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Better power supply for DAC unit

I have constructed many of your projects over the years; mostly audio and test gear. As I write, I am listening to a system with your 20W Class-A Amp­lifier at its heart. It’s a great amplifier that has enabled my old Tannoys to sing anew!

The reason for my email is in regards to a recent purchase of an Arcam “r-DAC” digital-to-analog converter that is powered from a cheap switchmode plugpack. I have read on the web that this unit is “much improved” with a better power supply. The r-DAC requires 6V at 600mA into a standard DC power connector.

Several years ago, I built your Studio Series Preamp Headphone Amplifier into a box with its own power supply. I can’t continue without expressing how much enjoyment that little unit and a pair of Sennheisers have given me over recent years! The power supply uses your circuit board (01109052 Rev B) which has a dedicated 5V supply branch circuit. That particular board uses a 5V, 5W zener diode (1N5338B) instead of a 100Ω 5W resistor before the 7805 regulator as was the case with a previous version.

So to my queries. Firstly, how critical are the voltage and current parameters to the safe operation and sound quality of the r-DAC? Secondly, can the present 5V circuit in the headphone amplifier power supply be modified to produce 6V at 600mA – or whatever is required to efficiently run the r-DAC? Finally, if the modification can be made, will supplying the results to the r-DAC via the standard DC connector improve the sound as is rumoured – or is it all nonsense?

The increase in popularity of DACs as stand-alone improvers of sound quality from computers is in no doubt. Nor is the fact that they actually work – there is a genuine improvement in sound quality from my laptop via the r-DAC. Coupled with your Studio Series Headphone Amplifier and good headphones, the improvement is nothing short of thrilling!

If improving the power supply to the r-Dac does in fact improve the sound quality and that can be done via modifications to the redundant 5V branch of the abovementioned supply, it seems to me to be the possible beginnings of a beautiful relationship that can be replicated by others. (P. G., via email).

• It is possible but unlikely that an analog supply would give markedly better performance. If that were so, wouldn’t the original manufacturer have done this and claimed even better performance?

You could modify the unused 5V supply in the Studio Series preamplifer to provide 6V at 600mA by replacing the 7805 with a 7806. But this would be very inefficient and you would need to fit a large heatsink to it. A better solution might be to use a 9V DC plugpack and connect a 7806 (with smaller heatsink and bypass capacitors) to its output.

12V 15A variable
power supply wanted

I am looking for a 12V variable motor controller to operate a battery supplied winch to 15A (approx.) that can have current limiting adjustable/set to match the motor at full load current (FLC). With this set, the motor won’t overload and rely on the fuse for protection. Which project can you can recommend? (D. O., New Plymouth, New Zealand).

• Funnily enough, it seems that the only speed controllers we have published which had over-current control were those for model train controllers. Typically though, these were only rated for up to 6A although it would be possible to increase that.

To take the simplest example, the Li’l Pulser Train Controller from the February 2011 issue was based on an MTP­3055 Mosfet and was limited to 2A. It would be possible to modify the basic circuit to provide a much higher current limit by reducing the current sensing resistor and using a much higher rated Mosfet. However, the PCB itself could definitely not withstand the higher currents.

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