Welcome to the ePortfolio Page!
This is where Bob's, or Bob's MEL works in the past 6 years are shown.
Past Projects and Experience
(Future) Website Update
Feeling this static webpage with only text and images can be as boring as textbook, I might redesign the page someday with Bootstrap or CSS learnt back when I am an undergraduate student. It could take a long time though, since the author of this website still prioritize audio projects over website maintainance. :)
(Present) Feedback and Circuit Architecture Studies (In progress, part of MAS)
Analog circuit textbooks usually introduce feedback as an effective way to control circuit parameters and Negative Feedback can aid circuit stability. However over the years some audiophiles insist the NFB can also negatively harm the sonic quality in aspects like Transient Intermodulation Distortion (TIM) or other Intermodulation Distortion(IMD). Such flaws are not harmonic - meaning that the resulting sound coming from the system with lots of NFB may sound unnatural and uncomfortable. Nelson Pass even quoted that some audiophiles think feedback can "sucks the life out of the music", added with support from studies of John Linsley Hood that NFB can reduce some lower order harmonic distortion content but adds up the higher content. These defects are much perceivable than lower order counterparts and hence making amps with NFB measures well but sound dull and lifeless.
Since I am in the quest of studying different circuit architecture and implementing them into the MAS design, I found that many famous commercial amp design still uses NFB, even when the journals about TIM was out since 1970s. Trained with an engineering mind, I feel like whether to use the feedback mechanism can have tradeoffs like many engineering problem. In fact, I have also read that for audio circuit certain degree of NFB is a must have (maybe in local NFB form instead of global NFB) or the amp would be so instable to control its output and turn into an oscillator.
Despite the fact that TIM are difficult to measure and rarely used as amplifier benchmarking tool, I decided to give it a preliminary test by listening to some design I have now and in the future. To make it easier for me to test, I would do these design in headphone amp. scale - meaning that the maximum output would be far below 1W or perhaps even lower in class-A region.
The circuit I am using to test is MAS-HPA2Pre - a Headphone Amp/Pre-amp Multi-purpose design with one single op-amp and single-ended class A (which some enthusiasts regarded as the best) output stage. By adjusting how the feedback resistors are placed we can control it to have local NFB or global NFB. Tried the local NFB version with OPA111 and I could say the sound is very different from traditional headphone amp - soft bass and perhaps too much focus on higher mids and treble.
You will need BP output cap if you want to try this design on your own.
One classical design implements FET Input Stage - FET Voltage Amplification Stage (VAS using Douglas Self's notation) - BJT Push-pull Output Stage. This one has global NFB - very commonly found approach to stablise the amp.
Another one is the FET - BJT - FET symmetrical push-pull design. When I started the design I immediately found that matched complementary pair of FETs for input stage is very hard to find nowadays - commonly used 2SK170/2SJ74 or 2SK364/2SJ104 has been obsoleted since mid 2000s and price have skyrocketed from $5/pair to about $40/pair. If we design a new amp with symmetrical FET input and do some cascoding that can be one expensive project.
(Present) Modular Audio System (MAS) - A complete audio system (In progress)
Thinking of making a complete audio system consists of DAC, Headphone Amp, Pre-amp and power amp, I stated a long journey of endless nights with PCB drawing and chassis building has been started from the next half of 2016.
I come up with the name MAS - Modular Audio System since I want to build an audio system that can ultimately feature very flexible, plug-and-play PCB (e.g. changing from op-amp to discrete design, BJT to FET input stage......) and as a fancy name representing what I have learnt from the previous years. This should help me to learn what component should be used in a specific spot of the circuit and how different building block can sound. The headphone amp of the same design proposed by roggom was completed before the end of Christmas and was installed in the new, separated chassis. The amp is powered by 2 sets of Jung super regulator (one for tube filament and another for FET output stage).
RCA 12AU7 Tube in place
Gold plated 2oz PCB with mixed silver chassis wires and so-called Western Electric wires. The silver input cap on the right is Russian PIO cap. The sound is suprisingly good and very warm.
HPA and dedicated PSU along with my reference headphone - AKG K712
Shielded Transformer to minimize any EMI noise induced to audio circuit
Side panels garnished with red sandel wood and gold-plated parts for classic looking
Carbon fiber top panel with custom-made metal sticker for better appearance
Some sample mix-and-match of wood and knob colours
(23/9/2017 update)
The project is still expanding and more ideas are being added into it. It can be pretty sure the project will last until years like 2019.
Recently I am focused on pre-amps and headphone amp (which I am interested in as always) had made 2 variations: PreAmp1 with Full Discrete BJT like Elliott Sound Products Project 37a and PreAmp2 with JFET unity-gain Buffer. This 2 preamp PCB can be interchangable and only needed operation to substitute one with another is screwing 6 terminal blocks.
I tried tuning the sound of preamp with different input caps with reference to Humble Cap-test etc. and found the Jensen copper-foil and Intertechnik Audyn True Copper very suitable for this highly demanding usage. Later I may do some experiment on DC-coupled pre-amp and see if I can build the so-called very transparent preamp.
(Prototype Created/Upcoming revision) TPA6120 Headphone Amplifier
The first prototype was built and tested in Feb., 2016, albeit without success. Both channels reproduced the music with heavy noise. The left channel also occasionally shuts down on its own.
Having found that the power supply section was flawed and cannot provide sufficient current to the TPA6120, I switched to use an NJM4556 Op-amp in series with the TLE2426 vitual ground IC to boost the current output of the ground channel.
This 2nd version is built successfully. Equipped with a current-feedback architecture it seems to have a more clear sound than ordinary audio single-ic design.
This design will later be modulized and put on a smaller board to form a fully modulized portable DAC, along with the SA9027 project below.
(29/3/2016 update)
With the arrival of the TM902 thermometer, the temperature of the virtual ground part is tested.
The graph below shows that the temperature stablizes at about 49 ℃. While this can be acceptable as long as the power dissipated is within the absolute maximum value given by the datasheet, extra care should be taken in future implementation if the same circuit are to be applied in portable amp, where the interior of the chassis can be tight and ventilation is inadequate.
(Room Temp. Under Test: 19.9 ℃.)
(23/5/2016 update)
As tested with the lately bought AKG K712 (about 62 Ω) this pure buffer seemed to have sub-optimal soundstage and sub-par details and positionings, which is an indication of not enough output voltage swing. A gain stage with 2 single op-amp will be added to test later again
(July, 2016) DIY 3.5-inch Full Range Speakers
I have always wanted to step into the world of speakers and not limit myself to earbuds or headphones. Making a basic yet rich-sounding full range speaker was how I started. The units I have considered are Peerless, Fostex and Markaudio due to the brand reputation, quality and availability. I have chosen the last one since it is purchasable on Taobao and many reported their units can reproduce outstanding sound for the size.
While I am not the best at acoustics and its related maths I feel like trying with different types of wools and absorbing materials can alter the sound quite a lot and is overall a very fun project. The finished speaker is now used in a near-field setup that I occasionally listen to with my PC and DAC.
(Present) DIY Discrete Operational Amplifier for Audio purposes (In progress)
Traditional operational amplifier features high gain input-stage and low-power class AB output stages. While these attributes are sufficient for most low frequency analog design, the low-impedance load driving capability seemed to be insufficient for headphone amplifier application.
Perhaps most consumer-grade headphones are rated for 32 Ω or well below 100 Ω. General purpose op-amps usually provide optimal driving power at 600 Ω, which can inflict impedance mismatch.
The Japanese reference link below enlightened me to create my own op-amp for such purposes. The actual gain might be lower in these customized op-amp than traditional FET-input devices, the output impedance may, nevertheless, be much lower and capable of driving headphones. These types of specialized op-amps can also be designed as a substitution for normal op-amps.
(Reference: Innocent Key (Japanese) )
(2016) DIY Jung Super Regulator (Completed, using in MAS-HPA PSU)
Power supply can significantly affect the performace of an amplifier or DAC. With the Jung Super Regulator design it is believed that the PSRR can be higher than normal LDO ICs.
The PCB was designed with a split ground planes that connect to the negative pole of the reservoir capacitor for lowest noise.
Recently the LM329 Precision reference and AD817 was used in the testing. The test of ripple and noise will be continued later.
In the recent state of design, remote sensing is disabled for the best simplicity.
(Reference: Tangentsoft.com)
(Reference 2: Walt Jung References & Regulators)
(Suspended) DIY SA9027-PCM5102 USB DAC
SA9027 is a 24bit USB audio decode chip produced by SAVITECH. This next project is designed to be the successor of the PCM2706 DAC module.
In the stage of implementation, the necessary components were purchased and the prior researches are still in progress.
A low-cost AKM4430 DAC modulized External I2S DAC Card is included in this project.
(2017 update)
Since newer and better solutions like Savitech SA9227, XMOS and Amanero has come to public in these 2 years I have decided to switch to these more advanced solution.
(April, 2016) DIY 8-strand 3.5mm to 3.5mm Cable
Equipped with 8 strands of silver single core wires, this most recently created 3.5mm cable secured its position as one of the best looking cables in my DIY projects and offered me several nights of pure enjoyment.
(2015 - 2016) DIY FET-input Differential Headphone Amplifier
Tired of building single op-amp headphone amplifier,I had been motivated to look for designing a discrete amplifier. Although single IC amplifier can be simple to establish and often offer low-cost solutions, using discrete parts can grant a specialized design that fit the requirements and offer better load impedance matching.
The Amp. Board was specially designed such that the power stage BJTs can be changed at will. Among 10 different sets of different chips tested the followings were the most astonishing:
Pre-amp stage BJT:
- 2N2222/2N2907: Very clean sound. (More explanation later)
- BC550/BC560: Vocal is clear and in the upfront. (More explanation later)
(Inspired by op316.com (Japanese) )
(2015) DIY Wooden Headphone (w/ 42mm speaker unit)
A new DIY headphone for audio testing purposes. The casing and the speaker units are purchased in the mainland China market.
The speaker unit is rated at 54 Ω. It features a large port for acoustic tuning. Covering the port with high-density wool can reduce the bass significantly. This tuning design is particularly fun to experiment with as a DIYer.
(2015 - 2016) DIY Audio Creation Triple-C Headphone Cable
The triple-C headphone cable is manufactured with a similar architecture of that in PCOCC. Very soft as if the music notes have no edges with a warm touch.
Audio Creation Official Website (Japanese)
(2015) DIY OCC Earphones Cable (w/ MMCX connector)
(2015) DIY PCM2706-TDA1543 USB DAC (w/ passive I//V stage)
A PCM5102 modulized External I2S DAC Card was also designed along with the project.
It turns out to have a smoothier, softer sound than TDA1543. The TDA1543 still excels in reproducing punchy bass and lower human voice.
(2014) DIY Sommer Albedo MkII RCA Cable
Yet another inexpensive cable that reproduce superb price for its cost. This RCA cable is used to connect the DAC analog output to my computer and is still used today in my CAS setup. The cable sounds slightly more vivid to the Galileo but it can be difficult to tell which is superior.
The nylon sleeves added here provide a clean appearance.
(2014) DIY Sommer Cables Gailieo RCA Cable
A good sounding pair of cables that is priced reasonably. This was used in the CD -> Amplifier signal path. The sound of the cable is generally neutral.
(2013 - 2014) DIY 12AU7-IRF510 Headphone Amplifier
This is perhaps my largest scale DIY project in a while. The project lasted for 1 year and technically the improvement on it is not ended.
From PCB preparation to the drawings and design of the aluminum chassis, I have learnt much from the Cadsoft Eagle PCB and Solidworks.
While the very first prototype was assembled successfully, a hint of thermal noise reminded me to pay extra attention to the heat sinking and routing design.
Eventually, another version with almost no noise(the one in the photos) were installed and the tube lit up the Christmas at that year.
(2013) DIY Szekeres Class A headphone Amplifier
(2013) DIY Sijosae Virtual Ground Circuit
The circuit was used to replace the resistor divider in the previous CHA47 amplifier. The class AB output stage provides a better current output capability.
(2012 - 2013) DIY CHA47 Headphone Amplifier + Ohman’s Crossfeed Filter
Arguably the first complete amp project that is more than just purchase PCB and do the soldering)
The painting on aluminum chassis was quite time-consuming. Several thin layers of metal primer were needed to be applied onto the surface before the color was sprayed.
The copper layer of the PCB was also routed using delicate silver-plated cables in a point-to-point manner. (Thanks to Sijosae schematics and point-to-point hints)
Although the amp was later replaced by the PCB-based version, this first amp is still carefully preserved until today.
(2012) DIY 6N3-6080 Class A Tube Headphone Amplifier
Interested in making headphone amplifiers, this 6N3-6080 amp is the second beginner kit I have purchased and built when I first entered the world of electronics.
Although at that time I have little background on tube amp, the project was overall fun.
With the pre-drilled case and PCBs, the tube amp was completed at one night and the result was astonishing.
The thrill of completing this audio project allured me to go further into the world of audio electronics.
Who am I?
I am Bob Chong, an audio enthusiast graduated from the Electronics and Information Engineering degree in Hong Kong Polytechnic University in 2016. Interested in Electronics and feeling comfortable working in the audio aspects of the discipline, I started building RCA cables and amplifiers since I was in high school. (Feb., 2011)
My recent projects include a full audio system called Modular Audio System (MAS) (modulized design is inspired by pavork.org) and discrete operational amplifier for audio purposes. (Reference: Innocent Key)
What makes me start DIY?
Back then when I was an ordinary secondary school student I was going to purchase a headphone amplifier to drive my Audio-Technica A900 closed-back headphones. I attempted to purchase one and had the plan suspended since I found that a decent quality amp would be priced for at least HKD$2000 at that time.
Knowing that building one may have much more fun than straight up buying one, I started to learn basic electronics and soldering with notes provided by my physics teacher in high school. This is how I started the DIY projects back in 2010 and 2011.
It would be a pleasure for me to purchase different kinds of amateur electronics kits and assembling them in the weekend. Despite having few creative elements involved, I had built up passion towards analog electronics and electroacoustics through soldering the components and grasped the basic knowledge before I attended the higher diploma courses in electronics engineering.
In last year, I succeeded in my 2nd trial to build my first USB DAC using PCM2706 and the antique yet good-sounding TDA1543. This is when I have started to expand my interest beyond analog electronics.