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Simple moving-coil pick-up amplifier.

Contents


Introduction.

This is a simple FET-input pre-amplifier for a low-output moving-coil pick-up.
The amplifier runs off a 3.6 V power supply and is suitable for DIY projects, but not for volume production as it requires adjustment of the bias point.
I have built a few of them, but have never got around to do any measurements, so the specs given here are from a simulator.


Circuit.

Simple MC pre-amplifier schematic.
Fig.1: MC pre-amplifier schematic.

Before you start building this circuit, make sure you can get suitable FETs.
The 2SK369 specified has been out of production for more than 20 years and most you find on web-shops and auction-sites are fakes.
If you can get the "real" ones, use those.
To use another type, look for following specification:
Input noise: < 1 nV / √Hz @ 1 kHz.
Idss: 8 mA..16 mA.
Reverse transfer capacitance ( Crss ) <50 pF.
Do not try with high-frequency FETs - their low frequency noise is too high.
Likewise, do not try with very low voltage-noise FETs designed for very-low-frequency applications - their reverse transfer capacitance is too high.
Some suggestions are: Linear Systems LSK170B, LSK170C or some of the types from Interfet.
It may be worth trying LSK389 with both FETs in parallel ( they can be paralleled without source resistors ). You will have to adjust the bias point to fit these.
If you buy FETs from an authorized reseller, you will get them from the same batch so there is no reason to match them between channels.

R2 should be selected so the voltage on Q5's emitter is around 1.5 V. Use same value in both channels. 240 Ω is suitable for a FET with an Idss of 8 mA.
Resistors must be metal-film types.
C1 must be an NP0 ceramic or a film capacitor.
C3 must be an X5R or X7R ceramic.
Electrolytics are general-purpose types.
You may want a ( NP0 or film ) capacitor ( around 1 nF ) across R1 to reduce HF pick-up.


Specification.

Supply voltage3.6 V to 3.8 V
Supply current40 mA
Input impedanceR1
Output impedance 100 Ω
Voltage gain @ 1 kHz20.8 dB
-3 dB frequency830 KHz
Maximum input voltage85 mV
Slew rate4 V/µS
Equivalent input noise @ 0 Ω source, 20 Hz3.44 nV/√Hz
Equivalent input noise @ 0 Ω source, 1 KHz880 pV/√Hz
Equivalent input noise @ 0 Ω source, 20 KHz750 pV/√Hz
Power supply rejection ratio, ref. input, 20 Hz91 dB
Power supply rejection ratio, ref. input, 1 KHz77 dB
Power supply rejection ratio, ref. input, 20 KHz51 dB
Maximum noise on Vcc, 20 Hz62 µV/√Hz
Maximum noise on Vcc, 1 KHz3.1 µV/√Hz
Maximum noise on Vcc, 20 KHz135 nV/√Hz

Maximum noise on Vcc is specified for a 1 dB degradation in equivalent input noise. A suitable voltage regulators is for example LP3982 from Texas Instruments, but these are only available in small SMD packages.


Voltage regulator.

Another way to make a regulator for this amplifier is to use a LM317 with output noise filtering ( LM317 output noise is around 400 nV / √Hz which is too high for this application ).
Note that this circuit has been simulated only - not tested.

Voltage regulator schematic.
Fig.2: Voltage regulator.

The components marked x1x is a 4.6 V regulator. The input to this regulator is a regulated ( as in: not rectified mains ) DC voltage of 7 V to 37 V.
If the input voltage is more than 12 V, you will need a small heatsink on U10.

The components marked x2x is a capacitance multiplier that will reduce the noise from U10 to an acceptable level.
Components x1x are common for both channels, x2x are repeated for each channel.

As an initial test, load each of the VCC output with 100 Ω to ground and adjust Vcc to 3.6 V to 3.8 V.
With the initial value of R11a, the output voltage should be a little on the high side. If not, replace R11a with 750 Ω.
Try with different values for R11b until Vcc is in range.
You may be able to use a BC546B for Q21, but I will prefer the higher current rated BC337 ( or similar ).


Copyright and disclaimer.

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Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: pp@poulpetersen.dk

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