pwr amp


Posted on Feb 6, 2014

This is an unusual circuit for an ultra-low distortion power amp. According to my original notes the circuit is dated January 1977 so the circuit is not exactly modern but it is still sufficiently different to be interesting. The circuit was designed and sold as a card by a purveyor of surplus components but, even using mostly manufacturer`s rejec


pwr amp
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ted transistors, we managed to get about 0. 02% total harmonic distortion at 30 watts with a ±25v power supply into 8 ohms. : no bad figure even in these days of MOSFET and ICs. In 1977 anything below 0. 1% was considered excellent. And this figure was pretty repeatable without doing much selection. The problem of course is that since I haven`t touched this amplifier for many a year I have absolutely no idea what modern transistor types one should use for it but they are not critical: output transistors and drivers need to be the correct type but the other transistors can be small signal types - as long as they can handle the full voltage between + and - supplies. Tr1 and Tr2 are a long-tailed pair (LTP to save typing). It is quite common to have a LTP in an audio amp but this is different: this is a complimentary LTP. As far as I am aware no one else had used a complimentary LTP at the time, though I have since seen it used in one other circuit. So I guess the circuit is unique to the author. One of the things that limits the performance of a conventional LTP is that the tail source loads the common emitters. In a complementary LTP this can`t happen as there is no tail current source so that all the current of one transistor has to flow through the other. Tr2`s collector current flows into D1 and D2 which develop a voltage: this is used to bias Tr8 as a constant current source for Tr4`s collector. The fact that Tr4 is...




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