class A eq amp - part 2

It has been a long and winding road. I built a prototype to be able to listen and measure.

I also added the transistor gyrator circuit in fig1 to provide the bass boost. I was pleased with the sonic results but I soon found out that I only achieved reasonable distortion levels with a power supply of at least 12V. At 5V distortion was a horrible 0.5% or more.

fig1: transistor gyrator circuit

Replacing the FET current source with a transistor current source improved the circuit a lot. Distortion levels dropped significantly but still at 5V it was nowhere good enough.

 fig2: gain block with transistor current source

I almost gave up the idea of  using a 5V power supply and I was very tempted to choose the high voltage route as the circuit works very well at 24V and I have this very cute TEAC transformer with 26V open circuit voltage.

That would really cut the mustard but further experiments revealed that the mediocre distortion results were caused by the gyrator circuit. Without the gyrator I could achieve distortion levels well under 0.01% by carefully adjusting the bias voltage even at 5V.

Finally I decided to stick with my design goal of 5V and I worked out a passive tone control circuit using the gain block as a make-up amp.

fig3: passive tone control

I worked out the tone control with some trial and error using spice.

low boost

low + high  boost

low + high cut

flat

Next step: listening and choosing  values.