Solid Stage Power Amplifier Design – Our Approach
We have designed tube and solid state amplifiers over the years. I have explored conventional approaches in both tube and solid state design. In all amplifier designs, I found that the simplest and most elegant solution is best. By challenging conventions, we have found or own solutions. Excluding global negative feedback allows us to streamline our designs and provide a sound that is more natural and true to the original performance.
Following, are the stages that make up conventional solid state power amplifier designs:
Input or Transconductance stage.
VAS or Voltage Amplification Stage.
Buffer stage.
Unity gain power stage.
The purpose of the transconductance stage is to convert input voltage to a high open-loop-gain current signal.
The Voltage Amplification Stage (VAS) is where the current from the transconductance stage is converted to a voltage signal of very high amplitude or swing, The ‘swing’ refers to the sine wave signal ‘swinging’ to positive and negative levels, as high as the +/- voltage rails. This is how maximum power is developed.
After this stage, there is generally also a buffer or follower stage that reduces the output impedance of the VAS, to allow it to best drive the Unity gain power stage.
The Unity gain power stage is essentially a current amplification stage. Gain has been established and the power transistors merely amplifying the current needed to drive the low impedance speaker load. This is where the amp does the Heavy Lifting.
Negative feedback enters the equation here. It involves feeding a portion of the signal from the output, or Unity gain power stage, back to the inverting, or out-of-phase input, of the Transconductance stage. This amount of feedback used results in an equal loss of signal gain. Most solid state amplifiers use larges amounts of negative feedback to reduce distortion, improve bandwidth, stability and linearity. This also means that there must be A LOT of gain from the preceding stages.
The problem with this approach, is that negative feedback is ‘baked in’.
We design our solid state amplifiers differently, using a single gain stage and zero global negative feedback. This means that we also don’t need to provide high gain, and this improves linearity and stability. Our input stage provides wide bandwidth and gain in a single stage.
After our input/gain stage, we do have a buffer stage(s), depending on the use of Mosfet or BJT transistors and our power output stage is standard. But, there is no global negative feedback taken from the output of the amp.
Negative feedback is a powerful tool for reducing distortion. In many applications, it has its uses. When it comes to audio design, I prefer to exclude global negative feedback completely. Shorter signal paths, simpler circuits and less manipulation of the audio signal allow us to provide a sound that is true to the original recording.