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2016/11/10 updated

PA-202: Tube Stereo Preamplifier

 PA-202
Description:
Tubed stereo preamplifier. Tube complement: six 12AX7. Circuit type: Active/passive mixed equalizer, closed-loop flat stage, Lux type tone control. Inputs: PHONO x2, LINE x2, TAPE x1. Outputs: REC OUT x1, PRE OUT x2. Frequency response: Unknown. THD: Unknown. MAX input level: 300mV (PHONO). Input impedance: 50k ohm(PHONO).
Dimensions: Amp unit: 400mm(W) x 120mm(H) x 200mm(D). Weight: ?kg. Power supply unit: 200mm(W) x ?mm(H) x 100mm(D). Weight: ?kg.
Cost: Approx. 40,000 yen.
History: Made in 1974. Discarded in 1980.


Concept

High-end preamplifier superior to Marantz model 7.
This preamplifier was built on condition that it would be paired with the power amplifier MA-201.

Circuit Design

I copied Yoshiro Uesugi's design (introduced in his book published in 1973; no English version) with some customization. I modified the power supply circuit. In the original design, Lux's mains transformer was used. It was not affordable to me. Instead of it, I used the cheapest mains transformer in Tango's product line. And I decided to place the power supply circuit in another enclosure. The power supply unit (PSU) was connected to the amp unit via a 6-core cable.

FYI: Yoshiro Uesugi is the founder of Uesugi Lab (known for U-BROS amplifiers). He was the president of the company until he died in 2010.

Since the original schematic has been lost, I restored the schematic based on Uesugi's one and my recollection. The PSU is integral to the amp unit in the schematic, as opposed to the real circuit.
[Schematic: drawn in 2016]

Mechanical Design

The mechanical design was based on Uesugi's idea. Two simple aluminum chassis were combined to form the enclosure. This construction offers separate compartments for the L/R channels, and results in good channel separation. I have repeatedly used this construction for later NOBODY amplifiers.
The difference from Uesugi's design was, as mentioned above, the PSU was implemented in the separate case.

Building

I was very careful in soldering, as in MA-201.

My Own Review

E (Worst)!!
Every kind of noise could be heard including microphonic noise. Particularly, Click noises caused by power on/off of other electrical appliances were the most serious. Moreover, PA-202 picked up a radio wave emitted by a nearby ham radio station as noise. The RF noise was demodulated somehow in the amplifier and voice could be heard so clearly that I could catch what they spoke.

This project resulted in a total failure as opposed to the catch copy "High-end preamplifier superior to Marantz model 7." By the way, the catch copy was not mine but Uesugi's.

Through this project, my overconfidence was shattered. I realized I had to learn a lot more to build high quality amplifiers.




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