I am puzzled why the idle current goes down rather than up as it warms. Usually its the other way round because of the negative temp coefficient of transistors.Actually this is pretty common with bipolars. They can get into a phenomena known as 'thermal runaway' if this is not well controlled.
However, my latest amps are Class A, and I suspect that a more realistic match is obtained by culling outliers by HFE, then match from VBE using the bias at constant potential and sufficient to generate the operating current. Finally, instead of using matched emitter resistors, I use emitter resistors tailored to the output devices, so that each emitter resistor sees the same potential drop.
Your thoughts? Any advice appreciated.
There's a difference between idle and dynamic operation. Matching the hfe over the range in which the device operates is going to be a better method. Otherwise what will happen is a particular transistor can 'hog' current at higher operating points and it will thus be the thing the sets the distortion of the amplifier at power, especially full power.
with Class D switching freq over 450/500 KHz these days do you still think output filtering is an issue?. How do I educate more about this ?. If the issue of filtering not a function of switching freq please correct me. Thanks again.
There's lots of information on the web! You might want to read the papers of Bruno Putzey who has done a lot to further the art. The filter is there to filter out the switching frequency, which it cannot do completely. What is left is a sine wave called 'the residual'. Usually the filter is set to about 60-80KHz, so as to avoid phase shift within the most sensitive area of the audio passband, but otherwise get the residual down as low as possible. The higher the switching frequency the easier this is to do- the speaker inductance itself starts to play a huge role at higher frequencies.
Mac Turner brought me in to consult on his. What disturbes me about them is that they all get their sound from the IC in the front end. Perhaps some are discrete front ends but I have not seen any.Actually you don't need any ICs in the front end prior to the encoding scheme, if you set things up right. The encoding scheme of course is going to use some sort of chip- our amp for example uses a high speed comparator chip. But the audio signal is applied directly to it.