Gain Measurements
I installed two 4K99 in series with the input and installed J1 (10Ω termination) to create a 60 dB pad.
I then input 0 dBu into the pad and read the out put level.
All linked except the ones listed...
J4 Open; Output = -54 dBu (Pad reality check: Front end unity gain, +6 in the double-balanced output)
J5 Open; Output = -20 dBu (VR set for +40 dB gain)
J6 Open; Output = -8.3 dBu
J7 Open ; Output = -4.2 dBu
J4-J7 linked; Output = +1.6 dBu
J6 and J7 Open; Output = -10.6 dBu
Frequency Response Measurement
Frequency response with the servo switched on and off.
With the servo off the preamp has response to DC.
The green trace is limited by the response of the Focusrite 2i2.
With the servo on there is a derived 12 Hz high pass response.
![Image](https://proaudiodesignforum.com/images/jpg/ZTX851_MC_Preamp_Frequency_Response_Servo_On_Blue_vs_Servo_Off_Green.jpg)
ZTX851 MC Preamp Frequency Response Servo On Blue vs Servo Off Green