Description: Tubed stereo preamplifier. Tube complement: two 12AX7 (PHONO stage), two 12AU7 (Flat stage). Circuit type: open-loop phono equalizer, Non-NFB flat stage. Inputs: PHONO x1, LINE x3, Tape monitor x2. Outputs: REC x2, PRE OUT x2. Frequency response: 20-20,000Hz +/-0.3dB (PHONO). Distortion: unknown. Maximum input level: 1000mV (PHONO). Input impedance: 47k ohm (PHONO).
Dimensions: Amp unit: 400mm(W) x 120mm(H) x 200mm(D). Weight: ?kg. Power supply: 200mm(W) x ?mm(H) x 100mm(D), Weight: ?kg.
Cost: Approx. 50,000 JPY.
History: Made in 1980. Improved (Rev. A) in 1995. Discarded in 2003.
This amp was built to replace PA-202, which was a trouble maker. I needed a high grade preamp as soon as possible,
so I decide to build it before building other components already planned.
Its concept is similar to PA-202, but simpler by omitting tone control.
The all stages of PA-203 were open-loop type to make this amp more stable
than PA-202, whose stability was not good. The distortion level of a tubed
voltage amplifying stage can be low enough without NFB. As for noise, open-loop
stage is rather better than a closed-loop stage, because the number of
stages can be reduced thanks to its high gain.
I copied a tubed preamplifier designed by Takashi Kubota with few modifications, which was printed in MJ magazine (a Japanese magazine on DIY audio). All the stages were open-loop type and the first stage was SRPP. It was perfectly fit with my requirements.
A major modification was the tape monitor switch; two tape decks could be selected, and a tape-to-tape dubbing feature was implemented. That's because I had two tape decks (compact cassette and reel-to-reel) at that time.
I omitted balance control and mode switch unlike PA-202.
Alps Detent VR (dual-unit) was used for volume control. It was the most expensive part in this amp.
The power supply unit (PSU) includes a mains transformer for power amplifiers and a smoothing network composed of two choke coils and three capacitors, which offer powerful electrical inertia. Besides, a bleeder composed of five 10W wire-wound resistors was added to improve regulation. They were all Kubota's idea.
[Schematic (L ch only, PSU excluded)]
Kubota's article included little information about mechanical design. So I designed this amp based on the construction of PA-202.
I could build this amp without any difficulties since its construction was similar to PA-202. It didn't take me much time to build it as far as my memory goes.
As in other NOBODY amplifiers, an AC line noise filter was added. The potential
of FG (frame ground) was set to the middle of the hot and cold of the mains
line (AC 100V).
A closed-loop voltage regulator, which was composed of high voltage transistors, was added to the DC supply for the first stage.
I didn't use PCB but 3t bakelite boards without mounting holes. The components were mounted by using silicon bond. The circuit was formed by soldering the leads of the components directly. The boards were fixed on the chassis with butyl rubber adhesive tape. Screws were not used. I expected this construction would reduce vibration of both the boards and chassis.
This construction have been employed on successive NOBODY amplifiers.
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