2011/04/03 created
2021/11/25 updated

Stereo Tube Preamplifier
High-class preamp with CR-NF type phone equalizer and Lux type tone control

Features Vacuum tube: 12AX7/ECC83 x6.
Circuit type: CR-NF type phono EQ, P-K looped flat stage, Lux type tone control
Separate power supply unit
Outline Specifications Functions: selector, tape monitor, volume contorl, balance control, tone control.
Inputs: PHONO (MM) x2, LINE x2, MONITOR x1.
Outputs: REC x1, PRE OUT x2.
EQ spec: Gain: +36.5dB. Input impedance: 50kohm. Max input: 300mV.
Dimension Amp unit: 400(W) x 124(H) x 200(D) mm (not including protrusions). Weight: ?kg.
PSU: 200(W) x ?(H) x 100(D)mm. Weight: ?kg.
Cost aprrox. 40,000 JPY
History Built in 1974. Discarded in 1980.

The following contents were copied from the page of PA-202 on my previous homepage 'Tonochi's Audio Room', and edited to fit in the new format.


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.


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; reference-3, pp.33-43) 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 (PA-202_Schematic.pdf)]

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.


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


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.