BrownoutOD - why the IC just for VREF?

[HamishR]

I was looking at the schematic for the Brownout OD (Subdecay Variac) because it sounds excellent. I noticed that it uses an entire JRC4558 dual opamp IC just for VREF. Why would it be necessary when in most OD circuits just a simple two resistor voltage divider and a filter cap is used? Is it for extra stability or quietness or something?

 

[Scruffie]

It does add extra stability (nice low output impedance) especially if you have a lot of VR sources that could interact or are going to pull a lot of current, it can be quieter although not always.

In that schematic, does kinda seem overkill especially when half the op-amp is wasted... you can also use a transistor to do the same job or as R.G. Keen was always keen on mentioning, an LM386 will self bias to half supply and do the job too, although I've never actually seen anyone use it for that purpose.