Large low frequency activity can be seen when galvanic voltages are generated by the electrodes. This can happen with a difference in contact quality between two electrodes on the scalp. An effort to move the balance light on the impedance meter to green can help resolve this issue. A bad balance in the electrode contacts can come from different age electrodes, or a manufacturing differences in electrode plating. An extreme example would be using different metals like silver and gold electrodes at the same time, this would create very large problematic galvanic voltages.

Solutions:
1. Try new electrodes
2. Use nu-prep, and work to improve the contact.
3. Improve the contact at all sites equally to maximize the balance between the electrodes.

If you cannot completely get rid of this low frequency activity, you can train through it without too much interference.