Testing
GKL recommends that you do your final testing of your BabyPre circuit before
mounting the PCB into the enclosure. This will allow easy access to the circuit in
case anything needs to be fixed.
Test the Power Supply First!! Once your PCB is fully populate (Be sure all of the
TPs have been soldered EXCEPT TP6 and TP7) plug the board into the power
transformer. Measure the Volatges at TP6, TP7, and TP8. You should have
approximately +15V DC, -15V DC, and +48V DC respectively. If you have the correct
voltages, install TP6 & TP7 and test the voltages again. If you suddenly DO NOT
have the correct voltages once TP6 & TP7 are installed, unplug the board
immediately and begin troubleshooting.
If the power supply voltages are correct, begin testing the audio circuitry. Even if you
do not have an oscilloscope and function generator or an Audio Test System, you can
confirm the proper performance of your BabyPre using a standard DAW and audio
interface. A Digital Multi Meter (DMM) is useful but not absolutely necessary.
Most common audio interfaces use a standard reference of -14dB FS = +4dBu.
+4dBu is line level for professional audio. You can confirm this by generating a 1kHz,
-14dB FS RMS sine tone in your DAW and measuring the RMS voltage between pins
2 and 3 (or between tip and sleeve of a TRS cable) of your audio interface with a
DMM. The RMS voltage should be 1.228 Vrms.
To test your BabyPre, send a -50dBu (-68 dB FS RMS) 1kHz sine wave out of your
DAW into the BabyPre’s mic input. Return the BabyPre’s output to your DAW and
adjust the gain of the BabyPre such that the signal returns at -8 dB FS (this will be
60dB of gain).
1) Record a few seconds of the output of the BabyPre. Inspect the waveform.
It should be a clean sine wave with no clipping or distortions.
2) Engage the Transistor Amp or Colour Module section. There should be no
significant change in the signal level returning to the DAW. Disengage the
Transistor Amp / Colour Module.
3) Leaving the BabyPre set to 60dB of gain, turn off the signal generator. This
will leave only the BabyPre’s Equivalent Input Noise(EIN) floor. The level of
the signal returning to the DAW should be less than -58 dB FS. This
equates to a noise level of -40dBu and, since there is 60dB of gain applied,
an EIN level less than 100dBu.
4) Change the frequency of your DAW’s signal generator to 20 kHz, you should
see no change in signal level. Change the frequency to 20 Hz, you should
see a 1dB drop in the signal level returning to your DAW.
5) With the signal generator still outputting 20 Hz, engage the HPF button.
There should be approximately a –18 dB drop in signal level at the input to
your DAW.
6) Return the signal generator to 1kHz. Engage the Pad button. There should
be a -19 dB drop in signal level at your DAW’s input.
If all of these tests produce the desired results on both channels of your BabyPre, you
can be confident that it is in good working order. If not, refer to the troubleshooting
section of this Assembly Guide.
Troubleshooting
The majority of problems we see with the BabyPre project are either components that
have been soldered in the wrong orientation or poor solder joints. Visually inspect the
orientation of all polar capacitors (Electrolytic and tantalum capacitors), diodes, LEDs,
voltage regulators, and Integrated Circuits (ICs). Visually inspect the bottom of the
circuit board for solder bridges or suspicious looking solder joints. Try reheating or
adding more solder to any questionable solder joints.
If you cannot identify the source of your problem, you can refer to the troubleshooting
section of the Complete Manual, available at www.gklaudio.com/gkit-babypre.The
Complete Manual contains significantly more detail on best practices while
troubleshooting. If problems persist, see the Policies section of this Assembly Guide
for information on how to contact us for help.
Enclosure and Final Assembly
Once your board is fully assembled and tested, it is time to mount the PCB into the
enclosure. Place the switch caps on the push-button switches and remove the
hardware from the potentiometers. Slide the PCB into the slots on the bottom part of
the enclosure. Put the washer and nut from the potentiometer back on but DO NOT
tighten them yet. Slide the top enclosure on and make sure the XLRs are aligned at
the back. Use the four silver 4-40 screws to attach the top and bottom enclosure
pieces together at the sides of the unit. Use the black self-tapping screws to attach
the XLRs to the rear panel (sometimes a little force is needed to pry the XLRs into
place. The XLRs should extend out from the back of the unit about 1mm. Once the
XLRs are tightened down, tighten the nuts on the potentiometers at the front of the
unit. Finally, turn the potentiometers fully counter clockwise and push the knobs on
making sure the pointers on the knobs are in the correct position.
CONGRATULATIONS! You're GKit should now be complete and ready to use.
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