This is again where the case and power supply eat up the costs quickly. Let's just say you picked this
http://www.digikey.com/product-detail/en/LNR1K103MSE/493-7694-ND capacitor.
Some choices are: Do you want two independent dual supplies or one single one. Custom transformer or two separate ones.
For a double dual supply/case might have:
2 rectifier bridges + mounting hardware and heat sink compound
4 capacitors ; That 4 * $20.00 USD and brackets
1 line fuse - holder
1 power cord
Some options are IEC cord with a lock
You can integrate with a power entry module
4 x 1 fuse per power supply rail and holders (probably chassis mount)
2 isolated RCA phono inputs
Speaker connectors: Binding posts, speak-on connectors, push in for wire leads.
1 Case + an internal chassis
If you create a frame internally, then the amp becomes stand-alone when you take the covers off.
I made the choice of adding an extension cable to work on the amp outside of the case.
Speaker protection kit - e.g.
**broken link removed**
amd/or AGX fuses.
Possibly a power LED, an over-temp LED, a protect LED and a right and left clipping indicator.
Power switch: The rear one can be a safety on/off. You can use a AC control input from the preamp. A possible way to bypass this. i.e. A locking toggle switch on the back. You can use a smaller toggle switch on the front that doesn;t switch the mains directly. DC switching has become popular in automation systems.
Possible switched and non-switched outlets for other components.
Surge suppression for mains and ZNR's across caps.
Thermal switches on heat sinks.
Either optoisolators or relays to cut the input sound or a version of my fancy one that ramps up the sound. I made the optos part of the case essentially.
headphone jack - I'd recommend trying to do it in a way that inserting the headphone reduces the gain of the amplifier. The resistor in series with the output doesn;t work well. Omitting is just as good. This is why you might want to power off the amp from the front.
You may need a small "always on" power supply.
A relay or triac for the power if needed.
Slow turn on circuit required for high power amps with a linear power supply. e.g. **broken link removed** I don't like the data sheet.
As a dumb example 1 or two transformers:
**broken link removed**
Clipping indicator circuit (optional).
bar graph displays for power into 8 ohms (optional)
Auto-brigtness controls for the LED indicators (optional)
This **broken link removed** sort of explains some of the stuff in a nutshell.
SMPS power supply vs transformers:
http://hifiduino.wordpress.com/2011/02/26/resonant-smps-for-audio/
**broken link removed**
Bridging module and support (optional): Turns amplifier into a mono bridged amplifier with double the voltage output. Speaker connected between the two positive terminals. Current may have to support bridgeing.
All of this stuff has to fit in an appropriately sized case.
It get's expensive "real quick" and no amp yet.