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Preamplifier Wo-1

Wo-1 is a high-end pre-amp of Miyajima-Lab.
This amplifier is the unique design that the signal of the sound does not pass resistance.
This amplifier controls the volume by a change of gain not volume resistance.
The phonopermission input is 1,800mV.
It is approximately 5 times of the conventional high quality vacuum tube amplifier.
The output impedance is 1,900 Ω. (plate follower)
Watt of the output is approximately 10 times of the general amplifier.
Wo-1 has a current meter to fine-tune the balance of the vacuum tube.

A phono is 2 circuits, and this amplifier can change input impedance.


Vintage vacuum tube:
Input impedance:
Input terminal:
Output impedance:
Output terminal:
PHONO allowable input:
Maximum input sensitivity:

AUX frequency characteristics:

Approx D460 x W480 x H175 mm (including terminals)
Approximately 21kg
E80F or EF86 x2, 6BX7 x2, 12AU7 x2, 6SN7 x2
Power supply custom order x 1, choke transformer x 4
PHONO A-47KΩ, B-620 / 15K / 30K / 47K / 68K / 100KΩ
1.9kΩ (plate follower)
1,800mV (1,000Hz)
PHONO 1.8mV (at output 1V), AUX 21.3mV (at output 1V)
PHONO amplifier about 38 times (1kHz),
sending amplifier 0 times to about 47 times (maximum)
Approximately 5 to 700,000Hz (-2dB)


The photograph is the inside of Wo-1.
The vacuum tube of the equalizer is
EF86 and 6BX7-1/2.
The volume circuit is 12AU7 and 6SN7.
The Output circuit is 6BX7-1/2.
Wo-1 uses a big power supply transformer
and four chalk transformers.
The design of Wo-1 is simple and bold.
Because all wiring is handmade,
the amplifier is longer life.

General volume Wo-1 volume

This is a simple image of the volume circuit.
Wo-1 does not use a partial pressure by the resistance.


Reason for making Wo-1
Forty years ago when I began making vacuum tube amps, when I designed the preamplifier equalizer circuit I learned an inexplicable reality.
At that time, I was investigating the very expensive and famous Macintosh C - 22 and Marantz # 7.
The output of the cartridge rarely exceed 100 mV.
The equalizer circuit of these two preamps distorts even if it calculates how much.
I was still immature in those days.
The two equalizer amplifiers are not distorted by the NFB.
At the time, these two amps were representative amps with good sound.
The reason for good sound is the size of the allowable input of the equalizer amplifier section.
When the average of other amplifiers was 100 to 300 mV, the Macintosh C - 22 was 390 mV and the Marantz # 7 was 370 mV.

* What is NFB? (The complete explanation will be long, so I will briefly explain the NFB here)
It is a circuit that cancels distortion and noise by feeding back a signal opposite to the circuit.
Distortion and noise are eliminated, the frequency characteristics are greatly extended, and it can be shown to a very excellent amplifier on the data.
In general amplifiers, NFB is used favorably, but when used with a large numerical value, the sound quality obviously deteriorates.
To design a real amp, you need high technology and expensive parts that do not depend on NFB.

I made up my mind.
"Make an equalizer amplifier that does not distort even without NFB"
I think that adding NFB to a distortion-free amplifier has no problem, but I thought that even if NFB was added strongly to a distorted amplifier, good sound would not come out.
However, considering common sense, there is a limit to increasing permissible input by all means.
The answer that arrived at the end of trial and error is to change the equalizer circuit to a circuit like a power amplifier.
I chose a vacuum tube that I never used in preamplifiers and I changed the impossible to possible.
The completed preamplifier did not distort even if it exceeded 100 mV.
After that, I added the necessary NFB.
And an equalizer amplifier with an allowable input of 2000 mV has been completed.
Currently, the allowable input of the equalizer amplifier of Wo - 1 is adjusted slightly and it is used at 1800 mV.

Wo-1 has further improved from there.
It is natural to use good parts to make a good amp, but to make it even better it is important to omit useless parts.
In the design of the amplifier, the sound quality is better if the capacitor and the resistance are less in the conductor through which the music signal passes.
There is no problem with a certain number of capacitors, but resistance has to be omitted.
Among them, the most influential one is the resistance of the volume.
The sound signal loses 50% to 80% of energy by this volume resistance.
Also, the volume resistance greatly increases the impedance of the amplifier.
Even if you use high quality resistors for volume, the sound quality will not be solved.

In September 1997, I tried to eliminate the resistance of the volume.
I thought of adjusting the volume by changing the gain of the amplifier, and Wo-1 was completed in 1999.
Wo-1 has been further improved by 2014.
The amplification of this amplifier is 0 times to about 45 times.
The meter of the front panel is attached to manage the precise operation of the vacuum tube of this part.
In the circuit of Wo - 1, there is no resistance in the conductor through which the signal passes.

Wo-1 has another feature.
The output of a general high-end vacuum tube preamplifier uses a cathode follower circuit of a voltage amplifying tube (low power).
It is output with apparent low impedance (less energy due to 100% NFB), but in the Wo-1 2014 model, an output tube (high power) is used to lower the output impedance.
It outputs music signals with high energy of the plate follower circuit while maintaining the frequency characteristics.
As for what this means, most of the general preamplifier pretends as a good quality low impedance preamplifier.
Wo-1 is a genuine low impedance that is not a pretense.

In order to make Wo-1 you need technology and quantity that can not be thought of with normal pre - amps.
The sound quality feels a sense of transparency, speed, volume, sense of force, space and makes me think of the reproduction of live performance, but since it is an amplifier I made by myself, I will refrain from the impression here.
However, if you understand the essence of the design, I think you can understand that this preamplifier naturally has the best sound quality beyond common sense.

Currently, Miyajima Laboratory's products are exporting overseas, so when we sell amps overseas we need to adapt the standards of parts used to the world standards.
I am using a black tubular black cat capacitor in the circuit picture of WO - 1, but it is unknown whether RoHS of European standard is compatible for now.
When adding the CE mark to the amplifier, we are considering changing this capacitor.
All other parts are RoHS compliant.