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Category: Preamps

Simple JFET Preamp

A JFET preamp for beginners. Simple yet elegant. Low noise.

The following is an example for a very simple and high quality JFET preamp. You can use this to amplify a very low signal sources. FET transistors are usually not used independently in preamplifiers. In most of the cases they are paired with bipolar transistors.

simpe jfet preamp

SSM2017 Line Preamplifier – High Quality Design

This page will show you how to use the SSM2017 as a line preamplifier. With some clever designing a very high quality preamp could be build at affordable price.
For those of you unfamiliar with the chip, here is the datasheet:
http://www.analog.com/static/imported-files/data_sheets_obsolete/139484016SSM2017.pdf

Now some few words about this device. It has been designed as a microphone preamp with selectable gain. It requires only one external component – the gain set resistor. As a microphone preamp it was designed to work in harsh environment, long cables, signal pollution, noise etc. Despite that the SSM2017 is able to achieve extremely low noise performance and high gain. This makes it perfect for a normal line preamp with selectable gain.

Now the SSM2017 is obsolete. Few replacements are available from few manufacturers.

AD’s SSM2019:
http://www.analog.com/static/imported-files/data_sheets/SSM2019.pdf

TI’s INA217
www.ti.com/lit/ds/symlink/ina217.pdf

THAT’s THAT1510/THAT1512
LINK

THAT’s IC’s have a superior noise performance at low gains so those are preferable. They are a drop in replacement to SSM2017/SSM2019 but a different values of the gain set resistor must be used because of the different input stage of THAT’s IC’s.

Whichever  IC you chose to use, a high quality line preamp is the result. Another interesting thing is the REF pin. It allows us to design a servo. This way we can null all the DC offset at the output and remove any capacitors from the audio path. For this purpose we need a low cost FET input operational amplifier. TL071 is the right choice. The servo is not in the signal path so the servo opmap quality is not critical.

Here is the schematic for this project:

The PSU is pretty straightforward. Two adjustable vregs and filter caps. Nothing fancy here. Use good quality industrial capacitors (Panasonic FC, NCC KY series, Rubycon etc.).

Now for the stage itself. The power supply goes trough an individual RC filtering to each IC. This helps reducing noise even further. Again good quality parts are used. All resistors are metal film to help keeping noise low.

The SSM output is forced into class A operation using a JFET cascodes ad the outputs. This increases the overall performance of the stage. A better sound is the result as well as better driving ability reducing the interconnect cables negative effects.
It is important to match the loading JFET’s so that the two SSM’s work under same conditions. Because the line stage is meant to drive high impedance loads, no high currents are needed. That is why the 2N5484 FET was used. It has a typical Idss of 1-5mA. For our application a value of 2-3mA is more than enough. It is important for the upper JFET to have a higher Idss rating. That is why the 2N5486 JFET was used in this place. It has a typical Idss rating of 8-20mA so no matching is needed here.

The whole project is assembled on a single layer PCB including the rectifier. The original PCB artwork is available for download here: >>> [LINK] <<<

Now some few words about matching JFET’s by Idss. This is actually a very simple job. All you need is a voltage source and a multimeter. Here is the testing schematic:

Simply tie together the gate and the source pins and apply voltage to the drain pin. Connect the  positive node of the ammeter to the gate and the source and the negative node to ground. The JFET will saturate at its own Idss. Please note that the Idss value is temperature dependable. Let the JFET to settle for about 10min and then note its Idss. Repeat the procedure until you find two closely matching devices.

Attention: This design was provided for personal usage only and is FREE for such purpose. Commercial usage is not allowed!

THANK YOU FOR READING THIS ARTICLE !!!

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