Quad Preamp – 4-Channel Analog Preamplifier

Published 24.11.2014 (built late-2014)

This design is to be connected to possibly lethal mains voltage. These are not detailed instructions, so do not try to build similar device without extensive understanding of electronics and electrical safety!

4-Channel analog audio preamplifier for 4-channel DIY audio system

  • Three 4-channel inputs: two balanced and one unbalanced
  • Balanced and unbalanced outputs
  • Comprehensive software features
  • Remote control possibility with any IR-remote control using NEC codes
  • Controls whole system using wired remote control interface to SharcDSP and Quad DAC
  • PGA2310 volume control, OPA2134 op-amps, THAT1200 balanced line receivers, DRV135 balanced line drivers

Operation and Design

Quad Preamp performs two basic functions of a preamplifier: input switching and volume control. Even input switching is rare in this unique 4-channel system using mainly digital sources connected to the SharcDSP where digital input selector is located. Basically Quad Preamp controls volume and input selector in the SharcDSP by wired remote control interface and provides extensive software-related features. Balanced and unbalanced inputs and outputs are provided. Block-level functional diagram is shown below and schematics are presented lower on this page.


This is a simple preamplifier with balanced and unbalanced inputs followed by relay input selector. Stepped attenuator IC is used for volume control, controlled by microcontroller (MCU). Buffers are provided with possibility for fixed attenuation and gain. Proper differential line receivers and drivers are used for balanced connections. MCU is also used to provide wired remote control functionality with Quad DAC and SharcDSP. Device is mains-powered with separate transformers and voltage regulators for analog and digital supplies. Only half of the preamplifier is shown, another stereo board is stacked on the one shown. Power distribution network is not drawn.


Design consists of two identical 2-channel boards organized as 4-channel amplifier. With both unbalanced and balanced connections, the inner structure is single-ended with proper line receivers and transmitters depending on the connection type. Most of the enclosure is filled with power supply related components since transformer and regulators are on separate boards. Not very space-efficient but there was plenty of room and I had designed these boards already for other devices. There are two transformers and power supplies: one for digital logic and one for analog preamplifier components. Combined display and microcontroller board is from DD Preamp and is mounted on the front plate.


Most of the enclosure is filled with power supply related components. It is not very space-efficient to have separate transformer and regulator boards but I had the boards and I had the space. Microcontroller board is mounted behind LCD with ribbon cables for control signals. Two 2-channel preamplifier boards are stacked behind connectors.


Software offers simple user interface but also quite many settings to change. User interface consists of a 2×16 symbol alphanumeric LCD, rotary encoder and push button. I like to use rotary encoders with integrated button but here normal one does well with a large knob, and there was an external button anyway. In basic use, knob changes volume and pushing button brings input selection, again chosen by the knob. By long button push menu opens with numerous settings. The features of the software can be summed up as follows:

  • Volume control
  • Input selector: 6 digital inputs in SharcDSP, 2 inputs in Quad DAC and 3 inputs in Quad Preamp
  • DSP filter bank preset
  • LCD backlight on/off
  • DAC filter roll-off
  • Channel pair 3&4 offset (compared to 1&2)
  • Channel pair 1&2 balance
  • Channel pair 3&4 balance
  • Input name for all inputs from predefined list
  • Input offset for all inputs
  • IR-code learning

Settings related to SharcDSP or Quad DAC use wired remote control interface. Input selector works based on an assumption that SharcDSP is connected to the input 1 of Quad DAC which is connected to the input 1 of Quad Preamp. Therefore, these inputs are kept static when changing input in SharcDSP. Also other inputs of Quad DAC and Quad Preamp can be switched to.

Input-wise offset values can be used to compensate louder/quieter inputs and balance may be useful in some cases. Channel pair 3&4 offset can be used to control subwoofer level when subwoofer(s) are connected to the second channel pair. Input name is shown next to the physical input name (“Cx1”, “Opt1” etc) and can be chosen from predefined list containing names such as “PC”, “CD”, “TV”, “AUX” etc (can be seen on the photo on this page). IR-code menu item is used to teach IR-remote codes. Due to the wired remote interface between devices it is possible to change all possible settings with one small remote controller commanding Quad Preamp. I am using small Apple remote from a dock of my ancient iPhone.

There is a video in DD Preamp page about a similar user interface.


Rear panel is packed with connectors. XLRs are on board but RCAs needed to be wired out of the boards. There are three 4-channel inputs: balanced input 1 (blue), balanced input 2 (green) and unbalanced input 3 (orange). Connector arrangement is this because of two 2-channel boards working as a 4-channel configuration. All 4 channels have balanced and unbalanced outputs. Remote connectors are for wired control between preamp and Quad DAC and SharcDSP.


Schematics

Here are schematics for preamplifier board, voltage regulator board, and transformer board.

Preamplifier is built around PGA2310 (U9) stepped attenuator IC designed for high-quality audio applications. It is a high-supply-voltage counterpart of PGA2311 and can operate with analog voltages of up to +-15 V, in practice the same supply with op-amps. Input switching is before the PGA2310 by relays (K5, K6, K7). Relay may not be the optimal switch for signals with very small current such as audio signals since there may be a threshold for conductivity. In practice, relays are widely used and sealed small signal relay should not cause any problems.

THAT1200 ICs (U1, U2, U3, U4) are used as balanced receivers. I found them mentioned in Elliott Sound Products site, this is a great source of information, by the way. They use bootstrapping to provide very high common-mode rejection ratio (CMRR) also with practical component values although usually CMRR of receiver circuits are very sensitive to component tolerances. More information can be found at the THAT1200 datasheet and ESP. Basically, the higher the CMRR (which is a function of frequency) the better the balanced interconnection works when it comes to noise and interference rejection. Components around THAT1200s are for boot-strapping and noise filtering, the circuit is from the datasheet. For unbalanced inputs there are filtering and protection components followed by OPA2134 buffer (U6). All single-ended signals are routed to relays and then to volume control.

OPA2134 (U5) buffer follows after PGA2310 along with resistors for various attenuation or gain possibilities. This buffered signal is connected to RCAs through some RF-filtering for unbalanced outputs. The same signal is fed to DRV135 balanced line driver for balanced outputs.

On the second page of the schematics there are possibilities for various routing options for data input SDI and data output SDO of SPI of PGA2310. By selecting appropriate lines it is possible to daisy-chain up to four of these boards. For example, with these two boards the data out of the first board is routed to the data in of the second board. There are also other resistor places where 0-ohm resistors can be used or leave them unconnected, depending on the desired grounding scheme and type of connectors used.


Two preamplifier boards are stacked to work as one 4-channel system. Relays are used for input switching and internal operation is unbalanced.


Power supply boards

Transformer board has footprints for few common PCB-transformer models, primary and secondary fuses and possibility for serial or parallel connection of dual-secondary transformer. Full-wave rectifier and filtering capacitors are also on board. Regulator board also has rectifier and filtering capacitors, since these are not specifically designed for this device, followed by 317/337-regulator circuits. In this preamp, part of the regulator PCBs have been cut out to fit in the enclosure and since rectifier/capacitor part is not needed anyway.

For digital part, 2x 6 VAC transformer has secondaries in parallel followed by LM317 adjusted to 5 VDC, providing power for microcontroller/LCD-board and digital parts of preamplifier boards. 2x 12 VAC transformer and LM317/LM337-pair are connected to provide ± 15 VDC for PGA2310s, THAT1200s and OPA2134s. Regulators are mounted on the enclosure.


Separate transformer + regulator board combination is used for both analog and digital circuits. Regulators are mounted on the bottom of the enclosure and holes drilled for screws. Obviously not the way I designed the boards to be used.


Microcontroller board

Microcontroller board is designed for DD Preamp and to be mounted behind LCD as is also done here. Used controller is 8-bit Atmel ATmega324A. The board provides JTAG-interface for programming and debugging (I am using AVR dragon), LCD socket, I/O-connectors which are used for rotary encoder, button and IR-receiver. Software functionality is described on top of the page.


LCD + microcontroller board stack is carefully designed to hold LCD at the right position still providing access to the board if necessary (nowadays I try not to cover everything with hot-glue).


Layout

Layout is rather simple. PCBs are big considering the amount of components, 10 cm x 15 cm, to fit all the connectors. RCAs outside of the board. Stackup is 2 layers, bottom ground plane and top routing/component plane. There is a ground fill part on the edge of the board which is effectively extension of the enclosure since it is connected to the shell and pin 1 of the XLR-connectors. It is used for chassis-connected filtering components. PCBs are designed with Kicad and ordered from Iteadstudio.

Software

Software functionality is described on top of the page. It is based on the software for DD Preamp. The microcontroller used is ATmega324A from Atmel AVR8 -family, and development environment Atmel Studio 6 with C-compiler. The program is FreeRTOS -based which may seem overkill but I wanted to learn some philosophy and operating principle of RTOS (Real Time Operating System). After getting familiar with it, it also made it easier to further develop the software. However, the structure of the software is quite a mess in DD Preamp and so is also here but I didn’t want to start from the beginning now. This is mainly because I didn’t know exactly what I was doing when starting to develop the program. Due to this reason I am not going to share the software at least yet, and also not going to explain the functionality in more detail. If you want to design something similar, a lot simpler software will be enough.

Enclosure

At first the enclosure I got from eBay seemed way too big, and I chose this one cause it had a front panel which I could use directly. The plan was actually to fit the Hypex amplifier modules also into this. Soon after fitting the separate transformer and regulator boards, which is not very space-efficient solution, it was clear that this will be actually full. Also the rear panel area is tight with all the connectors in place.

Two preamp boards are stacked with few large spacers in between to provide the necessary height and electrically connected with ribbon cable. Also transformer and regulator boards are firmly attached to the enclosure and regulators mounted to the bottom of the enclosure to take care of excessive heat. After all, the overall structure is quite sturdy with everything firmly in place with screws. As with all mains-powered devices, remember to firmly connect the safety earth to the enclosure! With anodized aluminium make sure it really has good contact to the metal. Microcontroller board and LCD are mounted on backside of the front panel.


I have not given references list but used links directly to references. However, there is a list of some documents and books I think are worth reading on the main projects page. Feel free to comment and ask questions so I can clarify some things and further develop the description. At the moment it is still quite vague. It will not become detailed building instructions though.

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