Friday, 8 February 2013

Quick update to explain what those regs are doing! Probably repeating myself a bit here but hopefully this will help in following the principle.
In the standard player, we have a distributed 5v rail fed from a single regulator. The reg converts an input voltage of approx 10v to a regulated 5v. There are 3 connections for the reg. IN/GND/OUT. THe std reg is a 7805 which is a fairly noisy device and hasn't changed in many years. Newer regs are much quieter which can only be a good thing. What we are doing by adding multiple regs, is to distribute the unregulated 10v rail from the IN pin of the single 7805, feeding into a separate regulator and using this individual supply to feed a descrete pin on a device. In this example we are talking about pin 12 of the SAA7350 DAC chip. The pin was originally fed from the shared 5v supply via a low ohm resistor. After the resistor, we had a 47uF cap to ground and in parallel a 4.7nF ceramic SMT cap to ground.

The small value cap is designed to shunt any trace noise to gnd. Ideally it should be as close as possible to the supply pins of the device and ideally it should span the relevant supply and gnd. Looking at the datasheet for the SAA7350, we can see the digial supply pin is pin12 and the digital gnd pin is 14 which is quite handy as we can attach an SMT cap (100nF PPS but C0G would also be good here) directly accross the pins with excellent effect. Next we can remove the original 47nF ceramic cap and replace with a larger value (the effect the original cap has on the rail has been superceeded by the 0.1uF directly on the DAC chip). I opted for a ceramic 2.2uF here. Finally the original 47uF electrolytic has been replaced for a much larger solid polymer cap (solid polymer have astoundingly low ESR).

Texus Instruments REG104

The regulator I have chosen to use here is not the std 3 pin type but don't let that worry you, its not difficult to impliment. The TI REG104 is a 1A reg with an impressive noise figure. It actually has 5 connections. In order pins 1 to 5 IN/OUT/GND/NOISE REDUCTION/ENABLE. The tab is also GND whihc i'm connecting to the top screen of the main PCB as this is player gnd in this example.

Hopefully the 1st 3 pins are obvious. The other 2 are pretty straight forward too.
NOISE REDUCTION - connect via 0.1uF (100nf) to GND - This reduces the noise on the output pin
ENABLE - Connet via 175k to IN - the resistor is required where the input voltage exceeds 10v

Before and after diagram

The process has been repeated for DAC Digital, DAC Analogue Left and DAC Analogue Right so far with the only difference being the use of normal electrolytics for the analogue rails as they tend to sound better then solid polymer.

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