SolderSmoke Challenge Alternate Audio Stage

 

The audio amplifier in the SolderSmoke challenge DCR is relatively easy to build - just three 2N3904 common emitter amplifier stages, and an audio transformer.  Builders in certaing parts of the world have had challenges acquiring the transformer and have asked for a design that does not use one.  Also, many builder struggle with oscillations and feedback.  To address this I replaced the final 2N3904 stage and transformer wiht this signficantly more complex final amplifier stage from Fig. 1.17 of Experimental Methods in RF Design.  I built it mostly to the orignial schematic with a few changes that I found in an archived article from Vasily (Todd) of Popcorn QRP.  

Q1 is the gain stage for the following NPN/PNP  complimentary pair emitter follower transistors (Q3 and Q4),  Q2 provides active DC decoupling and provides the floating voltage source that establishes bias for the complimentary pair.  It uses negative feedback via R3 to stabilize the circuit and to set the overall bias for the stage.  

Todd did some analysis that showed that as-designed there was significant crossover distortion in the output wave form and I verified this to be the case. 

EMRFD Fig 1.17 Amp showing crossover distortion

 The solution was to change R6 to 4.7K which cleaned it right up.

Changed R6 to 4.7K - Virtually eliminated distortion

Clean output aftger mod

The only other change was to change R1 to 1K which increased the overall voltage gain of the stage significantly.  Here is what the final build looks like in the DCR. I've kept the first two 2N3904 driver stages from the SolderSmoke DCR Challenge and substituted the new amplifier for the final stage.

KK4DAS DCR Final Audio Amp Mod

The amp sounds great and provide room filling audio, but ut it is not a beginners amplifier.  

So it is possible to build a great sounding, very stable, all discrete component transformerless audio amplifier.  It just takes a little more work than we wanted to subject new builders to and and with the addtion of a complimentary pair of transitiors, active decoupling and feedback it adds signficant complexity to the circuit.

73 from Great Falls,
Dean
KK4DAS