U87 Studio Test

Because it was national soundcheck day "12-12-12", we went into the studio to get a few samples of our U87 clone microphone detailed previously. Figured I'd stir the pot a bit Anyways, it was just acoustic guitars this time, but we were able to put a few mics up to compare. Among them, 2 original vintage U87's. We did not get the Klaus U87 up because our friend down the street who own it was having a pretty rough day. He had to put his dog down who he had been walking for near 20 years, so it was all-in-all a bad day to be borrowing mics.

Vintage U87 Circuit Clone Microphone Build

At the heart of every successful recording studio is a closely guarded vault of high quality microphones.  The ability to match a high quality microphone to whatever is being recorded is of critical importance to the recording process.  Many vintage microphones now cost tens of thousands of dollars and new production boutique microphones while not as costly as original vintage units in good condition also demand a high premium.  With the help of some enterprising people in the DIY community, it is now possible with a little bit of research and determination to build quite a formidable arsenal of clone microphones that faithfully reproduce the original vintage electronic circuits.  One staple large diaphragm microphone design is the original Neumann U87.  In this post, I will build a clone of the original U87 circuit. 

Classic Audio Products VC528 Build

After reviewing my in-depth photo documentation of several microphone preamp builds, Jeff Steiger, the owner and CEO of Classic Audio Products of Illinois decided to officially employ my photo-documentation services for his VC528 kit.  This post utilizes the photos I made from the official build documentation.

The VC528 is a unique 500 series or 51X module in that it is a "go-between" design.  While it is common to find preamp circuits from a variety of vintage consoles in lunchbox form, this module  re-creates the rest of the channel strip circuit of the vintage API console all the way out to the channel fader.  Many have referred to this module as "audio bacon" because it can be inserted after or before any other outboard device and add the analog goodness of a full frame vintage console at a fraction of the cost and space.  VC528 allows for a wide range of experimentation with gain staging when inserted into the signal chain because you can drive whatever previous device hotter or softer than normal and adjust for proper gain with the VC528 before hitting the recorder.  Another common use for the VC528 is inserting a pair on the main stereo mix bus.  For this application, Jeff offers stereo kits with closely matched resistors for optimal consistency of the left and right channels.

In this post, I will show a complete VC528 kit build from start to finish.  An assembly aid document as well as bill of materials, trouble-shooting tips, and calibration procedure can be found here.

IJ Research F76 Compressor Build

The Universal Audio 1176 FET compressor originally released in 1968 is broadly recognized as one of the all time great audio compressors.  It seems every major studio has a few of these in their outboard racks and the particular way the FET circuit compresses is distinctive and a "familiar" sound due to the widespread use of the 1176 through the years in the industry.

The I.J. Research F76 is a 51X format FET compressor designed around Universal Audio's revision F 1176 with a few very useful added features most notably, the addition of a 2:1 ratio, a side-chain high pass filter, and relay controlled true bypass.  The kit also allows for a myriad of build options.

Igor Kapelevich based in Haifa, Israel can only be described as a prolific designer, and he is the mastermind behind the F76 compressor.  Igor's primary business is building complete turn-key audio devices for various studios worldwide and his custom work is not cheap, but he is very active in the DIY community and makes his partial kits and PCB's available for purchase as a side business so that DIY people have an opportunity to build some fantastic sounding pieces of audio gear at very reasonable prices.  For support and technical questions regarding this project, please refer to the official support thread.  To purchase PCB's and font panel/hardware kits check out the IJ Research webstore.  Complete kits may be available again from time to time depending on Igor's current stock of parts.  In this post, I will try to show a complete build from start to finish.

VP312DI Microphone Preamp Build

One preamp that was very interesting to me from my initial DIY research was the VP312DI kit from Classic Audio Products of Il.  One of the main reasons I wanted to try this kit was the feature set seemed very conducive to live use for bass guitar.  It has some really cool modular features that allow for a wide range of customizations.  In particular, this preamp has easily detachable modules to allow for 4 completely different flavors of DI.  It also has a built-in switch to bypass the DI module and inject the signal directly into the opamp which yields a different sound option that is built-in and always available.  In addition, the opamp is a standard footprint 2520 opamp.  Because this preamp comes in a full 51X Alliance standard configuration, not only are API-type +/- 16V opamps accepted, but a number of +/- 24V opamps can also be used.  The DI section of this kit also has a selector switch for 16V and 24V operation depending on the module being used.

So, in short, there are a mind-boggling array of possible configurations for this preamp kit that can all be changed quickly and without soldering.  For this build, I will explore a few of them.

opamps:  GAR2520, APP2520E (+/- 24V)
DI modules:  FET DI, Picatron passive transformer DI, and the IC chip DI

The current revision of the VP312DI preamp has a new option on the PCB for stepped resistors on the gain knob, so you can now choose between the constant variable Bourns pot (as depicted in this older revision build) and the Grayhill switch with stepped resistors when purchasing this kit.


There is no in-depth step-by-step assembly guide for this build, but a lot of critical build information is consolidated in the official support thread on the http://www.groupdiy.com forum.  All of the critical build documents (schematics, bill of materials, and assembly guide) are also sent via email upon purchase.  In this post, I will attempt to detail every step of the VP312DI kit build.

GDIY 51X Power Supply Build

This post is part 2 of my GroupDIY 51X rack build.  The key benefit of the 51X platform is the design flexibility it provides with its 5 supply voltages +/- 24V, +/- 16V, and 48V phantom power.  The GroupDIY 51X alliance power supply is a robust design capable of powering 11 devices at +/- 16V to VPR specification.  It can also simultaneously power 11 modules at +/- 24V.   Not all modules draw the maximum specified current, and very few modules utilize all of the power rails, so with some planning, it is quite possible to power more than one rack with a single powers supply.  In this post I will walk through a build of the 1st generation power supply kit.  The circuit for the power supply has not changed, but the enclosure configuration for the current versions have changed a bit.

GroupDIY 51X Rack Build

The GroupDIY 51X Alliance is a collaborative, cross-continental effort within the DIY audio community to produce a new standard that is compatible with the popular API 500 series card format but adds some cool features to support a broader range of designs.  The original API 500 series rack utilizes a 15 pin edge connector and supplies +/- 16V power rails as well as +48V phantom power.  At the heart of the 51X standard is the use of an 18 pin edge connector that allows the top 15 pins to remain identical to the 500 series cards and retain full compatibility but the bottom 3 pins allow for the addition of +/- 24V power rails.  This innovative feature allows for the easy design and development of modules that natively run on +24V like NEVE, Neumann, and Telefunken based circuits.  It also allows for devices from Forsell, John Hardy, and APP Studio to utilize the higher power +/- 24V rails. 

The 11 space GroupDIY 511 rack is in its 4th production run, and there are now over 300 units in operation around the world.  In the wake the successful launch of this standard within the DIY community, a slew of new designs and kits have emerged.  For my part, I simply happened to be exploring the idea of DIY while the rack kit, power supply, power transformer details were being finalized by capable 51X Alliance members in Germany, UK, and the United States.

GAR2520 Discrete Opamp Build

One of the true gems in the sea of DIY recording equipment is the GAR2520 discrete opamp designed by Gary Barnett.  These are sold in kit form by Classic Audio Products of Illinois.  They are also available fully assembled and tested as an add-on option for CAPI's preamp kits.

This opamp circuit is a modern component interpretation of API's mid 1970's Huntington era 2520 and operates on bi-polar +/- 12VDC to +/- 20VDC power supply voltages.  For my studio, this righteously priced kit has been an absolute life saver due to the sheer number of opamps required for the various preamps, EQ's, and other outboard audio equipment we utilize.  While I have never been hands-on with a vintage Huntington 2520, many people who are intimately familiar with the vintage opamp feel the GAR2520 is sonically "hair-splittingly close" to the original that it is modeled after.  What I can say is in my experience, this opamp has sounded gorgeous in every device I have tried it in, and as such, it is my go-to baseline opamp for the many API-family devices I build for the studio.  There are many fine pre-built options currently available, and people often equate high cost to high performance, but make no mistake, a properly built GAR2520 is capable of holding its own when compared to any of the other top tier API-type opamps available.  Slight sonic variations and colors aside, the pedigree of this device is on par with the rest of the field.



For this article, I will build 2 complete GAR2520 kits to the best of my abilities.

VP26 Microphone Preamp Build

One of the most important components of audio recording is a high quality microphone preamp, and one of the problems with tracking drums and basic tracks is the need for a handful of quality preamps for all of the tracks being simultaneously recorded.  This is even more critical if you are not recording through a large format console.  Luckily, a number of companies and small businesses in the DIY audio community have created some outstanding cost-effective preamp kits.  If wisely chosen, these can save thousands of dollars on the way towards building a fully operational, professional recording setup.  One of these companies is Classic Audio Producst of Illinois.

In my opinion, the all discrete Classic Audio VP26 preamp delivers the highest cost to performance value among all of the kits currently available.  It also happens to be one of the easiest builds for a first time audio DIY project and was my entry point into the world of audio DIY.  In this post, I will try to show the entire build step by step.