Avoiding TDA7293 pitfalls
by Martin Clausen (mc AT rotgradpsi DOT de)
How to build a very simple high quality audio amp with TDA7293 chips and how to avoid its pitfalls
Most of the hints given here apply for the whole TDA729X family of chipy by ST.
Table of contents:
High operating voltage and output power
Mute and standby functions
No switch on/off noise
Very low distortion and noise
Clip detector (TDA7293)
Can be connected in parallel to drive low impedances and share thermal load (TDA7293)
Low cost, standard parts, low part count
A good amplifier begins with a high quality power supply. It should be capable to supply high currents while maintaining a stable voltage. Use therefore a toroid transformer. It should preferentially be potted to reduce hum. Long lasting low esr capacitors (e.g. BC components 051 or 056 series, Panasonic FC) should be used to smooth the rectifier output. The last important ingredient is proper grounding.
If in the application, the speakers are connected via long wires, it is a good rule to add between the output and GND, a Boucherot Cell, in order to avoid dangerous spurious oscillations when the speakers terminal are shorted. The suggested Boucherot Resistor is 3.9W/2W and the capacitor is 1µF.
TDA7293 can be connected in parallel to drive low impedances and to increase output power especially at high voltages. Furthermore the paralleling of chips reduces the total thermal resistance of the system and therefore reduces cooling demands to a level which is more likely to be handled correctly under all operating conditions.
Make sure to use the schematic from the datasheet from 2003. Slave mode is activated by pulling IN+, IN- and SGND to -Vs, the negative supply! In older versions of the schematic slave mode is NOT activated properly, which may lead according to reports on the internet to the destruction of the chip.
The low frequency performance of the TDA729X is poor, if used according to the datasheet. The sound is somewhat "thin" in the low frequency range. The reason is that the lower -3dB frequency of the input capacitor (C1, 470nF) and the corresponding resistor to ground (R1, 22k) is around 35Hz and that of the feedback network (R3, 22k; R2, 680ohm; C2, 22µF) is about 25 Hz.
If you increase the input capacitor C1 to 2.2µF the roll of frequency drops to 4Hz and with value of 100µF for the feedback DC decoupling capacitor C2 the -3dB point of the feedback network is around 3Hz. Now the frequency response becomes nicely flat:
It is recommended to increase the value of the bootstrap capacitor C5 to at least 47µF per chip to improve low frequency performance further.
Recommended capacitances for the input capacitor C1 include Wima MKS-2 2,2µF 50V DC (metallized polyester foil) and for the feedback DC decoupling C2 and the bootstrapping C5 Panasonic FC 100µF/35V (aluminium electrolytic capacitor, low esr, 105°C).
Disclaimer: No warranty at all!
Copyright by Martin Clausen, Germany.
Contact the author: mc AT rotgradpsi DOT de