Tda7294 blown in powered mixer

[megajocke]

I got a used Soundcraft Powerstation 350 mixer with blown power amplifier for about 40$... the mixer seems to work as it should, but the chips were removed from the power amplifier. One of the chips has a crack in it and is blown apart a bit where the leads exit

The person I got it from said they had turned it in for repair, and he got it back as not repairable. It is pretty obvious that they replaced the chips once but they blew again... (burn marks on some capacitors that are non-original in front of the chip) The opamp driving them was also replaced.

I have traced the important parts of the circuit and have found out that the power supply to the speaker protection, input opamp and fan control is missing. Found -8v everywhere around the ics.

Fusible resistors in the +17v and -17v feeding these have blown. This -17v rail is connected to the stby gnd of the ic's, 2 per channel so I guess this is the reason the resistors blew. TDA7294 has about 50 ohms between all pins. The LM2901 quad comparator in the speaker protection and overtemperature standby circuit is blown too... resitive between +vs, gnd and one of the inputs.

The school I go to has the same type of mixer, and I know it was sent for repair for a blown power amplifier so this seems to be common, maybe some design fault...

There are some things in the circuit that has caught my attention:

1. input capacitors are in different directions, I guess for lowering 2nd order distortion from the caps which sit with 0v across in normal cases. But if the opamp were to blow, one of the capacitors will conduct a bit backwards and make the output ICs drive current into each other.

2. using 1n and 1k in series from -in to gnd to lower feedback so it will not oscillate. The datsheet says minimum gain is 24dB. 1k and 22k will give about 27dB. But is the cap big enough to lower the feedback enough att middle frequencies. It will introduce some extra lag below the crossover point.

3. 22p across 22k feedback resistor. Maybe this is to counteract lag from the 1n capacitor?

4. STBY-GND is connected to -17 of *another* supply. I read this thread, and especially this post: http://www.diyaudio.com/forums/showt...005#post873005
If -Vp for the power part goes below -Vs for the signal part the IC will latch up and blow up. But bad things happen for any pin going below -Vs? What happens when standy gnd goes below -Vs, if it were to come up first. Can this be the reason for the blowup?

5. Another poster in the thread mentioned that bringing mute up before stanby would blow the IC. In this circuit mute is connected to gnd so it will always come up first.

I like the design that makes the speaker protection, maybe the other 3 TDAs and input opamp blow up if one of the TDAs blow... NOT...

Good thing they put those fusible resistors there though... This circuit is powered from the same regulator as the rest of the mixer... I guess the mixer wouldn't like -42V on its -17v rail

I'm thinking of installing 220 ohms in series with the stanby gnds and schottky diodes to prevent them going below -Vs. also something on the standby input. Or does this circuit need low impedance? Capacitor is now located far away on the PCB.

/Joakim

[megajocke]

Maybe this should be moved to solid state.

[pinkmouse]

Moved as requested.

These are not the most reliable bit of kit, an AV company I knew had constant problems with them. I would pull out all the power amp stuff, and install some new LM4780/LM3886 chipamp boards if I were you.

[megajocke]

Wouldn't giving the LM3886 +-42V be cutting it close though? "Max voltage with signal: 48V"

The transformer is pretty nice, has separate windings for powering the mixer and a temperature sensor that is connected to the output relay circuit...

I traced most of the circuit and it seems that what the standby pins are connected to is just the AC loss detector. Part of the comparator turns off the output relay when standby goes below 0V.Maybe I'll just add the things from the datasheet and not have to worry about standby gnd going below v-.

edit: Just read the TDA7294 datasheet: "Supply range +-10 to +-40V" (No load +-50V) It's a bit close there too. Maybe I will replace them after all with LM3886s which doesn't seem to spontaneously combust as easily...

[pinkmouse]

You could look at some of the class D modules as well.

[megajocke]

I'm going to try with 2 LM3886 parallell per channel. Will mostly be driving 8 ohms. Irritating though that the pinouts of LM3886 and TDA7294 are practically mirrored