Flexiguy FG500 Mic/Line/D.I. Preamplifier

Clients who loved FG8000 but couldn’t afford an eight-channel investment asked NLA to release the Flexiguy preamp in 500 Series format, so that they could buy one channel at a time. Said clients have since filled entire 24-slot recording consoles and 10-slot racks with FG500 modules, upon discovering "it's the only preamp I've come across that works on every source".

While re-drawing the PCBs, Jens added some unique rail-sagging circuitry so that FG500 would maintain its tonal and overload characteristics regardless of which third-party 500 Series power supply/enclosure it was plugged into.

A self-contained D.I. circuit was also added, with its own output transformer which drives the Flexiguy’s ZUTT 012J mic input transformer.

And John took this opportunity to revamp the LED signal-level metering circuit. Shown here as a through-hole daughterboard, it will be redone in surface mount for future production runs.

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

CurvOmatic FT501 Resonant Filter Set and Tone Control

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

Blenda PLP502 Parallel Loop Processor and Effects Interface

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 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

Candy Buss CB2 Complete 2-Mix Signal Path Upgrade

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 NonLinearAudio 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 nonlinearaudio.com.

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)

FLEXIGUY 8000 8-Channel Mic Preamp

John Klett conceived of an overkill platform for building heavy-duty eight channel mic preamps and other analog outboard processing, which NonLinearAudio (a joint venture of Tech Mecca, Inc. and Eisen Audio, LLC) calls the 8000 Series of products.

Jens at Eisen Audio was tasked with executing this concept, the initial results of which are pictured below.

Design Requirements Included:

• External 48VDC power supply must be fused/protected, filtered, distributed, and then re-regulated on each PCB according to the product circuit requirements.
• Robust modular steel chassis must be electro-plated and earthed throughout, to serve not only as structural support but also redundant signal grounding and shielding.
• All audio transformers mounted on internal bulkheads, so as not to put weight on any PCBs.
• PCB motherboard/backplane and I/O connectors have an underlying standard pinout, some of which may be jumpered or rewired according to specifics of product permutation. And motherboard connectors must mount at a diagonal!
• Unique product circuitry contained on daughterboard PCBs, eight of which mount to the product front panel and then plug in to the mating connector backplane in one fluid motion, for ease of assembly and service.
• Front panel thickness and mounting handles strong enough to support the great weight of chassis and internal components.

Flexiguy FG800, an eight-channel mic preamp, is the first 8000 Series product from NLA, designed by John Klett and Jens Jungkurth based on custom audio transformers and classic recording console circuit concepts. A companion preamp flavor called Orson OW8K was also developed but deemed unnecessary and did not make it to production.

This item is available as special-order with either black or silver (pictured) front panels. The backlit translucent pushbutton switch changes operating mode while also displaying signal level via varying color/brightness.

Details and sales at nonlinearaudio.com