Blowing Resistors in BLS pre-amp

Ok, this may be a trivial question and wonder if others are have the same problem

Lately when ever I turn the power on in the pre-amp, input voltage resistors R16 & R17 (both 22.1 ohms 1/4w each) blow up and smoke away - sometimes leaving a black stain on the pcb.

Now I believe the problem is caused by running higher voltages to the main board - roughly around 70 VDC instead of the 60VDC in the schematic (just had to be different by running higher voltage for lower distortion).

Will have to replace with higher wattage resistors say 1 watters? Or should I go with 3 watters?
 

paulb

Member
2001-06-01 4:53 pm
Calgary
Resistors can generally handle more than their power rating. To blow up and smoke away means a lot more than 1/4 W is being dissipated.
The article says 80 mA per channel; this should be about 1/8 watt in each of these resistors.
Something has gone wrong with the circuit. It sounds like the 22 ohm resistors are directly across the supply voltage. If so, putting in 3 watt resistors will smoke them too.
 
paulb,

I forgot to mention that the pre-amp runs perfectly when it powers up fine (for several hours). Running from a Variac at a much lower voltage also runs flawless. It's just that the odd time when I run straight from the wall outlet - the resistors blow when switching the pre-amp on. Just like when you flick on a light bulb and it blows right that second.

Also note that 80ma draw is assuming that the mosfets drains are at 30VDC. The mosfet drains on my pre-amp is 40VDC which is 1/3 more and would mean i'm drawing a LOT more current than 80ma.
 

grataku

Member
2000-12-31 9:31 am
-
Super_BQ
ever thought about changing your name to Super_BBQ?

I say better for the res. to blow up than the PS mosfets. How about ramping up the voltage slowly to the full 70V using the variac? Do they blow up then or they just get real hot or they look fine?
You could measure the current drawn at 60, 65, and 70 to find out if and when the bias current gets to alarmingly high values.
Maybe it's a combo of caps charging, higher bias current to cause the explosions.

Good luck and make sure to wear goggles while you listen to your stereo.