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Upgrading An Ultra-LD Mk.2 Amplifier To Mk.3 Standard

This teensy PCB lets you add the vital modifications to an Ultra-LD Mk.2 amplifier to bring it up to Mk.3 performance. We are doing this so that all those people who built the Ultra-LD Mk.2 from the August-September 2008 issues will not be too annoyed with us for superseding it with the Mk.3 version. After all, we want to keep our readers happy and content!

By Nicholas Vinen

this tiny upgrade board carries the new components, ie, the VBE multiplier transistor (Q16), the new driver transistor emitter arrangement and the 2-pole compensation filter. A few other changes are made by replacing components directly on the board. The upgrade board is mounted on the heatsink via the VBE multiplier transistor and wired to pads on the main amplifier board via flying leads.

In summary, the changes to upgrade an Ultra-LD Mk.2 to Mk.3 standard are:

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Fig.1: the upgrade board circuit. Q16 is the new VBE multiplier transistor, with the voltage across it adjusted by VR1. Next, the 220Ω resistor and parallel 470nF capacitor are connected between the driver transistor emitters for faster output transistor switch-off. And finally, the two 180pF capacitors and 2.2kΩ resistor form a double-pole filter across the transimpedance stage transistor on the main board, providing increased open loop gain in the audio band while retaining stability.

(1) Two of the ThermalTrak diodes in the bias voltage generator are replaced with an adjustable VBE multiplier, allowing quiescent current adjustment and providing better thermal stability. The constant current source resistor is also changed to 68Ω for correct biasing.

(2) The two driver emitter resistors are replaced by a single resistor, bypassed by a 470nF capacitor. This speeds up output transistor turn-off and so reduces high-frequency distortion.

(3) The 100pF Miller capacitor, connected between the collector of Q9 and the base of Q8, is replaced with two 180pF capacitors and a 2.2kΩ resistor. This replaces the single-pole compensation scheme with a 2-pole filter for more open loop gain at audio frequencies.

(4) The feedback capacitor goes from 220µF to 1000µF, which reduces distortion and flattens the response at very low audio frequencies. It also slightly improves the signal-to-noise ratio.

(5) The 820pF input filter capacitor is increased to 4.7nF, for more effective RF filtering.

(6) The output filter inductor and capacitor values are increased, improving magnetic field cancellation and thus lowering high-frequency distortion further.

Of these changes, the first three are incorporated on the upgrade board while the remainder involve component replacements on the main PCB.

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Fig.2: the parts layout on the upgrade PCB. Transistor Q16 is mounted on the component side, as far above the PCB as possible. Its leads are then bent around the board's edge so that it "hangs" upside-down from it, ultimately supporting the PCB on the heatsink - see photo.

Construction

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The first step to upgrade the amplifier module is to assemble the upgrade board. This is built on a PCB coded 01209111 and measuring 20.5 x 36.5mm.

Begin by fitting the four resistors. Check each one with a DMM set to Ohms mode before installing it. Follow with the two 180pF polypropylene capacitors, then fit the 470nF MKT capacitor and trimpot VR1. The latter should go in with its screw terminal to the right side of the board – see Fig.2.

That leaves Q16, the BD139 transistor. It should be soldered to the top of the board, with its metal tab facing away from the nearest edge and with its leads just protruding through the bottom of the board by a millimetre or two. Solder one pin, then ensure it is in straight before soldering the other two. With all three leads soldered, bend it around the edge of the PCB until its leads form a “J” shape, as shown in the photo.

Now solder lengths of wire to the pads marked “A” through “G”. The length required for each wire is shown in Table 1.

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