Created on 2010-11-23 20:50:00

Question:

I need to apply 48 V to the balanced lines of my 2722 analyzer’s inputs, in order to test a phantom powered microphone preamplifier. What is the recommended practice for this? I know how to apply phantom to the lines in the conventional way, but does AP recommend blocking capacitors, and if so any recommendations to the type and general selection would be appreciated.

Answer:

We recommend that you use a commercial 48 V phantom power supply between the microphone and the analyzer input. Almost all phantom power supplies use blocking capacitors to prevent the DC from affecting downstream circuits; if the supply you have does not use blocking capacitors (which is quite rare), be sure that the analyzer inputs are AC coupled (DO NOT have DC Coupling checked).

While it is possible to apply phantom power directly to the analyzer inputs using two 6.8 kW resistors and a 48 V power supply (and ensuring the analyzer is AC coupled), we do not recommend this approach because of the the noise introduced by this network of unshielded wiring.

CAUTION: Damage to the analyzer input(s) may occur if voltage is applied while the inputs are DC coupled.

CAUTION: APx585 and 586 analyzer inputs do not have blocking capacitors, and are always DC coupled. Be sure your phantom supply uses blocking capacitors when using an APx585 or APx586 analyzer.