Channel Amplifier

Eisen Audio’s owner, Jens Jungkurth, also heads Design Engineering for Awesome Transistor Amplifier Company (AwTAC). Together with AwTAC President, Dave Raphael, Jens helped develop the flagship Channel Amplifier product. Major contributions to all areas of product development, design-for-manufacture, and quality assurance were provided. Jens and Dave spent countless hours refining the circuits implementation and no-compromise component selection for this processor, and they consider the resulting sound quality to be an infallible artistic statement, on par with several classics.

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Channel Compressor

Eisen Audio contributed circuit design, prototyping, 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.


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.

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Fader Box, Mixer System

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. The top plate comes from AwTAC’s metalphoto art department and is also 1/8" aluminum. 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.

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