Bride of Zen

 

 

SE preamplifier with only a single active device (MOSFET) per channel

Used in the Attic

 

Associated equipment:                                                                                      Last updated: May 28, 2012

 

Homemade multimedia speakers (Monacor)

Homemade poweramp (Pass Zen Revisited)

Homemade FM tuner (based on Ambit modules)

Homemade phono preamp (with pencil tubes)

Pioneer PL-12D turntable with Audio Technica AT66 cartridge

 

The Bride of Zen (BOZ), designed by Nelson Pass and described in The Audio Amateur, 1994, is a very simple line-level preamplifier which contains only a single medium-power MOSfet per channel (see the schematic on this page). Mr.Pass has given a very lucid account of the design philosophy. Building instructions including the lay-out for a printed circuit board can be downloaded from Passlabs.

According to mr.Pass, the variable resistor P102 should be adjusted in such a way that the voltage drop over resistor R108 (in the absence of an input signal) is 4 Volts. However, in my own homebuilt BOZ I found this to be incorrect. The preamp sounded good using a cassette deck or an FM tuner as the signal sources. However, using a CD player a nasty form of distortion occurred during crescendos.

Since the volume control is after the gain stage, the preamp can be overloaded. I tried to limit the input signal by mounting a 470 k resistor in series with the CD input. A voltage divider is formed by this resistor and R107, resulting in a greater than 5-fold attenuation of signal input to the gain stage. Although the distortion had now disappeared, the resistors caused a severe limitation of the bandwidth of the preamp (-3 dB point about 15 kHz !). A lowpass filter was formed by the gate capacitance of the MOSfets in combination with the series resistors.

Help came from an unexpected source. Mr. Colin Rutter, of Spectral Technologies PTY Ltd., Australia, wrote: "The bad sound you experienced with the BOZ is due to the input signal being clipped asymmetrically as setting the bias current to 40 mA by measuring the voltage drop across the resistor may in fact not set the zero point correctly. A CRO (cathode ray oscilloscope) solves this problem or a bit of maths. My Bride of Zen has its idle current set at 32 mA for symmetrical clipping".

Colin was completely right. Watching an amplified sine wave on an oscilloscope, I carefully lowered the bias of my BOZ. In my case, symmetric clipping occurred at a bias current of 28-29 mA. Finally, I removed all series resistors from the audio inputs. Clipping is no longer audible on CD, even during powerful crescendos.

Although this is not shown in the schematic, Nelson suggests to include a mains filter in the power supply. I used a shielded filter (Schaffner FN2060-1-06, 1 A type) and mounted it between the mains switch and the power transformer. Before the filter was installed, there was a major transient upon turn-on and turn-off (loud thumping noise from the loudspeakers). After installation of the filter, there was no longer any transient. Thus, the filter is a major improvement. The preamp sound seems also to have improved (better resolution of low-level details, superior imaging).

Unfortunately I could not find a dual concentric 5 k log stereo pot which would allow adjustment of both volume and channel balance; therefore I used a separate mono potentiometer for each channel.

I have used the BOZ now for several years, in combination with various poweramps, including the Zen Revisited (Nelson Pass) and a Van Medevoort PA222. In all cases, the BOZ sounded very good. So this is a simple circuit of excellent quality, which can be easily built.

More construction details are posted at the DIY page of Jim de Kort.

The schematic of one channel of the BOZ is shown here.





Stephen Chan asked me to measure some data of my homemade BOZ since the voltages indicated in The Audio Amateur (Figure 1) may be incorrect. The voltages in my circuit were the following:

Supply rail (R101/R102 branchpoint)

Left 58.4 V

Right 57.8 V

Drain of MOSFET

Left 30.3 V

Right 30.1 V

P102/R101 branchpoint

Left 13.8 V

Right 13.6 V

R105 top

Left 6.86 V

Right 6.95 V

R105 bottom

Left 6.79 V

Right 6.89 V

R108 voltage drop

Left 2.83 V

Right 2.90 V

Bias current

Left 28.3 mA

Right 29 mA

Voltage gain (100 mV in, 1 kHz)

Left 8.2 x

Right 8.6 x



Frequency response is ca. 4 Hz...500 kHz (-3 dB), on both channels. Maximum output is greater than 40 V peak-peak (about 15 Veff). Clipping symmetrical, hum and noise inaudible.

Questions, comments: aren.van.waarde@hetnet.nl

 

(Use only the initial letter of my first name if you like a response to your e-mail)

 

Finally, a picture of my BOZ preamp:

 

 

 

Back to Main Menu