In DIY audio, its hard to find certain parts, the most difficult being chassis and large heatsinks. Over the years I've been stocking up on these whenever I run accross a nice set. After taking a few years off from making amplifiers I noticed a nice case in the garage with massive heatsinks that would be suitable for a diy class A amplifier. I looked around for a good design and found the "ExtremA," published by Hardware Analysis. Here is the link to the Hardware Analysis Article describing this amplifier. The amplifier is a balanced/ bridged design using 4 large Sanken output transistors per channel in Class A to 100 WPC. The design has incredibly low distortion and has fantastic measurements. Fantastic measurements meaning virtually unmeasureable distortion (like Halcro amplifiers). It was hard to find anyone who has built this design so what the heck, I bought the the boards and gave it a shot. Plus, I always wanted to make an amp with those big Sanken output transistors, they are kind of famous.
The first thing to note about the "ExtremA" is that its Extreme! Everything about it is extreme.
There are some extreme parts, but only a few (expensive and hard to find). Also, while you can buy the boards, the parts list supplied by Hardware Analysis is was not sufficently detailed to allow you to just start ordering parts. I spent a lot of time studying the schematics and measuring the circuit board layout and hole distances with a set of vernier calipers to figure out parts specs and sizes. I generated a parts list that will be of great help to anyone who tries to build this. I was unable to get confirmation on the parts so please judge for yourself; so far things appear to be working for me. If you are in USA some of the critical parts are difficult to source- 4 terminal capacitors and thermal insulators for the big Sanken transistors. Extreme parts include J510 and J511 which are current regulating diodes that are in low volume production, rare and expensive. To be fair most of the parts are typical and easy to find but if you don't get everything you don't finish the project, right?Here is my parts list- ExtremA Parts List Excel File (Nov. 2009)
Do not expect Hardware Analysis to provide advice and support. When I was almost done it became apparent that Hardware Analysis had sold their one working amp (or sections thereof) and could not even help with the DC values or startup sequence. I could not find anyone else who has built this. I couldn't even get a response to the draft parts spreadsheet. If you are able to get the boards you will be on your own with this one. If it does not work you are going to have a nice extreme boat anchor. In fact, this appears to be the first working ExtremA in existence (except for the original modules that were tested and sold). I don't mean to be critical, but that is the situation as I found it.
First you have to design the system and take care of the massive power surge that will blow your home breakers when you try to turn the ExtremA on. A soft start circuit is necessary and the power transformer AC wiring needs to be addressed. Then the parts are gathered and the heat sinks drilled and tapped/ threaded.
You see that I spaced out the big Sanken transistors from the board. This was because the heat sinks already had some tapped holes I wanted to use and because it allowed me to spread out the heat in a wider area. It looks ghetto but the temperature is pretty steady at about 54 degrees C.
The design uses a lot of crimp connections, 16 per channel. There are a lot of wires running around, and I tried to keep the AC spearate from the DC and to solder all crimp connections. I tried to braid all wires where possible but it still looks like a birds nest. The soft start circuit will present two CL-60 thermisters in series to the AC inlet to reduce the turn on current surge. After about 3 seconds a big 15 amp relay is energized to bypass the thermisters and directly connect the AC inlet to the primaries of the big power transformers. The two smaller power transformers are not subject to this procedure. The AC inlet is fused with a 5A fast blow fuse, and the smaller transformers are fused with a 2A fast blow fuse. So far there have been no surge/ fuse issues.
From the pics a few obvious issues (other than the messy wiring) are visible. On the left side A copper bar holds down one of the output transistors. I broke a tap in the hole and could not get it out, so this was my workaround. On the right side a driver transistor is not mounted directly into the board, there are jumpers. One of the pads on the top of the PCB came off so its jumpered in (the via and bottom are still ok). As you see I tried to braid the wire wherever I could but with all those crimp connections you really cannot braid because you need room to connect and to disconnect.
This is a "Class A" amplifier and the heat sinks get hot! I would like to enjoy the amplifier without worrying about how hot it is, so I included two temperature switches on each heatsink. These are Stancor Bimetal switches (the round things above each channel board) that open or close at certain temperatures. Each heat sink will activate a switch if the temperature of the heatsink exceeds 68 C, and illuminate a red LED, indicating overheating. Each heat sink also has another temperature switch to activate a green LED to showing that the amp is warming up. The right LED goes off at 45 C (this takes about 15 minutes), and the left LED illuminates at 57 C. So far it has never gone off, which is a good thing althiough the amp looks kind of funny with LEDs on only one side active.
A small perf circuit board on the front of the heatsink was carefully hand built with LEDs that exactly match up to the cutouts on the front panel. Yes, this creates more wiring but that is is price you pay for an informative readout.
So there you have it, I'm still testing and listening to it but so far it sounds great. People would expect an amplifier with such low distortion to sound clinical and dry. The ExtremA, however, has great body and detail, it is powerful and has a deep and tight bass response. It can shake the cushions on your chair, the windows and rattle the door. It has an ease of power delivery that makes it a pleasure to listen to a low volumes. There is no audible hum, hiss or noise. If I put my ear to the tweeter of the speakers during a silent passage, there is just the faintest veil of hiss. It does have a somewhat low input impedance so the preamp driving this should be able to ouput sufficient current.
Caution- when I first completed the amp, I started it up slowly with a VARIAC. Slowly turning the AC line voltage from zero up to 120V sometimes caused an issue. The + and - rails sometimes latch up causing the channel boards to output DC on the speaker terminals. And the DC protect circuit did not actuate. This also might happen if you power up very quickly after powering down. In such a case only 2 of the 4 bias LEDs on the L/R boards will illuminate so there is a visual indication of a problem.. Luckily, I have not been able to cause this behavior when the unit is connected to the wall and (important) I wait for over 10 seconds before applying power. So just FYI.
You can see that this has RCA and XLR inputs on the back panel. The amp itself works off of the XLR inputs, which are + and - and ground. When using the RCA inputs, a switch is provided that will connect the negative XLR pin to ground so that the amplifer has valid operational states. I do have a provision for an LED on the back panel but have not installed anything there.
Here are some measurements at high power. Clipping has some kind of oscillation but the square wave looks great.