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?
BrownoutOD - why the IC just for VREF?
- Thread starter HamishR
- Start date
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.
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.
YourGuitarist
Active member
BJTs do have the Vbe drop to contend with though and even then have emitter resistance to worry about. Opamps is best.
They do but it's just a case of tweaking the divider resistors to negate it, but yeah, not as good/simple as an op-amp but they do the job if you just need a little extra oomph in your reference.
There are also more complex versions that are better than just a standard common collector.
In this circuit? Probably be fine but an op-amp is easier... although there's nearly always a use for a spare op-amp half.
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YourGuitarist
Active member
The other thing about BJTs is current capability. They generally have more A on tap than opamps.
drew.spriggs
Well-known member
The normal reason you'd do it would be to use up the spare half of an opamp.
Rail splitter would be much easier and use up less room than an opamp, but they are getting a little pricey these days.
Rail splitter would be much easier and use up less room than an opamp, but they are getting a little pricey these days.