ePreamp01 – design and build

It’s been a while since my last post I admit. I was working for the last two years producing/co-writing/engineering/mixing/programming and mastering for the following album:


This album consumed almost every night over that two year period, but, just after I finished it, I got back into some long-awaited DIY audio!


I was contacted by a friend of a friend about the possibility of designing and building a two-channel microphone preamplifier.

Their specs included:

  • +48V phantom, switchable on both channels.
  • Phase reverse per channel.
  • Separate XLR inputs and outputs, fully balanced.
  • 20dB pad input for line-level signals.
  • Low noise.
  • High gain (up to 70dB).

So I set about designing and testing various circuits.

The design I settled upon was originally going to be 2 stages of BigFetBloke gain stages (designed by Tamas of GroupDIY).

Here are some photos of the prototypes I made of these Discrete Operational Amplifiers:



Unfortunately, I had quite a few issues powering these devices from +-24V rails derived from a custom PSU run from a Jaycar 17VAC plug-pack.

After quite a few extra LTSpice simulations and some thinking, I radically redesigned the entire device to use an e2510 and e2520 Discrete Operational Amplifier per channel, instead of two stages of the BigFetBlokes.

Here are some images of the gain stages (first the e2510 then the e2520):



These are customized DOA’s I designed myself for low current drain and lower +-16V power rails.

These fixed the power issue and ran beautifully off of the Jaycar plugpack power supply.

Here is a photo of the custom power supply fitted into the final unit using laser-cut perspex mounting brackets I made (they were the first thing I ever made using a laser cutter!):


Here are some images of the final build, which tested beautifully, and sounds completely amazing:

And here are some photos of the final device with custom front-panel fitted.

The custom is extremely happy with this design and the performance of the final unit.

He is using it to DI guitar, record microphones/keyboards and various other duties.

The final unit uses transformer-balanced inputs and outputs and 2 equal-stages of gain for a maximum gain of 70dB!

_MG_0024 _MG_0022 _MG_0021 _MG_0020 _MG_0019


If you have any questions about this build just let me know!

More projects coming soon….


Music should still be worth something

So far, I have used this blog to focus on my own electronics projects.

Admittedly, this is somewhat selfish, and self-serving, but, after all, it IS my website ūüėČ

I feel that it’s time I started also posting more about relevant topics that still relate to the original content, but expand upon it more, and let you all know more about me, and, more about the world that I see, that you may, or may not, also see.

Firstly, as you might know, I compose music. I’m currently working on an album for artist Lee Safar, due for release before the end of the year.

If you were not aware of any of this, then I would love if you had time to visit my soundcloud page and hear the material that my DIY gear gets used on: evolutionary theory soundcloud.

Those who follow me on twitter: etheory_music¬†or facebook: etheory_facebook, will know that I am very passionate about the very relevant topics of music and film piracy. I’ve discussed with various people either in short, or at-length, about why I feel so strongly. As someone who one day wishes to be regarded as a music artist by the general public, that the work that I do, and the hours I slave away in my pursuit of musical perfection, I want to be worth something, financially. Don’t get me wrong, I love what I do, and I don’t do it FOR the money, but I’m saying that as a skill, it should not slowly stop being WORTH something.

Since I’ve already written about this ad nauseum, I felt it better to defer to someone with a far clearer, and well-thought out discussion on this topic, and I suggest you do so also, here:

Letter to Emily White at NPR All Songs Considered.

I feel that the material, and the way in which it’s presented, excellently discusses the issues there are with the “popular” perception of the worth of music, and offers some social insight into ways we can all change this perception.

Thanks! and I hope you all stay posted for my next discussion when I return to what’s been happening in DIY land recently – there’s quite a lot to update you all on!






etheory Microphone pre-amp custom project ‚Äď part 2

OK, so I added the rest of the parts to the discrete opamps, and then did a couple of promising tests that indicate, at least to a very basic degree, that they are working properly. ¬†This is awesome news! ¬†Time to share the photos of the build. ¬†The first two are of the “bug-style” they start out as. ¬†Then I finished adding in all the resistors, then the final part, the 2SK170 JFET input part:


etheory Microphone pre-amp custom project – part 1

Since I needed to gather some more funds to finish the API Compressor project, I’ve decided to go into customized audio design for friends and also a studio I’ve just teamed up with.

The idea is that I take a bunch of information into account, then design (or more commonly re-purpose existing designs to save time), test, produce, improve, and finalize pieces of one-off bespoke customized gear for clients.

My first project is a set of 2 microphone¬†pre-amplifiers¬†for a private home-studio customer. ¬†Since this client is after a warm old sound, I’ve opted to build a transformer in-out balanced 2-stage design, with between 34 and 80 dB of gain in microphone mode, and at line input mode between 16 and 60dB of gain. ¬†I’ve very excited about the design I’ve come up with, which includes phantom power, 2-stages of balanced gain and seriously high-specification components throughout.

The gain stages are discrete, and are based on a design from prodigy-pro.com member Tamas.  The design can be referenced here: http://skipwave.net/images/bigfetbloak_v2.pdf

Everyone who has build that circuit comments on it’s sound quality and it’s power and high-current and voltage handling capacity and ability to resolve detail, so it seemed like the perfect fit. ¬†My extended mic-preamp specification also includes Sprague-Vishay coupling capacitors and ClassicAPI 2622 input and 2623-4 output transformers. ¬†It’ll be an absolute corker of a preamp to be sure.

Simulated frequency response is about 10Hz to 100kHz to within 0.5dB of a perfectly straight-line.

I’ve already started to build the 4 discrete operational amplifiers that are required (DOA’s), and wanted to share some photos of them here:









Custom Audio Compressor ‚Äď Part 3

After a rather long delay, it’s time for a BIG update!

First of all, the 2520 did work, perfectly, and here is what it looked like after I soldered it together:


Next up I did some testing.  Here I am using the Xprotolab miniature DSO for all of my testing (since it also incorporates an extremely useful function generator).  The power supply was on the breadboard, and the DOA being tested is connected to its off-board components via alligator clips.  I fed in a signal from the Xprotolab function generator using the 2520 amp in differential mode (from the compressor circuit), and got back the same signal that I put in Рperfect!:


The next piece of design was the 2510 discrete operational input amplifier.  This is basically a less beefy version of the amp above, with a much smaller DC offset, and less power handling capacity.  However, always wanting to improve things, I have modified (IMO, even improved upon!) the original design, and added beefier output transistors in an attempt to improve the life of this generally gutless amp (the originals apparently blow-up quite frequently) and also added some form of output short-circuit protection, should the need ever arise that is has to be used.  My layout, CNC, and subsequent physical build, looked like this:


In order to keep things moving along, and save cost at the same time, I decided to split the rather complex compressor side-chain circuit from the audio path, and build the audio path of the compressor first. ¬†This resulted in a partly populated, but working audio circuit layout, where the 2510 sits in front of a pair of 2180LB THAT Corp voltage controlled amplifiers (VCA’s), which are in charge of the actual gain change due to compression, and then through the output 2520 to drive the ClassicAPI 2503 output transformer. ¬†This circuit is both simple, with not that many components in the audio path, and extremely high-performance, and also a better systems test for my amplifiers, so, this is what it looked like after this board was CNC’ed:


And here the audio path is being populated with parts:


Next I fit the two DOAs to the audio path board, and then did a bunch of shots to show it off, cause I thought it looked pretty cool, and it was the most complex thing I’d ever designed at the time, so I was very proud of it!


Then it was the scary time were you have to test everything to make sure it all works!  Luckily it all did, and passed the signal precisely as calculated on the simulator:



After successfully showing their operation, the next step was to get the DOAs manufactured by a professional PCB fabrication plant, which will be the next part of this compressor series.



Custom Audio Compressor – Part 2

This is the second post along the road to DIY compressor bliss….

Picking up where I left off last time, I am starting by creating my own, super cheap, DIY version of the API 2520 Operational Amplifier. It will be used for the input and output stages of my take on an API 225L/2500 Frankenstein compressor. Initially I also planned on cloning the API 2510 used on original API compressor input stages, but my simulations have led me to believe that I will get better and more reliable performance from the same configuration, but using an API 2520 instead. All the basic resistors, capacitors and transistors arrived from element14 this week, whilst I was away for Christmas.  They are ready to be soldered:

After inserting some bits on the PCB I made in my last post, here are the results of the layout – everything fits perfectly – all of the pre-planning paid off ( it’s just missing 2 capacitors, which had not arrived when these photos were taken ):


This view from the top really shows how nicely the placement is working:


Tonight I’ll be soldering the parts to the board to see how it all goes ( I’ve never worked with a DIY CNC’ed PCB before ), and if I get time, I might even run a quick test on it. ¬†More to come soon….



My first large-scale DIY project – a custom audio compressor….

Hi everyone!

It’s been a while since my last post. ¬†I took a holiday and went to Italy for 5 weeks. ¬†It was completely awesome and gave me a lot of time to think about a lot of different things.

Whilst I was away a had quite a few crazy ideas, and this was one of them…. a custom compressor based somewhat on the API 225L:

My idea however is to take it further than the original, much much further. ¬†In fact it will probably more resemble the API 2500 when it’s finished:

I had to start somewhere – so I began by breaking down the problem, as I always do, into manageable chunks of work. ¬†The first was the famous API2520….

One of the key components of all of API’s gear is their Discrete Operational Amplifiers ( DOA’s they are called ).

In particular, the API 2520¬†( image courtesy of¬†http://www.silentarts.de/¬†)¬†DOA is extremely well known in audio circles and a key component in “the API sound”. ¬†So to begin designing this project I knew I had to start there. ¬†There are many commercial options out there for API 2520-style DOA’s, from kits, to full clones, or even buying directly from API themselves. ¬†After a lot of investigation I found the best option would likely be one I’d already used before – the gar2520 by¬†Gary Barnett of Barnett Industries, which I discovered as an optional extra with Jeff Steiger’s ClassicAPI kits. ¬†In particular, I remembered it from one which I had already built; the VP26 microphone preamplifier.

After some more investigations I quickly realized that the 8-10 2520’s I would need to buy to both test and then finish the compressors ( I intend to build 4 of them ) was going to be too high. ¬†So I decided to do something a bit crazy and see how much it would cost me to find the original circuit on the internet, tweak it, source the parts myself, do a custom layout of the circuit board, and then try and build it…. ¬†At the very least I thought I could build some test devices, and then once the compressor design was finished I could replace them all with the gar2520’s ( because I know they are of such high quality ).

The cost difference for me was quite compellingly in favor of jumping in the deep-end and building them myself – so that’s precisely what I’ve started to do.

Firstly I built the circuit in LTSpice¬†( based on the “original” I found here¬†http://www.waltzingbear.com/Schematics/API.html, along with various modifications and observations through many many hours of online research and forum snooping!¬†)¬†and simulated it, tweaking the layout and component selection for parts I could source directly and quickly in Sydney, Australia, from element14 ( I think I only ended up having to change one resistor value since they didn’t stock it…. ) and ended up with the following:

From there, I rebuilt the schematic in the free version of Eagle PCB, and came up with a PCB layout that was single-sided ( for ease of CNC-routing ) and matched the original API 2520 size and footprint:

From what I can tell, this might be the first single-sided PCB layout for a 2520-like circuit that doesn’t use links or off-board wires to complete any of the circuit paths…. ¬†If so, then I’m pretty proud of it!

From there, I took these files ( after a good friend of mine, and PCB-making-expert –¬†talsit¬†–¬†checked them for me ) to Robots and Dinosaurs ( a Sydney Hackerspace ). ¬†The CNC machine they have there is completely awesome. ¬†Even though this was the first time I’d ever used it ( I must have just gotten lucky…. ), after loading in the files, and asking Gav ( Robots and Dinosaurs supreme resident hacker! ) a lot of questions, I got straight to it, and the result seems pretty text-book….

Here was the progress after the first layer, which eagle labels an “etching” layer:

Secondly the holes get drilled:

Then the board is “milled” out to become its own entity:

And here is the result – a rather nice little amplifier PCB ready to be populated:

More to come soon ( hopefully in a couple of days ), when I actually populate the board and do some tests to see if all of my pre-planning has actually worked….





DIY Anodizing

Hi there!

To finally round off a good solid month or two of building audio electronics studio stuff for the album ( in this case some gorgeous pre-amplifiers for recording vocals ), I decided, just before handing over the final product to Lee Safar, to try my hand at DIY Anodizing….

So it was off to the wonderful Sydney Hackerspace ( Robots and Dinosaurs ), to get some help.¬† Macca, an already competent Anodizer ( ūüėČ ) gave me a hand understanding the process.

First we mixed the active ingredient ( an acid ) with demineralized water, sitting inside of a PVC tube that contained a cathode ( a piece of aluminium ), and then the anode ( the front panel of the pre-amplifiers itself ) was connected to complete the circuit through a constant-current 2.5Amp power supply.

We let this do it’s anodize thing for an hour, then immersed it in a very string mix of red clothes die and more water ( this time regular, but warm, tap water ).

From there, around another hour later, it was straight into a boiling water bath for around 10minutes, to fix the die within the molecular structure of the metal, and then BOOM, you go a red piece of aluminium….!

So enough nerd speak, here are some crazy-cool images of the final result – a DIY “red”(ish) coloured set of pre-amplifiers:

ClassicAPI VP26 and Sound Skulptor MP73 Preamps are finished!

Well, what a project!

For those who haven’t been following my progress lately, I built ClassicAPI VP26 and Sound Skulptor MP73 microphone pre-amplifiers ( pre-amps ) to use on the album I am currently producing for Lee Safar.

Whilst the pre-amps themselves were fully self-contained kits, the processing of putting them into a 1u rack case, along with some of my own customizations, was a much more time-consuming and complex project than I originally expected.

I am happy to say that as I type this, they are completely working and racked up beautifully in a case I cut with my own hands ( no CNC machines were used this time, just a good old drill press and a bunch of manual labour! ). ¬†The next time I undertake such a project, I think that I’ll be relying more on automation, but I wanted to get my hands dirty this time.

Picking up from my last pre-amp post, I rounded up the last part of the build by assembling everything roughly, and running through the MP73 calibration procedure for a few hours, running a bunch of test tones to and from the unit, and calibrating things like the clipping LEDs etc.

At that point in time, the whole thing looked a little like this:

Here you can see the test tones are being fed in to the MP73:

Since the MP73 is designed for a custom Sound Skulptor case, I had to improvise on some of the wiring:

I also had to ensure that the VP26 kept working with the MP73 running simultaneously, since the JLM Audio Powerstation power supply I used was providing all of the various voltages I needed:

Here was the first image I took of the clip lights on the MP73 operating correctly.  That was an exciting time!

From that point, all that was remaining was the hefty final wiring, which included an extremely cool customization feature that I added for the specific purpose of being able to get a bunch of different tones for the recordings we are going to do of Lee’s vocal.



The Customizations:

Basically, you can see here, an unlabelled little switch just to the right of screen:

When it’s sitting “up”, which is the “off” position – it has no effect. ¬†but when you flick it down to the “on” position, it does something pretty cool. ¬†It turns on a relay that bridges the two inputs ( if we label the VP26 circuit as pre-amp A, and the Sound Skulptor MP73 as pre-amp B ) so that the inputs for A and B are connected. ¬†This means that with the switch down, plugging a single microphone into input A, drives BOTH the preamps A & B, and produces distinct, isolated outputs from both the A & B channels.

This is an intentional cost-cutting feature, designed because we decided not to buy two microphones.  Instead, I wired in this bridging mechanism, allowing us to get the phase-accurate sound of BOTH the API and Neve-style pre-amplifiers at the same time, from one source.

This allows us to then record the two different flavors of sound to the Left and Right inputs of a standard sound-card. ¬†Cool huh? ūüėČ


The second customization again relates to the bridging swtich, but allows you to still safely use phantom power. ¬†With the bridging switch in the “off” position, channels A and B have separately controlled independent phantom power switches. ¬†However, with the bridging switch “on”, the phantom power is disconnected from the B channel, and instead, the phantom power from channel A is used only. ¬†Since the inputs are bridged, this just means that you get the correct phantom power, controlled by channel A’s phantom power switch, without the hassle of having to worry about the current setting of channel B’s phantom power switch.

It’s basically just a safety feature that allows phantom powered microphones to be used in the bridging confuguration.

I can’t WAIT to use this feature and be able to blend two independent colours of Lee’s voice together to find the right tone.



The Final Images:

Here it is, completely finished, but with the lid off:

And here are the carefully wired internals:

Followed by some close-ups, that show some of the features I’ve discussed:

Here you can see the relay driving the bridging input configuration. ¬†It sits right near the inputs to keep the length of cables as small as possible. ¬†I didn’t want to run audio cable all the way from the rear inputs, to the front switch, and then back again, so this was a good alternative solution:

And here are the rest of the internal pics:

Since Lee needs to take this back to LA with her, I’ve included the ability to switch between 120V and 240V input power:

Looking good!

And here it is, trying out a rack space for size, alongside my previous build, an SB4000 SSL-style Bus Compressor ( thanks ruckus! ):


For any details about this, please let me know and I’ll give you whatever information I can.

It was a really fun and challenging project and I can’t wait for Lee to arrive in Australia to start recording the vocals for her album through these.

Sound samples coming soon….

Updating a Legend – SEM transplant

I’d like to tell you all a little story about a recent experience I had with Synthesizer legend Tom Oberheim.

As you’d know from this website, I purchased an Oberheim SEM Pro synthesizer from him a while back. ¬†I had issues with it interfacing with my modern MIDI gear ( namely an RME Fireface 800 sound card ), which it just would not recognize.

I actually wrote directly to Tom about this, also mentioning that I’d figured out that running the Fireface MIDI out to my Yamaha s90ES keyboard, out through it’s MIDI Thru and THEN into the SEM actually fixed the issue. ¬†This got me thinking that the problem was to do with the fact that the original MIDI spec specified 18V signals. ¬†Most modern gear can only output 5V max due to USB power restrictions. ¬†After a few e-mail exchanges, I explained the issue, let him know that I had a theory on signal strength and the SEM being too correct in it’s MIDI implementation ( i.e. to spec, but therefore not working with modern gear ) and was only half expecting a response.

Well, it turns out I DID get a response. ¬†It was a very detailed and lovely e-mail, the first of a few I received from Mr. Oberheim. ¬†I went on to explain that I have significant electronics experience, and, a few months later, he told me that he’d organized a fix for me. ¬†The fix was a replacement Printed Circuit Board ( PCB ) for the entire Input/Output ( IO ) board.

Not only did he tell me in an e-mail that he was in the process of sending this replacement board to me, but that he also was including a screwdriver as it was the only tool required to perform the replacement.  WHAT a guy.  There are so few people around with such a kind soul.

Since this synthesizer is going to be used on the album I’m currently producing with Lee Safar, I sent Tom a link to our first single, I’m Here – evolutionary theory Remix featuring Lee Safar, and here was his response:

Great!  I love your track! Now I want to hear more.


…. Well, as the photos below show, the transplant of the new IO board was a success, and I documented and photographed the internals during the process.

What a lovely device it is! ¬†Really well made. ¬†You can see also in the photos ( the last few especially ), the old board sitting outside the case, with the new board having been transplanted to it’s new home.

The last photo is the SEM now all fixed, and working perfectly, interfacing beautifully with all of my modern MIDI gear, posing with Tom’s screwdriver ( which I still have BTW, it’s a freaking awesome screwdriver! ).

So, I hope you all enjoyed that little experience, and soon, you’ll also be able to enjoy the sound of this newly fixed synthesis legend.

If you have any questions about this please feel free to ask.