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Old 4th December 2001, 12:43 AM   #1
Won is offline Won
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Hi all. I'm about to start on some of my first DIY projects, including Rod Elliot's 100W amp (project 3A). I've got the PCB, and am about ready to start ordering parts. Any suggestions for good part substitutions or other nuggets of good advice on this project?

Thanks, Won
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Old 4th December 2001, 12:56 AM   #2
paulb is offline paulb  Canada
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I'm building it too, and have used substitutes for all of the parts. I'm using 2N3055/MJ2955 output transistors.
Rod's construction notes that he supplies with the project are very good. Use oversize heat sinks if you can.
I've built and tested the modules, and am in the process of mounting into an enclosure along with a headphone amplifier.
I suggest using a 10-turn pot for VR1.
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Old 4th December 2001, 06:31 AM   #3
Won is offline Won
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You replace all of the parts? As in resistor values and whatnot? Or did you just use higher quality passives components? Did you also upgrade the non-power transistors?

Thanks, Won
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Old 4th December 2001, 01:36 PM   #4
paulb is offline paulb  Canada
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Sorry, should have said all the transistors. It wasn't a matter of upgrade, just ones I had available. I don't see too many BD139/140s around, for instance, I think I used MJE340-type devices there.
I do recommend using high quality passives, and possibly bypass (parallel) the electrolytic caps that carry audio (C1, C3, and I think C5) with lower-value polypropylenes, say 0.1 uF. I'll try that next.
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Old 5th December 2001, 01:26 AM   #5
bawang is offline bawang  Malaysia
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Thumbs up Substitute parts for Rod Elliott's Project 3A

I used BC550C/560C transistors for the small signal transistors, 2SB649A/2SD669A for the drivers, a 3.3uF MKT input cap, MKT or polycarbonate bypass caps, and as the Toshiba transistors suggested by Rod has been discontinued, I used their new equivalents (also Toshiba; can't remember the part numbers). And the sound? Fabulous! Simple to build too, too me only an hour to stuff the board and install the heatsink.
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Old 5th December 2001, 06:27 AM   #6
Vivek is offline Vivek  India
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I am using BC 547s in place of the BC 546. I managed to get hold of some BD 139 and 140, so no problem there. For the output, I am using MJ2955-2N3055 pair.
Is it okay to use a matrix board instead of a PCB. I hope this will not cause a problem.
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Old 5th December 2001, 02:09 PM   #7
Won is offline Won
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I wonder if there's enough interest for a Rod Elliot forum. It would be quite a contrast from the Nelson Pass forum!

-Won
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Old 5th December 2001, 05:26 PM   #8
grataku is offline grataku  United States
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If you go through all the trouble to make your own amp why not spend the additional 20 bucks and buy the recommended power transistors instead of using crappy 2n3055-mj2955 (which by the way are undersized for a 100W amp)?
The design has been optimized enough to really benefit from the exceptional performance of the 1302-3281.
Toshiba and Sanken should be the only types of transistor used in power amp this day in age. Everything about these transistors but in particular bandwidth and hfe linearity is far superior to any of the motorola mj150xx and 2n-whatever available.
Don't take my word for it, all the major audio designers Dean, Clausen, Aloia, Self, not to mention the commercial hi-end brands are using almost exclusively toshiba and sanken transistors.
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Old 5th December 2001, 10:48 PM   #9
paulb is offline paulb  Canada
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I emailed Rod and asked him about the 2955/3055 and he said that's actually what he uses in his own amp. If you want high power, especially into 4 ohm loads or are bridging into an 8-ohm load, use the expensive ones, otherwise I would use the others. Cheap output transistors was one of the big attractions of this amp for me. There's enough feedback in this design I bet I would be unable to hear the difference.
An alternative would be to use pairs of output transistors in parallel, I believe Rod mentions this somewhere in the 3a or its predecessor, project 3.

Mix 'n' match: I intend to use this design to drive woofers, a Pass design for midrange, and an IC amp to drive the tweeters. Op-amps in the crossover, just need vacuum tubes somewhere to complete the circle. Maybe I'll just light up some filaments without actually having them in the signal path.

Paul
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Old 5th December 2001, 11:06 PM   #10
Won is offline Won
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But how expensive can transistors possibly be? The extra power headroom will probably be worth it, not to mention the potential for additional linearity. I guess feedback makes the latter feature less significant. Today, they may be driving the midranges, but tomorrow they might be bridged into a subwoofer.

Thanks, Won
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