PlusMin15V.pdf (26.52KB)
PlusMin15V.pdf (26.52KB)


Low Voltages.pdf (18.69KB)
Low Voltages.pdf (18.69KB)







BTpowerBoard.pdf (17.24KB)
BTpowerBoard.pdf (17.24KB)


Powersupply (Updated march 2018)


The required power supply rails for the Pre amplifier are: +/- 15V for the opamps, volume control and input selectors and +5V and +3V3 for the converters and optical I/O's. These supplies must all be low output noise supplies.
There are two micro controllers planned, one on the Pre amplifier board and another to control the display and IR receiver.
These controllers will be always powered and when the Pre amplifier is not operational the micro's are in a sleep mode to preserve power consumption. There is also a 5V supply required for the Display unit.

The initial idea was to build a traditional 50Hz transformer based power supply but I quickly abandoned that idea since it would be much harder to keep 50Hz residues out of the sigal chain, especially from the Phono pre amplifier.
I decided to use a wall adapter with 12V output voltage rated at 2A.
To keep the efficiency of the total power supply as high as possible, after all a "Green" Pre amplifier is what I wanted, I decided to use swith mode powersupplies wherever possible.

I used a design from the Texas Instruments website to use a buck converter (TPS54260) as inverting booster and use a coupled inductor to derive the positive voltage.
Basically it is a single to dual output boosted output switcher topology.
The negative output voltage is created first. The coupled inductor provides the positive output voltage. This topology allows for output voltages that are larger, both positive and negative than the input voltage.
The TI website also provides an Excelsheet for download to calculate the required passive component values based on input voltage, output voltages and output currents.

The schematic of the single to dual output converter is shown in Fig.1.

The output of the split rail supply is +/-16V and is followed by a positive and negative post LDO (TPS7A4901, TPS7A3001) to filter out switching noise and provide a clean +/- 15V supply.
These are low noise LDO's with a PSRR (Power Supply Rejection Ratio) of >40dB that extends beyond 10MHz.

The 3V3  "always ON" supply for the micro controller is implemented with a TLV70133 LDO that has a very low quiesent current to keep the overall power dissipation low in sleep mode.

A blocking diode plus holding capacitor in front of the LDO are connected directly to the 12V input of the wall adapter and keep the micro's alive for a short period of time after a 12V power fail, long enough to allow a controlled power down of the Pre amplifier.
The  12V input failure is detected with comparator U102, that is read out by the Display controller.

Fig.2 shows the complete dual supply plus micro controller supply and the 12V input fail comparator.

To build the 5V and 3V3 low noise supplies I used a TPS560200 (U200) switcher to create a 12V to 6V output supply. With post LDO's U202 and U203 (TPS79650 and TPS79633) the low noise 5V and 3V3 rails were implemented.
The 5V for the display was also created with a TPS560220 (U201) but there no post regulation is required.  The full schematic of the low voltage rails is shown in Fig.3. 

After I connected the Pre amplifier board to this power supply it proved to be not resistant against the inrush currents on the +/-15V supply so I had to adapt my design. I also implemented a sequenced start up of the different power rails to reduce the inrush current stress on the Wall adapter output as well. 
The 12Vin to +/-16V out inverting boost converter remained but I placed a circuit between the +/-16V outputs of the inverting boost converter and the post LDO's that limits the inrush currents at start up.

Fig.4 shows the dual supply inrush current limit circuit. U108, a REF200 is a 100uA current source that creates equal voltage drops over R126 and R127. Opamp U107 is a dual OPA192 that has a very low offset error and drift and is capable of handling voltages at its inputs that exceed its voltage rails. The outputs swing close to the voltage rails.
The two opamps drive a P-channel MOSFET (T103) in the positive and a N-channel MOSFET (T104) in the negative rail and form with resistors R124 and R125 constant curent sources.  When power is applied to this circuit and capacitors are connected to the outputs, they are charged with a constant current.
The charge currents are chosen to be higher than the highest currents the Pre amplifier board draws but lower than the current limits of the post LDO's.  When the output capacitors are charged to the nominal output voltage, the current sources can no longer maintain this high current, the current reduces to its nominal value and the opamp outputs drive the FET's into full conduction. The MOSFET's then represent a series resistance equal to their Rds-on.

Care has to be taken to select MOSFET's that are able to handle the power dissipation whilst in constant current mode. The power dissipation in the MOSFET's equals to Vin*Iconst. at startup and reduces when the output capacitors get charged. The higher the output capacitance, the longer this period is. Power dissipation of these MOSFET's must be matched with startup power dissipation (Vmax * Icharge). 
Fig.5 shows the TinaTI simulator wave forms. In this simulation the loads are 100Ohms and 600uF. The output currents remain constant at 220mA, set by the voltage drops of 22mV over R126 and R124, equal to the voltage drop over the current sense resistors R124 and R125. The sum of the series resistance and the Rds-on of the FET determines the voltage drop at the maximum output voltage when the constant current is reduced to the operating current.

Fig.6 shows the complete +/-15V circuit.  U101 is a TPS2421 adjustable inrush current controller that protects the 12V wall adapter against excessive start-up currents from the switchers. This type has no current sense output but similar functionality as the TPS54260 of the first design.

U101 is a simple sequencing device, LM3881 that is used to start up the switcher for the analog 5V and 3V3 power supplies first, then the +/-15V supplies and the 5V Display power supply last with an adjustable time interval. This spreads the inrush currents from the different switchers over time for the wall adapter.

The 3V3 "Always ON" supply for the micro controllers and the 12V fault detector comparator remained the same. 


Fig.7 shows the complete low voltage schematic with added "Fault" indicator to the Display microcontroller.

Between U200 (TPS560200) that generates 6Vout and the two post LDO's (U204, U205) for 5V and 3V3 out, I placed inrush current limiter U202 (TPS2553). U201, a TPS560200 converts the 12V input to 5V for the display power. The same TPS2553 inrush current limiter (U201) is used at the output of the 5V display power supply.

These TPS2553 devices also work on the principle of a constant current cource during startup. 

Fig.8 shows the measurement results of the power supply.


On the Pre amplifier board is a connection for a headphone amplifier, that I decided to not use. In stead if I want to use a headphone I can connect a Bluetooth wireless headphone via the optial output of the pre amplifier.
To power the headphone transmitter a 5V power supply is required. If I want to connect a Bluetooth receiver, to play music from a smartphone or tablet I also need a 5V powersupply.
I created a 12V to 5V-2A USB output that can be switched ON and OFF by the Display controller.
The display user interface provides an option to switch the USB power on when a specific input is selected in the case of a BT receiver or switch the USB power ON for all inputs, all the time the Pre amplifier is active, in case of a BT headset.

Fig.9 shows the schematic of the USB 5V powersupply.

U2 (TPS561208) is used as 12V to 5V output Buck converter. The 5V output is protected with U4 (TPS2553) that provides constant current inrush protection at startup and limits the output current to 2A.
A small 50mA LDO U1 (TPS71533) is used to power the sequencer U3 (LM3881) that switches the TPS561208 and TPS2553 sequenced ON and OFF.
I also added a manual ON/OFF switch that is read out by the Display controller in case a manual override is needed when the IR receiver isn't working properly.
The PCB of this Bluetooth powerboard is located in parallel with the power supply PCB at the backside of the Pre amplifier.  

These power supplies work as can be expected. During test en software debugging it was switched ON and OFF may times and in operational use it shows no problems.