Audio Amp Resistor and Transistor

[amdkicksass]

Audio Amp Resistor and Transistor & Heatsink ?

Hey guys i am building this amplifier and while i ran into a few problems here and there, i have ran into a problem that i cant seem to trace or reason.

The amplifier circuit i am build was the ESP P3A
**broken link removed**

The problem at the moment i am having mainly is resistor R12 gets so hot that it burns and smokes. I originally thought there was something wrong with the transistor placement but everything seems correct as i have checked a few times.
An interesting observation, is i get sound from the output which is actually pretty decent, the LED blinks to the sound, Transistor Q7 gets a bit warm but i believe thats normal as its the output driver. Interestingly transistor Q8 is rock cold not even warm a tiny bit, as a output driver i was wondering why its not even warm- maybe that linked to the burning resistor.

Any thought or suggestion would be helpful.

Thanks
pete

 

[TrevorP]

Your using a resistor with proper watt rating I assume?

 

[amdkicksass]

Well the article specifies all resistors are 1/4 watt except a few in which this isn't one of them.

Another interesting observation i made is that the amp operates weather Q6 and Q8 are there or not. That leads me to beleive that whole "stage" on the bottom right of the schematic is not operating correctly.

I am not sure why this is happening ?

Also i am correct to believe that R11 & R12 is 220ohms, i ask because the R isn't there.

??
Thanks
pete

I have added the schematic here for ease. Courtesy ESP.
**broken link removed**

 

[Nigel Goodwin]

If R12 is smoking then something is VERY wrong!.

Bear in mind most faults in DC coupled amps will blow transistors all over the place - so many components have probably being killed.

 

[audioguru]

R12 has the base-emitter diode of Q8 limiting its voltage to only 0.7V. Then R12 dissipates only 0.002W.

I think Q8 is connected backwards or it is blown up.

 

[amdkicksass]

Ok i will try rebuilding the circuit tonight with new components. A question i had is the LED, can anyone tell if the led brightness should fluctuate with the audio or should it just be one steadily ?

Thanks
pete

 

[ronsimpson]

Problem: Q8 cold, R12 hot, D1 LED flickers.
Q8 is wrong type NPN/PNP or bad. (The voltage across R12 cannot be more than 0.7 volts if Q8 is good.)
D1 LED should not flicker. Maybe the –35 volts is not good.

 

[amdkicksass]

Well guys i am happy to say i rebuilt the circuit with new components and it works well. The LED is on and both transistors get a bit warm, so i guess that means they are doing something.
My next problem is that i am using a TO-220 heatsink on them at the moment and i beleive they re getting a bit to warm for my liking ? I am limited to room on the sides of the transistor but anything that goes up will be perfect.

This is what i am using at the moment..
https://www.radioshack.com/product/...origkw=heatsink&kw=heatsink&parentPage=search
TO-264 Dimensions..
https://www.google.com/url?sa=t&ct=..._Ji4eru56BO8JWGkg&sig2=us5v0KQ7weT6tGW4l0gtgg

I cant find anyone who selss heatsink made for the TO-264 package, its possible that i am just looking in the wrong places or am searching for them incorrectly.

Thanks
pete

 

[audioguru]

As usual, RadioShack doesn't have any spec's for the heatsink.
1) What is its thermal resistance (the number of degrees it rises per 1W of power).
2) What is its size (so we can compare it with heatsinks the same size that do have spec's).

It is probably much too small and is rated for only 5W.
The article says that a heatsink that has a thermal resistance of 1 degree/W can be used for two amps. It says to use a fan if the amps are driven hard.

The amp has an output of about 66W into 8 ohms or about 120W into 4 ohms. Then its dissipation is 53W or 96W. Then each output transistor dissipates about 26.5W or 48W at max continuous power.

 

[Nigel Goodwin]

Yes, use good large heatsinks! - and fans if you're running them hard.

 

[amdkicksass]

The dimensions of the heatsink from radioshack are:
1.5(L)X1.5(W)X2.5(H) cm

I think something with the same width and length but a bit taller would do the job just fine.

Thanks
pete

 

[audioguru]

You have a dinky little heatsink rated for about only 4W. You need a BIG heatsink.

I looked at heatsinks at Digikey. They don't have big ones.
Then I looked at manufacturers Wakefield and Thermalloy. Their standard heatsinks are too small.

They have aluminum finned extrusions that are sold in long lengths that amplifier manufacturers cut to the required length.

 

[Speakerguy]

There are some extrusions on Ebay that aren't to expensive.

 

[kchriste]

Or find some old computers and scam the CPU heatsinks with fans.

 

[Sceadwian]

I've snagged some pretty hunky aluminum heatsinks from moderatly old monitors.

 

[amdkicksass]

I kinda was hoping to find something already with a hole in and cut to size ???

pete

 

[audioguru]

Wakefield still sells many heatsinks. www.newark.com has a good search engine for them. I found one that is much bigger than yours and it might be able to hold both your output transistors.

It has a thermal resistance of 2.4 degrees C/W and the transistors have a thermal resistance of 1 degree C/W so with 50W of output power both transistors will dissipate about 40W and their internal temperature will be about 161 degrees C.

 

[amdkicksass]

Hey audioguru can you link me to waht you fond on newarks site ?

Also what do u guys think of these if i just drill a hole thru them ?
https://www.allelectronics.com/cgi-...SINK,_2.59"#34;_X_1.20"#34;_X_0.37"#34;_.html
This place is local to me, so its easier ?

pete

 

[audioguru]

Go to www.newark.com and enter Heatsink in the search box.
On the search page highlight a thermal resistance of 2 degrees C/W and highlight the manufacturer Wakefield.
Click on Apply and you woll see that heatsink that is much bigger than the tiny ones you found.
Click on the link to Wakefield if you want to see all the heatsinks they make.

Your new heatsink is too small for the output transistors in a power amplifier.