The Perfect 25 W Amplifier

How should it be done ?

Base it on the Special Version schematic.  That should be the starting point. DON'T attempt to modify an existing amp, it should be built from scratch !

Then follow the points below:

  1. Change the power supply as follows:
  2. Increase the pre-stage voltage from 30 to 35 V. 
  3. Place regulators on the pre-stage voltage supply, allow 5 more volts to drop here, taking up the unregulated  DC voltage to 40V.
  4. Add a Cascode stage on the 3rd amplifier stage.
  5. Replace the output transistors with modern japanese types, f.ex. the ........
  6. Refine the other stages, shown in the suggested schematics.
  7. Design a new PCB layout, follow the PCB rules detailed below.
  8. Use heavy gauge wire on all supply lines and loudspeaker outlets. If possible, use steel/copper bars between the elctrolytics. 
  9. Keep all wires close to the chassis, glue them onto the metal.
  10. Place emitter followers before the 3rd stage
  11. Use shielded wires from the input signal jacks to the board. Use two signal wires, one for ground and one for active, with the shield connected only in one end.

 

PCB Layout rules:

  1. Keep input stage seperated from output stage
  2. Serial resitors placed as close to the base/gate of the receiving transistor
  3. Keep EVERYTHING SYMMETRICAL
  4. Short leads everywhere
  5. The higher the network impedance, the shorter the leads
  6. Seperate input and output stage at the connection between 3rd stage emitter followers and 3rd stage common-emitter stage.
  7. Use THICK traces on all parts of the output stage.  KEEP IT SYMMETRICAL.(Any asymmetrical trace routes here will cause an unbalance, with correcponding lack of distortion cancellation, you won't have that, will you ?)
  8. And, don't use a ground plane, but guard rings may be useful. (Never tried them thought.) A ground plane adds capacitance between all traces and ground, thus reducing high frequency performance.  Remember, this is not a radio, and the signals are NOT dependant upon RF reflection and things like that. Capacitance is MUCH worse !!!!

Then you'll probably have some questions.  Before you e-mail me, I will try to answer some anticipated questions up front:

FAQ:

  1. Can I sell amplifiers based on this schematic ?    NO, they are intended for your personal use.  And, for your close friends, if that helps you financing the thing. If you are a company and want to make money on this, you should contact me, before my lawyers contact you !
  2. Can you provide PCB's or components ?  I can not provide PCB's nor  mechanics but I may have some suitable elctronic components.   I will put up a list of these later.  Check back.
  3. Can I increase the power output ? Well, you can increase it slightly, up to perhaps 50W.  All output stages including the 3rd stage should have a corresponding voltage increase, and perhaps you should add more output transistors in parallell. Note however, that the more transistors in parallell you add, the more capacitive load you introduce.  3 or max 4 in parallell.
  4. What quiescent current should I use ?  The quiescent current should be calculated from each transitors Hfe versus Ic curve.  It should be set to no more than   sligthly less than half the current at which Hfe has its maximum.  Lower limit - don't push it too far down .....  Note then that more transistors inparallell means more power loss.