High-End-Preamp （5）

Published:2011/7/24 22:37:00 Author:Amy From:SeekIC

By Benjamin Hinrichs

Channel switching

The input channels are switched on the relay board. The circuit shown in Figure 3 allows two different wiring options.




With the traditional option, all eight inputs from the Cinch sockets are used, with a single output to the main circuit board. This requires JP2 and JP3 to be fitted. Each relay is then responsible for one stereo channel. If standard twin screened audio cable is used, one channel is connected to A and the other to B, and the screen braid is soldered to the bottom of the circuit board. Ground isolation, which may be desired to improve channel separation or avoid interconnecting the grounds of different items of equipment connected to the preamplifier, can be achieved by switching not only the signal lines, but also the associated ground potentials. In this case, for each channel the ’live’ lead is soldered to A and the ground lead is soldered to B. In addition, jumpers JP2 and JP3 are left open. In this mode, each pair of relays (RE1 & RE5, RE2 & RE6, RE3 & RE7, and RE4 & RE8) belongs to a single signal source. This yields four stereo inputs and two sets of outputs (OUT1A & OUT1B and OUT2A & OUT2B). Here again A corresponds to the ’live’ lead and B to ground. The only other thing you have to do is to change the Input Type option in the Set-up menu to ’Double’.

In order to handle balanced signals, we simply switch two relays simulta­neously. In this mode, you also have to add a second, parallel volume control and then connect the balanced signals to one of the input channels (such as Left In) on both volume controls. This works well-quite well, in fact.

Fujitsu (formerly Takamisawa) type RY-5W-K relays are used here. Naturally, other pin-compatible types could also be used, but the specified line has excellent characteristics and is avail­able for less than 2 euros (approx. £1.40), for example from Conrad Elec­tronic (www.conrad.de). This is a plas­tic-encapsulated relay, which prevents any dust from accumulating inside. The contacts are made from a silver-palladium alloy with supple­mentary gold plating.

Power for the relay board is taken from the main circuit board via connector Kl (which requires jumper JP1 to be fit­ted). If you wish to treat the channel switching board to its own power sup­ply or you want to use this board in a different project, you can also connect a 5-V supply to the K2 terminals.

In order to avoid overloading the microcontroller outputs, transistors with base resistors are used as relay drivers. Diodes D1-D8 act as free-wheeling diodes, while LEDs D9-D16 provide a visual indication of the relay states. The LEDs do not necessarily have to be fitted to the circuit board (Figure 4); they can of course be fitted to the front panel instead if you want to do without the display, or they can simply be omitted (along with then-series resistors).




JP1 can be implemented as a jumper, but JP2 and JP3 should be implemented as wire bridges. Otherwise there’s not anything particularly remarkable about populating the circuit board, particularly if you use the components specified in the components list and everything fits perfectly.

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