Cost optimized active crossover
Posted 7th April 2015 at 07:03 AM by abraxalito
I've driven the BOM cost of this board down quite hard by choosing some 'disrespected' opamps to build it around - TL084s. They go for 0.5RMB on Taobao (about $0.08, same as a recycled TDA1387). Listening so far is positive but I've got no audiophile opamp reference to check it against. In terms of bang for the buck though, its going to be very hard to beat. All filter caps are NP0 ceramics, resistors are bog-standard thick film 0805s.
What's different about this build for me is its fully balanced and classA operation in order to cancel power supply noise. To get classA operation from TL084s means the feedback/filter networks must be very high impedance so as to draw under 100uA peak currents. Opamps aren't used as output buffers, rather I've gone for emitter followers loaded by two-transistor CCSs.
The picture is my second prototype, using the best (aka lowest ESR) caps I could get hold of - Nichicon HZ and Nippon Chemi KZE. All the copper wires sticking up around the edges of the board are to heatsink the resistor droppers feeding the TL431 shunt regs. Since the output buffers are feeding step-down trafos and running in classA they draw about 10mA bias each. Total current draw is around 130mA, rails are +/-8V because that's the rating of the HZ caps. I reckon the components can all be bought (on Taobao) for under $6.
What's different about this build for me is its fully balanced and classA operation in order to cancel power supply noise. To get classA operation from TL084s means the feedback/filter networks must be very high impedance so as to draw under 100uA peak currents. Opamps aren't used as output buffers, rather I've gone for emitter followers loaded by two-transistor CCSs.
The picture is my second prototype, using the best (aka lowest ESR) caps I could get hold of - Nichicon HZ and Nippon Chemi KZE. All the copper wires sticking up around the edges of the board are to heatsink the resistor droppers feeding the TL431 shunt regs. Since the output buffers are feeding step-down trafos and running in classA they draw about 10mA bias each. Total current draw is around 130mA, rails are +/-8V because that's the rating of the HZ caps. I reckon the components can all be bought (on Taobao) for under $6.
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