PI2Media 502HTA
Hardware Reference Manual – P3 - 8/31/2017
PAGE 8
control various options as described further below.
4. Although GPIO5 (MUTE) has a default pullup on the RPi of about 75K,
which over-rides the 1Meg pulldown on the 502HTA, SW should drive this
pin high.
5. Software should drive GPIO13 high to enable the Low Latency Filter if
desired. It is set to normal latency by default with a pulldown resistor.
6. Software should drive GPIO13 high to enable the 44.1Khz De-Emphasis
function if desired. It is disabled by default with a pulldown resistor.
7. The DAC, when at maximum level, outputs 2Vrms to the Dual Triode Tube
Amp.
4.3
DUAL TRIODE AMPLIFER
A Dual Triode Amplifier provides the voltage amplification of the input signal (DAC or
Line In). This circuit designed to provide 10dB (3.3x) of overall gain (with volume control
at 100%).
4.3.1
DUAL TRIODE AMPLIFER NOTES
1. There is an attenuation switch which, when enabled (up position) reduces
the input to the Amp by 10dB, thus reducing overall maximum gain to
~0dB. The actual gain is set by the tube and it’s cathode resistor. It may
vary as much as +/- 1dB.
2. The Amp is driven from either the PCM5120A DAC or, when a plug is
inserted, the 3.5mm Line In jack.
3. An ALPS 10K Dual Potentiometer provides volume control and a power
on/off switch.
4. If the output is to be driven to a line level input on another device, the user
should enable the high attenuation. With this enabled (switch up) and the
volume control at 100% the overall gain is ~0dB.
4.4
CLASS-A MOSFET BUFFER
A Class-A Buffer consisting of the IRL510 MOSFET and LM317 Constant Current
Source provides up to 125ma (peak) of output current to drive Headphones from 32 to
600 ohm.
4.4.1
CLASS-A MOSFET BUFFER NOTES
1. A three position slide switch adds 0, 47 or 100 ohm in series with the
output. This allows low ohm headphones to be driven with much lower
