Hi

Yes it is a pain to implement dual supplies. I ponder that issue every time
I build one of these setups. I’ve built a lot of them …. If you are going to
do a single supply, setting up a “virtual ground” is probably the best way
to go. Do it with a drive circuit to provide very clean 15V off of a 30V supply
then tack everything ( including all the mixer grounds to that 15V supply.

Keeping the signal undistorted before you check the beat note and use it
to drive the EFC does keep you out of various issues. You do not want to
deal with possible clipping / saturation artifacts getting into either process.

With devices having positive side EFC, negative side EFC, and “both
sides” EFC, it’s hard to get around a dual supply in any sort of general
purpose device. A 15V center point is not going to fit any EFC that I’ve
seen :).

Struggling with the ground loop problem is always the big deal in any
setup. Trying to rule out / take out line noise is usually the final straw in
any series of tests. Doing that with everything at “real ground” is just a
bit easier.

Part of the calibration is measuring the beat note as it goes past zero.
The ’scope gets cranked up and you look at a bit of the crossing right
at ground. Keeping the device happy while doing this is much easier if
the chassis does not need to float at 15V.

Whatever is used as a supply turns out to be a dedicated device. The
same ground loop / isolation stuff get in here. An old style non-switching
design is just about mandatory. Keeping switching artifacts out of things
is almost impossible. All of this makes a “build from scratch” approach
less and less crazy. Old style three terminal regulators ( so 78x18 / 79x18 )
are not as easy to find these days. They do fine if you happen to have a
pair …. There’s really not much power used by any of this. The need for
anything massive. 100 ma out of each side is overkill ….

As you build things up, you eventually come to the realization that a big
sheet of brass is a good idea for the ground. Tie this and that to the sheet.
Keep everything non-essential away and likely keep it turned off. Tying
a dedicated supply to that sheet along with the amp and EFC stuff is not
at all unusual.

Bob

On 7/10/22 9:07 AM, Bob kb8tq via time-nuts wrote:

Yeah, but that virtual ground brings with it it's own set of problems.
For instance, it has to both sink and source current, so you can't just
use a 3 terminal regulator to create the midpoint, although I've seen
schemes with a resistor from virtual ground to negative supply, but
that's not very power efficient - the resistor needs to see, say, 10x
the maximum sink current.

Now that I think about it, some sort of op amp driven appropriately (or
a complementary PNP/NPN pair?) might work - but then you're concerned
about the output Z of the op amp, and how it varies over frequency. And,
of course, the bias current.

We face this in designing low noise instruments for space. You're
running off a DC bus of some sort, and DC/DC inverters tend to be
noisier than straight out buck converters or linear regulators.

I've ALWAYS used an op-amp to nail down the half rail supply, I never
use the two resistors and cap idea directly  unless the whole thing is
quite basic....

Just don't put bulk capacitance on the output !

like I have seen in some bad designs. ROFL.

Some op-amps are better at this than others- IE dead zones in the
source-sink changeover curve.

On 11/07/2022 3:14 am, Lux, Jim via time-nuts wrote:

On 7/10/2022 10:14 AM, Lux, Jim via time-nuts wrote:

You might look at the LT1118-2.85 "Supply Splitter".  It is able to do
either sink or source, as needed, unlike an ordinary regulator.

Rick N6RK

Hi

I seem to recall TI having similar parts.

The big gotcha is tossing large chunks of C onto the ground to rail
connections. A typical op amp is not at all happy with this.

Bob