Bo Deadly
Well-known member
I just got this cheap active DI. Aside from the fact that you can't drive it into an unbalanced input (pin 3 is AC coupled to op amp out) I did some tests into my MOTU and it works fine. See plots below.
Obligatory inside shot:
Parts clockwise are:
9V battery
sw ground lift
sw power
sw phase
balanced XLR out
1/4" in
-20dB pad
-20dB pad (together makes -40dB)
1/4" thru
Input Z only ~175k.
Op amp is NE5532. Steel enclosure and controls are very nice.
UPDATE:
I have reversed the schem:
Note: Cannot output to unbalanced such that pin 3 would be grounded.
UPDATE 2:
Here are some performance plots.
Note: The increase in LF noise on 48V seems to be from my MOTU Traveler because it is not present with my API pre. Although, in general, this device seems to work quite a bit better on 9V largely because most mic pres (supplying the 48V) tend to actually have a little more noise in low gain settings (60Hz was only ~85dB down on my API pre).
UPDATE 3:
Here is a more complete schem with a mods (in green) for impedance balanced output (meaning pin 3 is disconnected from the op amp output and connected to pin 1 instead so it's just ground -> cap -> 68k -> pin 3). This is necessary to drive unbalanced inputs:
This also shows different values for resistors R5 and R6 to reduce series resistance and increase the AC input impedance to 730k. I want to use this on my bench so I want high Z, low noise. And it's probably better for DI-ing anyway. I don't want to plug guitar into 175k.
In general, this is a nice form factor with room for a switch if you need a box with an op amp to do something specific. Quite hackable.
Obligatory inside shot:
Parts clockwise are:
9V battery
sw ground lift
sw power
sw phase
balanced XLR out
1/4" in
-20dB pad
-20dB pad (together makes -40dB)
1/4" thru
Input Z only ~175k.
Op amp is NE5532. Steel enclosure and controls are very nice.
UPDATE:
I have reversed the schem:
Note: Cannot output to unbalanced such that pin 3 would be grounded.
UPDATE 2:
Here are some performance plots.
Note: The increase in LF noise on 48V seems to be from my MOTU Traveler because it is not present with my API pre. Although, in general, this device seems to work quite a bit better on 9V largely because most mic pres (supplying the 48V) tend to actually have a little more noise in low gain settings (60Hz was only ~85dB down on my API pre).
UPDATE 3:
Here is a more complete schem with a mods (in green) for impedance balanced output (meaning pin 3 is disconnected from the op amp output and connected to pin 1 instead so it's just ground -> cap -> 68k -> pin 3). This is necessary to drive unbalanced inputs:
This also shows different values for resistors R5 and R6 to reduce series resistance and increase the AC input impedance to 730k. I want to use this on my bench so I want high Z, low noise. And it's probably better for DI-ing anyway. I don't want to plug guitar into 175k.
In general, this is a nice form factor with room for a switch if you need a box with an op amp to do something specific. Quite hackable.
