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Author: Ventsislav Simonov Page 5 of 11

Power Amplifier Design Considerations by P. Baxandall

Articles describing particular amplifier designs, or advocating specific solutions to design problems, abound in the literature, and it is evident that some quite conflicting views exist on certain topics for example, concerning the amount of negative feedback that should be used. The present approach is of a fairly broad nature, and aims to elucidate and compare various familiar and unfamiliar circuit techniques in such a way that their advantages and disadvantages may be clearly and logically appreciated.

In exploiting the very great virtues of negative feedback, the problems and difficulties that arise are largely those associated with obtaining adequate stability margins under all conditions of, operation. In a.c. coupled amplifiers, there are stability problems at both low and high frequencies, but the elimination of output transformers, together with the adoption of d.c. coupled circuitry in most modern designs, has virtually removed the low-frequency problems.

Audio Power Amplifier Design by P. Baxandall – >>> LINK <<<


Record Dyanmic Range Database

Here is a large database helping in finding good recordings with low compression and high dynamic range. For those familiar with “the loudness war” this would be a nice place to visit!

It’s interesting to know that “loud” recordings are never good recordings. They might sound very good to the listener at the beginning. Listening to such a recording for a longer time reveals its defects. Quiet passages sound as loud as the loudest ones. That makes the recording sound “flat” once we adjust the volume down to more listenable level. The impact of the kick-drum is gone or the large orchestra has no fortissimo any longer.

A good and more technical explanation could be found in this video:

So be sure to check out this large database of recordings here – >>> LINK <<<

RIAA Considerations

The following link is a great source of info about RIAA designs. A great collection of schematic topologies and measurements. A must read material. The site includes variety of industrial schematics. Some of them are pretty old but could be easily adapted and brought back to date using the modern component base.

Now this one is a real gem:

It’s a design by Linsley-Hood. This one could be easily optimized for even lower noise. What comes to mind is the Toshiba’s 2SK170 JFET. This will drop the input bias circuit and the need of input capacitor. Plus – more JFET’s can be paralleled to reduce the noise floor even further at no real penalty. The only problem is the hfe matching between the JFET’s.

Be sure to read the article here – >>>  LINK  <<<

Small-Signal Distortion. Amplifiers. Audio.

We examine how intermodulation distortion of small two-tone signals is a ected by adding degenerative feedback to three types of elementary ampli er circuits (single-ended, push-pull pair, and di erential pair), each implemented with three types of active device (FET, BJT and vacuum triode).
Although high precision numerical methods are employed in our analysis, the active devices are modeled with rather simple models. We have not investigated the consequences of more elaborate models.
Though negative feedback usually improves the distortion characteristics of an ampli er, we nd that in some cases it makes the distortion “messier.” For instance, a common-source FET ampli er without feedback has a distortion spectrum displaying exactly four spurious spectral lines; adding feedback
introduces tier upon tier of high-order intermodulation products spanning the full bandwidth of the ampli er (as suggested by Crowhurst in 1957). In a class-B complementary-pair FET ampli er, feedback mysteriously boosts speci c high-order distortion products.

Small signal distortion in feedback amplifiers for audio by G. Boyk and G. J. Sussman – >>>LINK<<<


Here is an interesting schematic diagram for USB DAC based on the famous PCM2706 chip by Texas Instruments.

The design incorporates opto isolators to reduce the high-frequency garbage and prevent it from entering the analog part of the DAC. Like most of the DIY minimalistic designs, this one uses passive low-pass analog stage at the output of the DAC chips. The current to voltage conversion is done passively (duh) thanks to R19 and R17. Those are subject to experimentation to obtain the optimum value. Too high the resistor value and you get increased THD. Too low means you get lower output level.

PCM2706 + 4 x TDA1543 – LINK

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