Hmm, upper Op-Amp noise gain = 6dB (gain also 6dB and attenuation on the input by 6dB, lower Op-Amp noise gain = 6dB.
Thus noise (ignoring resistor noise) is 9dB worse than a simple voltage follower.
I don’t know why I was thinking that the outputs cancel by more when they are summed back to single ended at the receiver.
ohhhhhh i know what it was. different topology:
with a double balanced output made from two equal-value diff amps, a SE or differential signal comes out differential and 6dB higher. but an equal value diff amp has 3dB noise gain (plus johnson details). so gain is commensurate with noise gain. do tell me if i have that wrong.
you can make a double balanced output with an INA1620. the load on the preceding stage is 6??R, so you need a buffer (a fader buffer, for instance). resistors in that package average 965R.
965R || 350R = 257R
10V / 257R = 39mA
39mA x 10V = 390mW…nope
7V / 257R = 27.2mA
27.2mA x 7 = 190mW…close
6.2V / 257R = 24.1mA
24.1mA x 6.2V = 150mW…bingo, full power at 1.6MHz BW. 24dBu differential into 600R with 50R buildouts.
But into a 5K differential load, it’ll do 28dBu. Just that dual amp and two 100p caps, no resistors to speak of.
I'd do follower + inverter. Follower noise gain 0dB, inverter noise gain 6dB, circuit gain 6dB, so noise stays lower at around the 6dB of the inverter.
I think we could find a way to lower noise in the inverter, with a noise servo, to the same level as the follower. Then we may end up with something closer to Birt.
I’d like to see what you mean on the back of a napkin.
In your other Birt suggestion, were you saying to feed signal to the inverting input and DC servo the non-inverting input via the existing voltage divider, or were you saying to drive the circuit differentially and come directly in to the inverting input with a high-value resistor off the servo output? Where are you drawing the DC sample from?
if you wouldn’t mind showing these two Birt-based circuits it would be helpful…
If it is not obvious, Birt has the inverter in the feedback loop of the first Op-Amp but noise from this becomes common mode (as does DC offset on the positive in of the inverter). This lowers differtial noise of the Birt Circuit to close to single op-amp levels.
Nope, not obvious. Also, referencing the typical higher value resistors for inverter Rf and Rg, wouldn’t you induce additional noise if you use, say 5K/5K rather than 2K/2K as drawn?
But honestly, I fail to see why we need an active balanced output. It has no benefit whatsoever, other than that it simulates a center tapped (or floating) transformer input.
That I would agree with. People generally know enough to lift a cold when need be.
I can see why commercial designs need to offer that (compatibly with the one or two oddball pieces that needed it).
I think it’s pretty important to provide an input and output that can at least accommodate a converter running at 24dBu = 0dBFS. That’s why I’m okay with the 2210 — 25dBu across the hot and cold. There are a few amps that can put out 11 or 11.5Vrms on 18V dual supplies, but there’s always some caveat from what I’ve seen.
I’m willing to bet there are broadcast and educational clients who don’t buy anything that doesn’t have capabilities that an impedance balanced I/O can not provide.
Anyways, I don’t wanna get too in the weeds here. Optimizing a Birt output for low noise with acceptable drive would be pretty on topic tho.