PH2S

A MM Phono Amplifier

This is a description of a phono amplifier with passive RIAA equalization for moving magnet (MM) cartridges. It is built up using two linear amplifier stages where the equalization network lies between these two stages, see the figure below.

The gain is set to about 30 dB for each gain stage to give a total gain of 40 dB at 1 kHz. It is, however, possible to increase the gain without altering the RIAA network.

The gain stage is shown in the figure below. It is a two stage amplifier (complementary feedback pair). The first stage is realised by the JFET Q₁₀₃ and the second stage is realised by Q₁₀₆. The input impedance (without R₁₀₁) is very high thanks to the JFET and the feedback. The resistor R₁₀₁ define the cartridge loading in the first gain stage. The resistors R₁₀₇ and R₁₀₂ is defining the closed loop gain as R₁₀₇ /R₁₀₂, about 30 dB with the resistor values shown. Increasing R₁₀₇ will give a higher gain. The capacitor C₁₀₅ limits the bandwidth. The output impedance is quite low, thanks to feedback.

The choice of JFETs is important, the JFET should be low noise with high transconductance. In my own amplifier, I have used 2SK170V transistors (no longer in production). However, LSK170B,C or D can be used. The same is to be said about 2SK170BL (no longer in production).

The figure below shows one channel of this phono amplifier. My PCB contains two channels, the two channels are identical. Essentially the components R₁₀₈-R₁₁₄ implements the equalization. However, R₁₁₆ acts as a load for this network. This makes the calculations somewhat less straightforward, but measurements confirm that the accuracy is good with the values shown.

As mentioned, R₁₀₁makes up the resistive loading for the cartridge. The capacitor C₁₀₀ is shown to be 220 pF in the schematic. Together with the cable capacitance and the equivalent input capacitance for the gain stage, about 10 pF, this makes up the capacitive loading for the cartridge. But, be sure to use a value of C₁₀₀ that suits your cartridge.

At the output the capacitor C₁₂₃ is quite high, so the amplifier may be loaded with values down to 10 kohm without any problems. The power supply is not critical; a supply voltage in the region 22-30 V can be used. The components C₁₂₅ to R₁₃₃ give a heavy RC-filtering of the supply voltage. This ensures that the amplifier has a very quiet power supply. The only drawback with all this filtering is a slow start-up at power-on.

A two layer PCB has been made for the amplifier. As mentioned, the two channels are identical, the component numbering is 100-etc for one channel and 200-etc for the other channel. The component placement is shown in the figure below.

As mentioned, the gain at 1 kHz is 100x (40 dB), so a 5 mV cartridge will give an output of 500 mV. There is low distortion for an input signal up to 100 mV. The RIAA accuracy is very good using components with 1 % tolerance for the resistors and plastic capacitors. The figure below shows the 3d PCB.

The figure below shows the populated PCB before the transistors are mounted.

Bill of Material

0.6 W Metal film resistors with 1% tolerance have been used. The capacitors in the equalizer network also have 1 % tolerance. Other types of both resistors and capacitors are of course also possible as long as they fit in on the PCB and don't have other values (with the exceptions mentioned in the text). The bipolar transistors Q₁₀₆/Q₂₀₆ and Q₁₂₁/Q₂₂₁ are low noise types, but it is much more important that the JFETs are low noise types. In the following the BOM is for one channel only.

R101 47 kohm (See text)
R102 47 ohm
R104 680 ohm
R107 1.5 kohm (See text)
R108 51 kom
R113 6.8 kohm
R114 1 Mohm
R116 560 kohm
R117 47 ohm
R119 680 ohm
R122 1.5 kohm (See text)
R124 470 kohm (Not critical)
R127, R129, R131, R133 22 ohm

C100 220 pF 1 % Rect Film L 7.0 mm W 2.0 mm P 5.0 mm (See text)
C105 10 pF NP0/C0G P 5.0mm
C109 15 nF 1 % Rect Film L 7.2 mm W 5.5 mm P 5.0 mm
C110 1 nF 1 % Rect Film L 7.2 mm W 4.5 mm P 5.0 mm
C111 100 pF 1 % Rect Film L 7.0 mm W 2.0 mm P 5.0 mm
C112 47 nF 1 % Rect Film L 7.2 mm W 7.2 mm P 5.0 mm
C115 100 nF Rect Film L 7.0 mm W 2.5 mm P 5.0 mm
C120 10 pF NP0/C0G P 5.0mm
C123, C126 10 uF Rect Film L 18.0mm W 9.0mm P 15.0 mm
C125, C128, C130, C132 4700/2200 uF Electrolyte Radial D 18.0 mm P 7.5 mm

6 pcs Screw terminal 01x02

Please notice:
This project description is for non-commercial use, only. Using this document on a site and charging a fee for download is vialation of non-commercial use and prone to demand for payment. So, for commercial use, contact me for agreement of terms. This page, however, can be downloaded for own use, and linked to, not violating term of non-commercial use.

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Knut Harald Nygaard