UcD400 review High-Fidelty No.4, 2006

[peranders]

The original (high res 8.6 MB!) can be found here:
http://sjostromaudio.com/ucd400_review.pdf

I will make the file smaller later.

Since English is not my native language I'll appreciate if you correct me if you see any obvious errors.

PAGE 48

TEST
"Times are changing"

A class D module which meets up to our expectations - plus more there is

By Thomas L Madsen and Michael Madsen (measurements)

Picture text: Hypex' test set-up of two UcD400 modules forming a stereo amplifier.

It has taken quite a while but now it looks like the class D modules are coming in everywhere. You can see them in "all in one" surround equipment you can buy in your local grocery store, over to more ordinary  receivers to all the way up to high-end which now has taken the class D technology to their hearts.

One of the areas where they have been fast to adapt the class D technology is the "semi" DIY market. "Semi" is to be interpretated as ready made modules, just add a power supply and a case. LC Audio was the first one out.

"Flat as Holland"
The subject for this test is a class D module with the type name UcD400AD (audiophile version equipped with AD8620). It is manufactured and marketed by the Dutch company Hypex, a firm which is already known as manufactures of subwoofer modules of high quality sold to the DIY market.

Development of class D amps is a task for specialists. Hypex was well aware of that  but found the right man for this task. His name is Bruno Putzeys and has under many years worked for Philips and class D technology. One of them is Universal Class D (UcD). Hypex has acquired licence for using this technology. Hypex has also hired Bruno Putzeys which is responsible for the development of the class D modules.

At the moment there are three types of modules available for the DIY market, UcD180, UcD400 and UcD700. The numbers indicates the output power in 4 ohms. We have chosen to take a closer look at UcD400, which we imagine is the most popular module. The module consists of a printed circuit board with the dimensions 82 x 72 mm, containing all necessary parts. The height is under 30 mm which means that it is extraordinary compact if you bear in mind how much output power it is capable of.

On the pcb there is a T-shaped cooling profile which makes the module ready to be mounted on some type of heatsink. The modules seems to be very well considered and professional. As an example a big Faston contact can be seen for connecting the supply voltage and the output and a smaller Molex connector for the input. You don't need to fire up the soldering iron in order to connect the module. Soldering (and especially desoldering) thick wires is notoriously hard and this solution shows that Hypex has paid the problem attention. According to Hypex, they also deliver a substantial amount of modules to OEM customers and this may imply that the product is well tested.


Picture text, bottom page 48.
Simplified circuit over UcD400. Compared to more convential class D amplifiers this amp has feedback after the mandatory LC-filter at the output.

"Easy to get working"
Without going into detail I must mention that I'll notice one thing in the principal schematic.

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The modules fetches it's feedback from the output, after the mandatory LC filter. This ought to reduce one of the biggest disadvantages of class D and that is a output signal which is load dependent. The technique doesn't only require deep insight in general class D theory but also in advanced servo theory. ICEpower uses also an advanced servo solution which looks both at the signal before the filter and after. You can read more about the theories at Hypex website.

UcD400 is available in two versions, a "standard" version (UcD400ST) with OPA2134 at the input and a more audiophile version (UcD400AD) using AD8620 (the reason for this was that many DIY'ers over at www.diyaudio.com changed the opamp in the UcD180 which used NE5534. This opamp is not very well regarded in certain circles but not particularly bad according to me. This is more like a rumour than a fact. /P-A) This is this version we have listened to in this test. The input can handle both unbalanced and balanced input signals.

You can also put the module in standby mode which can be very practical  if the modules are mounted in active speakers and subwoofers.

In order to ensure optimal working conditions we asked Hypex of a ready made complete amp with the chosen modules. This will make it less challenging for a reviewer but on the other hand it will ensure a proper function when the manufacturer itself is responsible. Hypex has mounted the modules in a 1U (44 mm) high 19" case with heatsinks at the sides. The enclosure is 260 mm deep and of a regular type you can buy almost everywhere. The only unconventional thing is that the fins of the heatsink are horizontal instead vertical. This would create a better cooling. The frontpanel has only one thing, a blue LED, a "ON" indicator. The back panel has only the necessary, two XLR input connectors, two lovely big WBT speaker connectors and a mains inlet. There are all there is needed, not more, not less.

"Implementation"

When you for the first time see this amp and know that it is capable of delivering 2 x 200 Watts in to 8 Ohms, you think that "Class D amps must be terribly effective, because it's small and the heatsinks are very tiny!". The first surprise comes when you try to lift the amp. The total weight is only 8.5 kg (19 lbs). When the modules themselves only weight 150 g (5 oz.) a piece, there must be something else that weights. When we lift the lid we'll get the surprise no. 2. Both modules are mounted on the same side! The left heatsink isn't in use at all.

The modules excluded, the inside is dominated by a huge toroid transformer of 1200 VA. On the opposite side to the transformer there are rectifier bridge, smoothing caps and a softstart module. The power supply delivers +- 60 V DC to the modules which is a bit from max +- 75 V DC.

You could think that due to the high efficiency a class D amp ought to manage a smaller transformer than a conventional class A/B amp. But OK, here you are in no doubt that the transformer is too small and the power supply won't set the limits. The energy which is delivered to the speakers is the same regardless of amplifier technology. The difference is the power losses in the amp itself.

Picture text, above: Hypex UcD400. Notice how compact the module is and the blue T-shaped cooling profile which makes it easy to mount the module to some kind of cooling arrangement.

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"We must have current"

In this implementation  there were no doubts regarding the power supply due to the huge transformer. Therefore the amp was put to drive a bass section consisting of 2 x 10" in a bandpass system with active equalization, down to 20 Hz. A system which really lifts off with a decent amp. Using a less powerful amp may sound like a mop dragged up and down in the bucket.

In order to make this test more perfect we have used a speaker system with an impedance of 4 Ohms, which often removes the last shininess of the hopeful candidates.

Hypex UcD400 did manage this task without putting down the foot in the wrong place. Frankly, it was real horrifying how good this little module did succeed to control the bass elements in our system. You must almost hurt yourself in order to admit that those modules did manage the situation splendidly.

I can recall that only one amp which did perform equally well driving this system and the price for this amp was 10 times the price of a complete Hypex amp.

"Lukewarm, not more"

My introductory reservations regarding the size of the heatsinks did I have to eat up. The heatsinks became never warmer than lukewarm, through the whole test. Even not after a whole CD with a real high sound pressure a la "We are going a rock festival this year", the heatsinks became not hotter than a "burn the fingers" temperature.

Without doing too much calculations I think a heatsink half the size Hypex is using would have sufficient. Hypex do call some attention about the cooling, some is actually needed.

Funny, we noticed that the amp got at least equally warm on idle as a conventional amp. This you have to be aware of if you build a multi channel amp but in return the amp did not get warmer when more power was put out. The amp performed really well under our test period.

"Coherent sound"

One of the first things you do notice when you start listening to this amp is  that all frequencies are treated equally. The whole frequency is hold together were nicely. Many amp do focus at a certain range, but not this amp. After a while you get almost addicted to this very convinced tonal balance.

Listen to the tune "Same Damn Time", Mike Andersen Band from the album "Tomorrow". Al Agami's mellow voice has often a tendency to dominate the soundstage. Here is his voice as deep and impressing as it use to be but in a more natural relation to the rest.

It seems that this amp neither adds nor subtracts anything. It is like the Englishmen says: "wire with a gain". If you are used to amps which colouring the sound, the Hypex modules may sound chilly but this is more like a matter of taste. No particular frequency areas are accentuated, all is there.

PAGE 51

The U2 song "A Man and a Woman" from "How to Dismantle An Atomic Bomb" gets it's transparency which it sounds as weightlessly and dearly as it should sound.

Caecille Norby's "First Conversation" from the album with the same title did land in the CD player. Also here the very coherent playing style shines through. The lightness of the single instruments is really good but maybe a minor position under the very best. It seems that it decays a but faster than the reference and may therefore feel a but dry. Bear in mind that we are talking very small marginal and the reference costs 10-15 times more.

"Conclusion"

If we for a second forget the price of the Hypex modules, we are talking about very high performance. They do nothing wrong and the coherent play style make us addictive to this amp. If you take the price into account the "value-for-money" is extremely high.

Most people may use these modules to active speakers or to subwoofers but are you after a full-range frequency amp and not are afraid of putting it together by yourself, it would a shame if you didn't take a look at these modules. They deserve all the attention they can get.

Picture text: The gigantic toroid transformer dominates the inside. The small UcD400 modules are the two small printed circuits boards located at one side.

Measurements:

Page 50
Left, top: The control circuits gets it's feedback signal after the output filter. This ensures a controlled roll-off.

Middle, top: The filtering and feedback technique unsure not only a stable and decent bandwidth but also a suitable low output impedance.

Right, top: With 8 Ohms load there will be roughly 100 watts with low distortion and 200 W before the curve is heading sharply upwards. In 4 and 2 ohms 300 W is delivered with less than 0.1% THD+N.

Left, bottom: At 100 W is the distortion behaviour more or less in parallel. Over 50 kHz the amp has a heavy task.

Middle, bottom: 1 W, 8 Ohms is delivered with only 0.003% THD+N and the distribution of the 2nd, 3rd and 4th harmonics are appealing. The rest is under -100 dB.

Right, bottom: At 100 W, 8 Ohms THD+N is only 0.009% and the 2nd harmonics is dominating but now in return, the rest of the spectrum is stronger.

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Left: At 100 W, 4 Ohms do the 3rd, 5th and 7th harmonics increase. THD is still at respectable levels, 0.01%. From the 8th harmonics and up  are 100 dB down.

Middle: At 100 W, 2 Ohms we'll get 0.022% THD+N and all higher harmonics are present but in a controlled fashion.

Right: Under impulse conditions, the full voltage swing is put out which corresponds to 8 and 16 A in the peaks. In 2 and 0.6 Ohms the current is limited to 19.2 and 20.6 A.