The T-Phonum MKII is a high performance RIAA phono preamplifier for moving magnet (MM) and moving coil (MC) cartridges.
The T-Phonum MKII was designed to obtain the best performances in the reproduction of vinyl records. Using a novel two-stage RIAA network, which consists in an active and a passive stage, the main problems of accuracy, noise and headroom related with the traditional purely active or passive networks have been solved.
A very high accuracy in the RIAA equalization has been obtained through an accurate design, ensuring a very high fidelity. Furthermore the signal to noise ratio and the headroom are considerably greater than the traditional circuitry through the use of the two-stage RIAA network.
|Gain||MM: 40 dB
MC: 60 dB
|Adjustable to 46/66 dB (opt.)|
|Signal to noise ratio||MM: 92 dB
MC: 72 dB
|Op-amp LT1115, A-weight|
|Maximum input level||MM: 230 mVpp (81 mVRMS)
MC: 22,2 mVpp (7,8 mVRMS)
|Headroom||27 dB||1 kHz|
|Total harmonic distortion||~0,002%||Op-amp LT1115, 1 kHz|
|RIAA accuracy||±0,05 dB||Typ., 20 Hz ÷ 20 kHz|
|Input capacitance||0 pF, 100 pF, 150 pF, 220 pF, 250 pF, 320 pF, 370 pF, 470 pF|
|Input resistance||47 kΩ, 100 Ω|
The input capacitance and resistance are adjustable using high quality DIP switch for maximum accuracy using different cartridges. Finally, a dual-mono signal design reduces crosstalk.
All these aspects are designed to get an absolute fidelity in the reproduction of vinyl records and the best musical performances.
1.1 Preamplifier stage
The T-Phonum MKII makes use of a low impedance RIAA network composed by two separate stages: an active low frequency network and a passive high frequency network. This approach solves effectively the noise and headroom problems related with the use of traditional RIAA networks. This also improve the operating condition of the op-amps maximising their performances.
The circuit uses four single op-amps for audio purposes, as the followings:
|Op-amp||Equivalent input voltage noise|
|LT1115, LT1028, AD797||0,9 nV/√Hz|
In order to obtain the best signal to noise ratio, the use of a low noise op-amp is actually required only in the first stage because the noise introduced in the second stage is negligible respect the overall noise. Anyway, is necessary to use op-amp able to drive 600 Ω loads in the first stage, because of the low-impedance of the network (all above mentioned op-amps are suitable).
The high accuracy of RIAA equalization is obtained using low-tolerance parts, for example 1% or less resistors and 5% or less capacitor (MKT, MKP or metallized polystyrene are suitable).
The coupling capacitors (C23, C24, C31 e C32) are film type with capacitance between 2,2 and 10 µF. The schematic shows the suggested polarity when using electrolytic capacitor (25 V minimum).
DIP switches select the MM/MC operating mode (S2 e S4) and input capacitance and resistance (S1 e S3). The latter can be variously combined to obtain eight different capacitance values and 2 resistor values.
By-pass capacitors (C9, C10, C11, C12, C13, C14, C15 e C16) are ceramics for low high frequency impedance.
1.2 Option: 46/66 dB gain and custom input resistor
The T-Phonum MKII was designed to provide a 40 dB gain for MM and 60 dB gain for MC. This ensure the best compatibility for almost all the common cartridges. Anyway it’s possible to modify the gain to adjust the output volume according to own preferences. For example, if you want a 46 dB gain for MM and a 66 dB gain for MC it’s only necessary to change six resistors in this way:
- R15, R26 = 10 kΩ;
- R16, R27 = 1 kΩ;
- R17, R28 = 100 Ω.
The input stage of this phono preamplifier allows a selection of eight capacitance values from 0 to 470 pF for MM cartridges, It also allows to reduce the input resistance to 100 Ω for MC cartridges. This value is virtually suitable for any kind of MC which required a low load resistance. Anyway, if you want to customize this resistor value you can use a machined PCB connector. This ensure a fast input resistance replacement according to own preferences, without using a soldering iron.
1.3 Power supply stage
The board includes a regulated power supply stage that provides the double voltage required by the four op-amps. This stage works only with an alternating input voltage from 12 to 15 VAC provided by an external transformer of 6 VA minimum (do not use a DC voltage!). The using of an external transformer is the best way to avoid electromagnetic interferences. Anyway the transformer can be included in the same chassis using a appropriate positioning.
C1 and C2 reduces the reverse recovery noise of the fast rectifier diodes D1 and D2. C5, C6 and C7 are ceramic capacitors used for by-pass of the voltage regulators. C8 is a tantalum capacitor used for stability of U2. These types of capacitors ensure the best stability and a low noise.
This circuitry was designed in order to ensure a low noise power supply for the op-amps. The diodes D4 and D5 prevents the latch-up of outputs. The four resistors R4, R5, R6 and R7 (10 Ω or less) and bypass capacitor reduce also crosstalk, particularly at high frequencies where the PSRR of op-amp decreases.
1.4 External power supply option
Starting from rev. 6, the PCB includes a 3-pole connector (E6) for a direct DC power supply. This is useful if you want to provide directly a DC voltage to the op-amps using a pre-existing DC regulated power supply or two batteries. If you want to use this connector, you must not mount all power supply parts except the four resistors R4, R5, R6, R7 and the connector E6.
2. The two-stage RIAA technique
The main distinctive characteristic of T-Phonum MKII is the hybrid two-stage network. In fact, differently to almost all the RIAA preamplifiers which use only purely active or purely passive networks, in this preamplifier the RIAA equalization is obtained using two specialized RIAA networks: an active network for low frequencies, and a passive network for high frequencies. The advantages provides by this approach are very interesting, because they eliminate definitively the limitations related with the use of purely active or passive networks.
In fact, is known that high impedance network in series with signal introduces a certain thermal noise. When we use purely passive network, we usually use big resistors, for example ~50 kΩ, to ensure an accurate equalization avoiding the overload of the driver stage. But this causes a reduction of the signal to noise ratio, introducing the main limit to the preamplifier dynamic.
Using the two-stage technique it’s possible to minimize the value of this resistor (in this case the series resistance is only 2 kΩ) making negligible the noise contribution and increasing the dynamic range of the circuit respect passive designs.
Furthermore, the traditional RIAA passive networks introduce a 20 dB attenuation at 1 kHz. This attenuation must be recovered with ad extra gain provided from the amplifier stage. This introduces severe signal to noise ratio and headroom limitations. Contrary to this, the two-stage design doesn’t introduce this attenuation, reducing noise and maximizing headroom.
Otherwise, the use of a passive high-frequency network allows the correct roll-off even at ultrasonic frequencies. This would be impossible using only an active network in a non-inverting topology. With an accurate dimensioning, this ensure an extraordinary accuracy in the RIAA frequency responce at any frequency.
This also ensure the correct ultrasonic frequency attenuation reducing intermodulation distortion due to op-amps slew rate limitations and ensure their best operating condition.
From this point of view, the two-stage RIAA network provides the benefits of both technologies without their limitations. A complete mathematical analysis is provided here: two-stage network analysis (in Italian).
The PCB is contained into a metal chassis in order to avoid electromagnetically coupled noise. The chassis is made of black anodizing aluminium, where the logo was engraved with a CNC laser engraver. The PCB is 10 × 12 cm and was mounted using the tracks of the chassis. Anyway the PCB includes four holes for M3 screws (for use with insulating spacers).
The connection of the ground plane to the chassis was made using the power supply connector. In fact, the external metal cage of this connector was connected to the pin 3 (GND) of E1. The inner pole of the panel connector is connected to the pin 4 (12 VAC) of E1. The LED diode is connected to pin 1 and 2 (anode and cathode respectively).
It was used isolated RCA connector to avoid ground loops. It’s convenient to use shielded cable (for example RG 174 type) for signals to avoid crosstalk. The turntable chassis cable is connected to the central GND connector in the board. Its position has been chosen to obtain the best noise performance and the minimum crosstalk.
The PCB was silk screened to make parts positioning easier and the solder mask in the bottom side protects the copper from oxidation. The hole are internal metallized to facilitate perfect soldering.
4. DIP-switch configuration
|Input capacitance (S1 and S2)||Switch 1||Switch 2||Switch 3|
|Input resistance (S1 and S3)||Switch 4|
|47 kΩ (MM e MC)||Off|
|100 Ω (solo MC)||On|
|Gain (S2 and S4)||Switch|
|+40 dB (MM and high output MC)||On|
|+60 dB (MC)||Off|