1
\$\begingroup\$

I'm trying, for MONTHS (obviously not everyday, but I spent at least more than 80 hours on this) to do a 15 W audio amplifier with two TDA 2002's in a bridge configuration, following the circuit provided in the datasheet itself, but it never works.

This is the circuit:enter image description here

(Note: both the TAB and the 3rd pin are the same thing)

The circuit is only stable in one configuration: with the potenciometer P1 completely grounding the non-inverting input of the chip on the right, setting its output to 0 V.

This causes the chip on the left to work like it was alone in the circuit, with a speaker directly on its output with no DC decoupling. It sits around half VCC with no signal, causing a constant ~1.2 A to flow in a 4 ohm speaker.

Other than that, if the P1 is far from grounding (ie, if its on the opposite end), the circuit is wildly unstable, it keeps making loud clicking sounds and making the speaker jump from end to end, like if its output was a square wave going from one extreme to the other.

If it ever stays stable for a while, any input signal makes it go back to that unstable state.

In some of the circuits I made, it was so ridiculuously unstable that clapping my hand or screaming loud near the speaker would make it start again (just because of the small fluctuation in voltage caused by the sound reaching the spaker, making it behave like a microphone).

I've checked every single component, specially the capacitors many times, all of them are working and correctly valued.

I've checked my power source and even used a car battery to ensure it wasn't the cause of the problems.

I've disassembled the circuit and reassembled it TWICE (all components are soldered in place) and checked them to make sure no connection was wrong or failing. I also tested changing the load from my speaker to an equivalent resistor to avoid unstability caused by its inductance.

I even changed the chips 2 times. I even thought they could be fake, so I did a test circuit for the single chip amplifier configuration and all of the ones I bought work correctly on it.

I've come from the "something maybe wrong with the design" to "something is surely wrong with the design".

What's curious is that the bridge configuration for other chips like tda 2003 is almost identical, I doubt they would make the same error twice.

But I can't see any way the problem is in any of my equipment or pieces.

Do you have any idea what the issue could be, and how to solve?

Thanks!

\$\endgroup\$
  • \$\begingroup\$ Where did this design originate from? It is not what I normally see for a bridge configuration. P1 is to set the bias voltage, which is normally not needed. I do not think this is a good design at all. There are many bridge designs for that IC alone. Often the mandatory fewest parts works the best. \$\endgroup\$ – Sparky256 Dec 22 '18 at 1:58
  • \$\begingroup\$ @Sparky256 It came from the datasheet itself, from Fairchild semicondutors. I know this potentiometer is used to set the bias voltage, but it never really manages to do so. The output of the chip on the right never reaches half VCC as it should, and its unstable. Not using this potentiometer doesn't help either, it still is unstable as hell without it. Maybe I'm using fake parts that get to work in the single chip configuration but are too unstable to work in bridge configuration? \$\endgroup\$ – user2934303 Dec 22 '18 at 2:07
0
\$\begingroup\$

The power supply rejection is not very good (25dB at low frequencies).

Run separate HEAVY wires to each amplifier, and put 10,000uF capacitors on each of the VDD pins.

Also, this chip has 80 dB gain. That means a 0.001 ohm feedback resistor, shared in the GND daisy-chain between PowerRTN and input, with 10 ohm (or smaller) Rload, will cause oscillation.

I suggest you reduce the resistance of the GND wiring at least 10:1.

Remember that ONE MILLIOHM of Ground resistance (or impedance) will be a problem, given your open-loop voltage-gain is 10,000X.

============================================

Here is a sketch of the GROUND-as-feedback situation

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ Although Aol is 80 dB, Acl is 40dB with R3/R1, R8/R5 but I wonder if they are wirewound.and not sure about R8/R6=46 dB, I wonder what layout differences in inductance exist. 1 mΩ ? not sure about that \$\endgroup\$ – Sunnyskyguy EE75 Dec 22 '18 at 3:01
  • \$\begingroup\$ @ Tony As you know well, 4" of wire is about 100 nanoHenry. At 1KHz (assume a risetime of 0.5 millisecond, to produce that 1KHz), the Zl is 2 * pi* F * L = 6.3 * 1,000Hz * 0.1uH= 6.3 * 0.0001 = 0.6 milliOhms. Yes, that is my expectation. \$\endgroup\$ – analogsystemsrf Dec 22 '18 at 8:49
  • \$\begingroup\$ From the OP "... it was so ridiculuously unstable that clapping my hand or screaming loud near the speaker would make it start again..." \$\endgroup\$ – analogsystemsrf Dec 22 '18 at 8:50
  • 1
    \$\begingroup\$ I was thinking more along the lines of reducing the phase margin to create the instability that draws current with some hysteresis as well to cause the observed bistable instability \$\endgroup\$ – Sunnyskyguy EE75 Dec 22 '18 at 8:55
  • 1
    \$\begingroup\$ Look at all the power snubbers with 10ohm RC’s to prevent oscillation with a gain BW product of 100MHz. WW Resistors. could wreak havoc. \$\endgroup\$ – Sunnyskyguy EE75 Dec 22 '18 at 16:57

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.