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Old 1st February 2004, 06:09 PM   #1
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Question Building P3A Amplifier

I noticed with previous projects that the mechanical design of a project is even impotant as the electronical design. So I want to do it good this time.

This is my design for now. The P3A PCB bought from Elliot is under the TO-3 MJ15003/ MJ15004 output transistors:
Click the image to open in full size.
Any comments?

The rail voltage is 42 volts.
This is the front view of the heatsink for now:
Click the image to open in full size.
Any comments?

I want to use MJE340/ 350 as drivers.

Before I post all my questions here I will tell you for what I'm going to use this amp. I want to use it in my livingroom as Hifi amplifier. And sometimes when I give a party I want to use it as PA amplifier. In my livingroom I will use only 50watt, while I will use the full 100watt on parties. That is why I build in a fan. I can turn it off with a switch on the back if I use it in my livingroom, but when it's time to party, I turn it on.

I want to build the amp into a 19" 2HU rack case of 1.2mm steel plates. Is 1.2mm strong enough for this heavy amp?

Are 2 of these heatsinks with an aluminium plate between them overkill? 0.9K/W EACH! I don't know the weight of these things.

I connected all the GND's to one point of the chassis. And with ground. Is this done correct?

Should I mount the 35A rectifiers like on the drawing?

I want a nice frontplate, so I want a nice switch on it. The only nice thing I can find is a button. I want to turn the amp on by pressing the button for 2 seconds. Than the main relays switch on. Does anybody has a schematic for this 2 second delay? I want a 5V TTL signal.

What do you think about the new Onsemi MJL4281A (NPN) and MJL4302A (PNP) output transistors? Should I use them instead of the MJ15003/ 4?
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Old 1st February 2004, 06:53 PM   #2
sam9 is offline sam9  United States
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The heatsinks sound ok. You probably don't need a fan. In fact, loose the fan if at all possible as no mater how quiet it is, it will eventually come to irritate you unless you hide the amp in a closet.

Leave a bit more space for auxilliary boards such as turn on delay. You will probably find it hard to find a nice front pannel switch that can also handle the current safely - they usual solution is to use the the front pannel swithch to control a relay that can take the current. This is probably safer as well since you can then put a very low value fast blow fuse (<=1/4A) between the mains and the from panel switch - I think this is very often the commercial approach. Note this is IN ADDITION to the big fuses from the mains - not "an instead of".

I'll bet that if you built one amp using MJL4281A/MJL4302A (which are still hard to find an another with MJ15003/ 4 you wouldn't hear the difference under most circumstances. If you were going to use 4 ohm speakers that draw a lot of current, (such as Magnepans) the MJE might even give a bit more headroom.
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Old 1st February 2004, 08:50 PM   #3
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I want to do it like in the image here below, but I don't know how to make the delay circuit. Do you have schematics for this?

I put only a fan in for parties. When the amp is used for 90% 6 hours long. I can turn it off with a switch on the back of the amp.
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File Type: jpg front plate button.jpg (7.2 KB, 933 views)
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Old 1st February 2004, 09:47 PM   #4
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Nagard posted this a while back...simple solution...
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File Type: jpg onoff switch copy.jpg (32.6 KB, 926 views)
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Old 2nd February 2004, 08:39 PM   #5
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Has anyone comments on the design of the amp and the placing of the components? Is the grounding good in the first pictures?
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Old 3rd February 2004, 04:15 AM   #6
sam9 is offline sam9  United States
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A. It may be better if ground/common for the two caps should ties the caps together right at the two caps and then run as a single wire to the star ground. Aupossedly at the point where the common of the filter caps tie together there is a lot of noise - by doing ithe way I've described, the star ground a at distance from this point and thus quieter.

B. It's been recomended somewhere that the ground for auxillary devices such as the Velleman speaker protection go directly to the common point between the filter caps rather than star ground. This helps keep any noise from the protection circuit away from the star ground. I've done it both ways, and it didn't seem to matter.
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