Confessions Of A Lifelong DIY Audio Nerd

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Intro

As an electronics engineer I've always been interested in audio. I have built my own audio equipment for many years. Speakers, Preamps, Amplifiers, you name it, I have built them. In the early years, I was always on a budget. Quality HiFi equipment has nearly always been outside my reach, financially. When I was a poor high school or college student, then a new grad, I lusted after quality audio equipment. I set about designing, building, fixing, and copying quality equipment. I got in the habit early on and kept it up well into retirement. At first it was a necessity, but it evolved into a great hobby.

Back in the sixties, I collected junked tube radios, shortwave radios and record players. My parents had an old RCA record player with maybe 10 albums. Mostly Perry Como and other 50s performers that they loved. The band that turned me on to music listening was Herb Alpert and the Tijuana Brass. In junior high and high school I collected old hand-me-down radios, audio equipment and parts from the side of the road, all tube based.

In 1970, my older cousin Joe returned from Vietnam with a very nice Sony component stereo system that he bought in Japan. He also had a decent music collection and I would go to his house with my three-and-a-half-inch reel-to-reel tape recorder and record some of his albums.

In college in the mid-70s I had an old tube console record player. I couldn't afford albums so I borrowed LPs and 8-tracks from my friends, sister, and cousins. My sister had good taste in prog-rock and had Jethro Tull and Yes albums, and 8-tracks. In college I built my first real audio system, a portable boom-box.  It used some 6x9 car speakers driven by an 8-track tape deck. It was powered by a 12V motorcycle battery. We would bring this to the beach, on camping trips, to spring break, or whatever social events and it was great to listen to. We called it the party box. I wish I had a photo of it.

In college at WPI in the early 70s a few of my fellow dorm residents had decent stereos and I learned to enjoy them. FM Radio had finally caught on and there were some decent College stations in Worcester and Boston that we listened to. That was the end of AM radio and Top 40 for me.

When I started my first real job at Hewlett-Packard Medical in Waltham, my first roommate had a nice stereo that I listened to. After 1 year I needed a new apartment and moved in with my current wife as roommates. I asked if she had a stereo and she said no, but she had an awesome record collection, so I was motivated to get a stereo going. I started with a junky receiver that I bought for $10. We used it at work to listen to FM and had a rudimentary headphone Network wired into the building so you could plug in a pair of headphones with a volume control to listen to it. When I brought it home though, the phono preamp was not working. I traced the problem to bad electrolytic capacitors in the phono preamp section. But when I replaced one of the bad capacitors, the volume increased slightly but it was still very low on that channel. I soon realized that all the capacitors in the phono preamps were bad and so replaced them all in both channels. I forgot what I was using for speakers and a turntable but I'm sure there was something I scrounged up for almost free. Finally Alex and I could listen to her album collection and and began buying more albums.

DIY Bose 901s

Then one day at work I learned that some of my predecessors had built Bose 901 speaker clones. I asked for the plans for the enclosures and the equalizers and they showed up on my desk. I looked them over and realized that building the speakers and the equalizers could be done fairly cheaply. I asked around to see if anyone else was interested in such a project and sure enough 10 people signed up. I end up making 12 pairs of the Bose 901 clones and equalizers. I volunteered my dad, a carpenter, to build the cabinets. He along with my friend Dave cranked them out over a few weekends.

For the equalizer PCBs, I enrolled one of the guys in the PCB department at Hewlett Packard. I laid out the single-sided PCB and he built 12 bare boards. All of the components were in Hewlett-Packard stock and HP had an arrangement that employees could buy components from stock at their Cost Plus 10%. I remember that the components cost about $12 per equalizer. Everyone built their own circuit boards. I ordered 12 * 18 = 216 of the 4.5 inch speaker drivers, 4 1/2SR10B from CTS. A week or so later an entire pallet of drivers was delivered to my desk at HP. I built up the first set and they worked really well. I distributed all the parts and cabinets to all the other builders. Everybody was happy with what we built. I personally listened to them for about 25 years. Here is one of those first Bose Equalizer boards, installed in my first DIY preamp.

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Advent 300 Receiver

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I bought an Advent Model 300 receiver to use as a preamp. It only has 30 watts per Channel which is not adequate to drive Bose 901s. My intention was to use it as a preamp and to drive a equalizer and an external power amplifier for the Bose 901s. The Advent 300 has a state-of-the-art audiophile phono preamp design by Holman, similar to the one in the Apt Holman preamp. Unlike many 2 transistor Phono preamp designs, this one used a proper transistor amplifier and achieved low noise and better than 0.01% distortion.  

But when I brought the receiver home and connected it to the speakers, I discovered that there was a fairly loud hiss. I could hear the noise with headphones and speakers. At first I thought maybe it was only my unit, so I went back to the store with my headphones and plugged them into the store demo unit, and sure enough it had the same noise. I traced the noise to the tone control section of the preamp. I noticed that the volume control was before the tone control section, not after. Typically this is bad practice because any noise generated in the tone control section feeds your amplifier directly at full gain, and so can be audible. The noise was pretty bad with the built-in 30 watt-per-channel amplifier and even worse when feeding an external high-power amplifier. So after thinking about this for a while, I rewired the volume control in (my precious) A300 receiver, to move it after the amplifier preamp stage and before the amplifier. At least at quiet volume levels, the noise was reduced significantly by the volume control. Important lesson learned. My intention was that I would use the A300 as a preamp and would design a high power amplifier stage to drive the Bose 901s. Note the Preamp Out  / Amp In connections on the A300 schematic below.

Here is the Advent 300 Phono preamp, Tone stage, and Amp schematic.  Note the volume / loudness control located just before the tone section. I rewired mine to place it after the tone control and before the amp. I also "borrowed" their excellent low-noise Holman-designed phono preamp circuit and built several of these boards over the years.
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BPA and Preamp

Bose 901s are some of the least efficient speakers ever made. Each speaker uses 9 identical 4.5" drivers, which are good midrange speakers. To compensate for the low response at low and high frequencies, an active equalizer is used to boost highs and lows considerably. The active equalizer has up to 14 dB of bass and treble boost to compensate for the driver frequency response so to get 1 watt of sound out of the speakers requires about 25 watts of power. The minimum recommended amplifier is 100 Watts but 200 to 300 watts is preferred.

At Hewlett-Packard in the 70s, I worked with some pretty great engineers. Two of our EEs were Mel Clarke and Rich McMorrow, who founded Dunlap Clarke. Dunlap Clarke built the Dreadnaught 500 and 1000 Watt high power amplifiers for dance clubs in Boston and everywhere else, and for crazy audiophiles. Mel was a great guy who I got to hang out with on a number of occasions. Inspired by Mel and Rich's amazing design, I thought I would give it a try. I aimed for a reasonable 100 Watts per channel into 8 ohms to drive a pair of Bose 901s. This was my first high power amplifier design and I used components that were available from Hewlett-Packard stock. For heat sinks I pulled some dual TO-3 heat sinks from a scrap bin. For a transformer, I needed 52 Volt center-tap at 6A, and the transformer shop at Hewlett-Packard Waltham made two of them for me even though it was for a home project. I remember bringing the new transformers home to my apartment in Brookline where I tested them with a voltmeter and they measured 52 volts. I was a bit disappointed that they measured exactly 52 volts because I thought that the voltage would drop below 52 volts when I put a full 6A load on them. I didn't have much test equipment in my apartment, just my analog meter. So I loaded it with my toaster, and sure enough the voltage did not drop. Nor did the transformer even get warm. These transformers were really well built. I built up the first amplifier. Ran it with full power sine waves, and discovered that despite the load resistors getting very hot, the transformer, capacitors, diode bridge, output transistors and heat sinks all ran cool. I had designed it to drive 100 watt sine waves continuously. Because real music is nowhere near the crest factor of a sine wave, this thing was massively over-designed. I decided that using the same power supply components and heat sinks, I could double the number of amplifier boards and output transistors, and run the amplifier in bridged mode to get well over 200 Watts into 8 ohms. I built up an amplifier and it drove the Bose 901s quite nicely. Meanwhile my friend Dave had built up two pairs of '901s and was interested in the amplifiers so that he could run parties in the basement of his dorm at U-Mass Amherst.

Here is my current BPA in all it's dusty 1970's glory. It has worked well as my main amplifier for these 45 years. The beer-can size capacitors still are fine.

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When I finally changed my main speakers from Bose 901s to my new tower speakers in 2005, I could hear a low-level hum when the output should be silent, and some distortion when listening at very low volumes. Finally I had efficient speakers and some of the long-standing amplifier limitations could be heard. The hum was caused by using cheap RCA cables which let AC in thru their inadequate shielding. The distortion was amplifier crossover distortion. The original design did not have a bias adjust, and was running with little or no bias, pretty much Class B. I added a temperature compensated bias adjust, set the bias current to about 50mA, and that took care of the crossover distortion. I ran the gain of the amplifier at the original 51x (34.3dB) for a while just recently in 2023 I reduced R5 and R4 to 30K, so the gain t to about 30x (26dB). Works and sounds swell.

Preamp

In 1978 my system then consisted of the Advent 300 receiver driving one of my BPAs and the Bose 901 speakers. Sadly some teenage thugs from my neighborhood broke into my apartment and stole my Advent receiver and DIY power amplifier.  I needed a new amplifier and a proper preamp for these speakers. I liked the Holman phono preamp design in the Advent 300,  and had the schematic for it. I laid out a circuit board to build my own. I also designed a simple Bass/Mid/Treble tone control circuit.
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Disco System

My friend Dave and I built up a disco mixer consisting of two of these phono preamp boards, a mixer, tone controls, and the ability to drive two of the BPA amplifiers. For parties he would use two turntables, the mixer, and two BPA amplifiers driving four Bose 901 clones. The system sounded awesome and his parties soon developed quite a good a reputation at UMass. A single large MDF box contained a two channel bridged BPA and a second non-bridged BPA. We needed a bullet-proof heat sink for the 8+4 TO-3 output transistors. We bought a surplus flow-through style, air-cooled 16 TO-3 heat sink that used a 5.25" cooling fan. This thing was a serious beast that we found at Eli Hefron's electronics surplus store near MIT. I wish I could find a photo of one.

Home Preamp

I needed a new home preamp to replace the stolen Advent 300. I planned to use one of the Holman Phono Preamp boards. The enclosure would also house a Bose Equalizer. These were the days of cassette tapes so it needed a tape recorder output. For input source selection in my home system,  I decided to use CMOS switches instead of mechanical rotary switches, and instead of just using the single source control to drive the tape output like most commercial stereos, it would be handy to have a second source control so that I could listen to one source while I was taping something else. I designed a buffered input section that could drive '4051 8:1 CMOS switches. These were driven by keyboard encoder ICs, 74C922. The switches would be momentary and illuminated. I found some surplus switches to use. 

There is a simple trick to obtaining low distortion and crosstalk using CMOS switches: Drive them with a low impedance, and receive the output with a high impedance.  Each CMOS switch fed into a high input impedance buffer. In this way, the CMOS switches had very low currents running through them, so any non-linearity in the CMOS switch resistance was mitigated, and very low distortion of about 0.01% could be achieved. Cross-talk is roughly the ratio of the switch off impedance divided by the switch on Resistance. The off impedance is mostly capacitive, and so crosstalk is slightly higher at high frequencies.

I used this preamp for the late 70s, 80s and 90s until I moved into my own house and decided that I wanted a whole house audio system.

Whole House Audio System #1

In the early 90's  I built my first Whole House audio system. It used a TDA7318 preamp chip per-channel, and two MT8809 8 x 8 CMOS crosspoint chips, controlled by an MC68hc11 microprocessor and a graphics LCD. Since I had successfully designed an 8x2 stereo matrix switch on the previous preamp, I felt confident to expand it to an 8x8 stereo CMOS crosspoint for the new system. 
The CPU (left front) and crosspoint (center-front) boards were wire-wrapped by hand. The main preamp board was wire-wrapped (right-middle). The other channel preamps were small, home-etched single-sided PCBs, one per zone. These  were mounted to the rear panel on their RCA output jacks. The phono preamp (top left) is one of my Holman boards.

I redesigned the Bose Equalizer (bottom, right) to make it smaller than the original transistor design board. I used NE5532 op-amps instead of transistors (right front).  For the main channel amplifier I used my beloved BPA. For the other channels around the house, I used a motley assortment of external amplifiers, collected over the years, and stacked in the basement.

For firmware, I had developed a few small projects using the Motorola 68HC11 processor with  LCD graphics,  and written using the Motorola "Small C" Compiler. Which was downloaded from their BBS. Ah, the good old days.  I would download the code to the Motorola "Buffalo Monitor" during development, and then burn an EPROM. The LCD controller was an Actel FPGA that I designed. For a while in the early 90s, I had a small business building monochrome LCD controllers. 

Here is the original system, now dusty and neglected. I used this system from the late 80's until 2010 when I redesigned it. 

Original

Bose 901 Equalizer Redesign

My first Bose Equalizer clone was an electrical copy of the original Bose 901 Series 1 Equalizer from the 70's. I wanted to redesign it to shrink the board size and to change the required power supply from +18V unregulated to +/- 12V.  The original design was often coupled directly to a high-power amplifier and caused a huge 9V step !?! to be applied to the power amplifier input on power-on. The original design consisted of a simple transistor voltage-follower and a simple transistor gain stage with frequency-determining components. I felt that low-noise audio op-amps should do the job. Here is my redesign. The frequency determining components are the same as the original Bose design. I was happy with the performance of this new circuit.
 
Here is the original Bose 901 Series I Equalizer schematic circuit I used to build the first batch of equalizer boards. In the late 80s, in my redesigned version, below, I replaced the transistors with modern op-amps. The 2 transistor darlington input buffer was replaced with an NE5532 follower. The 3 transistor discrete amplifier was replaced by another NE5532 in an inverting configuration. By using bipolar power supplies, everything can be biased to GND, removing large (100uF) bias bypass caps from the original design. By using linear regulated +/-12V supplies, the power supply RC filtering can be replace with simple bypass caps. I replaced the treble contour rotary switch and its resistors with a potentiometer. The 14 pin connectors for the input and output are what I used for power and for connection between the crosspoint and the preamp boards.  The power supply components were optional, so the board could be used stand-alone in a separate enclosure.

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bose 2

I was messing with REW (Room Equalization Wizard) audio test software. It is quite nice. I use it with an OK older 24 bit USB sound card. The distortion floor is about 0.004% and the noise floor is about -100dB, pretty decent.  I needed something simple to test with it, and blew the dust off an old Bose 901 equalizer I wanted to test it and then sell it with a spare pare of my Bose 901 clone  speakers. I've built 2 generations of these equalizers, but have never used or measured the original Bose one. 

The L+R audio outputs had +2-4V of DC, and the bass boost was gone on one channel. Of the 6 electrolytic caps in the signal paths, 4 were dried up, dead and high leakage resistance. I replaced all 6, cleaned the switches, and now it's good as new. Nice to generate proper distortion plots and frequency response automatically and with no hardware cost. Here's the Bose Eq with various settings.

Someday I'll measure my DIY Bose Eq's as well as other projects. Confession: I generally just do a quick check of the frequency response on most of the equipment I build, and I listen for noise. Rarely do I plot frequency response, or measure noise and distortion. No excuse now that I have REW.

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New Tower Speakers

In 2005 I upgraded my 1977 DIY Bose 901 speakers. Here is the Tower Speakers project page.

PC Preamp: PC audio interface

In 2008 I decided to build a PC Preamp. It would interface between a PC that used for music listening, and the rest of my whole-house audio system. It has volume and analog tone controls, source selector, a Phono Preamp for ripping LPs to .MP3, and audio transformers to provide isolated audio to and from the Whole-house system.

Whole House Audio System #2

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In 2010 I wanted to improve my coding skills, and get into a new processor. So when the opportunity to enter the STM32 Challenge appeared in December '10, I decided to do it. Winter is a perfect time to do nerd projects. So what to design? I wanted to do something with both nerd and general appeal, something that I was familiar with and could use myself. My original house-wide home stereo was showing its age. The parts and technologies I used for the original 1990 system were mostly obsolete. The 68hc11 microprocessor, LCD, FPGA, preamps, and other key technologies had gone end-of-life. I wanted to update this system to newer technology and new PCB designs using ExpressPCB. Here is the page for the new Whole House Audio System.

2013-2016 Synthesizer projects

More audio projects, this time from the sound creation point of view. In 2013 I dove into building a music synthesizer. Fun projects, and I learned a lot.

Built a handful of custom eurorack modules: VCO, ADSR, VCA, sequencers.

Built a handful of modules from kits and boards: 2 VCFs, MIDI->CV, Turing Machine, Mixers.

Cloned a handful of Mutable Instruments modules from bare boards: Braids, Elements, Ripples.

I built 2 Hammond Organ simulators based on Teensy Audio.

Then I tried my hand at building a modern Prophet-5-like polysynth.

Built a Mutable Ambika mixed digital / analog Polysynth.

2022 Music Server Project

I wanted a convenient way to play digital music through the Whole House system without requiring a full PC. And one that could be controlled by a phone or tablet.  In 2022 I built a Rasbberry Pi based Volumio Music server. Fun project.

Board Construction Techniques

In the 70s, the early BPA amplifier boards were built in my basement using hand-taped artwork at 2:1, photo negatives made by a local print shop, and photo-resist boards, which were then hand etched. These worked quite well. I built several of the Holman Phono preamp boards this way as well. The other boards in my early preamps and mixers were mostly hand-wired and wire-wrapped since they were one-offs. That too worked well.

Later, I tried my hand at toner-resist boards and used them for the first Whole-House system. But the quality of these boards was never great. I couldn't get the toner to deposit cleanly on the boards. So I reverted to hand-wired and wire-wrapped boards for a while. The original Whole-House system was built with a mix of wire-wrapped, and hand etched boards. Then in about 2004, ExpressPCB arrived on the scene. I built many, many of their Mini Boards: $70 for 3 boards, 3.8" x 2.5". They do larger boards for a higher price. My early synthesizer boards and the second Whole-House system were built with these. The good part about ExpressPCB is that their CAD tools are extremely easy to use and their boards show up in 3 days.  The downside is that it once you design a schematic and board with their proprietary CAD software, you are pretty much stuck ordering boards from them.

But now the Chinese offer much lower price boards. Now I use Diptrace to lay out boards, and PCBWay in China for quality, low cost boards.

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Last Updated: 4/3/2024