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Power for
multiple DI boxes

I am interested in the Direct Injection Box project featured in the August 2001 issue. I would like to build four of these DI units and house them together in a single metal box.

My main concern is whether I would be able to power all four units from a single on-board power supply instead of separate plugpacks, as described in the power supply section of the article. At this stage, I also feel that I don’t want the 9V battery supply as I will either power the units from the 48V phantom or on board power supply.

My thoughts for the on-board power supply are to use the ±15V Universal Power Supply (SILICON CHIP, August 1988) and a 12-18V multi-tapped 1A transformer. This would be wired as shown in Fig.4 (unregulated single supply version) and the corresponding component layout shown in Fig.9. My hope is that the said power unit will supply the required power to run the combined four units.

Would the stated power unit combination be satisfactory and capable of supplying the necessary power? If not, what would I need to meet my requirements? (D. W., via email).

• Each DI circuit draws about 9mA and so four such units would only require 36mA. This can be easily supplied via a single 9V DC plugpack that can supply 36mA.

If you decide to use a mains transformer and the universal power supply arranged in a single-ended DC output, then a 12V 150mA transformer is all that is required. We do not recommend using a mains transformer supply as it would introduce hum into the DI signal. A single external DC plugpack is a better way to power the four units.

Question on
fluorescent lamps

I’m asking if you could help me with this question. I do not understand why I cannot put two fluorescent lamps in parallel. (F. M, Tunis, Tunisia)

• Have a look at how a conventional fluorescent lamps works on

As you will see from the description, a fluorescent lamp needs a ballast (ie, an iron-cored inductor) to generate the high-voltage spike needed to start the discharge. If you had two lamps in parallel, only one is likely to fire.

Adding A Mains Transformer To The DAB+ Tuner

I plan to build the DAB+/FM Stereo Tuner but would prefer to fit a toroidal transformer inside the case and connected to an IEC socket. Would putting a transformer inside the case compromise the performance? If so, would an aluminium sheet cage around the transformer (or some other approach) solve the problem? (B. D., via email).

• The DAB+/FM Stereo Tuner was designed to be able to accept a centre-tapped transformer as well as an AC plugpack as the power source.

If you are going to use a transformer, it should be centre-tapped and you should fit an additional two 1N4004 diodes below D1 and D2 to provide the full bridge rectifier. The centre of the centre-tapped transformer then connects to GND while the other two leads connect to the outer two pins of CON1 (ie, one connects to the anode of D1 the other to the anode of D4).

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