Non oversampling push-pull DAC with 12 TDA1543

I like amplifiers without global feedback. It's not many posibilities how to make such amplifier with acceptable amount of THD (1-2 %). One of them is push-pull topology. When I made my first PP amplifier I was not satisfied with his sound comparing to SE amplifier. I had problem with the way of making the diferential signal. I tried various spliters and inverters but no one liked me. Even very often used schematics has problem with symetry of diferential signal. The best solution for me was using a special wound transformer, but this is too much work ... . I decide to make the diferential signal for PP amplifier directly in DAC.

The idea is very simple. Commonly used DACs work with binary two's complement coding. I use two DACs. Second DAC get inverted data signal. This is not the correct way of making the two's complement of a binary data signal, but the error is very small and can be compensated. Realising the "value 1 adding" would too much complicated the curcuit.

Schematics of non oversampling push-pull DAC. Input of I2S signals is optocoupled with 6N137 - not mandatory, but this option has advantages. E.g. cutting the ground loops, filltering the signal. For edge stabilisation is used the Inverting Schmitt Trigger 74HC14, which should drive max. 8 pcs TDA1543. I made stack of 6 TDA1543 for one signal polarity in Push-pull mode ( Switch SW1 ) or totaly 12 pcs of TDA1543 in Single ended mode. At the output is not the obligatory condenser, therefore I can use this voltage for biasing the following amplifier.

Power supply voltage is 8,5 V, the diference of THD by higher value is not big - I prefer safe operation of TDA1543, because in my case of mechanical realization it will be not easy to change damaged piece of TDA1543. First step of DAC adjustment is the setting of minimal THD ( Rbias, Rbias' ). Then should be set up signal symetry. This can be done by changing the value of RR, RL, RR' or RL'. I added paralel resistor to one of this resistor.

The most dificult work on this DAC was a stacking of 6 pieces TDA1543 together. For better cooling I used copper heatsink. Whole DAC together with power supply is made on universal PCB.

	I am very pleased to have very good source of diferential signal for driving Push-Pull amplifiers. I got very good results with simple amplifier( 2x EL84 ), not loud - only 0,7 W but 0,25 % THD without feedback. Clear, bright and very nice sound.
	21.11.2005