Sunday, 20 January 2013

Power Supply Mods

Made a few changes to the power supply.

First off I identified the audio analogue supply. There is a separate winding on the transformer dedicated to this which is pretty normal. The winding supplies the rectifiers which in turn supply +/-24v dc to the +/-15v regulators via smoothing capacitors. I've seen anything from 470uF right up to 9400uF per rail as standard for this job in CD players and DACs. This is a quick win normally governed by space. Here I've gone from standard 470uF up to 2200uF Panasonic FC (great pre reg where space is an issue) The rectifier is replaced with 11DQ09 schottky type. Post the regulators were some rather weedy 47uF caps. These have been replaced with 470uF Rubycon ZLH low impedance, low ESR caps which I often use on analogue rails.

Next to address is the +/-10v. The same principle can be applied here but due to the heavy load put onto the positive rail (its used to supply the 5v reg which supplies most semionductors in the player) I've gone from 4700uF to 6800uF. Again we could go bigger but space is a issue. On the negative rail from 1000uF to 3300uF. Schottky diodes replace the original bridge too. Post regs I've gone from 47uF on the negative rail and 220uF (Black Gate!!) on the positive to 470uF ZLG again. These mods bring the PSU's closer to that of a much more expensive player. The restrictive part of the PSU is now the transformer which we may address later depending on budget. We'll save the Black gate for later!

I've also identified all of the local PSU caps around the DAC, filter, receiver, ram etc for swap out later. AS usual, I'll go for solid polymer on any digital rails and standard construction low ESR (probably Rubycon ZLG) on analogue rails.

Next easy win is the opamps. As standard, the PSU caps are Elna Simlic so not bad. I've swapped out the standard opamps for gold rolled pin sockets so I can play with opamps. I've popped some LME49720HA for now just to see where we end up. I've also swapped the green Nichicon BP elco's blocking DC on the output for some MKP type. It may be that we don't actually need any at all but I'll measure the DC offset and check later.

I also have a few 11.2896Mhz oscillators so I paired one up with one of my regs and couple of nice caps a 470uF Nichicon solid polymer bypassed with a 1uF PPS on the reg and 0.1uF PPS on the oscillator. Should be a pretty good clock. 

Its starting to sound quite acceptable! The clock makes a big difference on this chipset and right now, its only clocking the DAC. I will direct clock the filter and decoder chips later which should improve things again.

I've also been looking at low noise LDO chip type regulators. I would like to separately regulate various components including isolating the DAC analogue and digital rails as I think this will bring vast improvements, but thats another job for another day. I'll leave this lot to settle for a few days

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