Hi,
kt88seamp wrote:How will using two gain stages instead of a phase splitter (the signal coming in is already split) not provide good CMRR without a transformer?
The nature of the beast!
At best, a design like this can provide when tweaked perfect, 60dB of CMRR and in reality, probably 40dB. Even a $10 Edcor WSM can give you 100dB.
I studied active vs. passive balanced inputs in detail before taking on a tonne of commissions demanding balanced inputs. I had been up to eight tubes (practcally an op-amp) before I was at 70dB CMRR.
The invention of active balanced was on wide bandwidth oscilloscopes. HP was hitting 100MHz before the rest were breaking 5MHz. CMRR was not that big of an issue, since the deflection plates were insensitive to common-mode signals. But we did get some bang-up balanced amplification techniques out of their research and the above circuit is a greatly simplified version of it.
I thought a design like this is fully balanced innately and transformers are used when the design is not truly balanced.
If that were true, Bogen and other PA equipment and studio broadcast mixing panels wouldn't have transformers in them
Cheers!