Thursday, October 4, 2012

Simple active crossover and chip amplifiers


I built numerous active crossovers in the search for perfect one. By the perfect one I mean the most transparent or pleasant sounding. This is very simple one, yet it has pleasant unobtrusive sound in comparison to commercial offerings or the OPA based active crossovers I built.



There is more than one way of using active crossover. First, is the normal, or typical way you wire the crossover. Selected signal enters the crossover, signal is split for high pass and low pass, in case of 2-way crossover, or in 3-way or even more, as you desire. Split signals are then fed into separate amplifiers. Lower power more refined amplifier is usually used for midrange/tweeter section, more powerful high current for woofer section. The diagram is shown below.

 The advantages of using active crossover are numerous, no need to mention them. However, there are disadvantages too. I found the signal to be altered or deteriorated to some degree if numerous operational amplifiers are in signal before the amplifier. I found simpler crossover to sound better. However, there is another way of using active crossover. In my case, the midrange/tweeter section is most of the time open baffle. They naturally roll off. Even if small closed boxes are used for mid/tweeter, the natural roll of can be seen as natural crossover. One capacitor is all it takes for mid/tweeter section. Thus, signal selected is fed full range into high quality low power amplifier and to the mid/tweeter (or one full-range). Sound is unaltered, clear, free of grunge of pesky operational amplifiers. Speaker output signal is simultaneously fed into active crossover, directly or through some resistors. Only low pass section is being utilized now, feeding big power amplifier and woofer. Master volume control is on low power amp now, not on crossover. Low pass signal is well filtered and allows to omit passive crossover, with all the benefits of no power loss in big indictors and better control of the woofer. Plus subwoofer section often allows variable crossover frequency and phase, adding more flexibility in adjustments. Ears are more forgiving in bass section, so even if operational amplifiers are used in low pass active crossover, it has much lesser impact. Second type of approach to active crossover is shown in second diagram below.
Hence, I was lately interested only in subwoofer sections.
  




Well, this was a short post so I thought I will add a section about chip amplifiers. While there is a number of chip based amplifiers, the first and most famous gainclone was the LM1875. More powerfull version was LM3875. Quite a lot was written about chip amplifiers, so I am not going to repeat that. Only thing I will share here is the experience with three types of circuit used with chip amps. Firts is non-inverting, simple, good sounding circuit, recommended by chip maker National Semiconductor / Texas Instruments.



























Next is inverting circuit. This one sounds way better, but it needs a buffer on the input.
Last one is transconductance amplifier, or current feedback. This one sounds best. Highly recommended. Sounds as good as good tube amp. Liquid, smooth, engaging, musical.