blow transformer

[ricrado]

HI everyone;

I new on this and I am going crazy.
i made a gainclone .It sound ok very nice no hum at all.
but everytime i put it on the transformer get very hot eveen with no sound at all.
I blew two of them in .
any suggestion.
i look at the boards and I do not see anything in short cut.The last time I take and amperimeter
and it was going up until 4.5 amp.
we use here 110v.
any ideas?
thanks ricardo

 

[Netlist]

Ricardo,

I think your amp is oscillating. You need a scope to check.
How big (VA) is your transformer?

/Hugo

 

[ricrado]

what do you mean sscilating"
And what is doing this?

The transformer is 20+20 .
The rare thing is that the sound is good and there is no Humm.
do you think it can be a bad chip?
ricardo

 

[Netlist]

In simple terms, an oscillating amp is producing high frequencies that you cannot hear.
If it puts much of his energy in doing that, a lot of current is running all the time even when no music plays.
A common cause is too long wires in the signal path or a bad board layout. Can you post a schematic and a picture of your amp?

If it plays fine, the chip will be fine, too.

/Hugo

 

[pinkmouse]

I'm with Hugo, definitely sounds like oscillation. What is the current rating of your transformer?

 

[runebivrin]

Well, if the chip doesn't get warm, it's probably not oscillating. Could just as well be an incorrect rectifier connection.

 

[Netlist]

Indeed, that is why a picture, showing the heatsink size could be handy. They could be massive.

/Hugo

 

[pinkmouse]

Whenever I wired up a rectifier incorrectly, the failure was usually instantaneous.

 

[BWRX]

I had a similar instance once and believe it or not a power supply capacitor was bad and measured a low impedance. The amp worked and sounded ok but the rail voltage was low and the supply got very hot because the bad cap was a low impedance across the supply rail.

 

[pinkmouse]

Yup, never happened to me, but that certainly sounds possible. Of course, it could also be a reversed electrolytic...

 

[Netlist]

Of course, it could also be a reversed electrolytic...

In that case, it wouldn't have survived two transformers.

/Hugo

 

[BWRX]

That too. Polarized electrolytics usually explode if subjected to reverse voltage, especially if the cap's voltage rating is close to that of the rail voltage. If the rating is substantially higher they can take reverse voltage for longer than you'd expect!

 

[Peter Daniel]

Reversed electrolytic is usually instantaneous as well.

Does the transformer get hot without amp circuit connected (just rectifiers and smoothing caps)?

 

[john65b]

My guess is that the Transformer is wired incorrectly to the PS board (Center tap vs dual secondaries).

I ran into this problem this first time I built a BrianGT GC. On the instruction on the LM3875, it goes into detail on how to wire up either a CT or dual secondary, while on the LM3886 instructions, just the dual secondary wireup is mentioned. Dual secondary requires two Full Bridge Rectifiers while CT only requires one, so half of the eight MUR 860's are not used....

If you have a high enough fuse or no fuse at all, maybe the transformer may slowly burnup??? .

Do you have a Center tapped or Dual secondary tranny?

 

[Nordic]

I think you are on to something.... my money is on him haveing no fuses... not that that is the problem... but anyhting that potentialy eats transformers is bad news... yes you need fuses to protect your transformer too...even if your fuse was way too big, it would probably have burned out in about 10 seconds protecting the transformer.

 

[ricrado]

thanks for your ideas.
I have a fuses but the transformer keep blowing out.
I think that the problem is as john65b said that I have the 3 times a ct transformer.
I looked the LM 3875 manual and I do not understand clear.
I think that I have to use only one of the secondarie + and one - ( acin)
is this correct.?
Thanks in advance it is been very helpful the forums and your opinions.
Ricardo

 

[Magura]

Ricardo,

We need some info if you want us to help you.

First and foremost, the brand, and product code of the transformer you use. Second a photo of the setup. If you provide those two things, you will most certainly get around this problem of yours.

Magura

 

[john65b]

If you only have three wires on your transformer secondary, it is center tapped and you would only require one full wave bridge rectifier.

If you have four wires off your secondary, you hvae a dual secondary, and you can use two or one full bridge rectifiers

In order to help, need to see a pic or diagram of the power supply circuit. Is this a BrianGT or Perter Daniel kit?

 

[Nordic]

Remember to measure your speaker impendance with a multimeter... I have had instances with thermal runaway, due to a speaker that must have developed a short in the coil... measuring something above 2.5ohm instead of the 6. something you'd expect from an 8 ohm speaker... I remeber ripping that PCB up probably 3 times before I zoned in on the problem...

Useing an LM3875, the chip can handle i nthe area of 7A (too lazy to check it up again right now... but it is in that vicinity...)

the system will fry the weakest link in the chain... if your transformer has a lower current rateing than the chip.. the chip can easily outlive the transformer during an error condition...

I would at a minimum use a 160VA transformer... and prefferably something in the 250 to 300VA range

Of course if you make a mess with the diodes I suppose it could increase the workload on the transformer... but I'd expect it to blow pretty fast...

please tell us more about the tranformer connections