High-End-Preamp （6）

Published:2011/7/24 22:54:00 Author:Amy From:SeekIC

By Benjamin Hinrichs

Stable supply

A large part of the stabilisation, decoupling and hum suppression for the supply voltages takes place locally on the main circuit board, for both the digital portion (+5 V) and the ana­logue portion (5V). The only other thing you need is a sufficiently stable source of symmetrical DC voltages (which shouldn’t be a problem with the low current consumption). This provides an opportunity to use a gen­eral-purpose low-power mains power supply (Figure 5).




The power supply is designed to use Talema 10-VA encapsulated low-pro­file toroidal transformers (see refer­ence [2]), which can be obtained from DigiKey (among others). Partic­ularly for audio applications, toroidal transformers are preferable to other types of transformers due to their low stray fields.

The power supply circuit board is gen­eral-purpose because it can be config­ured to provide either an asymmetric supply voltage tV+ and ground) or symmetrical supply voltages (V+, ground and V-) by simply fitting either wire bridges ’A’ (for asymmetric out­put) or ’B’ (for symmetric output). Nat­urally, C6 and R2 are not needed if it is configured to provide an asymmetric supply voltage.

The output resistors (Rl & R2) should be wire-wound types, since this fur­ther reduces the hum voltage. Fast diodes with a recovery time of less than 200 ns are used as rectifier diodes (D1-D4) — for comparison, the recovery time of standard 1N4004 diodes is 1.2 |/s, which is more than six times as long. If you wish to use ultrafast diodes (< 50 ns), you are of course free to do so.

Small foil capacitors are connected in parallel with the rectifier diodes to effectively suppress high-frequency noise, which primarily arises from diode switching transients. Each cir­cuit board is protected by a slow-blow fuse rated at 125 mA.




Fitting the components to this board (Figure 6) is also about as simple as could be imagined. First solder the wire bridges in place (one circuit board with the three ’A’ wire bridges and a second one with the three ’B’ wire bridges). Then fit the remainder of the components in any desired order. Mount the series resistors vertically.

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