My son and I are building a pair of monoblock amps from Ron Elliott project 101 as our first DIY project. They are small, relatively inexpensive to build and easy to start up and dial in. Before I buy the boards and components, I want to be sure that the design is appropriate for my application. We are building it for fun mostly, and if it stinks he can use them in his bedroom for his guitar hero music. But my hope is that they will sound as good as Mr. Elliott claims, and I can use them in my hifi system at least for a while
. I have written to Ron, but not heard back yet, so I thought I would ask the geniuses here on the BIG AC
My biggest concern is about the current output of the amp for my particular speakers, Legacy Audio Focus 20/20. Here is impedence/phase curves per freq:
http://www.stereophile.com/images/archivesart/LEG20FIG1.jpg. As you can see, it is brutal in a couple places, especially near 25Hz. The speaker freq response is 26Hz - 20kHz+/-2dB. They will play low, but with such a tough load in the bass, they need a lot of current. My Cary SLI-80 tube amp sounds wonderful higher up but is almost silent below 45Hz on test tones. If you have never heard these speakers, they are worth whatever it takes to get them to play, making any music sound huge, natural and exciting. I have only heard one other speaker match them, costing 6 times what I payed for these. No, not Wilson MAXX2, haha... As reviewed in Stereophile, Paul Bolin called the Focus 2020 "the finest value that I have ever encountered in high-end audio." Feeding these monsters is story. I have heard commercial amps I would like to own, like Butler Monad 100s, but saving up for those is a separate project.
The Elliott MOSFET amp design uses 2 pairs of Exicon lateral MOSFETs per channel:
http://www.profusionplc.com/pro/gex/pcatdtl0?ipartno=ECX10N16. Here is the schematic of the amp:
http://sound.westhost.com/p101-f2.gifI intend to use one 300VA (or larger) tranny for each amp. I am trying to decide what I should use for rail voltage to the amp. I have recently learned that a lower rail voltage will allow greater current to the speakers than running a maximum specified rail voltage. The amp design specifies +/-70V maximum when using dual pair of output transistors, but suggests using +/-56V for higher current applications. Says Ron of the low power version with only one pair output devices per channel, "using only 1 pair of MOSFETs, it is essential to limit the supply voltage to +/-56V so that it can drive both 4 and 8 ohm loads without excess dissipation....Naturally, dual MOSFET pairs may be used at this voltage as well, providing much better thermal performance (and therefore cooler operation), far greater peak current capability and slightly higher power."
I listen to acoustic music, chamber and symphonic classical as well as small combo and big band jazz so dynamic accuracy is important. I also listen to decent rock and pop recordings when I can find them. We listen at natural levels, similar to live performance. This is pretty damn loud with symphony stuff, but still mostly on the 1-2 second peaks. The speakers are relatively efficient at 95 dB/W/m, and so half volume setting on my Cary is usually enough to fill the 10,000+ cubic foot listening area nicely, so high continuous power is not so important as musical dynamic peaks and low frequency levels being preserved.
I would like to use the amps full range, but might use them just on the lows if they don't do highs as nicely as my current tube amp.
My questions are all related to using this amp design with my speakers:
1. Should I use +/-56V rails for greater current potential into the low end of my speakers, or just go for the max power rating with +/-70V rails?
2. Is Ron's recommended 300VA big enough power transformer for these amps, which are rated as follows:
AC Volts DC Volts VA Power (8 ohms)
20-0-20 +/-28V 100 40
25-0-25 +/-35V 100 50
30-0-30 +/-42 160 80
40-0-40 +/-56V 200 150
50-0-50 +/-70V 300 240
The amps ratings are 180W RMS into 8 ohms, and 250W into 4 ohms with +/- 70V rails, with peaks of 240W into 8 ohms and 380W into 4 ohms depending on power supply reserves. He warned to use these figures as minima when using the amp for subwoofer duty, which I think Focus qualifies for, with three 12" drivers per speaker and 26Hz at -2dB.
I plan to build the power supplies based on the TNT Audio article
http://www.tnt-audio.com/clinica/ssps1_e.html, which gives a good design and some advice on sizing transformers. That article recommends 500VA per side. Any other good references you can recommend on Power supply design for a beginner?
3. Does anybody in US make amplifier cases that are attractive? Not the steel rack spinoffs, but a really attractive extruded aluminum case that I might let my wife see instead of having to build a new closet too. We can do some nice woodworking here, and that's what I plan to do for lack of nice boxes available, but maybe you know some resources I have not yet found. I have seen some nice cases from european dealers, but I like to keep my hobby money onshore.
Thanks in advance for any advice and encouragement on our first DIY project. My son is 8yo and is very interested in electronics. He loves to learn about components and basic circuits. He can't wait to get his hands on a soldering iron! It is worth a few hundred bucks to get him permanently addicted to DIYaudio, don't you agree?
4. Last, and less important, I have never heard a SS amp that can do justice to high frequency signals like bells, chimes, tinkly percussion, as well as accordion and other screechy acoustic instruments. Trumpets and trombones have a lot of that chimey tone and sound more natural on tube amps, IF the recording is excellent. Is it possible that a SS amp of any pedigree can do justice to an orchestral triangle? I am not assuming this project will do that.
Thanks so much for your replies!
Rich
Raleigh, NC
