Design: Product – Hairball Audio Elements

Using our experience from DIY500 we helped Hairball Audio design the Elements series of mic preamps and DIY kits.

Hairball stocks all of the necessary components and provides all of the necessary support, making Elements Silver much more affordable and accessible than DIY500 ever was. Upgraded features include an active DI which travels through the entire mic input path, transformer included, and a buffered output fader for Gain–>Master-Volume style overdrive variations. Moreover, we are confident that these Hairball Gold, Copper, and Bronze preamps meet Eisen Audio’s very high standards for sound quality and build longevity.


Design: Product – DIYRE TM79 Colour

Peterson Goodwyn at DIY Recording Equipment developed the Colour 500-Series Palette Kit and invited Eisen Audio to create a hardware “plug-in” for this open source signal processing platform. Our first contribution, manufactured and sold by DIYRE, is the TM79 Colour, which you can read all about.


Design: Product – AwTAC Channel Compressor

Eisen Audio contributed design engineering, and helped set up the manufacturing process for Awesome Transistor Amplifier Company’s Channel Compressor module. This device uses a JFET-controlled amplifier to achieve dynamic control, surrounded by four other purpose-designed discrete transistor amplifier stages. Only the LED meter circuit is monolithic (an array of comparator ICs), because this section makes no sound (zero audible effect) and a discrete meter driver would’ve added unnecessary expense for our customers. Of particular note is an 11-transistor op amp that Jens designed for the sidechain, which has DC accuracy comparable to a uA741, such that it won’t contribute appreciable offset when used as a precision rectifier or difference amplifier.

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Design: Product – NonLinearAudio CurvOmatic FT501

Often times when mixing multi-track recordings, you can achieve most of the necessary equalization with just some filtering and a bit of tone control. Recognizing this, the design team of of Jens Jungkurth and John Klett created the CurvOmatic console EQ module in 500 Series format, combining some very powerful high and low pass filters with a very forgiving baxandall tone tipper.

Although these circuits are active, they were influenced by passive inductor-based filters on the 1960s, and painstakingly massaged over a substantial period of time until they sounded just as nice. This involved auditioning more than fifty different op amps and a dozen different series of capacitor for each position, and in some cases sourcing New Old Stock components when there wasn’t a better sounding substitute.

One of John’s favorite mix tricks when using an old filter set like Pultec HLF-3C is to remove the 600 Ohm termination resistor, replacing it with a high value rheostat, and followed by a high-Z buffer amp. If high Q, the inductors will ring once undamped, causing a resonant bump at the cutoff frequency. This can be exceedingly useful (i.e. to emphasize the shell resonance and beater click of a close-mic’d kick drum while removing unwanted frequencies below and above), which is why we included resonance knobs for each of the CurvOmatic filters, making filter shape adjustable from 0 dB flat to as much as +15 dB of resonance.

Details and sales at (NonLinearAudio is a joint venture of Eisen Audio, LLC and Tech Mecca, Inc.)

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Design: Retrofits – Replacement PCB

Often times in recording studios you can find vintage equipment which is still very valuable and in demand, but has long since been discontinued (and unsupported) by its manufacturer, or whose manufacturer is long since out of business. Sometimes when replacement parts are unavailable, we have to build our own.

This example shows (below) an original PCB which was irreparably damaged by contaminated flood water, and (above) a copy that Eisen Audio was hired to draw.


Design: Retrofits – 1272 Sensitivity Switch

Converting a Neve 1272 Buss Amp for use as a mic preamp has been a controversial issue at times, but it need not be, because there is a right way to do it: following the exact Neve spec as spelled out in a 1073 schematic, except omitting the high-gain positions where the missing amplifier stage would be switched in (1073 has three, but 1272 has only two stages). Glide On Fade recognized this and hired Eisen Audio to help design a sensitivity switch kit for implementing this electronically authentic upgrade.

Details and sales:

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Design: Retrofits – Neve B100 PCB

A colleague was rebuilding some very rare vintage Neve modules (I think the model was 1058? Germanium transistor, powered by negative 24V, with black panels and black knobs), and they were missing their B100 preamp PCBs, so he hired Eisen Audio to layout a replacement. Note that we didn’t just scan and trace the original card, but rather completely re-drew it, resizing and repositioning the component footprints as we saw fit. This was done to keep all holes on a 0.1″ grid for modern fabrication techniques and automated assembly.

Neve_B100_repro-bot Neve_B100_repro-topNeve_B100_original copy

Design: Product – AwTAC Panner

As Head of Design Engineering for Awesome Transistor Amplifier Company (AwTAC), Jens helped create the Panner/Mute/Summing module for AwTAC’s Table Top Mixing System. This product turns a Sweet Ten 500 Series enclosure into a stereo mixer, expandable up to 64 inputs.

As per AwTAC’s usual 1970 style manual, the circuitry employed had to be exclusively discrete transistor. Another requirement was 100% silent operation, to ensure that “on the fly” mixing would be free of clicks, pops, scratching, and any other audible noise. We satisfied both requirements in our opto-isolator mute block, coming up with a mosfet circuit that would convert the momentary switch pulses into a flip/flop ramping signal to drive the photo resistors, turning changeover between mute/unmute into a 10mS crossfade rather than an abrupt cut. A couple of timing capacitors enabled primitive logic, allowing end-users to “play the mutes” (quick touch on the beat) or “count out a bar” (hold and release). Meanwhile, thirteen varieties of high quality pan pot were evaluated so that we could use the one with least amount of track noise, and Jens’ cascoded JFET input buffer measures quieter than any IC op amp that we could’ve used instead.

For details and sales please visit


Design: Product – Eisen Audio DIY500

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. They 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.

This template was also used to build our on-demand custom preamp series, EAC, as well as the limited edition EAC312V, which is regarded as the most sonically authentic 312 clone in recent history.

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Design: Product – AwTAC Fader Box

Jens was tasked to design and prototype a 10-channel passive fader box for use with the AwTAC Table Top Mixer System. Each fader is wired to a standard 1/4″ TRS jack, for compatibility with the TRS fader loop in certain 500 Series rack mount enclosures (e.g. Purple Audio Sweet Ten). A recessed mounting plate allows clearance for inserted TRS plugs when this fader box is rack mounted using optional rack ears (not shown). Each piece of metal is cut from standard 1/8″ aluminum stock, so that these fader boxes can be easily built by a local machinist, one at a time, with easy customization options, and the top plate comes from AwTAC’s metalphoto art department. Were we to mass produce these fader boxes, they would be redesigned for sheet metal construction like our other rack mount enclosures, and we would subcontract the wiring to our friends at Coral Sound or RedCo, who would do a much nicer job.

Details and sales:

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