Secret Diagram

Because of high input impedance required to maximize CMRR, High impedance balance output circuit shown in figure below , has been used for the input impedance is determined solely by the input bias resistance R1 and R2. High impedance balance output circuit also useful for interfacing with valve equipment in the strange world of retro-hi-fi.

High impedance balance output circuit

Adding the output cathode followers for valve circuits are expensive and consume a lot of extra energy, so that the output is often taken directly from the anode gain-stage, as a result, even loading bridge the so-called 10 k distortion can seriously endanger performance and output swing available from the source equipment.
All balanced phase dealt with until now have their input impedance is determined by value input resistors, etc., and this can not be raised without lowering the noise performance.
High impedance balance output circuit diagrams above shows one answer to this. Input op-amp itself is quite a lot has infi nite
Impedance in terms of audio, so the input impedance is determined by the need to R1, R2 bias non-inverting input. A property of remarkable and very useful from this circuit is that the addition of Rg resistance increased profits, but maintain the balance of the circuit. This confidentiality guration can not be set to weaken for the advantages of an op-amp with feedback on the series can not decreases below unity.