Power Amp -  Class A or Class B - Your Choice !

The Trimodal Stereo Power Amplifier is based on the design by Douglas Self  & approved kits supplied by the Signal Transfer Company including  the protection circuit to give full speaker protection in the unlikely  event of a fault.

This version features many component changes and my own power supply design  using dual transformers & bridge rectification to give dual mono  performance.

This outstanding design is the result of many years analysis of the various  distortion modes that can occur in the various stages of power  amplifiers. The design adopted uses 3 stages, each stage has been  optimised for low distortion and works with local feedback. Some global  feedback is also used to lower output impedance amongst other benefits.

A key aspect of the design was to ensure the amplifier remains stable  into a wide range of speakers and using a wide range of speaker cables.  Instability in power amplifiers is not uncommon because they have to  drive speakers which can present a very reactive load. Symptoms of  instability can be high levels of distortion, overheating of components  or just momentary distortions resulting in poor sound reproduction.

At the press of a button the amplifier can be changed from Class B to  Class A operation. In Class B power consumption is only 7W (no music  playing) and yet distortion is only just measurable. In Class A the very low level of cross over distortion inherent with Class B is eliminated  but power consumption is much higher. Sound reproduction is very similar in either mode and may not even be detectable in some systems.

Output power is 30W per channel, which is adequate for most domestic  situations and often preferable to high power amplifiers. Speaker loads  down to 4 ohms nominal impedance are OK.

Input impedance is selectable at either 2KR or 8KR. 2KR gives lowest noise  level and should be OK with most active pre-amplifiers. 8KR has slightly higher noise level but probably not audible.

Price is £990.

The amplifier is similar to the one reviewed here but gives better sound reproduction (in my opinion) REVIEW

More detailed description by the designer:-www.douglas-self.com/ampins/projects/trimo.htm