TDA7293 troubles, DC on output

[huricaine]

Hello guys I have been working on a portable stereo(mono but I use stereo for lack of a better word). I am testing rightnow with 4 6v lantern cells. I have the 6vs in a dual 12v configuration. I have the board etched...I have made about 3, one I didn't like the layout because I had too many jumpers, the next I blew the ICs for some unknown reason but obviously the layout was probably bad.

So now I have a circuit ready on heatsink and everything but I get DC on the test speaker when I apply the power. I am using my ipod straight to the input, it has a low 100mv peak RMS output as measured by my oscope. When I have the ipod at max volume and apply power I get the DC which pushes the speaker out and hear VERRRY faint vocals hardly noticeable. When I remove the power you can hear the music good and with decent output like it should be but this lasts a about a second obviously since the power is being removed. I have 4 large 10,000uF capacitors for my rail caps. 2 in parallel per supply side(+/-).

My theory is that the batts do not have enough ability to power the circuit. All I wanted was atleat 5watts or so so I can hear it and know it woks and move on from there. But this dosnt explain DC offset does it? Maybe the batts are horribly imbalanced they are very cheap rayovac batts.

I refuse to believe an IC or two is blown(shorted). These ICs are new. They do not get warm what so ever though. Check attachment for PCB layout. Btw I added a cap for dc decoupling even though the circuit from the datasheet seems to have one...I do so because ESP Elliot sound products says to add one.

keep in mind how I mentioned that I get fine output for a second after removing the power wires(I apply them by hand). This issue seems so familiar on circuits I have had troubles with in the past but I cannot think of what I have done in the past.

 

[4pyros]

Do you have a schematic?

 

[huricaine]

yes here is my Cad schematic: see attached

 

[huricaine]

oops...here

damn my crappy cad program only takes a snapshot instead of exporting the whole image...

 

[4pyros]

Ya that didn't work, try again.

 

[audioguru]

You forgot to tell us how much was the output offset voltage.
Please post the schematic as a normal PNG, GIF or PDF file, not a CAD file.

The MINIMUM supply for the IC is plus and minus 12V so get rid of the weak batteries and use a proper and powerful power supply instead.
Sorry, I didn't look at your pcb.

 

[huricaine]

The only thing I can do is zoom out and capture the small image which is a little hard to make out I blew it up some so it dosnt look pretty. Its probably useless....Anyway I started this thread for some more simple theories..I don't think its the layout to be honest. I am trying a small 50 watt transformer rightnow and will see if it works.

Audioguru I noticed the voltage sagged down to +/- 11v so maybe this is why...before loading, the batts are at +/- 13v.

 

[huricaine]

Well, after connecting my 50va transformer(+/- 12.6v before rectification) I got the same think with no music at all. after removing the power, the seaker sort of resonates for a second, strangely.

 

[audioguru]

Your schematic is .... shall I say it?
A mess of wires, parts and text all on top of each other.

With the power applied, please tell us how many volts and which polarity is the output offset voltage.

 

[huricaine]

btw, I tied the mute/standby to +vcc because it didn't seem to do anything at all using the soft start method shown in the datasheet. I read that the ICs varied to where some simply may not power up with the mute/stby components shown in the datasheet....so I have them tied to +vcc as I was told that it would disable the feature and power it up.

 

[huricaine]

I have 33V across from rail to rail, so +/- roughly 16.5v. Its very strange because when I was using the batts the speaker moved outward. Now with this transformer it is inward. Also, the ICs do not get warm whatsoever...they are the same exact temp as not being powered up.

Since I'm going mono I tied both of the stereo channels from the ipod cable to the input while using the ground for the ground. Is it wrong to put both channels together?(common)

 

[huricaine]

I just figured out that pin 6s are not tied together. I will tie them and then back.

 

[huricaine]

Same damn problem...

 

[audioguru]

The amplifier will play when the MUTE pin is + a few volts or more. If you do not use the soft-start RC parts shown on the datasheet then it will probably make a loud POP when it is powered and unpowered.
Why do you have two ICs connected "modular" master and slave? Do you use the amplifier as a welder?
For the THIRD time I am asking how much is the output offset voltage (and polarity) when it is powered??
It might be bad to short together the stereo outputs of an ipod because the outputs might be destroyed. Connect a 1k resistor in series with each output then connect both free ends of the resistors together at the input of the amplifier.

I wish your schematic will be readable. Maybe you can copy then modify the schematic from the datasheet instead.

 

[bchris]

OK, here is what I believe to be the uploaded schematic
Forgive any errors, but it was next to torture reading this schematic...

I myself see a lot of design errors in there.
In fact,I am suprised it worked at all.

Compare the above with the one in the datasheet.

Merry Christmas to y'all.

 

[huricaine]

You are right audio guru. I really do need to change from parallel. I will be powering a 100w tweeter and a peerless hds at 50w or so. I may split 50 and 50 or 150w/2 in btl mode(bridged). Or use 50w each with 150 total for headroom for eq. When I measured the voltage on the driver I got 100mv which seems small given how far the driver moves out.

In bridged mode I expect to get about 150w after losses and use maybe 200w. What VA rating should I order a transformer in? I was also thinking about designing a buck smps from AC mains down to the voltage needed for efficiency. What do you think?

For smps I have a few varieties of fets I will list and a few already wound toroids, a few cores for inductors and some wire I can list specs on later.

 

[huricaine]

Ill have to check it out later when I get to my computer. It looks similar. What did you find incorrect?

 

[huricaine]

OK, here is what I believe to be the uploaded schematic
Forgive any errors, but it was next to torture reading this schematic...

I myself see a lot of design errors in there.
In fact,I am suprised it worked at all.

Compare the above with the one in the datasheet.

Merry Christmas to y'all.

Actually it didn't work at all. Sorry for the crazy schematic, I just draw quick schematics to translate to pcb layout. Have you guys not seen the data sheet? I can copy and paste the sch image no problem..

 

[audioguru]

The datasheet for the TDA7293 amp has useless spec's.
No graphs. Power output at a horrible 10% distortion only at two very high power supply voltages.

I will do some arithmetic:
1) With a 90V supply its output is 140W into 8 ohms with 10% distortion.
Then maybe its output is 112W (80% of 140W) with low distortion.

2) 112W into 8 ohms is 29.9V RMS which is 84.6V p-p.
The losses are 90V - 84.6V= 5.4V p-p.

3) The 33V supply that is used minus maybe 5V is 28V which is the peak-to-peak output with low distortion which is 9.9V RMS which produces the power of 12.3W into 8 ohms. If two of these amps are paralleled then the output might be 13W.

You can do the same kind of arithmetic for two bridged amplifiers with your 33V supply and 8 ohm speaker. I guess 42W into 8 ohms with low distortion.

 

[huricaine]

I would not use 33v p-p. Maybe +/- 33. I was just using the small supplies incase I made an error so the ICs would have less of a chance blowing up. And to simple determine if everything is working.

The ICs produce about 50wrms cleanly according to users experience, this is for one simple IC arrangement, not bridged, parallel or anything but one IC and a good enough voltage/supply.

I have a 6ohm tweeter and an 8 ohm peerless driver I wanted to use so for some reason I wanted to go parallel but I realize it is useless in parallel.

Today after getting home from work I got my double sided PCB in the mail. It is large and I can cut it into multiple pieces if I need to make more boards due to error or something.

So, set me on the right track, should I bridge it, or run one IC circuit per driver? I would like to have that little extra headroom from bridging since I will get more out of it but making two separate single IC circuits seems so much simpler. If I do bridge it, I have heard that there is a BTL schematic in the TDA 7294 data sheet(this one is 7293) and the ICs are almost the same.