BA512 Discrete Operational Amplifier

Ed Anderson and Hairball Audio, LLC commissioned Eisen Audio to re-engineer this classic discrete op amp, which was originally designed by the BBC for use in their 1970s Neve broadcast consoles. The transistors were substituted, the circuit reconfigured for operation from bipolar power supplies (instead of +24VDC), the broadcast filtering was removed for extended bandwidth useful in music recording, and of course the PCB layout was completely redone conforming to a standard 990/2520 footprint for use in DIY projects. The class A/B bias and heavy-duty heatsink means this amplifier can drive remarkably low impedance loads. Indeed, we generated a spec sheet via standard op amp testings which shows impressive numbers.

Manufactured by Eisen Audio in New York, and sold exclusively through Hairball Audio in Seattle.

Jensen™ Bump Box

Eisen Audio initially built this utility device, which converts 1/8″ headphone output to balanced studio line-level, for internal use. Clients took note and asked us to build more.

Detailed information here.

LILPEQr LE60 Program Equalizer

John Klett had a large stockpile of vintage Marinair and St. Ives input transformers, left over from custom Neve console projects, so he suggested Eisen Audio design a product in which to utilize them. 31267 is a very colorful sounding transformer, with a rich low end, so Jens at Eisen Audio took this opportunity to make solid-state 500 Series EQ module which would emulate some of his favorite vacuum tube program equalizers. This input transformer was paired with a Crimson Audio output transformer, which is “bigger sounding” than most, interfacing with a purpose-designed discrete transistor amplifier and some carefully selected film capacitors. Jens deliberately chose/designed each of these elements to impart an overt and complementary tonal response, trying to achieve a spectral distribution of low-level harmonic distortion which was similar to that of the classic devices which inspired this one. A limited edition of 60 units was sold direct through Eisen Audio during the ’07-’08 holiday (one-time only, discontinued). 

EAC Series Custom Preamps

Back when there were few choices in 500 Series preamps, people really appreciated the many permutations offered by our DIY500 template. For those who were not DIY-savy, they'd write with a specific request and Jens would create a custom preamp module trying to address this request. Each one was unique, built-to-order, and assigned a serial number (EAC###) and a brief descriptive name. This series came to be called "Eisen Audio Custom". The service lasted from 2006 through the end of 2010 when we discontinued our DIY500 product. Dozens of different modules were made, including a limited edition of fifty EAC312V. A few examples are pictured below.

DIY500 and DIY500 mkII Minimal Kits

In April of 2006, Eisen Audio released the first-ever Do It Yourself kit for 500 Series. At that time, only two third-party manufacturers were producing mic preamps for API’s popular modular format. We found 500 Series to be a sensible platform for auditioning different mic preamp designs, and the EXILE tech shop where Eisen Audio was headquartered contained lots of old parts to try. DIY500 (and later DIY500 mkII) was essentially a PCB containing dozens of superimposed transformer footprints (for input and output balancing), as well as a discrete op amp for variable gain, and the standard mic preamp feature set. Now discontinued, these were sold as minimal kits through 2010, including the custom metalwork and standard hardware, switches, pot, and connector required. You can read a Tape Op review.