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Old 19th June 2012, 05:31 PM   #11
AndrewT is offline AndrewT  Scotland
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Seems very much like fakes.

All BC without the L are CBE.
The BC L versions are ECB, the same as 2sa/sc and the same as To220, To247 & To264
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Old 19th June 2012, 08:39 PM   #12
denry12 is offline denry12  Estonia
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Amp runs just lovely. Once again, thank you all for your help.
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Old 20th June 2012, 01:49 AM   #13
wahab is offline wahab  Algeria
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The MJE3055 & MJE2955 heatspreaders are way too small , even with +-12V
supply voltage , Q9 should be in thermal contact with one of the heatspreaders.
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Old 20th June 2012, 04:26 AM   #14
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Quote:
Originally Posted by wahab View Post
The MJE3055 & MJE2955 heatspreaders are way too small , even with +-12V supply voltage , Q9 should be in thermal contact with one of the heatspreaders.
That's a real problem. With heatsinks about 5% of required rating, it will soon be toast with any kind of output transistor
Actually, as Sakis has demonstrated, placing the bias transistor alongside or on a driver transistor gives a better tempco and safety for P3a CFP design.
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Old 20th June 2012, 09:52 AM   #15
AndrewT is offline AndrewT  Scotland
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the P3a has a CFP output stage.
The temperature compensation should not be fitted to the output devices. The temperature compensation must be fitted to the two driver devices.
Placing all three in a row and coupling them to a strip of copper ot aluminium should be sufficient.

A CFP output stage can run with very low crossover distortion with as little as 5mA of output bias current. The drivers could have a similar bias current, but more usually a bit less (2mA to 3mA the same as in the sch.) for a total output Pq of 120mW @ +-12Vdc. The driver Pq total is ~70mW

The heatsinks on the output devices are perfectly adequate for +-12Vdc and music reproduction during testing.
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Old 20th June 2012, 01:21 PM   #16
denry12 is offline denry12  Estonia
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Currently I can't complain about insufficient cooling. However this is because I am trying to end up with an amp that runs off of 13.8V - I made the power supply quite poorly (whipped something up quickly) and it heats up a lot more than the amp (as a sidenote, amp and power supply each have a fan blowing on them (2 fans!)). Apparently the part that heats up the most in the PSU is the transformer. Probably unsuitable amount of turns for that specific operating frequency. But this is not exactly a talk fitted for this sub-forum anyway. When I get more spare time I may rebuild the PSU and try to do it properly this time.
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Old 20th June 2012, 06:03 PM   #17
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Ian Finch View Post
Actually, as Sakis has demonstrated, placing the bias transistor alongside or on a driver transistor gives a better tempco and safety for P3a CFP design.
If the drivers heatspreaders are little enough such that their
temp is higher than the one of the power devices heatspreader
sure that it will work better....

But not in the particular case of the amp above, isnt it ?...
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Old 21st June 2012, 02:36 AM   #18
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Quote:
Originally Posted by wahab View Post
......But not in the particular case of the amp above, isnt it ?...
I take the point that this driver stage needs no heat spreader at all. However the tiny output transistor spreaders fitted will likely cycle wildly and indeoendently in temperature and the bias with it. Fan cooling will add to the variables. I would seriously rethink the assembly, parts and compensation arrangements for the OP 50-100W version that surely should be the model for this test assembly.

Sensing the output transistors is notorious for problems with unsuitable tempco on CFP designs. It's a matter of the slope which only need compensate for half the number of junctions of an EF2 design, where the drivers are forced to track the main heatsink temp. The problems were evident with commercial products and it is better, I think, to go with the designer's scheme for DIY builds. According to D. Self also, the drivers are the way to go, and just one driver transistor and tiny spreader may be sufficient here. Further, I can't see that it's necessary to use both devices to provide the same information - unless the notion of symmetry is more important.
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Old 21st June 2012, 04:35 PM   #19
Vostro is offline Vostro  South Africa
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Interesting, I'm new to Audio, but I agree with what I think I understand
from what AndrewT is saying.

From the attached drawing, It seems that EF output amp bias is affected by Drivers, but mainly heavier biased and warmer final outputs.

Its my opinion from looking at the circuit,
am I must state again, I don't have experience in this, so its only an opinion,

That the CFP output's bias is mainly influenced by the temperature differential of the driver pair, at least for bias transistor to be in thermal contact with drivers.

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Old 21st June 2012, 05:31 PM   #20
AndrewT is offline AndrewT  Scotland
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All the output devices change temperature from start up to warmed up quiescent to hot just after a high power series of transients.
Theoretically all these Vbes need temperature compensation.
From what I recall Self said that the the outputs of a CFP stage required only 1/6 of the tempco compared to the drivers. He and most others choose to ignore this small tempco requirement and concentrate their efforts and the very much needed tempco of the drivers.

I have a couple of amplifiers where the two drivers and the Vbe mulltiplier, All To92 devices are attached to a common heatsink. These two designers appeared to have agreed with Self's observation.

On that basis, the CFP on the right is correctly shown with the tempco around the Vbe + drivers.
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