Aspects of the subject disclosure provide a reader device for managing operation of an automatic gain control (AGC) sub-circuit on a host device computing device. In some implementations, a reader device of the subject technology can include a controller, a noise generator, noise coupling circuitry, and a 3.5 mm audio plug including an audio bus that is configured for insertion into a headphone port of a host device, such as a smart phone or tablet computer. Upon connection of the reader device to the host computing device, the reader device can provide an intentional noise signal to the host device via a signal path where the intentional noise signal is configured to establish a substantially stable gain amount at the AGC sub-circuit.