Update to the Musiland Power Mod
Minimizing Switching Regulator Residue in Linear Regulator Outputs is an application note describing the use of ferrite beads to eliminate high frequency spikes from switching supplies.
Designers frequently use linear regulators to post-regulate switching-regulator outputs. The benefits of this approach include improved stability, accuracy, and transient response, along with lower output impedance. Ideally, markedly reduced switching-regulator-generated ripple and spikes would accompany these performance gains. In practice, all linear regulators encounter some difficulty with ripple and spikes, particularly as frequency rises.
The USB power feeding the Musiland MINI in my setup comes from switching regulators inside the computer and thus prone to have spikes [I can’t measure these with my low-end scope, but lets assume they are there].
These spikes cannot be filtered/attenuated by the linear regulator because of their high frequency. The reason is the following: ripple rejection of linear regulators can be 70 db or more at low frequencies, but this capability decreases as the frequency of the disturbance increases. In the MHz range, the ripple rejection is down to 20 db or less. Spikes at this frequency can pass right through. The following diagram shows the ripple rejection as a function of frequency for the LT1963 regulator. As you can see, at the higher frequencies, the ripple rejection is just not too great.
Low frequency ripples can be filtered with input capacitors. In my mod, I used 1000 uF capacitors and that should virtually eliminate all low frequency ripples.
For the high frequency ripples, the application note recommends the use of ferrite beads. I added ferrite beads to the inputs and outputs of the regulators. The photo below shows the ferrite beads to the inputs, one to each regulator.
[If you are starting from scratch, you can pull the ferrite off its lead, leave the positive lead of the capacitor long and just insert the bead to the lead the capacitor]
It is hard to say… But looking at the dropout (unlocks) behavior of Buffalo II, I would say there is some improvement on the frequency of dropouts. In any case, this is good practice (the use of ferrite beads). Twisted Pear Audio uses them extensively at the outputs of the on-board regulators.
- The elegance of ferrite beads as a circuit design and problem-solving component [link]
- Vishay ferrite beads app note [link]