Technical Bulletin

Adjusting the Oscillator Output Level of the Qbox

(Applies to Qbox Rev1 ONLY)

The Qbox 440Hz tone oscillator is intended to be used in pass/fail situations and is not designed to be used as a precise calibration tool. The specification of the tone oscillator's output is:

"+4dBm, +/-2dBm with a 2000 Ohm load connected between pins 2 and 3 of an XLR connector"

The original Qbox presented some interesting problems during its design.

We didn't want to separate the female XLR from the male XLR because the multi gender I/O was a desired feature. However, this means that the output XLR is in parallel with the input XLR and the output impedance of the tone oscillator is in parallel across the input impedance of the headphone/speaker amp.

The consequence is that the output impedance of the tone oscillator must be set intentionally high, in fact about 900 Ohms. Because of the parallel connection to the input of the headphone speaker amp, lowering the impedance would create adverse loading on audio sources driving the Qbox headphone/speaker section. Another benefit, wiring the XLRs in parallel allows 2 Qboxes to be connected directly to each other with one mic cable, providing a hands-free 2 way intercom.

For this reason we do not recommend using the Qbox to calibrate levels as the oscillator's output level will vary in level depending on the input impedance presented to it.

If a known level is desired regardless of the load impedance, one can construct a terminating cable or adaptor with a 2k Ohm resistor wired across the + and - terminals. Then, when connected to impedances greater than 4k Ohms, the level will remain constant regardless of load. (This will work with most line inputs.)

If reduction of the output level of the oscillator is desired, this can be accomplished by adding a 4.7 MegOhm resistor to the foil side of the PCB in parallel with R3. (Refer to graphic qboxoscadj.pdf for the location of this resistor.) Removal of the PCB from the Q-box is not required.