TEST PROCEDURE - SPARTAN 30 High impedance headphone and line amplifier

Start with headphone output tests. Use 150 ohm load box and adapter cable. RCA input 3.


STARTUP TESTS

Unit powers on within 4-6 seconds, restarts without issue.


TRACKING TESTS

Set generator level to 400mV at 1kHz, CTA to gain, and FFT to channel balance (all dB scale). Default bandwidth settings.

Measure gain at maximum setting: 17.3-17.6dB

Channel balance should be within:

    - 0.3dB from +17.3dB gain to -20dB
    - 0.5dB from -20dB to -30dB
    - 1dB from -30dB to -40dB

Adjust gain to 0dB - potentiometer should be around 1 o'clock position for current builds


HEADPHONE OUTPUT NOISE TESTS

Set CTA to noise measurements (dB mode). Output noise should be less than...

    - -108dBV at 0dB gain
    - -116dBV at minimum gain
    - -93dBV at maximum gain


HEADPHONE OUTPUT DISTORTION TESTS

Set analogue inputs to tied at 34dBu, set generator to 4V RMS 4kHz. FFT distortion should be less than...

    - 20ppm (usually around 15) at 10.3V RMS, 700mW into 150 ohms
    - 5ppm (usually around 4) at 6.2V RMS, 250mW into 150 ohms
    - 10ppm (usually around 7) at 8.7V RMS, 500mW into 150 ohms

Check THD+N in CTA at 20Hz, less than 10ppm?


FREQUENCY RESPONSE AND CROSSTALK TESTS

Connect balanced leads to d-Scope, auto-ranging enabled, set FFT to THD+N relative.

Set CTA to gain mode (dB), set bandwidth to 'DC Block' to '40kHz'

Set frequency to 300Hz at 7.5V RMS, adjust gain to 9dB to give 21V RMS output.

Gain should not change by more than 0.1dB, and THD+N should be less than 5ppm at

    -15Hz
    -30kHz
    -5kHz
    -10kHz

Adjust the gain to give 15V RMS output (6dB) with generator set to 10kHz channel A and B crosstalk attenuation should be less than -74dB (for 80dB separation at 10kHz).


SWITCHING RELAY TESTS

Adjust generator level to 1V RMS, check headphone output muting: at least -115dB gain.

Check all inputs are switching correctly and off-ness is <40dB at 10kHz, moving from input 3 to 1.


FINAL NOISE TESTS

Set CTA to Noise (dBV). Noise at potentiometer settings should be less than...

    - -112dBV at minimum
    - -104dBV centred
    - -87dBV full gain

FINISH


MISCELLANEOUS NOTES

135 ohms and 1OV RMS is too spicy. Regulators starting to drop out (19.5V and 1V AC component).

135 ohms at 10V RMS stabilises to 12ppm THD.

10.5V RMS into 150 ohms works nicely. 735mW maximum output power. 19.8V and 0.92V AC component on rails.

Best rating should be 720mW into 150 ohms as maximum power at 20ppm THD full bandwidth.

Lower distortion possible with 700mW into 150 ohms at 16ppm.

THD is less than 10ppm throughout the audio band at 500mW into 150 ohms.

Will do 10.5V RMS into all loads above 180 ohms with 15ppm THD.

Max continuous power into 32 ohms is 160mW less than 20ppm THD.

Do not ask for more than 70mA RMS simultaneously from each output or the rails will drop out.

Headphone output noise

At minimum gain it's 1.2uV (1.4uV shown)

At -20dB it's 1.6uV noise

At -10dB it's 2.1uV noise

At 0dB gain it's 3.6uV noise

At 10dB 9.3uV

Noise is therefore lower than 108dBV on the headphone amp output at 0dB gain

Max gain is 17.5dB on headphone out which results in 20uV of noise.

0.1dB response

24Hz to 35kHz at maximum gain

15Hz to 40kHz at 0dB gain

Line output stays below 10ppm THD at all gains 20Hz to 22kHz

Line distortion stays below 5ppm THD 20Hz to 22kHz