To build stereo amplifiers using LM3886 chipamp ICs, I decided I needed a small PSU board, quite a bit smaller than the "Universal Power Supply PCB" sold in the diyAudio store. This is the result. The new board is 78mm by 98mm and its mounting holes are (7.0 cm by 9.0 cm) apart. Its holes line up with the 1cm grid of holes in the chassis floor plates of Modushop / diyAudio Store chassis.
The PSU circuit schematic is shown in Figure 1, and its parts placement is shown in Figure 2, below. The PSU connects to a dual-secondary power transformer and provides dual rail (± XX volts DC) DC output voltages. The capacitors are rated for a maximum of 50V, and the diodes are rated for a maximum of 100V. This is plenty, since the chip amps themselves (LM3886) are only rated for an absolute maximum of ±42V supplies. Your power transformer's secondary voltage should be 30V AC (rms) or less, to avoid violating the LM3886 maximum rating on its DC supplies.
I have specified 10 ampere discrete Schottky diodes for the bridge rectifier, mounting two diodes per heatsink (Aavid 5132 or 5131) with thermal insulator pads between each diode and the heatsink. Schottkies were chosen because of their significantly lower forward voltage drop, which produces significantly lower power dissipation. Power dissipation is a concern because the diodes are mounted next to the large electrolytic filter capacitors. High temperature reduces capacitor lifetime, so we want low temperature, so we want low power dissipation in the PSU, so (I conclude) we want Schottky diodes.
THESE THINGS GET WARM!
Figures 3, 4, and 5 show infrared camera images of this PSU board operating at above-maximum load. It's delivering +27V at 4 amps, and also delivering -27V at 4 amps. I took the photos after 90 minutes of running at this condition. With ambient air at +27C, the heatsink temperature was +61C and the hottest spot on the electrolytic capacitors was about +50 or +51C. In my opinion, this is comfortably below the maximum operating temperature of the caps (+85C) and they will live a long and healthy life. Especially when you consider that the test conditions (constant max-DC current into a load resistor) are much worse than anything possible when playing music into loudspeakers. I've attached an application note by Cornell Dubilier (a well-respected manufacturer of electrolytic capacitors) that walks through Operating Life calculations for capacitors at elevated temperatures, if anyone wants to pursue that further.
I recommend using the two inch tall heatsinks called "Aavid 5132". Although DigiKey stocks and sells them, regrettably Mouser has a minimum order quantity of 750 pieces (!). Mouser does stock and sell the 1.5 inch tall version called "Aavid 5131". It does a good job but of course the taller heatsink does a better job. If you plan to run your chip amp at very high loudness for long periods of time, I recommend you expend the extra effort to buy the two inch tall 5132's from DigiKey. On the other hand, if you don't forsee blasting your chipamp at max loudness very often, I think you will be just fine with the 1.5 inch tall heatsinks that have no minimum order quantity.
OTHER COMPONENTS
Resistors R3 and R4 are "bleeders", which discharge the capacitors to zero volts when the power is turned off. This is a safety measure to protect repair personnel. R5 and R6 are current limiting resistors that set the current in power-OK indicators LED1 and LED2.
Each transformer secondary is connected to a snubber, which prevents LRC oscillatory ringing. The snubbers are (MOV1 + C1 + R1) on the positive supply, and (MOV2 + C2 + R2) on the negative supply. {"MOV" is an acronym for Metal Oxide Varistor; it's a bidirectional voltage clamp used in surge surpressors.} Snubber component values were chosen after testing on the Quasimodo test jig {described elsewhere on diyAudio} and verified on a completed amplifier using two LM3886s, playing music at full volume. MOV1 and MOV2 each have a capacitance of about 550 picofarads, which is the "Cx" component of the snubber. In addition to snubbing, the MOVs also provide a measure of protection against surges on the AC mains. However, if you are uncomfortable using MOVs in your PSU, feel free to replace them with 100V, 680pF film capacitors.
CHECK OUT THE DETAILED PARTS LIST
There's a spreadsheet attached to this post, which contains a very detailed Parts List for the PSU for Chipamps. Extra detail has been included to make it easy for you, the builder, to find parts whose electrical performance is acceptably good and whose physical size will fit the PCB. If one or more parts are out of stock or backordered, the Parts List is your savior. It will help you find a replacement that's just as good or better.
Also attached is a web link to a Mouser.com preloaded Shopping Cart. A so-called "one click" Shopping Cart. BEWARE: don't shut off your brain when using this; check to be sure that every line item is in stock and not backordered. Check to be sure you're not accidentally ordering 1000 of something. Be mindful.
HOW AND WHERE CAN YOU OBTAIN THE BARE BOARDS
I've got a few dozen extra boards that I'll sell for $2/board plus shipping. They're 1oz copper with gold "ENIG" pads. Maybe you might want to also order some Super Gain Clone chipamp boards and/or soem Compact3886 chipamp boards at the same time. At the moment (Nov 2019) I'm charging $2/board no matter which board you're buying. Shipping is $6 per order to USA addresses, and $16 per order to rest of world. Max 10 boards (total) per customer. PM me with quantity desired and to get my PayPal info.
If those boards sell out and there is additional demand, I'll organize a Group Buy in the GB section of the Forum. And if there is LOTS of demand, I'll show the numbers to the diyAudio Store people, maybe they might want to eventually offer these boards through the Store.
_
The PSU circuit schematic is shown in Figure 1, and its parts placement is shown in Figure 2, below. The PSU connects to a dual-secondary power transformer and provides dual rail (± XX volts DC) DC output voltages. The capacitors are rated for a maximum of 50V, and the diodes are rated for a maximum of 100V. This is plenty, since the chip amps themselves (LM3886) are only rated for an absolute maximum of ±42V supplies. Your power transformer's secondary voltage should be 30V AC (rms) or less, to avoid violating the LM3886 maximum rating on its DC supplies.
I have specified 10 ampere discrete Schottky diodes for the bridge rectifier, mounting two diodes per heatsink (Aavid 5132 or 5131) with thermal insulator pads between each diode and the heatsink. Schottkies were chosen because of their significantly lower forward voltage drop, which produces significantly lower power dissipation. Power dissipation is a concern because the diodes are mounted next to the large electrolytic filter capacitors. High temperature reduces capacitor lifetime, so we want low temperature, so we want low power dissipation in the PSU, so (I conclude) we want Schottky diodes.
THESE THINGS GET WARM!
Figures 3, 4, and 5 show infrared camera images of this PSU board operating at above-maximum load. It's delivering +27V at 4 amps, and also delivering -27V at 4 amps. I took the photos after 90 minutes of running at this condition. With ambient air at +27C, the heatsink temperature was +61C and the hottest spot on the electrolytic capacitors was about +50 or +51C. In my opinion, this is comfortably below the maximum operating temperature of the caps (+85C) and they will live a long and healthy life. Especially when you consider that the test conditions (constant max-DC current into a load resistor) are much worse than anything possible when playing music into loudspeakers. I've attached an application note by Cornell Dubilier (a well-respected manufacturer of electrolytic capacitors) that walks through Operating Life calculations for capacitors at elevated temperatures, if anyone wants to pursue that further.
I recommend using the two inch tall heatsinks called "Aavid 5132". Although DigiKey stocks and sells them, regrettably Mouser has a minimum order quantity of 750 pieces (!). Mouser does stock and sell the 1.5 inch tall version called "Aavid 5131". It does a good job but of course the taller heatsink does a better job. If you plan to run your chip amp at very high loudness for long periods of time, I recommend you expend the extra effort to buy the two inch tall 5132's from DigiKey. On the other hand, if you don't forsee blasting your chipamp at max loudness very often, I think you will be just fine with the 1.5 inch tall heatsinks that have no minimum order quantity.
OTHER COMPONENTS
Resistors R3 and R4 are "bleeders", which discharge the capacitors to zero volts when the power is turned off. This is a safety measure to protect repair personnel. R5 and R6 are current limiting resistors that set the current in power-OK indicators LED1 and LED2.
Each transformer secondary is connected to a snubber, which prevents LRC oscillatory ringing. The snubbers are (MOV1 + C1 + R1) on the positive supply, and (MOV2 + C2 + R2) on the negative supply. {"MOV" is an acronym for Metal Oxide Varistor; it's a bidirectional voltage clamp used in surge surpressors.} Snubber component values were chosen after testing on the Quasimodo test jig {described elsewhere on diyAudio} and verified on a completed amplifier using two LM3886s, playing music at full volume. MOV1 and MOV2 each have a capacitance of about 550 picofarads, which is the "Cx" component of the snubber. In addition to snubbing, the MOVs also provide a measure of protection against surges on the AC mains. However, if you are uncomfortable using MOVs in your PSU, feel free to replace them with 100V, 680pF film capacitors.
CHECK OUT THE DETAILED PARTS LIST
There's a spreadsheet attached to this post, which contains a very detailed Parts List for the PSU for Chipamps. Extra detail has been included to make it easy for you, the builder, to find parts whose electrical performance is acceptably good and whose physical size will fit the PCB. If one or more parts are out of stock or backordered, the Parts List is your savior. It will help you find a replacement that's just as good or better.
Also attached is a web link to a Mouser.com preloaded Shopping Cart. A so-called "one click" Shopping Cart. BEWARE: don't shut off your brain when using this; check to be sure that every line item is in stock and not backordered. Check to be sure you're not accidentally ordering 1000 of something. Be mindful.
HOW AND WHERE CAN YOU OBTAIN THE BARE BOARDS
I've got a few dozen extra boards that I'll sell for $2/board plus shipping. They're 1oz copper with gold "ENIG" pads. Maybe you might want to also order some Super Gain Clone chipamp boards and/or soem Compact3886 chipamp boards at the same time. At the moment (Nov 2019) I'm charging $2/board no matter which board you're buying. Shipping is $6 per order to USA addresses, and $16 per order to rest of world. Max 10 boards (total) per customer. PM me with quantity desired and to get my PayPal info.
If those boards sell out and there is additional demand, I'll organize a Group Buy in the GB section of the Forum. And if there is LOTS of demand, I'll show the numbers to the diyAudio Store people, maybe they might want to eventually offer these boards through the Store.
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