David Berning Co Quadrature Z –– Prototype!

Here's an opportunity to own a one-of-a-kind pair of amplifiers made by the legendary David Berning. I purchased these from the original owner, an authorized dealer, in June 2022. Here's some information about the history of these amplifiers:

"In 2007, I approached David Berning asking him to design an uber-high amperage Output Transformerless amplifier. David seems to enjoy a challenge and accepted. I told David I wanted enough current to weld if needed. I was sort of joking, but serious at the same time. It made my point that I needed a high amperage OTL/great sounding amp. Something unheard of at the time. It took David a lot of thinking, bike riding time and experimenting, but he succeeded. And he succeeded beyond expectations. These are outstanding amps. Especially these particular amps. Later production Quad Z amps were tuned to develop lower wattage for production. Production units were choked down because David wasn’t comfortable with so much voltage in the hands of civilians. I convinced him to leave these wide open. And they are. They should drive any speaker with ease. That has been my experience. David said these amps have well over 250 watts each at 8 ohms load. More importantly, they have a LOT of current."

One of the many brilliant features of the Quadrature Z is an extremely sophisticated auto-biasing circuitry that uses fast sample and hold techniques to measure and automatically adjust the tube bias for all tubes, thus eliminating the need for the user to make any type of calibration adjustments. In fact, there are no calibration adjustments in the amplifier. This auto-bias circuit is linked to the power supply, in that if a defective tube prevents proper biasing, the power supply is cut back to prevent the defective tube from causing other component failures and/or speaker damage.

These first Quad Z’s were shipped with the Berning designed Faraday cage around the circuits and power supply but no decorative outer case as per our original agreement. I had a sheet metal shop build the case. I also added a great damping material to the base. The outer case is made out of the same aluminum as street signs. Very very strong and not prone to resonance issues as thinner everyday aluminum tends to be. Never had any issues whatsoever with these amps.

They have a switch on the front panel for three different feedback settings. A great feature. Makes these amps play nice with any bass driver. In other words, these amps let you fine tune the reflex control of the drivers. True balanced circuitry with both RCA and XLR inputs.

Other features and specifications include:

• Both single and balanced input, all balanced amplification.
• Special triode-like screen-drive output stage.
• Adjustable feedback, no phase compensation used.
• Constant output impedance over entire audio spectrum.
• Built-in five-stage power-line filter and surge suppressor.
• Non-magnetic chassis prevents skin-effect induced distortion.
• Separate shielded enclosure for power supply reduces noise. (Faraday cage)
• Power consumption: idle-125 W; max-500 W. Power factor corrected.
• Signal to noise: broadband (typical) 55 dB, all feedback settings. 20 kHz bandwidth (typical): 98 dB, NORM feedback; 92 dB, MED feedback; 86 dB, LOW feedback. All figures unweighted.
• Distortion (typical, at 8 ohm loads at 200 W): 0.3%, NORM feedback; 0.5%, MED feedback; 1%, LOW feedback. These double at 4 ohms.
• Typical output power at onset of clipping: 8 ohms 270 W.
• Frequency response at 5 W, 8 ohms; +0, -3 dB: 2 Hz to 90 kHz, NORM feedback; 2 Hz to 60 kHz, MED feedback; 2 Hz to 40 kHz, LOW feedback.
• Frequency response at 5 W, 4 ohms; +0, -3 dB: 2 Hz to 50 kHz, NORM feedback; 2 Hz to 40 kHz, MED feedback; 2 Hz to 30 kHz, LOW feedback.
• Full power bandwidth (-3 dB), 4 ohms: 20 Hz to 50 kHz, NORM feedback; 20 Hz to 36 kHz, LOW feedback. Low frequency bandwidth limit intentionally set by speaker protection (0dB down at 20 Hz).
• Typical output impedance (measured at 1 amp, 60 Hz): 0.8 ohms, NORM feedback; 2 ohms, MED feedback; 5 ohms, LOW feedback.
• Sensitivity, 200 W, 8 ohms: 1V, single-ended input. Feedback gain compensated.
• Input impedance: 35 k -50 k, depending on feedback setting, both single-ended and balanced inputs.
• Tube complement: (4) 33JV6 output; (2) 16GK6 driver; (3) 12AT7 input and gain.
• Size: 48 cm wide, 48 cm deep, 20 cm high, (19 X 19 X 9 inches), including connectors and feet.
• Net weight: 40 pounds each.

These run cool and put no serious load on any of the tubes. I will include my spare NOS tubes in the sale. I have the original boxes.