Direct Box – Active Circuit
This is a simple active Direct Inject Box with some very nice features. It is similar to The active box featured in the Boscorelli Book The Stompbox Cookbook. The circuit has a charge pump in order to provide more headroom for the initial signal and outputs the signals to the XLR format jack.. It also features a Gain control, Polarity switch and Ground lift. If built in a metal enclosure (recommended) you should use a XLR jack with a plastic casing so that the ground lift to work properly.
- General Build Instructions
- Active Direct Box Build Instructions
- Active Direct Box Bill of Materials
- Active Direct Box Schematic
- Active Direct Box Parts Layout
- Active Direct Box Ready-For-Transfer Printed Circuit Board Layout
There seems to be a lot of different opinions about the Pros and Cons of Active verses Passive Direct boxes. Here are some notes from a studio pro:
No load on source (best for high impedance sources like old Rickenbacker
Generally neutral frequency response
Active output will drive long distances at high level.
Limited headroom, distorts easily on hot input signals
Background noise in active circuit noticeable on quiet sources, or gaps
Phantom powering affects common mode rejection at mic input.
More prone to grounding issues.
100% Galvanic isolation, completely decouples source from destination.
NO self noise.
No power required.
Large headroom, with transformer saturation instead of hard distortion
(think tape vs digital here).
Transformers can be selected for colour.
Source loading, high impedance sources suffer from high frequency loss.
Non linear frequency and phase response, varies with transformer (again
tape vs digital).