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Ultra-LD Mk.3 200W Amplifier Module, Pt.2

The Ultra-LD Mk.3 amplifier module introduced last month is by far our lowest distortion Class-AB amplifier design. This month, we present the construction details and give some additional information on its performance.

by Nicholas Vinen

WARNING!

High DC voltages (ie, ±57V) are present on this amplifier module when power is applied.

In particular, note that there is 114V DC between the two supply rails. Do not touch the supply wiring (including the fuseholders) when the amplifier is operating, otherwise you could get a lethal shock.

Building this new high-performance amplifier module is really quite straightforward although there’s a fair amount of work involved. When building a high-power amplifier like this, it’s important to take your time and double-check each stage of the assembly as you proceed.

The double-sided PCB shown in the photos is critical to the performance of this module. It not only simplifies the supply wiring but has also been carefully designed to largely cancel the magnetic fields produced by the asymmetric currents drawn by each half of the class-B output stage.

In addition, the double-sided board eliminates the need for wire links, the exception being a couple of 0Ω resistors.

We’ll describe how to assemble the module shortly but first let’s take a look at a few more aspects of the design.

Increased fuse rating

The previous Ultra-LD Mk.2 had two on-board 5A fuses which we changed in the Mk.3 to 6.5A (7.5A is also OK). That’s because the amplifier is capable of delivering around 300W of music power into 4Ω. Under this condition, each fuse carries around 4.4A RMS (including the quiescent current). It could be even higher with a highly reactive speaker load.

While this is unlikely to blow a 5A fuse, it could eventually lead to fuse failure due to thermal stress. We have not had any reports of blown fuses in the Mk.2 version but we thought it best to make the change anyway.

Click for larger image
Fig.10: follow this parts layout diagram to build the Ultra-LD Mk.3 Amplifier module. Note that you should install a tinned copper wire feed-through in the middle of each group of five vias. Note also that this overlay shows the arrangement for BF470 & BF469 transistors for Q7 & Q9 while the photo at right the alternative arrangement when using 2SA1837 & 2SC4793 transistors.

Inductor value

The Ultra-LD Mk.3 amplifier uses a higher value inductor (10µH) than previously for better distortion cancellation. This results in slightly worse high-frequency response for 4Ω loads (the difference with an 8Ω load is insignificant), the response being -1dB at 20kHz compared to -0.7dB.

We feel that -1dB at 20kHz is acceptable. If you really want the extra 0.3dB though, you can have it in exchange for slightly higher distortion (peaking at 0.0048% at 20kHz rather than 0.0038%). All you have to do is wind the inductor with five fewer turns of wire.

Either way, its orientation and wind-
ing direction are very important. The reason is that the current flowing through this coil creates a magnetic field that partially cancels the magnetic field generated by the speaker current loop on the PCB.

With a positive output voltage, the speaker current flows in a clockwise direction around the loop which includes the 10µH inductor and the speaker. However the current in the coil itself flows in an anti-clockwise direction and so the magnetic fields partially cancel. This reduces the magnetic coupling back to the input stage.

As described later, the inductor itself is actually wound in a clockwise direction. It must also be installed exactly as shown on the PCB layout diagram (Fig.10), otherwise the distortion will be higher than it otherwise would be.

If you have already built an Ultra-LD Mk.2 amplifier, check its inductor orientation. If it’s the wrong way around, its 20kHz distortion figure will be around 0.012% rather than the specified 0.006%.

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