PI 2 Design 502HTA Quick user guide

PI2Media 502HTA Hardware Reference Manual – P3 - 8/31/2017
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502HTA
Hybrid Tube Amp Shield
Hardware Reference Manual
© 2017 PI 2 Design

PI2Media 502HTA Hardware Reference Manual – P3 - 8/31/2017
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Table of Contents
1 Warranty......................................................................................................................................................................................... 3
2 Operating Specifications................................................................................................................................................................. 4
3 Overview ........................................................................................................................................................................................ 5
4 On-Board Functions........................................................................................................................................................................ 7
4.2.1 PCM5102A 24-Bit 192Khz DAC Notes............................................................................................................................. 7
4.3.1 Dual Triode Amplifer Notes .............................................................................................................................................. 8
4.4.1 Class-A MOSFET Buffer Notes ....................................................................................................................................... 8
4.5.1 6.3V Heater Supply Notes................................................................................................................................................ 9
5 RPi GPIO...................................................................................................................................................................................... 10
5.1.1 RPi GPIO Notes............................................................................................................................................................. 11
6 502HTA Power ............................................................................................................................................................................. 12
7 502HTA Software ......................................................................................................................................................................... 14
8 Connectors and Switches ............................................................................................................................................................. 15
9 Test Results.................................................................................................................................................................................. 17
10 Document Revisions................................................................................................................................................................... 30
11 Errata.......................................................................................................................................................................................... 31
List of Tables
Table 1 – 502HTA Operating Specifications ...................................................................................................................................... 4
Table 2 – RPi to PCM5102A Connections ......................................................................................................................................... 7
Table 3 – CPU GPIO Pin Assignments ............................................................................................................................................ 11
Table 4 – Document Revisions ........................................................................................................................................................ 30
List of Figures
Figure 1 – 502HTA Block Diagram..................................................................................................................................................... 6
Figure 2 – 502HTA Power Hold Circuit ............................................................................................................................................ 13
Figure 3 – 502HTA Connectors and Switches.................................................................................................................................. 15
Figure 4 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, 0dB, 600 ohm ...................................................................................... 18
Figure 5 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -3dB, 600 ohm..................................................................................... 19
Figure 6 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -12dB, 600 ohm ................................................................................... 20
Figure 7 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -60dB, 600 ohm ................................................................................... 21
Figure 8 – 502HTA Headphone Out, 11.025Khz, 16[email protected], Jitter Test, 600 ohm ................................................................. 22
Figure 9 – 502HTA Headphone Out, 12Khz, 24-Bit@96Khz, Jitter Test, 600 ohm ........................................................................... 23
Figure 10 - 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, 3dB, 600 ohm..................................................................................... 24
Figure 11 - 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, 0dB, 600 ohm..................................................................................... 25
Figure 12 - 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -3dB, 600 ohm.................................................................................... 26
Figure 13 - 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -6dB, 600 ohm.................................................................................... 27
Figure 14 - 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -12dB, 600 ohm.................................................................................. 28
Figure 15 - 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -60dB, 600 ohm (Generator set to 50mV rms) .................................... 29

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1WARRANTY
The enclosed product ("the Product"), a part of the PI2MEDIA Shield/Hat series, is
warranted by Pi 2 Design for a period of one year for reasonable development, testing
and use, all as further described and defined below. This warranty runs solely to the
individual or entity purchasing the Product and is not transferable or assignable in any
respect. This warranty is valid only for so long as the product is used intact as shipped
from PI 2 Design. Any attempt or effort to alter the Product, including but not limited to
any attempt to solder, de-solder, unplug, replace, add or affix any part or component of
or onto the Product, other than components specifically intended for the user to plug
and unplug into appropriate sockets and/or Connectors to facilitate user programming,
development and deployment, all as specifically described and authorized in this
Product Hardware Reference Manual, shall void this warranty in all respects. Coverage
under this warranty requires that the Product be used and stored at all times in
conditions with proper electrostatic protection necessary and appropriate for a complex
electronic device. These conditions include proper temperature, humidity, radiation,
atmosphere and voltage (standard commercial environment, 0C to +70C, <60%RH).
Any Product that has been modified without the express, prior written consent of Pi 2
Design is not covered by this warranty. The use or connection of any test or bus
Connector, adapter or component with any device other than a Pi 2 Design Connector
or adapter shall void this warranty and the warranty of all other components, parts and
modules connected to the rest of the system. Pi 2 Design shall not be responsible for
any damage to the Product as a result of a customer's use or application of circuitry not
developed or approved by Pi 2 Design for use on or in connection with the Product.
This warranty does not cover defects caused by electrical or temperature fluctuations or
from stress resulting from or caused by abuse, misuse or misapplication of the Product.
Any evidence of tampering with the serial number on the Product shall immediately void
this warranty. This Product is not intended to be used on or embedded in or otherwise
used in connection with any life-sustaining or life-saving product and this warranty is not
applicable nor is Pi 2 Design liable in any respect if the Product is so used.
Notwithstanding anything to the contrary herein, Pi 2 Design expressly disclaims any
implied warranty of merchantability or implied warranty of fitness for a particular purpose
in connection with the manufacture or use of the Product.

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2OPERATING SPECIFICATIONS
2.1 502HTA OPERATING SPECIFICATIONS
The 502HTA conforms to the following specifications:
Specification Value
Dimensions 65mm x 56.5mm – Hat Compliant
Weight ~8oz (with tube)
Storage Temperature 0C to +70C
Operating Temperature 0C to +50C
Humidity 0% to 95% RH, Non-Condensing
Input Voltage (VIN) +24V +/- 5% 500ma Peak
Power Consumption 8W Typical, 10W Maximum
Table 1 – 502HTA Operating Specifications

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3OVERVIEW
3.1 INTRODUCTION
The 502HTA, designed and manufactured by Pi2Media (a division of PI 2 Design), is a
High Quality I/O Shield designed to bring “Sweet Tube Sound” to the Raspberry PI ®
family of Single Board Computers. This HAT allows the Raspberry Pi ® SBC to drive
Headphones from 32 - 300 ohm with ease and fidelity.
The On-Board PCM5102A, 24-Bit @ 192Khz DAC drives the single stage Dual Triode
Tube. The Class-A Output Buffer consists of the Audiophile grade IRL510 MOSFET and
LM317A Constant Current Source. A dedicated 6.3V Heater supply insures lowest
crosstalk.
In addition, the 502HTA can be operated as a Stand Alone Headphone Amp via the
3.5mm Line In jack. The major features of the 502HTA are as follows:
•FORM FACTOR – Raspberry Pi ® Shield w/40-Pin Connector
•24-Bit DAC – The well regarded -112dB SNR PCM5102 is used to convert the I2S Stream to drive the Single
Triode Tube Stage at 24-Bit up to 192Khz Frame Rate
•TUBE ROLLING – The 502HTA supports both 6DJ8 (default is JJ 6DJ8) and 12AU7 type tubes. Individual
Anode Bias Adjustment Pots plus selectable Series/Parallel Heater Configuration insure the perfect setup for
virtually any tube!
•HEADPHONE OUT – Rugged 3.5mm Jack plus selectable output resistance between 1.5, 47 and 100 ohms.
•VOLUME POT AND GAIN SELECT – High Quality ALPS Potentiometer and dual Gain settings allow the user
to adjust the output voltage to the optimal level for each Headphone.
•SINGLE POWER INPUT OPTION – A Single 24V @ 2A input powers the 502HTA and delivers 5V@3A to the Pi
via Low Pass Noise Filter. Power can also be isolated between the 502HTA and the Raspberry Pi for the lowest
possible noise.
•STAND-ALONE OPTION – The 502HTA can also be operated in stand-alone mode using the 3.5mm Line In
Jack. A dedicated stand-alone version, 502HTA-SA, without the Raspberry Pi interface or the PCM5102A DAC is
also available.
3.2 BLOCK DIAGRAM
Refer to the following figure for a block diagram of the 502HTA.

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502HTA BLOCK DIAGRAM
PCM5102A 24-Bit
192Khz DAC
ULTRA-LOW
NOISE LDO’S
ANALOG RIGHT
I2S INPUT
USB-A
HOST
USB-A
HOST
USB-A
HOST
USB-A
HOST
10/100 RJ45
w/Transformer
HDMI
40-Pin Expansion Header
ANALOG LEFT
LOW JITTER PLL
Raspberry Pi ®
w/40-PIN GPIO Header
HW ONTROL
½ DUAL
TRIODE
IRL510/
LM317A
IRL510/
LM317A
WIDE INPUT
6.3V SWITCHING
SUPPLY
24V @ 2A
Power Brick
Backpower
Protection
Circuit
5V @ 3A
LDO
40-Pin Socket
+5V
GPIO
I2S
OUTPUT
RESISTOR
SELECT
OUTPUT
RESISTOR
SELECT
SERIES/
PARALLEL
HEATER SEL
3.5MM
Headphone
Jack
3.5MM
Stereo
Input Jack
ANODE/
CATHODE
BIAS ADJ
½ DUAL
TRIODE
GAIN SWITCH
& VOL UME
CONTROL
Volume Control
On/Off Switch
Figure 1 – 502HTA Block Diagram

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ON-BOARD FUNCTIONS
4.1 OVERVIEW
The 502HTA interfaces to the RPi via the 40-Pin GPIO Connector. This section
describes in detail the functionality located on the 502HTA.
4.2 PCM5102A 24-BIT 192KHZ DAC
At the core of the 502HTA is the Texas Instruments PCM5102A 24-Bit 192Khz DAC.
This device interfaces to the Raspberry Pi via the I2S. The signals used to interface with
the PCM5102A are shown in the following table.
PCM5102A
Signal
RPi
Signal
Description
MCLK - Grounded at PCM5102A
BCLK BCLK I2S Bit Clock from Pi
LRCLK LRCLK I2S Word Clock from Pi
SDIN SDOUT I2S Serial Audio Data from Pi
DEMP GPIO13 Pulled High to Enable De-Emphasis, Drive Low
to Disable
FILT GPIO6 Pulled High to Enable Digital Filter, Drive Low to
Disable
*MUTE GPIO5 Pulled Low to Mute, Drive High to Enable
Table 2 – RPi to PCM5102A Connections
4.2.1 PCM5102A 24-BIT 192KHZ DAC NOTES
1. The PCM5102A is not populated on the 502HTA-SA.
2. The 502HTA is designed for the Raspberry Pi to operate in master mode,
supplying the Bit Clock, Word Clock and Audio Data to the PCM5102A. In
this mode the PCM5102A derives its master clock from an internal PLL
using the Raspberry Pi Bit Clock (BCLK) as the source. The PCM5102A
automatically detects the incoming frequency of BCLK and adjust its PLL
accordingly.
3. The PCM5102A is Hardware controlled and thus has no direct SW visible
functionality. But, three GPIO’s (5, 6, and 13) are connected in order to

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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

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noise and overall power consumption. For the best experience, the user
should start at the highest resistance and reduce only if the output levels
are not satisfactory.
2. There is a 25 second delay from power up before the headphone output is
enabled. This allows the tube to warm up and reduces power on thumps.
The output is disabled immediately when power is removed.
4.5 6.3V HEATER SUPPLY
A low noise, high performance switching supply provides the 6.3V for the Tube Heaters.
This supply also feeds the 5V LDO used to power the RPi (up to 3A).
4.5.1 6.3V HEATER SUPPLY NOTES
1. A 3-Pin Jumper is used to select how the power to the Tube Heater is fed.
In 6V mode a jumper is placed on pins 2-3, while in 12V mode a jumper Is
placed on pins 1-2. Note that 12V mode only works with tubes that allow
6.3V parallel operation, such as the 12AU7, not true 12V heaters. The
following circuit illustrates this:

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5RPI GPIO
5.1 OVERVIEW
The 502HTA uses a number of signals from the RPi GPIO header for control and status
purposes. This usage is defined in the following table. Note that on the 502HTA-SA the
Raspberry Pi interface is not supported.
RPi
PIN
DIR AF PUP/
PDN
502HTA
Name
Description/Notes
1 - - - - RPi +3.3V - Unused
2 - - - +5V +5V Power to/from the RPi
3 I/O Y PUP I2C_SDA I2C Bus Data - unused
4 - - - +5V +5V Power to/from the RPi
5 OUT Y PUP I2C_SCL I2C Bus Clock - Unused
6 - - - GND
7 - - - GPIO4 Unused
8 - - - GPIO14 Unused
9 - - - GND Unused
10 - - - GPIO15 Unused
11 - - - GPIO17 Unused
12 IN Y - BCLK I2S Bit Clock to pCM5102A
13 - - - GPIO27 Unused
14 - - - GND
15 Out - - GPIO22 Power Hold – Drive high to keep power on
16 - - - GPIO23 Unused
17 - - - - RPi +3.3V - Unused
18 IN - PUP GPIO24 Power Switch – Read state of power on/off
switch. Low = on.
19 - - - GPIO10 Unused
20 - - - GND

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RPi
PIN
DIR AF PUP/
PDN
502HTA
Name
Description/Notes
21 - - - GPIO9 Unused
22 - - - GPIO25 Unused
23 - - - GPIO11 Unused
24 - - - GPIO8 Unused
25 - - - GND
26 - - - GPIO1 Unused
27 - - - ID_SDA Unused
28 - - - ID_SCL Unused
29 OUT - PUP *MUTE GPIO5, 0 = Mute PCM5102A Output
30 - - - GND
31 OUT - PUP FILT GPIO6, 1 = Enable PCM5102A Filter
32 - - - GPIO12 Unused
33 OUT - - DEMP GPIO13, 1 = Enable De-Emphasis (44.1k only)
34 - - - GND
35 OUT Y - LRCLK I2S Left/Right Clock to PCM5102A
36 - - - GPIO16 Unused
37 - - - GPIO26 Unused
38 - - - GPIO20 Unused
39 - - - GND
40 OUT Y - SDOUT Pi I2S Serial Audio Data to PCM5102A
Table 3 – CPU GPIO Pin Assignments
5.1.1 RPI GPIO NOTES
1. DIR is from the point of view of the RPi.
2. Y in the Alternate Function (AF) column indicates that the use of this pin
requires the pin to be assigned to the function as defined by the RPi
specifications.
3. PUP/PDN indicates if the GPIO should have its associated Pullup (PUP)
or Pulldown (PDN) resistor enabled.
4. An asterisk ‘*’ at the beginning of the name indicates a low true signal.

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6502HTA POWER
6.1 OVERVIEW
The 502HTA is designed to be powered from an external 24V +/- 5% source via 5.5mm
x 2.1mm Jack.
6.2 SOFTWARE CONTROLLED POWER HOLD
GPIO22 allows the RPi to hold the power on even after the user turns the power switch
to the off position. After initialization, software can drive PWR_HLD (GPIO22) high, thus
holding the power on regardless of the state of the on/off switch. Driving this low
removes this hold condition. Note that for proper operation this requires user software to
disable the internal pullup for GPIO22.
6.3 SOFTWARE VISIBLE POWER STATE
The 502HTA allows the RPi to keep the system powered for clean shutdown. As part of
this, PWR_SW (GPIO24) is used to sense the state of the ALPS Volume Control
Switch. When high it indicates that the user has turned the volume control all the way to
the left (counter clockwise) and thus turning the unit off. For proper operation, this circuit
requires user software to enable the internal pullup for GPIO24.
The following schematic illustrates the Power Hold Circuit:

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Figure 2 – 502HTA Power Hold Circuit

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7502HTA SOFTWARE
7.1 OVERVIEW
Due to the various resources interfaced on the 502HTA, both internal and external to
the Raspberry Pi, it is necessary to initialize a large number of CPU registers and
external devices before correct operation can begin. These values and their proper
sequencing are beyond the scope of this document. See our web site at
http://www.pi2design.com or email us at [e-mail] for example operating
and initialization code.
7.2 DRIVER SOFTWARE COMPATIBILITY
The 502HTA is compatible with the standard Raspberry Pi PCM5102A driver. An
example is the Hifiberry DAC. Using this driver will support all the audio functionality of
the 502HTA, but will not enable the Power on/off control. The Volumio 2 Audio Player
(www.volumio.org) supports the 502HTA by way of the 503HTA plugin.

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8CONNECTORS AND SWITCHES
8.1 OVERVIEW
This section provides the type, location and pinout for the various connectors and
switches on the 502HTA Top Side. These are shown in the 3D rendering below.
Figure 3 – 502HTA Connectors and Switches
8.2 J1 – 3.5MM HEADPHONE JACK
J1 is a 3.5mm Stereo Jack for Headphone Out
8.3 J2 – 3.5MM HEADPHONE JACK
J2 is a 3.5mm Stereo Jack for Line In
T1 – Dual
Triode Tube
P2
5.5mm x
2.1mm
DC JACK
J1 – 3.5MM
Headphone
RP5 – Volume
Control & Power
Switch
SW1
SW2
Attenuation
J2 – 3.5MM
Line In
RP3 RP1 W2
R G L

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8.4 W2 – 3-PIN BIAS ADJUST HEADER
W2 is a 3-pin .1” single row header. It allows access to the Left and Right Channel Bias
points. The pins are marked R-G-L for Right, Ground and Left respectively.
8.5 P2 – 40-PIN GPIO HEADER
This is a standard 40-Pin .1” Dual Row Female Header. It is designed to plug into the
40-pin GPIO Header on a Raspberry Pi single board and conforms to the standard
Raspberry Pi pinout. It is located on the bottom side of the 502HTA. P2 is not populated
on the 502HTA-SA.
8.6 P2 – DC JACK
This jack is designed to accept a 5.5mm x 2.1mm center positive plug for external
power in. Voltage must be 24V +/- 5%, 2 Amp minimum.
8.7 P3 – 3-PIN HEATER VOLTAGE SELECT
P3 is a 3-pin .1” single row header. It is used to select between 6V or 12V (in series
mode) tube heaters.
8.8 RP1 AND RP3 TRIMPOTS
RP1 and RP3 are the bias adjustment potentiometers for the Left and Right Channels
respectively.
8.9 RP5 –VOLUME POT WITH ON/OFF SWITCH
RP5 is an Alps RK09 Low Noise, Conductive Plastic Volume Control with built-in On/Off
Switch.
8.10 SW1 – ATTENUATION SWITCH
SW1 – a Single Pole, Double Throw (SPDT) toggle switch that controls the attenuation
relay.
8.11 SW2 – OUTPUT RESISTANCE SLIDE SWITCH
SW2 is a Double Pole, Three Position (DP3T) Slide Switch. It adds 0, 47 and 100 ohm
headphone output resistance when in the Left, Middle and Right positions respectively.
8.12 W2 – BIAS ADJUST HEADER
W2 is a 3-pin right angle header. It carries the Left (L) and Right (R) Bias signals along
with Ground (G). This allows the bias voltage to be measured while adjusting for the
current tube.

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9TEST RESULTS
9.1 TEST RESULTS SUMMARY
502HTA test conditions were:
•Raspberry Pi 2 running Volumio Version 2.23, 503HTA driver selected
•External 24V from low noise benchtop supply
•Raspberry Pi powered from standard USB 5[email protected] supply
•All Test Files were encoded as 24-Bit@96Khz unless otherwise noted
•0db reference = 2Vrms
•dScope IIIe Analog In Load resistance = 600 ohm
•dScope IIIe Analyzer FFT = 32K, THD w/no weighting, Residual Noise w/A-
Weighting
502HTA-SA test conditions were:
•External 24V from low noise benchtop supply
•All Test Files were encoded as 24-Bit@96Khz unless otherwise noted
•0db reference = 2Vrms
•dScope IIIe Analog Generator set to 2Vrms Out, Floating Ground
•dScope IIIe Analog In Load resistance = 600 ohm
•dScope IIIe Analyzer FFT = 32K, THD w/no weighting, Residual Noise w/A-
Weighting
9.2 HEADPHONE OUT, DAC DRIVEN TEST RESULTS
Using 1-meter 3.5MM to XLR Adapter cable. Attenuation Switch High, Volume at 50%.

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Figure 4 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, 0dB, 600 ohm

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Figure 5 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -3dB, 600 ohm

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Figure 6 – 502HTA Headphone Out, 1Khz, 24-Bit@96Khz, -12dB, 600 ohm