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Old 2nd April 2002, 08:34 PM   #1
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Location: Boston, MA
Default Increasing power output of Rod Elliot amp

Hi, I'm new here and interested in building an approximately 200w/4ohms power amp primarily for floor monitors in a small PA system.

I have extensive DIY experience with tube guitar and hi-fi amps, guitar effects units and mic preamps. For what it's worth, I have the Slone book, the JL Hood book, and have studied the Leach amp and Self's Blameless design.

I built Rod Elliot's Project 3 a few years ago, and was very satisfied with the results. Compared to the "Blameless" and Leach designs, Project 3 is a lot simpler and cheaper to build.

Rod has a 100 watt guitar amp project on his site (Project 27). The power amp section is basically the same as Project 3, with a protection circuit added. He has PCBs available for this project.

My thoughts:
1. Increase rail voltages to about +/-50volts. I will use a chassis from a Gallian Kreuger bass amp that has a 70vct transformer.
2. Replace the MJE3055/2955 with something like MJ10003/4.
3. Increase bootstrap resistors to 3.3k
4. I'll probably use MJE340/350 instead of BD139/140
5. Most amps have a small inductor and damping resistor on the output. Should I add this? Rod says you don't need it, but the amp will be driving fairly long cables into cabinets that may potentially have piezo tweeters.
6. Recalculate the resistor values in the overload protection circuit. Actually it might be best to modify the protection circuitry to a "multislope" style as found in the Slone book.

Any reasons why this wouldn't work? I realize there are several designs in the Slone book that would work, but I do not like the higher complexity. Again, the intended use is to power some floor monitors, so I don't feel I need anything spectacular.

Thanks,
Mike D.
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Old 2nd April 2002, 09:29 PM   #2
djdan is offline djdan  Romania
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Join Date: Dec 2001
Location: Romania
If you replace power devices with 2 pairs MJ21193/MJ21194 or MJL21193/21194 you can run safe at +/-50V but you must recalculate protection circuit for 10A . If you have a very good power supply ( 400VA toroidal transformer and 8 x 4700uF/63V ) the amplifier can run very well at 190-200w/4ohm).

Just an opinion: My recomandation is to use for drivers 2SC5200/2SA1943 if you want to push very hard and Rod Elliot's simple limiter.This combinations give better results for PA use.

Another posibility it would be to make two amps and run at +/- 28-30V in bridge mode . You will have one big advantage : no need to match power devices becouse it can be used one pair MJ21193/MJ21194 for each amp.

A small inductor and damping resistor on the output it is a very good idea for PA use.
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