There�s a ballast resistor that loads the capacitor board on the 861, 808 & 800 power supplies, and it�s been bugging me. I assumed it was there to keep the SMPS happy, but I�d like to remove it with the linear supplies (and I wanted to make sure there wasn�t a good reason why I shouldn�t).

It�s a small 33 ohm power resistor that�s bonded to the base of the chassis, and connected to the capacitor board by two short, green wires. It�s not visible in many of the internal pictures, but you can see it clearly in this one:

http://www.Meridian-audio.info/viewphoto.php?compid=50&photoid=2600

The resistor is pretty much in the centre of the picture, between the PSU, fan and capacitor board. It sits on one of the 12V lines, so it pulls about 360mA and dissipates over 4W. With a linear regulated supply, the total dissipation is higher than that due to the inefficiency. My estimates of actual power consumption (of the newer units, at least) are going down, so this load constitutes a significant fraction. My V4 consumes no more than 15W internally, so removing the resistor would make it 11W, tops.

Linear supplies work OK with no load � I tested mine the other day when I was measuring the no-load consumption. However, SMPS often need a minimum load just to keep them ticking over, and the start-up sequencing may be a critical factor here. The +5V line has no ballast resistor, so I presume that comes on line first, and the delayed load from the 12V line might cause the SMPS some problems. If you look at page 2 of this XP Power datasheet you can see that multi-output models like the ECM40UT31 need to have a minimum of 100 � 500mA load on one or two of the lines.

http://datasheet.octopart.com/ECM40UT31-XP-Power-datasheet-7541498.pdf

I think this is a particular requirement for SMPS, so I think it�s safe to leave the resistor disconnected when fitting an LPS. That will leave the equipment running cooler, and it also means the real power consumption of the newer equipment (ie: not the early 800�s & 861�s) may be quite low. This gives more flexibility with which power supplies can be used.

Nick