This is the white paper for Classique – Phono Valve Amplifier With Passive RIAA Using 12AX7 / 12AU7 – DIY Kit. Please note that the schematics/components presented on this page might differ to the product itself. The updated components lists can be found on the product page itself.
The aim for this design is to produce a moving magnet phono pre-amplifier with passive RIAA correction (no feedback). It uses a two-stage amplification using two 12AX7 (double triode) valves and a single (double-triode) 12AU7 for the buffer stage.
The two triodes inside a single 12AX7 are responsible for one stage amplification for two channels. This configuration ensures that each channel (left and right) will be better matched as any two triodes in a single valve will be (by default assumption) a better match compared to two halves from other valves.
The third valve, the 12AU7, is also a double triode. However, this valve does not provide amplification but increases the current output capability (in another words lowers the output impedance). This is a very important stage as it makes sure that there is no (or minimal) degradation in the output audio signal produced by this phono valve amplifier.
One of the important factors in the design of any phono pre-amplifiers is that the whole system has to be noise-free. Just about every component, device and design decision has to be made with that extra caution, as any noise introduced to the system WILL be amplified at a later stage. This is due to the very high amplification factor in the pre-amp stage.
The phono pre-amplifier board draws about 24mA (both channels) when running at 300Vdc supply voltage.
Valve Working Voltages and Currents
We will now be discussing the loadlines to determine the idle current and voltages for the valves in the phono pre-amp.
The setup for stage 1 and stage 2 (the two 12AX7s):
- Supply voltage : 250VDC
- Cathode resistor (un – bypassed on the first 12AX7) : 2Kohm
- Plate voltage : 172VDC
- Idle current: 0.8mAmp
Referring to the graph above, we can see that a relatively linear region has been chosen as the idle operation point. The operation point, marked by the red dot, is at the grid voltage of -1.5V.
All resistors are of type metal film. Metal film resistors are known to have less noise and retain their resistance values for a long time.
The RIAA Equalization
The RIAA correction is done by a group of resistors and capacitors in between the two stages of the 12AX7 valves.
Please note that the exact resistance and capacitance values for these components are crucial in order to perform the reverse RIAA function. Again, they have been carefully selected according to their specifications. They are built up by mainly Vishay MKT 1822 polyester capacitors, which have been our favourite capacitors for audio signal.
The Result : RIAA Validation and Verification
We are proud to say that frequency response graph measured from the phono amp is very accurate.
Graph below shows the measured frequency response tested using 2 x Electro Harmonix 12AX7 and 1 x Electro Harmonix 12AU7:
From the measured frequency response graph, the system is shown to have produced +/- 0.5 dB variation from 40Hz to 30kHz.
There are two transformers included in the design. One for supplying the B+ (high voltage) to the amplifier board and another one for powering the valve heaters. Having two transformers (instead of using a single transformer with multiple secondary windings) might not make sense to many as it almost doubles the cost. However, this ensures that there are no coupled noise from one to the other (especially the diode switching noise in a high current environment of the heater filament supply). Two separate transformers ensures maximum separation in terms of noise and any high load (demand) on one transformer will not degrade the other.
The specifications for the power transformers are:
Transformer 1 (B+):
- Rating: 43VA
- Primary : 115 VAC, 230 VAC
- Secondary: 230 VAC, 190mA (two secondaries in parallel)
- Low flux density with a flux band
Transformer 2 (Valve heater):
- Rating: 43VA
- Primary : 115 VAC, 230 VAC
- Secondary: 14 VAC, 3.0A (two secondaries in series)
- Low flux density with a flux ban
The output from these transformers are connected to the integrated power supply PCB. The integrated power supply PCB contains two sections for powering the:
- Main Power Supply (B+) – around 300VDC
- Heater Filament Supply – around 12.6VDC
Main (B+) Power Supply
This linear power supply board takes the AC input (230VAC) and produces about 300VDC for the main phono pre-amplifier board. At this voltage, the phono pre-amplifier board draws total of about 24mA (about 12mA per channel).
The first part of the circuit is the bridge rectification which converts the AC to DC voltages. The DC voltages are then fed into three sets of RC filter networks to smooth out DC ripples leftover from the rectification stage.
All components on the power supply board have carefully chosen based on their abilities and specifications. For example:
- Rectifier diodes: Wolfspeed Sic Schottky Diode 600V 2A
- Reservoir capacitors: Nichicon Aluminium Electrolytic Capacitor
- Output capacitor: WIMA Polyester Capacitor
All the capacitors are of the long-life and high maximum working temperature (105degrees C) to ensure the durability and longevity.
The power supply schematic also contain a voltage divider (with a dedicated reservoir capacitor) for the purpose giving a raised voltage reference to the heater filament power supply. This is required as the cathode/heater potential difference exceeds the maximum level (on the 12AU7 valve), the heater voltage is raised to about 90VDC above ground (from the lower side of the filament).
Valve Heater Regulated DC Power Supply
- output voltage: 12.4VDC to supply 450mA
- ripple voltage: 0.0017v rms at 450mA
The sole purpose for this power supply section to provide the three valve heater filaments with a DC voltage. Using DC voltage (as opposed to AC) gives users a number of advantage, mainly the ability to suppress noise produced by alternating current.
At the heart of the power supply circuit, is the LT1084 chip which is a Low Dropout Positive Adjustable Regulator. This regulator outputs a fixed 12VDC to the circuit. On top of supplying the needed voltage level, the regulator also has a very small ripple voltage – which is what we need. The LT1084’s output current capability is 5.0 Amps, which is more than our requirement.
There is also a common mode filter on the board (FL1). The common mode filter is excellent at rejecting common-mode noise that might be picked up from the power transformer from primary to the secondary. It will also filter out the recovery noise created by rectifier.
A power transformer with 14VAC secondary was chosen to power up the heater filament supply. The decision to go for the 14VAC (rather than 13 or 12 VAC) was chosen was so that it would provide a safe margin for the regulator to work (in case of local wall AC voltage fluctuation). The LT1084-12 regulator needs about 1.2VDC above 12V to function.
Taking the voltage drops in the diodes and the fluctuation of our national grid providing the primary voltage, it was decided that 14VAC would a safer bet.
The downside of the “safer bet” is that in some occasions, the voltage at the input pin of the regulator might be a bit on the high side. Therefor, it will have to perform a bit more work to regulate the voltage down to 12VDC and will be emitting higher heat. For this reason itself, a heat sink has to be used with the regulator to suck up all the excess heat from the regulator.
Additionally, there is a diode connected to pin 1 (ground) of the regulator to raise the output of the regulator by about 0.6V (0.61V is the specified voltage drop for STMicroelectronics 60V 3A, Schottky Diode). The common mode filter, with its internal resistance, will drop the voltage by about 0.1V, which makes the end result to be 12.5VDC (12+0.6-0.1).
Putting It All Together
Please note that there are some extra ground connectors on the boards that might not be used in the final wiring. For example, there is a ground terminal (J6) on the main power supply PCB which is not connected to anything in the final wiring. The connector is here to give end users other ground wiring scheme.
We can see in the diagram above that there are two separate transformers used in this configuration. This is optional as you can use a single with two secondary windings however using two dedicated transformers proves to perform better and has less risk of noise contamination.