100W Audio Amplfier By Smart Kit

[faizanbrohi]

Hello ,

my name is faizan brohi ,

There is a 100W Audio AMplifier Project in the Audio Section of the projects By SmartKit. The PRoblem That i am facing is that the power supplies neutral wire goes red and then breaks in the PCB Board. Ofcourse this means that the amplifier circuit is short , but i have checked everything and nothing seeems to be short. please can you help me with this , any help on this would be greatful . i have worked on this for 7 days and still no results ..plzzzz...help.......  :'( :'( :'( :'( :'(

 

[audioguru2]

Hi Faizan,
Welcome to our forum. ;D
We have a few 100W amplifiers, but maybe you are talking about this one:
http://www.electronics-lab.com/projects/audio/017/index.html

Its schematic has errors that I corrected before, but maybe its pcb layout is OK.
Which printed wire on the pcb burns?

View attachment 38068

 

[faizanbrohi]

The Power Supplies neutral rail is getting burned out , not the amplifier's neutral connection. i think that there is something wrong at the amplifier end , but the thing is i have double checked every short connection , only one transtor Q6 seems to be short , but otherwise i have checked the DC Input stage . everything seems to be fine... what could be the problem ...  ??? ??? :'( :'(

 

[audioguru2]

There are only a few parts connected to the amp's 0V (neutral):
1) The speaker. Disconnect it and measure that the amp's output is close to 0V.
2) C1 and C9. Maybe one or both are shorted or backwards.
3) R13 through C8. Maybe C8 is shorted or backwards.
4) R28 but it is 1.2k.
5) C6 but it is through R21 which is 220 ohms.
6) R6 but it is 2.2k.

Even if Q6 is shorted, very little current will flow in the neutral without a speaker.
Isn't your power supply +44V, 0V and -44V? I guess you call its 0V the "neutral".

Your power supply should have fuses or circuit breakers for the first tests of a high power amp. I would put 60W/120V lightbulbs in series with both voltages. If the current is normal, they will be a very low resistance when cold. If there is a short, they heat and increase their resistance which limits any fault current.

 

[faizanbrohi]

I have disconnected the 0V neutral lead , nothin heats up except the goddamn 0.47 ohm (5W) R2 And R16 Resistors but the amplifier loads up perfectly , not even the transistors heat up . maybe i shoould place 1K resistor of 1 Watt in parallel with them  ??? as written on the manual.
But is this heating up of the 0.47ohm (5W) resistors normal when everything is fine. ??? ???

I made a discovery that the transistors body where the screw is mounted and touches the heat sink was shorted, so the transistors collector was actually shorted with the heat sink . i corrected that problem by applying a plastic washer in between them . Could this be the reason that the neutral wire actually burned up ????  ??? ???.

now i am putting the goddamn 1 k 2W resistors in parallel with the R2 And R 16 .. ok

 

[audioguru2]

I have disconnected the 0V neutral lead , nothin heats up except the goddamn 0.47 ohm (5W) R2 And R16 Resistors but the amplifier loads up perfectly , not even the transistors heat up . maybe i shoould place 1K resistor of 1 Watt in parallel with them

 

[faizanbrohi]

Thanks for the information ,

Actually how do i in actual connect the power supply in series with the millimeter. can you show me diagrammatically .. plz  . and secondly did you make the 100W Audio Amplifier by Smart Kit Yourself .. thnx... 

 

[audioguru2]

Hi Faizan,
Here is how to measure the current.
I didn't make this amp, I have lotsa amps. ;D

View attachment 38070

 

[faizanbrohi]

Hello again ,

When i add a bulb in series with both the +40 And -40 of 100W/220V , the bulb heats up and there is no voltage drop across the amplifier and only the bulbs , i did also one careless mistake that i put a AC ammeter in series which burned  one capicator at the input . But what does the bulb shows that something is short in the circuit . i think the two resistors have more current flowing , so they may be virtually short , i am going today to buy the stuff ,

1 ohm 2W x 2
1K pot x 2
10uF / 63V x 2
2A x 4 Fuses
3A x4 Fuses
BC548 x 2
BC546 x 2

and by the way i connected the neutral wire , it's working now with the neutral wire plugged , and the problem was that the heatsink was shorted with the transistor's body

 

[audioguru2]

I think the parts list of this project is a mess. Read the note at its bottom: "For R2 and R16 if you don't find a 0,47Ohm place two of 1 Ohm parallel". So the parts list is wrong with 1 ohm/2W for R16.

Use 0.47 ohms/5W for R2 and R16 or two 1 ohm/2W in parallel for both to make your own 0.5 ohm/4W resistors.

The bulbs light with no voltage across the amplifier, so the amp is drawing a current much too high. Make sure that the pot is set to its highest resistance.

 

[faizanbrohi]

The 1K pot is causing some problems no matter how much i adjust it , there is no effect on the bulb. but when i checked the pot with its common at one end and the other two pins one by one , so i found that all of them are short. this means the pot is gone. ??? ???

 

[Decade]

Hi faizanbrohi !

If the pot wiper is at it's end position all pins will be shorted, check if Q7 is shorted between B-E pots don't usually get shorted, when resistors goes bad the value increase or they just go open circuit.
Q7 is supposed to work as a variable zener diode and rotating P1 will set quiscent current

 

[faizanbrohi]

I just want to confirm that when i apply the 1 ohm in parallel with 0.47ohm . it will not heat up and i can set the pot . because i have tried a DC millimeter but it didn't give any reading and it's fuse was broken . this can mean there is alot of current flowing through 40V lead

 

[audioguru2]

I just want to confirm that when i apply the 1 ohm in parallel with 0.47ohm . it will not heat up and i can set the pot . because i have tried a DC millimeter but it didn't give any reading and it's fuse was broken . this can mean there is alot of current flowing through 40V lead

No, don't add 1 ohm resistors. The confusing parts list says to use a 0.47 ohm/5W resistor for R2 and another 0.47 ohm/5W resistor for R16. Four 1 ohm resistors are used by people who can't find 0.47 ohm resistors.

Your amp's very high current is caused by a shorted P1 or Q7.

 

[faizanbrohi]

Today i am going to the electronics market . I think iwiil buy two pots . and by the way q6 is short so that might be the problem . what do you think , i will change the pot today and will change q7 and q6 ok , but i forgot to tell you that i am using BD825 and BD 140 in place of BD829 and BD830 and BDW84A and BDW84A instead of BDW84D and BDW84D , because i could not find the original. ok

 

[audioguru2]

Your substitute transistors BD825 and BDW84A are rated for only 45V! They will blow-up in this 80V circuit! The original transistors are rated at 80V.
Where in the world are you that you can't get common parts?

 

[MP1]

but i forgot to tell you that i am using BD825 and BD 140 in place of BD829 and BD830 and BDW84A and BDW84A instead of BDW84D and BDW84D , because i could not find the original.

What more is there to say? If you are going to substitute parts, you need to compare the specs.

MP

 

[faizanbrohi]

Actually i am getting BD829 and BD830 but they are second hand , and i am only getting BDW84A , not BDW84D . and by the way i live in pakistan.

 

[audioguru2]

The BDW84A will blow-up in this circuit because its voltage rating is too low.

Go to www.farnell.com and look for your country's flag. They have online electronic parts ordering in many countries.
Can't you buy a Smart-Kit there? They are from Greece but I can buy them here in Canada, nearly on the other side of the planet.