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Ultra-LD Stereo Preamplifier & Input Selector, Pt.2

In Pt.2 this month, we describe the assembly of the Input Selector module and its companion Switch Board. We also show you how to make the IDC cables that link the modules together and give the remote control set-up and test procedure.

By John Clarke & Greg Swain

Click for larger image
Fig.13 (above): follow this diagram to build the Input Selector PCB. Make sure that the two header sockets are correctly orientated and note that Q5-Q7 are BC327 PNP transistors while Q8 is a BC337 NPN transistor.

THE 3-INPUT SELECTOR board (Fig.13) is straightforward to assemble. Install the resistors and diodes D1-D3 first, then install the ferrite beads, the IC socket and the MKT capacitors (do not use ceramic capacitors on this board).

Once these parts are in, install the four transistors, noting that Q5-Q7 are BC327s while Q8 is a BC337. The two electrolytics can then go in, followed by the 10-way and 14-way header sockets. The header sockets must be installed with their slotted key-ways towards the top.

Finally, complete the assembly by installing the relays, the three stereo RCA input sockets and the two vertical RCA output sockets. Note the left and right labelling for the output sockets – this is not a mistake and arranging them this way gives the optimum layout for the PCB.

Motorised Volume Pot.

Altronics has advised that they can supply a dual-gang 5kΩ log motorised pot (Cat.R1998) for the preamplifier. This should be used in preference to a 20kΩ pot and the 4.7kΩ shunt resistors omitted.

Switch board assembly

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Fig.14: the three switches are mounted on the front of the Switch Board while the header socket goes on the back (key-way towards S2). Take care with the switch orientation (see text).

There just four parts on the switch board – the three pushbutton switches on one side and the 14-way IDC header socket on the other (see Fig.14).

The three pushbuttons can go on first but note that they must be installed the right way around. These have “kinked” pins at each corner plus two straight pins for the integral blue LED. The anode pin is the longer of the two and this must go in the hole marked “A” on the PCB.

Once the pins are in, push the buttons all the way down so that they sit flush against the PCB before soldering their leads. The IDC header socket can then be installed on the other side of the board, with its key-way notch towards the bottom.

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