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Class-A 12AX7 MM Phono Stage (47dB, compatible to 40/51/56dB)

12AX7 Phono
Inside 12A7 phono

 

Back of 12AX7 phono
Frequency response of 12AX7 Phono
Distortion measurement of 12AX7 phono

Price includes four tubes and capacitors upgrade.

Standard package is 12AX7 + 12AY7 to get 47dB gain.

You can replace 12AY7 with 12AU7, 12AY7, 12AX7 to get 40, 51 or 56dB gain. See below for more information.

 

Design Concept 

Pure Class-A all vacuum tube design with zero negative feedback

Compatible with 12AX7 + 12AU7/ 12AY7/ 12AT7/ 12AX7 for different gains and different sounds.

 

Premium Quality Parts
  • Premium power transformer to ensure high performance, low noise and high reliability,
  • Kiwame resistors at important postions
  • Obbligato premium film gold capacitors to improve tranparency and clarity

Adjustable Gain 

Why do we need adjustable gain? Phono cartridge manufactures produce their own specification with different cartridge’s output level and impedance.

How to make gain adjustable? Typical approaches usually sacrifice sound fidelity. (Note 1)

Therefore we use a different approach – adjusting gain level by changing tubes, therefore there’s no uncessary components that usually distort the sound.

Tubes Gain level Application Sound Availability
12AX7+12AU7 +40dB May need another line preamplifier (with +10dB to +20dB gain) 12AU7: linear or well balance sound. Popular NOS or old production are still easily accessible
12AX7 + 12AY7/6072A +47dB Suitable for most MM cartridges 12AY7: very warm and smooth
12AX7 + 12AT7 +51dB Suitable for low output MM cartridges fast dynamic sound with punchy bass
12AX7 + 12AX7 +56dB Suitable for ultra-low output MM cartridges and some MC high output cartridges of +1mV. 12AX7/ECC83 Telefunken is still available though hard to find

 

Zero Negative Feedback topology 

Simple one way feed through circuit arrangement to preserve simplicity in design. By not aiming to lower the distortion with negative feedback we instead design the tube circuit to work into pure class-A and let this linear spot bring out the best no manipulating sound.
The harmonic distortion is less than 0.1% at 4mV input and distortion figure will reduce nicely in linear proportion to the input signal level.

Layout

Internal construction of this phono is very simple, elegant and carefully laid out for symmetrical signal between left and right channel.

 

Dimension

19 x 26 x 10.5cm (W x D x H) *

Shipping Weight
4.0kg *
* All specification are subject to change without prior notice.

 

Specification

Frequency Response according to RIAA standard
+/- 0.5dB from 20Hz to 20KHz
Total Harmonic Distortion with 4mV input signal
less than 0.1%
S/N ratio (SINAD 20mV input, 400Hz & 30KHz filter on)
-82dB
Input Impedance
47K ohm
Output Impedance
600 ohm
Gain
+40dB to +56dB adjust by changing output tube (standard +47dB from 12AX7 + 12AY7)
Circuit arrangement
All vacuum tube 2 x 12AX7 (input), 2 x 12AY7 (output)
RIAA network
R/C zero negative feedback type
Bias Operation
Pure Class-A
Maximum input level
20mV from 20Hz to 20KHz
.

Note 1: Typical approaches usually sacrifice sound fidelity.

1) adjust negative feedback level. The higher dB in negative feedback, the lower the gain. This works like a dream, a nightmare dream. Not only it changes the gain, but it also change the output impedance, frequency response, harmonic distortion, phase shift etc, but because it only requires a few resistors (or trimmer pot) to implement, the cost saving is abundance, thus utilize in many phono stages nowadays.
2) attenuate the signal path of the circuit. This strategy is “Lazy Man” solution that works very much like first method but instead of intruding NFB circuit, it just like adding potentiometer to signal path. Dumping signal away from the next stage and add unwanted impedance into the signal path. Increasing signal impedance means increase noise, reduce bandwidth, reduce current drive. 

 

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