Transconductance Preamp

After I have designed my Line Transceiver, i wondered how much gain these kind of circuits can achieve. My aim was to build a discrete preamp that has:

  • High Forward Gain without any feedback loop
  • High Bandwith
  • Low noise
  • and of course with an Current Output Driver

During my experiments i had to learn the hard way what temperature coefficient does mean. Surpassing 20dB of Gain makes the circuit extreme sensitive to any change of the surrounding air temperature. The DC-Operating point of my circuits swinged across the whole range only by blowing some air over it! So at least it was necessary to implement a DC-Feedback path to compensates the temperature drifts. The graphic below show the circuit in simplified form:


The Preamp is now build for a maximum gain of 86dB, sufficient for tape cassette replay.
The base-voltages at the Inputs lie about 2 Volts below the operating point due to the high input current and the 1 megaohms resistors. This has the advantage that the cascode stage (not shown here) can be tied directly to ground. The pot between the input transistors controll the sensivity of the Input stage. A higher impedance permit higher Input signals without raised distortions!

The passive EQ network can be adopted for all kind of equalizing curves, like reel-to-reel tape machines, vinyl players and others…

The following Image shows my implementations for the MPP-Stack. It uses one of my Switched Micro-Power Supplies to generate the necessary bipolar voltage of +/- 5 Volts. The free space on the right  is the recording amplifier that has not been mounted yet.


In the picture you can see it connected with a disassembled cassette tapedeck. But I also have used it to digitize some Vinyl records…


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