Design: Retrofits – Replacement Patchbay

Vintage Trident consoles often featured integral patchbays built from PCB-mounted ADC jacks. 30-40 years later, said jacks are often in awful shape and causing intermittent connections, which makes the console impractical to use. Since cleaning or trying to replace said ADC jacks can be futile, Eisen Audio is sometimes hired to create entirely new jackbay cards as shown below. We use the highest quality nickel plated jacks which should last virtually forever. These retrofit cards can be used-installed as a direct replacement for the vintage cards, however a new top panel must be machined to hold the cards in place, accounting for the difference in the physical footprint of new jacks versus old jacks. This is not an inexpensive process, but it typically costs less and requires significantly less work (and down time) than the alternative approach of removing the Trident patchbay altogether and rewiring/reterminating the multipin connectors for an external wired patchbay. Contact us for an estimate if you’d like to purchase a similar retrofit for your console.


Design: Product – NonLinearAudio CandyBuss CB2

As studio techs who maintain vintage large format analog mixing consoles, we are occasionally asked to install upgrades (aka “mods” or “retrofits”) in certain consoles. Once such popular upgrade is to replace the IC op amp-based mixbuss in any virtual earth console (which includes most consoles from mid 70s through present) with a set of discrete transistor amps for perceived sonic improvement. After doing this several times as a custom one-off, John Klett decided it was time to make the end-all PCB solution and that NLA would manufacture and sell it as a kit product to be installed by us or other qualified techs, so he specified systems requirements and hired Jens to draw something beautiful. CandyBuss CB2 is the result, pictured below. You can buy one at

In addition to discrete current summing, CB2 includes several other useful features and a heavy-duty build throughout. Design goals met include:
– 100% common through hole components for easy field repairs in the future
– transformer balanced throughout, and capable of driving very demanding loads
– generic 2520/990 footprints for main summing and output amps
– an insert loop with controls for bypass and A/B audition
– return circuitry is full removed when insert is bypassed
– easy to reverse polarity throughout, to maintain phase coherency in any console system
– separate monitor amp outputs to feed control room section
– additional buss drop-ons for aux sources and cascading mixers (e.g. sidecar sum).
– discrete logic and JFET switching (used very carefully for lowest distortion) – no fancy ICs, nothing obscure
– incredibly desirable sound quality (which is a given)


Design: Product – NonLinearAudio Blenda

NLA design duo Klett and Jungkurth originally developed the Blenda PLP500 module in 2011 as a simple means of adding a parallel insert processing loop to Purple Audio Sweet Ten racks. It was exhibited at AES NY in October of the year, after which a small batch on units was produced and sold. Unique features included Hi-Z balanced inputs (comprised of ultra-transparent incredibly quiet discrete transistor followers) which could accept any source device without loading, and polarity switch on the processed return input for nulling and phase cancellation effects.

Sales were incredibly limited as Blenda only worked in Sweet Ten racks (because of the second set of 1/4″ I/O on each Sweet Ten slot, which no other 500 series enclosure had) and Sweet Ten had not yet reached market saturation at the time. This meant that most 500 Series owners could not use a Blenda.

Furthermore, we noticed that most Blenda customers were using theirs primarily with instrument effects pedals, and this didn’t work very well as the original Blenda PLP500 had only a line-level send output.

Wishing to address these issues, NLA hired Eisen Audio staff engineer, Abby Echiverri, to do a complete re-design of circuits and redraw the PCB, which has resulted in the Blenda PLP502 module now shipping as of October 2014. The new Blenda features front panel normalling jacks which break connection to the rear panel jacks when a plug is inserted. Also, and most importantly, there are now three operating modes for the parallel loop: line, synth, and pedal. Line mode is balanced lo-Z line-level, whereas synth mode raises the send impedance to 1k (compatible with most analog synths and modular systems) and drops the operating level by -10dB. Pedal drops the operating level to -20dB and further raises the send impedance to about 3k, which is similar to many hum bucking pickups. In synth and pedal modes Blenda’s return input becomes a Hi-Z unbalanced D.I. input with corresponding makeup gain so that the loop is effectively unity gain regardless of operating mode. This lets you optimize (or creatively compare the tonality of) operating level/impedance and makes Blenda compatible with virtually any class of analog signal processing device. While the remaining PLP502 controls/functions are the same as PLP500, its I/O circuitry has been further optimized for greater headroom and transparency. It is fundamentally a very high-fidelity (in that musically-relevant kind of way) 2×1 mixer with several extra features and secondary utilitarian applications such as reamp plus DI.

Full Disclosure: Eisen Audio co-owns and shares staff/facilities with NonLinearAudio

IMG_1274 PLP502_lazyiphone-01

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.

awtac-channelcomp-headon compressor-inside1

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.)

curveo-03 curveo-04 curveo-08ft501-hpf-vari ft501-lpf-vari ft501-tilt-hi


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:

1272mod 1272modtop

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