1 - Ideas and Design, 2 - PCBs, 3 - Amps, PSU and Grounding, 4 - Digital Control and Software, 5 - Results and Pictures

Part 1 - Ideas and Design

So, my 'TDA' surround sound amplifier was built originally in 2001, upgraded in 2003 and became my secondary amplifier after completion (and several upgrades) of my main hifi surround sound amplifier.

It did me through late secondary school, all of university, and became the bedroom amplifier when I moved into my own apartment. It was faithful, and worked for 19 years, although I did do a bit of ground bus re-writing in 2016 to reduce hum, as well as add a PT2399 digital delay, and a power-on mute circuit for the speakers.

With a bedroom redesign though, and removal of the TV (we decided it's not healthy to have a TV in the bedroom), the amplifier was looking for a purpose to hopefully make it survive past 20 and then another 20 years!

This v3 rebuild actually turned into a lot more work than I originally anticipated when I decided to tear it down and upgrade it, but here were my new goals:

• Remote control - I wanted the amplifier power, volume, mute and input to be controlled remotely, allowing it to be placed in the room and operated without having to walk over to it.
• Bluetooth - with no TV as the input, I wanted to provide a convenient way to connect Smartphones and Laptops, without having to hook in long 3.5mm audio cables (especially as these days phones are even now missing 3.5mm outputs)
• Fix noisy controls - by implementing a proper pre-amplifier and using appropriate op-amps which don't have DC offset on their outputs
• Fix/greatly reduce hum
• Leave the surround sound capability and allow for a potential surround pre-in for full 5.1 sound control

My intention was to reuse as much as possible, but upgrade with parts that I already had. This was all done during the Coronavirus pandemic in 2020, in which the lockdown meant absolutely no going out to purchase parts, terribly slow national mail deliveries and as I write a number of components shipped from Hong Kong and China still hadn't shown up!

I was fortunate during the pandemic to be able to work full time from home, so I didn't have lots of time free. But it did mean I could spend a little time on it in the evenings, and at least 5 hours a day on each weekend day. Even so, the build took me about 6 weeks.

I ended up doing much more work than I expected though, as during the teardown, my plans became a little more ambitious. Here's what was kept and changed:

• Amplifiers: Front TDA1514A - Kept, but slightly modified. This front channel left/right amplifier sounds good, but I modified it slightly to use larger 2.2uF polyester capacitors and I split the ground trace connecting both amps so each would have their own individual run to the star ground. I also changed the resistors the datasheet recommends for 4 ohm use.
• Amplifiers: Rear TDA2040V - Kept chips, remade board. The old board (which was from the 2001 original) was initially kept, but I ended up making a smaller one so I could move the amplifier further away from the mains input and transformers, for noise reasons. The TDA2040 chips, plus a few other components were de-soldered and soldered into a new board, with other components renewed.
• Amplifier: Centre TDA2040V - Gone. In its history, I only used the centre channel speaker in its early years. After I moved into my flat, and TVs technology changed to flatscreen LCD, placing a centre channel speaker became challenging and to be honest doesn't make much difference unless you're using a true 5.1 source (I'm not).
• PSU - 18V 80VA transformer. Kept
• PSU - 12V 80VA transformer. Replaced with a smaller 60VA transformer as that's all the rear amps and preamp needs, but mainly so I could fit both transformers on one side of the case
• PSU rectifiers and smoothing - Reused bridge rectifiers but replaced the PSU board (with capacitors) with a larger one.
• Surround sound decoder - replaced. I did add a PT2399 audio delay board later but would rather this be integrated to the same PCB as the decoder.
• Preamp - replaced. Well, there wasn't one really. Whilst there was a stereo width controller originally, that was removed earlier.
• Tone controls - replaced. The old board has a design flaw and the potentiometers also became really noisy
• Potentiometers - all replaced. All of them sounded incredibly noisy throughout their range.
• Switches - all removed. The only switch needed on the front now would be a power switch, and it's momentary instead.

To complete the objectives of the rebuild, 7 new PCBs would be constructed:

1. Preamp - this board would contain:
   • ESP Project 97 Hifi Preamp for the front left/right channels. This includes tone controls.
   • 2x ESP Project 88 High Quality Audio Preamp (Mk II) for the rear left/right, centre and subwoofer channels (the latter two are for future proofing really and currently unused)
   • A 6 gang ALPS motorised potentiometer for the master volume of all 6 preamp channels
   • A PIC16F627A microcontroller circuit for control of volume, mute, input selection and power on/off
   • Simple Motor H-Bridge for potentiometer, based on ESP Project 110 Infrared Remote Control (the receiver part)
   • 5 relays - 3 controlling the L/R input, and 2 for switching the decoder to internal or external decoding
   • ESP Project 05-Mini Power Supply for Preamplifiers based on 7812 and 7912 regulators giving +/-12V regulated from the 12V AC transformer.
   • Front L/R balance control (part of the project 97)
2. Preamp daughterboard, containing:
   • ESP Project 188 Surround Sound Decoder (Mk. II). The decoder (with inversion of one rear channel) and digital delay based on PT2399 on here.
   • Potentiometers for front volume adjustment, real volume adjustment and rear balance
3. PSU - one board containing both the +/-25V and +/-17V supplies. Each supply contains one 10000uF per rail, so the board has 4x 10000uF capacitors in total. Additional ripple filtering is included too using 1 ohm resistors. The star ground for each PSU is at this board too
4. Mains board - contains terminal blocks for mains in, switching relay, mains out (switched and non-switched), fuse for low voltage transformer, and capacitor/resistor for earth loop breaker.
5. Board for LED indicators and IR sensor. This would be designed to have the three LEDs aligned with the existing holes in the front of the amplifier. Two RGB LEDs would be used for Standby/Active/Muted and Input 1/2/3. The third is for IR activity.
6. Small board for mounting the Bluetooth module, and relay to power it on/off
7. Stereo (2x) TDA2040 revised and smaller board

Read on to find more details of the PCBs.

References and more reading:
The Audio Pages - Elliott Sound Products

Part 2 - Building, PCBs...