Reread the datasheet, but now the small print, not just the large letters (which say "20W")
http://www.onsemi.com/pub/Collateral/MJE340-D.PDF
You'll find this:
which means: "this transistor will dissipate 20W and not burn, IF you can keep the Case temperature (TC) at 25 deg centigrade, no matter what"
The problem is that to keep such case at 25 DegC you must bolt it to a huge aluminum cube, 500Kg would not be too much, and keep it below 25 degC (say, under a water faucet or something) .
Under real world conditions it will dissipate much less than 20W, emphasis on much.
So why do they bother to post it at all?
Because it's useful to calculate the entire overheating, including mica+grease and the actual heatsink used.
EDIT: Hey !!!, BEAUTIFUL City !!!!!
Last edited:
Thank you Ian Finch.Ok i will take a picture of my transformer.Yeah i know it is small heatsink.I will cut myself from aluminuim.Good thing is Mosfets are not getting hot only.MJE350 and MJE340 gets abit hot.I searched for heatsink everywhere,i couldnt find
Sound is very clear and nice to listen im very happy.Thanks for Holton really nice amp
Mosfets are not IRFP9240\240.I mounted irf540\9540 Four 540 and four 9540.IRFP240 too expensive maybe later i will switch output mosfets.Right now it is connected to 3300uF.Can this be reason that im loosing bass?What you see in picture it is stereo.when i turn off right or left chanel then sound is ok no matter how increase the volume.
Sound is very clear and nice to listen im very happy.Thanks for Holton really nice amp
Mosfets are not IRFP9240\240.I mounted irf540\9540 Four 540 and four 9540.IRFP240 too expensive maybe later i will switch output mosfets.Right now it is connected to 3300uF.Can this be reason that im loosing bass?What you see in picture it is stereo.when i turn off right or left chanel then sound is ok no matter how increase the volume.
Last edited:
Just a quick reply:
If the transformer is the same, we know the details already, no need for more detail.
If you have both channels in that tiny block, you are being very silly with the heat dissipation requirements, fan or no fan. Look at how thick the backplate of a commercial heatsink is, in order to spread the heat to perhaps 25 cm wide x 12cm high by 4-5 cm depth (for just one such amplifier ) with convection cooling.
I'm sure you understand that IRF540/9540 are not rated to dissipate as much power as IRFP240/9240 - they have smaller heat spreaders to begin with. You could still use them but it is appropriate to reduce the power supply voltage to match the smaller Mosfet power dissipation limit. For example, 55V rails would be much safer and the power may be 125W, still too much for what I think are high efficiency speakers shown in your pic. You haven't said anything about them yet, like the marked power rating or how loud you were playing them.
250W RMS is not peanuts - it's serious power and when connected to 4 ohm loads or less, the power almost doubles and more! This is where simple designs become complex in order to protect and limit damage when things go wrong as they inevitably will when we don't understand the design requirements.
Note: We need to know exactly what you describe in your question but you still haven't spelled out what levels of power you refer to.
If the transformer is the same, we know the details already, no need for more detail.
If you have both channels in that tiny block, you are being very silly with the heat dissipation requirements, fan or no fan. Look at how thick the backplate of a commercial heatsink is, in order to spread the heat to perhaps 25 cm wide x 12cm high by 4-5 cm depth (for just one such amplifier ) with convection cooling.
I'm sure you understand that IRF540/9540 are not rated to dissipate as much power as IRFP240/9240 - they have smaller heat spreaders to begin with. You could still use them but it is appropriate to reduce the power supply voltage to match the smaller Mosfet power dissipation limit. For example, 55V rails would be much safer and the power may be 125W, still too much for what I think are high efficiency speakers shown in your pic. You haven't said anything about them yet, like the marked power rating or how loud you were playing them.
250W RMS is not peanuts - it's serious power and when connected to 4 ohm loads or less, the power almost doubles and more! This is where simple designs become complex in order to protect and limit damage when things go wrong as they inevitably will when we don't understand the design requirements.
Note: We need to know exactly what you describe in your question but you still haven't spelled out what levels of power you refer to.
Last edited:
A To126 is usually 10W to 20W rating.
With a heatsink they can dissipate 1W to 5W and leave some margin to handle the audio signal.
Without a heatsink they can dissipate 100mW to 500mW and leave some margin to handle the audio signal.
If the Designer has DESIGNED the circuit to work without the added cost of a heatsink, then believe him.
If it is a cobbled together contraption, then someone should check the competence of the assembly.
With a heatsink they can dissipate 1W to 5W and leave some margin to handle the audio signal.
Without a heatsink they can dissipate 100mW to 500mW and leave some margin to handle the audio signal.
If the Designer has DESIGNED the circuit to work without the added cost of a heatsink, then believe him.
If it is a cobbled together contraption, then someone should check the competence of the assembly.
See a second set of TO-126 on that board. He is most likely using a driver
running the latfet in EF2 "style".
This would allow for the VAS to run @ 3-5ma (100's of mW) instead of W's.
Anthony's (your)amp drives the latfet gate capacitances directly requiring more wattage.
OS
I said "100's" of mw , actually 350mw @ 70V. But running my EF3 - >14K impedance ...
Running those gate capacitances might be a tougher job than running an EF3.
Mje 340/350 or 2sc3503/2sa1381 running 5ma = barely warm.
PS - at 7-10ma (forum's badger amp) , the to-126's will get warm... requiring a small heatsink.
-like a <10C/w PCB mount or 25 X 75mm AL plate....
OS
Running those gate capacitances might be a tougher job than running an EF3.
Mje 340/350 or 2sc3503/2sa1381 running 5ma = barely warm.
PS - at 7-10ma (forum's badger amp) , the to-126's will get warm... requiring a small heatsink.
-like a <10C/w PCB mount or 25 X 75mm AL plate....
OS
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.