stock Hypex SMPS1200:
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It is remarkable that a high-performance amplifier could be built in such a small enclosure with so few subcomponents.
In use, the evaluation unit would barely get warmer above room temperature. Heat dissipation is simply not an issue here. You could easily put these amps in an enclosed cabinet and not worry.
Note that the main offering here is the 1ET400A. The Amp Connector board is offered as convenience. In that regard, I tested the amplifier with and without the extra gain that the connector board provides. Alas, I wish there was something in between. You get full power with as little as 1.5 volt with the extra gain but need 11+ volts without it. I like to see manufacturers build a gain stage that produces full power at 4 volt which we routinely get out of source products (e.g. DACs).
Amplifier Audio Measurements
As provided, the extra gain stage was active producing this dashboard view at 4 ohm producing 5 watts:
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Focusing on FFT spectrum of a 1 kHz tone, we see stunningly low distortion product. 2nd Harmonic is barely visible which may actually be the contributions from my source signal. The third harmonic hovers around -130 dB which again, is almost at the limit of what we can measure. Given the ear's 116 dB or so dynamic range, you are assured zero audibility of these distortion products.
THD+N and hence SINAD (signal over noise and distortion) is substantially lower though at 104 dB. Since THD+N is sum of distortion products and noise (+N), the latter is the culprit here. We can see this reflected in SNR measurements:
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As you see on the left, our noise contributions is essentially what we see for SINAD. Fortunately at full power (right), performance shoots way up to fantastic level of 123 dB.
We can do better though by disabling the pre-gain stage:
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We gain 4 dB in SINAD resulting this summary ranking:
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The Benchmark in its low gain mode maintains the championship status still but the 1ET400A gives it serious competition here.
SNR naturally improves with lower gain:
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Drilling into the distortion (with pre-gain active) we see the vanishing harmonic distortion more plainly:
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Power is everything in amplifiers so let's see how we do with 4 ohm load:
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The tables are turned and the 1ET400A produces 257 watts of exceptionally clean power. More could be had if I had run this test with finer resolution and/or allowed the distortion to climb more.
We see the effect of lower gain with reduction of noise at lower power levels although even with the extra grain stage, we are talking very quiet here.
Switching to 8 ohm, the results are similar:
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Notice the big difference between a commercial amplifier (QSC DAC2422). They have more brute-force power but nowhere as clean as the 1ET400A amplifier module.
Intermodulation distortion versus power gives us another shot at analyzing distortion, this time what is in audio band:
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The Benchmark AHB2 maintains its less noisy baseline but as we reach full power, the 1ET400A catches up to it and keeps going with more power.
Distortion+noise versus power and frequency yields this (with pre-gain stage on):
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I was surprised to see the rise in distortion with frequency. I had hoped that the super high gain-bandwidth of the 1ET400A would do away with this. Not an audible concern though as the distortion products here are all in ultrasonic range.
I ran this test multiple times, tuning the input voltage. This eventually upset the amp and caused it to shut down when producing 274 watts into 4 ohm before our sweep completed (in green). Documentation clearly states that extra cooling is needed for continuous power.
The Benchmark AHB2 does a lot better since it has much cleaner ultrasonic spectrum:
Frequency response is exceptionally flat for a class-d amplifier:
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The output filters in class-d amps can cause variations as we get close to 20 kHz but we see none of that here. Response smoothly rolls off. At 50 kHz, we are down 1 dB.
Crosstalk using my non-optimal setup is still exceptional:
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I had to pull up the graph to show its crosstalk at 1 kHz. The 1ET400A is that good!
EDIT: here is the broadband spectrum of a 1 kHz tone:
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We have our switching frequency around 500 Khz and its harmonic at 1 Megahertz. Otherwise pretty clean.
It was just a few years ago that people scuffed at class-D amps not being very clean or good for audiophile use. How the situation has changed. First with Hypex modules and now with the Purifi 1ET400A. Audiophile myths are shattered with use of large amount of feedback and high bandwidth to produce an amplifier which brings transparency to anything you throw at it.
Importantly, the 1ET400A does all of this while producing a ton of power and staying cool and efficient to boot.
There is a subjective aspect to measurements that doesn't come across in the graphs. When I run these tests with switching amplifiers, I often watch the analyzer struggle to get reliable reading, or there are jumps and glitches in measurements. None of that was here. The amplifier basically acted like a traditional class AB amplifier. Indeed, I measured it with and without my AES-17 40 kHz filter and the analyzer was happy both ways. This is when I know there is quality engineering that has gone into design of this amplifier.
Overall, it is my pleasure to strongly recommend the Purifi 1ET400A to DIY and OEM manufacturers.
As always, questions, comments, recommendations, etc. are welcome.
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