16ohm/4ohm ? output overload?! please help

[Morbid]

Hi i've built this couple of monthes ago and it worked well
(schematic included) based on STK4046XI (same pinout)
the problem i have is when i use the amp for a while the resistor (mentioned in the schematic) is burned...twice and also the output fuse (before the speaker)...now i use 4OHM configuration (2x8ohm 50w in parallel) ...it is better for the amp to be 2x8ohm in serial ? 16ohm? less load on the output resistor?
i think maybe i need to change him to 4.7ohm 9W resistor....
i dont know what to do
please help!!

[Vikash]

my first guess would be that 2w is no not enough.

[Stuart Easson]

Hi,

A likely source of a smoked resistor in this position is unintended oscillation of the amp. At a high enough freq the coil will have a high impedance relative to the resistor, and the current in the load will end up going through the resistor.

You should consider using a scope to monitor the output before connecting more speakers, or you may end up losing more than the resistor and fuses. Long, highly capacitive cables will make the problem more obvious.

Check your layout and schematic to make sure you have all the small compensation caps in place and wiring is clean and direct.

Good luck

Stuart

[Stuart Easson]

All the datasheets I have for the 40xx series of chips show the same network on the output, but they have a 3uh inductor, not 4, this could make a small difference. The wire the inductor is wound with needs to be heavy so the DCR of the coil is negligible, as in 0.5-0.8mm or so, normally the coil is a number of turns around the body of the resistor. At audio freq the coil is supposed to be a dead short...

Stuart

[Morbid]

should i change this ? :

* bigger cables for the output
* 4.7ohm 9W resistor
* 10 turns around the resistor with isolated metal-wire
( i saw somewhere this gives u between 3-4uH)

optional changes:
* change the 4ohm output load to 16ohm ? (parallel to serial)?
* reising fuse ratings ?

checks:
* check the Voltage of the output on load?

thanks!

[Stuart Easson]

If you go for a 16ohm load, you'll get half the power in the output, 4 ohms will give you twice as much, relative to 8ohm, assuming the wower supply you have can supply the additional current... ie 16ohms->100w, 8ohms->200w, 4ohms->400w. Power supplies all have some internal resistance, causing voltage to drop somewhat with current which will give you less than the ideal doubling.

For 300W+ RMS into 4 ohms you are looking at >8amps RMS, choose your fuses, and all associated wiring, accordingly. Higher impedance speakers will demand proportionally less current...

Using a higher power resistor cannot hurt, try and make it non-inductive, and there will be a larger body to wind the inductor around.

With no signal look for any waveform on the output that would be indicative of oscillations, also check for heating of the module, warming of the output inductor/resistor, then before connecting a speaker apply an input signal and see if anything warms up, with no output load there should be no heat in any of the output network components.

Have fun

Stuart

[carlmart]

Originally posted by Morbid
should i change this ? :

* bigger cables for the output
* 4.7ohm 9W resistor
* 10 turns around the resistor with isolated metal-wire
( i saw somewhere this gives u between 3-4uH)

optional changes:
* change the 4ohm output load to 16ohm ? (parallel to serial)?
* reising fuse ratings ?

checks:
* check the Voltage of the output on load?

A few things you state are a bit confusing. What is in parallel? Your speakers? What kind of speakers are they: tweeters, mids, woofers? What size if woofers?

Some amps have trouble handling 4 ohms loads, and you shouldn't use those at maximum level.

Some woofers are specified as 4 ohm, or even 8 ohms, but have dips that get to 3 ohms or lower.

Fuses shouldn't be changed, except if you know what you are doing, or you may blow the amp. To do that you should know your load quite well, then measure currents and clipping points on a scope.

The output resistor is specified as 2w, so if it burned perhaps it was due to oscillation. Can you have a look at the output with a scope? Does it burn with music or just by connecting the speakers?

If you increase the resistor's wattage and the series fuse you may blow the speaker.


Carlos

[Morbid]

ok first the speakers are all-tone (for guitar use)
2x50W...

now the amp is very strong (120W@8ohm) so 60W will be enough...

thats why i dont care changing the speakers to 16ohm configuration (8ohm + 8ohm in serial ) instead of 8/2 in parallel...

the module have a major heat sink that doesnt get anymore than room temp... even after long time. its working alright...

[Stuart Easson]

...I think you have your answer. The amp and it's power supply will be less stressed driving 16 ohms. If the speakers are identical they won't alter each others sound too much and if they are physcially close together you will get a sensitivity increase as well.

Be careful about the wiring internal and external. If the amp begins to oscillate it can destroy itself and the speakers attached very rapidly. For 60w RMS into 16 ohms you only need 2 amp fuses, keeping these small will protect the amp and the speakers if the worst should happen.

Stuart

[Morbid]

thanks i will keep updating from the surface