The "SymAsym" is an amplifier design created by diyer Michael Bittner. His design can be found on his Web Site and additional commentary is available here. My SymAsym is based on boards from diyer "AAK." This version has some slight modifications and two output pairs of transistors for higher power. It remains a class AB amplifier and so should normally run cool. With two pairs of output devices per channel, we are looking at 100 WPC.
The case is from my disassembled F5 Power Amplifier. To summarize, from my F5 page, the chassis is from an old Onkyo receiver with the internals removed. Here are the pics (from the F5 link).
The problem with the chassis was that the 600 VA transformer I was using was so heavy it caused the bottom plate to buckle. I reinforced the chassis with square steel tubing to make it massively strong. The case is now strong enough to handle almost any weight. The sides of the case bend and overlap the bottom, and are joined by screws. Because the tubing is screwed into this overlapping region it makes the case very strong. The only problem is that the feet had to be extended. I installed some wide spacers under the soft rubber feet.
Raw steel is not so nice to handle (dirty/ messy) so I spraypainted it black. I also drilled out a countersink to allow the screws to mount flush. Not bad now.
This amplifier uses over 120,000 uF of filter capacitors and a 600 VA transformer. I therefore put in my standard Soft Start Circuit. The transformer has unused 12VAC taps that are perfect to run a delay circuit and regulator which actuates a large relay after a few seconds. Initially the AC line voltage goes through two CL-60 (10 ohm 5 amp) negative temperature coefficient (NTC) thermisters which absorb the turn-on spike and then gradually reduce their resistance as they heat up. After a few seconds these thermisters are bypassed by the relay. A 7.5 V regulator is also hacked onto this board in order to power the fans and front panel LED.
The F5 had two fans because it was a Class A amplifier and got very hot. My F5 required a voltage regulator to drive the fans, but I had already used that regultor to run the display for my Krill Power Amplifier. In the SymAsym, I am using the F5 case with two attached fans, Do I use the fans or not? I figured what the heck- might as well keep them. The soft start circuit has a Bridge and a regulator to provide a delayed voltage to power a relay. I added another regulator fed from the unregulated DC voltage to power the fans with 7.5 VDC (so they are quiet and slower speed than the default with 12 VDC). The fan circuit will feed unregulated 18 volts through a Stancor bimetal thermally sensitive switch. When the temperature on one heatsink hits 37 degrees C, the switch flips and the 18 volts pass into a 7.5 Volt regulator which outputs 7.5 volts to the fans. If the sinks are not hot, or if the sinks cool down, the bimetal switch is open and no voltage is fed into the 7.5 volt regulator.
The voltage from the switch also goes to the LED Above the power switch. This is 3 color (red/green/amber) three terminal common cathode LED. When power is first switched on the Green anode gets voltage which illuminates the LED green. When the fans activate the voltage going to the fans is fed through a resistor to the red anode, which illuminates the red LED. Since the green LED in the same casing also remains lit, the color is actually amber. So if the LED above the ON/OFF switch is amber you know the fans are running and that the heat sinks are getting hot. A nice little informative display.
I reused the big CRC power supply from the F5 for the main power supply. The SymAsym on-board power supply is supposed to be four capactitors at 4,700 uF each. Because I needed the space, I could not use the tall 4,700 uF capacitors, so I used 2,200 uF capacitors. I don't lack capacitance since there is 120,000 uF in the CRC power supply.
A CRC power supply has a resistor in series between two capacitor banks. It is suitable for class A amplfiers, since they have constant current draw you don't need to worry about surges through your power supply. The resistor drops the ripple from the power supply substantially, and thus helps reduce noise. However, the resistor in a CRC power supply limits the ability to instantly draw power from the wall socket. A class AB amplifier needs to instantly draw current from the wall, which is somewhat curtailed by the CRC power supply, and is why you don't normally see CRC power supplies in Class AB Amplifiers.
Since I had this already built, and since the part after the "R" is 60,000 uF, far above the design capacitence of the SymAsym, I left the CRC in place. This can easily be undone and turned into a standard power supply by cutting/ jumpering the resistors. For now I will try it out with the CRC to see how it behaves.
As stated above, the boards came from diyer "AAK" and once again I found myself cramming a ton of components into a small space. A lot of resistors have to go vertical and everything is really packed in there. This makes for a more compact layout, smaller with less chance of external influence on the circuit.
I jumpered the on-board diodes as I was using an external bridge with external capacitors. I did not use an input capacitor to block DC as the only cap I had was too big to fit, and I am careful to monitor DC to and from the amp anyway. The heat sinks are about 8.5" x 5" x 2" and get only slightly warm. The DC offset is 8 mV and the bias is set to 14.0 mV across each emitter resistor with 115 VAC input, with 120 VAC input the bias is about 14.8 mV on each emitter resitor.
The input BC550 transistors need to be matched to reduce DC offset. I looked at this web page to get an idea how to match. I was not sure what to do so I built a jig of the "Calculating hfe" circuit. I measured the voltages across all the resistors, and grouped the transistors according to these voltages. In the end I had some groups of 4-6 transistors that all "matched" so I put the transistors in the groups into a DMM with a hfe tester, and selected the pairs with the closest hfe data. You cannot touch the transistors with your fingers because they are very sensitive to temperature. You have to use a tweezer to pick them up by the leads and deposit into your jig. I'm not sure this is the right way to do it so don't trust me on this. One pair came up good (only a few mV offset) the other pair has about 45 mV offset, which is ok but not great.
The front panel was from the F5 and fit perfectly, though this time I used an illuminated on/off switch.