Modify P3A

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TedPolzin

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Joined 2002
#1
Hi I wanted to look at making some mods to Project 3a and I wanted your opinions.

I want modify it so it can support 90-100watts with 8 ohms but still be able to handle a 4 ohm load. This would basically be a +-31 toroid.

If I replace q7 and q8 with mj21193/4, and prehaps ran them parallel. Replace q4,q6 with MJE15030 and q5 with MJE15031.

Would I have to make any other mods? Would q1,2,3,9 still be able to run bc546? Or should I replace that one as well?

The link to the project with schematic is

http://sound.westhost.com/project3a.htm

Thanks a lot

Ted
 
#2
I'd be a little careful about pushing this amp too hard. There's no protection in the design.
How about building 4 boards, and bridging 2 per channel to drive the 8-ohm load? For 4 ohms, just connect one end of the speakers to ground (ignoring one of the amp outputs per channel).
 
#3
I have built three 3A's running @ +/- 42 volts. I have two teenagers that tend to play them loud and forget to turn them off. There have been no problems. They were built about 5 months ago. Two have MJL1302, 3281 transistors, one has 2sa1386 and 2sc3519. I don't think you have to change the driver transistors, just put heatsinks on them.
 
#5
My speakers are 6 ohms, but I didn't put any heatsinks on the drivers. If you are going to run them @ 4 ohms you should. If you are going to run high volumes for long periods into 4 ohms it will shorten your amps life. Check with Rod.
 
#6
I built a 4-channel P3A with 42 v rails. Running NC Borealis on two channels and two subs (4 ohm subs) on the other two channels (I had forgotten they were 4 ohm speakers). The two sub channels died and I have had to replace a few components. I

4 ohm speakers should be avoided with the P3A amp.

Steve
 
#8
Hi,

"My speakers are 6 ohms, but I didn't put any heatsinks on the drivers. If you are going to run them @ 4 ohms you should. If you are going to run high volumes for long periods into 4 ohms it will shorten your amps life. Check with Rod."

The P3A amp should be OK for full power into 4 ohm loads with an additional set of output devices, as detailed in his original P03 article:

http://sound.westhost.com/project03.htm

Hope this helps,
Tim.
 
#9
ThingyNess-
When I ordered the pcb's the assembly list used 2sa1492 and 2sc 3856. These are what I used. I have also bought some of the other output devices mentioned on Rod's website but have yet to implement these.
Steve
 
#10
i built the 3a with it bridge and it run under 4 ohm due to bridging. No explosion, smoke or funny sounds. Make sure you heat sink the driver transistor and add more transistor like rod's advice. I used 2 sa1302 and 2 sc3281 per channel so for the bridge amp, total of 8 transistor and the emitter resistance were used ala Rod's advice. Follow the advise and i don't think a problem can occur.

PS
Rod specifed 35v for 4 ohm. mine wer 38 v due to usage of a salvage trans. work fine
 
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