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Old 21st October 2009, 05:21 PM   #1
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Default 20w Class A amplifier!

I'm trying to build a Class A power amp, but I'm facing some problem here, the MJE2955 is keep burning. When I switch it on, the speaker will come out loud humming sound. I have checked everything on the PCB ad the connection, but everything just seem fine and follow everything on the schematic diagram. Can any expert out there tell me what is going on here? Thank you.
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Old 21st October 2009, 05:36 PM   #2
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You probably need to show us some good pictures of your construction.

I took a quick look at the schematic and see no obvious problems. Rod Elliot doesn't publish things that are inherently dysfunctional, so it is likely that some aspect of your construction is problematic.

You are going to see pretty big amounts of power being dissipated in the output transistors, about 30w each, so big heatsinks are a necessity. Each (and probably every) output transistor will fry itself in under a second with no heatsink.

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Old 21st October 2009, 05:48 PM   #3
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I have checked the MJE2955 and measured the temperature, the problem is when I switch on the amp, the MJE2955 did not even get hot, when heard the humming sound, I straight away switch off the power and the MJE2955 already shorted circuit in between B C E.
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Old 21st October 2009, 05:51 PM   #4
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FIRST rule.
Start up all new or modified projects with the mains light bulb tester inserted into the Live mains feed.
SECOND rule.
Do not connect any speaker until after you have proved to yourself that the amplifier is working properly.

Try changing the CCS 0r25 to 1r0 to get you started and able to measure some voltages.
Then we can try helping.
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Last edited by AndrewT; 21st October 2009 at 05:56 PM.
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Old 21st October 2009, 06:04 PM   #5
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and don't forget pictures...

I can only speculate, but my experience suggests that instant failure of the output transistors with no heat means mis-connection...until we can see how things are wired it's all random guesses.

Stuart
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Old 21st October 2009, 07:35 PM   #6
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Quote:
Originally Posted by Matthewong View Post
I'm trying to build a Class A power amp, but I'm facing some problem here, the MJE2955 is keep burning. When I switch it on, the speaker will come out loud humming sound. I have checked everything on the PCB ad the connection, but everything just seem fine and follow everything on the schematic diagram. Can any expert out there tell me what is going on here? Thank you.
first step is to remove all MJ2955. Enhance the Ube resistor from 0R25 to 25R (25 ohms). Now you must have 26mA instead of 2,6A idle current (you have basicly now a preamplifier line stage).
In this mode you can make your troubleshooting (without risc of damage of your pnp power devices) until you have the right dc voltage values at all places. You cannot use as load resistance your hifi loudspeakers in this mode. Instead of this one use a load resistance of 1K. Check out with your scope, if there are unwanted oscillating effects. If it so, you must find the reason therefore in any case. Only about this way you can find out, if the reason for the damaged power devices comes from errors of your voltage driver.
Good luck

Last edited by tiefbassuebertr; 21st October 2009 at 07:47 PM.
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Old 21st October 2009, 08:30 PM   #7
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Make sure that your total supply isn't floating over 60V. Those transistors will permanently do funny things with only slightly more than that Vce.
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Old 21st October 2009, 08:41 PM   #8
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Quote:
Originally Posted by Andrew Eckhardt View Post
Make sure that your total supply isn't floating over 60V. Those transistors will permanently do funny things with only slightly more than that Vce.
If this is from Rod Eliott site - there should be a description of how to test all electrical parameters before connecting a speaker/source for listening test ?

kannan
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Old 22nd October 2009, 10:20 AM   #9
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Originally Posted by kannan_s View Post
If this is from Rod Eliott site - there should be a description of how to test all electrical parameters before connecting a speaker/source for listening test ?
kannan
you are right, here the regarded weblink:
20 Watt Class-A Amplifier
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Old 12th December 2011, 11:58 PM   #10
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Hi there Matt'

I reckon you have probably sorted out that amp by now but I thought I would throw my 'two bob's worth'.

One discouraging thing that I have picked up on my latest amp is a connection between ground and my heatsink - despite using silicon rubber mounting washers on the TO-3P output devices and the other transistors that are mounted on the sink. If I measure the resistance from the sink to ground I can see that one way it is over 2k but when I reverse the leads on my multimeter I get about 30-to 40 Ohms, a quite significant amount of conductivity.

From this observation I get a mental image of output devices 'frying' like your pnp's did.

I know this has turned into another long story, but It may be worth looking at the conductivity of the heatsink itself, I know that some people skip the washers altogether and make a substantial gain in heat dissipation/transfer, but this would need to be factered into the design and construction from square one.

Getting back to my amp, - My thought is that the silicon washers have picked up some aluminium 'filings' from the drilling and tapping of holes inside the enclosure.

Have you/Did you sort that amp out? It looks very interesting an I have a picture of it on my hard drive. I would be keen to hear about it's performance and the power supply.

Cheers and good luck to you Matt.
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