Grounding -ve input to Vee=0V disables it.
You can use a differential input to Vin+ and Vin- but ONLY if the DC gnds and supplies are floating with respect to each other. Otherwise do not! Also do not put a cap across the Vin- to the rail. That will destabilize the internal feedback and it may oscillate. You can't AC ground the Vin+ and use Vin- either. This amp or the source must be "floating" and no use of SMPS either.
There are other methods, but this will work with 4 stages of gain being up 80 dB open loop with at least 20dB to 30 dB of AC/DC internal feedback.
It is a high gain low impedance current limited with fixed gain without Feedback added to reduce the gain but the internal 12V reference for the output and inverting input so and inputs and outputs must be AC coupled, but not Vin- which is low input impedance. That must be direct-coupled in differential mode to the inputs provided, yet floating. The input cap provided is to decouple from the inverting input ( the emitter) not isolate from the source.
The input level must be on the order of 1 mV and up and the source impedance determines the gain down to ohms for max gain!! So padding the source might be necessary for a large signal to a very low input impedance (< 8 Ohms for max gain) but I suspect <1K source impedance for reasonable gain is what it was designed for. ( I wonder what the user's manual says)
The lack of current sources everywhere with only 5 transistors relies on the high supply voltage for reducing the current modulation. So the output swing will likely be only a few volts max which is pretty loud for a headset.
Reducing the internal R feedback with the external FB connection to Output will reduce the gain very little (?). The feedback goes to the emitter so that attenuates the feedback with low impedance already, but current limited so it is intended for Sennheisers with high impedance. = 300 ohms not 8 ohms. With
that load, I expect it should be high-quality THD ~ 0.1%
It is not your typical Op Amp design. "It's a NEVE"
- Your Voltage measurements are normal, but a more detailed explanation is needed to explain why) See my simulation for more accurate results
here is an accurate simulation of how to use the amplifier with a low impedance floating differential source and thus high gain.
You can add series source resistance (using thumbwheel over the 330 R source R) and experiment with observing the ratio of output to input gain with the output Vpp probe.
The schematic uses labelled Nodes to connect the inputs. Here the input is 1mVpp and the output is 2.134 Vpp with a clean undistorted gain of 2134. Vfb is left open.
Vfb (pin 7) is at a midpoint for constant current DC Voltage between Pin 2 (Vin-) and Pin 5 (O/P) but that ratio does not define unity gain. Rather the ratio of (R15+R4/ (Rsource/hFE)) emitter low input impedance defines the gain with both R5+R14 for AC feedback and DC self bias.