What if you use your second (or indeed first, or indeed any of all the other devices) with a device made by someone thinking the same as you with a shared supply in some larger encompassing device? Call the person and ask if the one ground is the same as the other ground? i.e. Is signal ground the same level as power ground?
I can tell you, in "cheap" designs many do it the same way as you, but often use the full power input to divide. So you are going to compete. Violently, if their reference is 12V/2.
If you want to make a good design that's universally compatible, you make sure that the external grounds are all actually the same hardwired level. So if you output a +5V and a 0V only, that 0V should be the same as the 0V on all the audio plugs. That's a good design.
So in this set-up, what would have been better is to output your original 5VDC as a +/- 2.5VDC balanced around your 'weak' virtual ground in a three pin plug. Then suddenly you have a balanced power supply to your second box and you could even devise a system where if the middle pin is not lifted to half VCC, then you make it yourself if needed. Or for separate use make a second DC plug with a switch built in that activates the divider and disconnects the 5VDC lines (for safety).
The high-end or 'common rail' design thing if you have a DC input jack, is to use a voltage inverter to obtain your negative rail, +9VDC --> -8VDC ; +9VDC --> +5VDC ; -8VDC --> -5VDC (or 2.5V for each, but then, use 5VDC in, for efficiency). Or even better a fully isolated balanced DC. Unfortunately nobody else who makes $10 gadgets does this, so you can't even assume it.
Want to use a divided VCC as a virtual ground with an external DC adapter and stay safe and compatible to "shared power situations"? One of two options:
- You'll have to decouple and route the hard external ground through input and output or just force your own ground on the output again. It's not awesome, but it's what it is.
- Add the DCDC isolation on the DC input I discussed before. If your consumption is as low as 1W DC/DC with +5V and 0V out or +3.3V, 0V and -3.3V out is about $5. Using the balanced with two low-drop 2.5V regulators, one negative one positive, will even get you much better thermal stability on the ground and its relation to your supplies. Plus, it sources and sinks the maximum available current with no problems or aberrations. In fact, doing that same trick with your original DC input would have been possible by first dividing the 9V to virtual ground and then dropping the original power lines to +2.5V and -2.5V.
If you go down the rabbit hole of "Oh, I'll just assume it's only a 0.1V difference", in six months you'll be kicking yourself, because you have a 9VDC system with a 4.5V virtual ground, etc.
Your 2nd circuits (with the NPN and R divider) is a good approximation of a log circuit. This is because when the input opamp generates a voltage substantially above 0.6 V, the current in the 4.7k is proportional to the voltage, and so the V across the NPN is proportional to the log of that current. The 100k & 10k give a multiplier effect, so in fact, your transfer function is closer to
VOUT = K*26mVln(Iin/Is), where Iin = (VIN-0.7)/4.7k. 'Is' is hard to find directly, but if you measure (guess) VBE at 1 mA (say 0.6 V), the equation can be rewritten as
VOUT = K[26mV*ln(In) + 0.6], where 'In' is in mA.
K is the gain from your R divider -- with 'Contour' =0, it's 1; with 'contour' = 20k, it's 3.
Note that this circuit will change over temperature --if the NPN heats up by (say) 30 deg. C, that's equivalent to about a 10 dB reduction in volume (when you go through math).
Best Answer
The simplest way to balance an unbalanced signal is with an isolation transformer. Unfortunately, a transformer that works well across the entire audio frequency range is going to be expensive ($>100 from Jensen).
Another way to do it is to use a chip similar to the one you found from THAT Corp. Is it overkill, probably. But it'll be cheaper and perform better than a transformer (assuming that you don't really need the isolation that a transformer will give you).
The "typical" way would be to use two op-amps. One is set as a mostly-unity gain non-inverting buffer and drives the + input of the amp. The other opamp is set as a unity-gain inverting buffer and takes it's input from the other op-amp and drives the - input on the amp. Vary the gain on the first op-amp if you need to.
Of course, the real question for this is: do you need to use a balanced signal or can you get away with going unbalanced straight into the amp? Of course, a balanced input will give you better audio quality but the op-amps or THAT chip will degrade quality. If you don't do a good job, the net effect could be a decrease in quality. And then you have to wonder if the change in quality (for good or bad) is even noticeable to your ears. It's very likely that simply going unbalanced into the amp is going to be the better choice.