If you are experiencing a low-level noise/hum with your preamp, follow these instructions to make sure the hum control is at its best setting.

Hum Control Locations:

The heaters of these preamps are AC. The designers included a hum control to minimize AC noise introduced by the heaters. These controls may have to be in different positions depending on your conditions, and will be very different if adjusted for 60Hz lines and you are using them for 50Hz lines.

The 2 potentiometers are located behind the grill that covers the front of the tube area. If you look at the front of the unit just above the meter and to the right you will see 2 pots with screwdriver slots. The top one is for the EF86 heaters, the bottom is for the 12AX7 heaters (and the other tubes).

To adjust this using a DAW, without the use of an oscilloscope:

1. Disconnect any cable going to the inputs.
2. Connect the EF86 output to your interface.
3. Turn the gain of the EF86 stage to max.
4. Record this signal with your DAW. Important: zoom in as far as you can while looking at the waveform. Most of these noises will not even register on the meters or show up with standard recording settings.
5. While watching the waveform, vary the pot (top one for EF86) to the right and left slowly, find the point where the noise is at its minimal.
6. Move your output connection to the 12AX7 out, Repeat for the 12AX7 stage

Before Zoom: 

Same recorded wave form as above, after Zoom: