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The Super-regulated ezDAC | |
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Why the "Super-Regulated" ezDAC? Because all the voltages that are required for the operation of the ezDAC board are now being stabilized by a Super Teddy Regulator (STR). In this newly built version of the ezDAC, instead of the on-board regulators, four Super Teddy Regulators are used to supply the DAC-board with the necessary voltages. In the first built version, which can be found on the first ezDAC page, the LM317 and LM337 regulators that are originally present on the board were used. In this new version, the standard voltage regulators are removed, together with the surrounding components that are needed for their operation. The four Super Teddy Regulators are connected to the point where normally the outputs of the regulators enter the board, and supply the ezDAC directly with the needed 3.3V, 5V and +/-12V. Because there wasn't any space left in the former housing to accommodate the four extra circuit boards of the STR, the ezDAC is completely re-built in a larger case. The extra space also creates the opportunity to revise the rest of the parts. The case that is used is type GX383 from the series "Galaxy Maggiorato" by Hi-Fi 2000, an Italian company with a good webshop and prompt delivery. Super Teddy Regulator??  The Super Teddy Regulator is a discrete two-stage voltage regulator, that doesn't use any feedback or an active regulating element like an opamp for example. The circuit has been designed by Teddy Pardo, and that immediately explains the name as well. It is a further development of his first design, the Teddy Regulator. An extensive description of the circuit can be found on his website, together with tips for building and component selection. The regulator could perhaps be described best as an advanced voltage-follower, also known as a gyrator or a capacitance multiplier. The first stage of the circuit consists of a constant current source and a discrete adjustable zener diode, that provide initial regulation. The voltage from the zenerdiode is filtered by a passive low-pass filter, with a corner frequency that is well below 1Hz. This voltage is buffered by a 'fetlington': a darlington that's constructed with an N-channel FET and an NPN transistor. In the second stage, a divided voltage is created from this pre-regulated voltage with two resistors, and this is where the output voltage is eventually derived from. The divided voltage is filtered by a passive, two-stage low-pass filter, which attenuation at 100Hz is already 80dB. A second 'fetlington' buffers the filtered voltage and then it is available at the output. Super-Regulated ezDAC: the new features and changes - each supply fitted with a Super Teddy Regulator instead of the on-board LM317/337's - new pre-regulator PCB's for the analog and digital supplies - three digital inputs, selectable with relays by a single SPDT switch - input selection with LED-indicators on the front - BNC connectors - LOCK LED on the front indicates the presence of the S/P-dif signal - adjustable analog supply-voltage enables optimum setting for various opamps - changed resistors in the output filter for better symmetry Digital section Two supply-voltages are needed to feed the IC's in the digital section: 3.3V and 5V. To ensure proper operation of the Super Teddy Regulators, a minimum voltage difference of 6V between the input and output is chosen. By choosing the right value for the resistors of the voltage divider in the second stage, the voltage drop is distributed equally between the two stages. This means the input voltage of the Super Teddy Regulators for the digital section has to be 9.3V and 11V minimum. The AC voltage that comes from the supply-transformer is rectified, buffered and stabilized on two pre-regulator boards. The output voltage is set to 12V for both pre-regulators, so the minimum voltage for the Super Teddy Regulators is easily met. The circuit consists of four schottky diodes, a 4700µF buffer capacitor and a standard 7812 voltage regulator. A 1000µF Elna Silmic II capacitor is used at the output. Between the rectifier and the buffer-cap, two 220µH inductors are inserted, and the buffer-cap is bypassed with a 100nF MKP capacitor to improve filtering at higher frequencies.
Input selection An external D/A-converter usually has a central position in the audio-setup, and to enable the connecion of multiple digital sources, two extra digital inputs have been added. By means of three small high-frequency relays, one of the incoming S/P-DIF signal can be selected. The relays are controlled by a single switch with centre position (ON/OFF/ON) on the front. An additional transistor and two diodes on the relay-board make sure that in the centre-position the second relay is energized. For the input connectors, isolated 75Ω BNC connectors are used. The well-known RCA connectors are not suited for high-frequency signal, despite that they are often used for connecting digital signals.
 To indicate the presence of an S/P-dif
signal, an additional LED has been placed on the front of the DAC. To
realize this, a PNP transistor that drives the LED is fitted on
the ezDAC board. The empty space of LM317 regulator U8 is used
for this. The emitter of the transistor is connected to the 3.3V
supply-voltage by placing it in the middle connection of U8. The base
goes in the right hand connection and is connected to pin 14 of
the CS8416 receiver through a 47k resistor and a wire bridge. This pin
(NV/RERR) is
driven low when a valid
data-stream is detected. The collector is left floating, and is
connected to the LED through a wire, with the usual current-limiting
resistor in series. The SMD
resistor R11, that normally connects pin 14 to ground, has to be
removed from the board.Analog section To supply the opamps, a symmetrical supply-voltage is needed somewhere between ±12 and ±18V. Some opamps have a maximum of ±13V (AD8610), others ±15V or ±17V (LME49710). To enable some experimenting with the value of the voltage, the Super Teddy Regulators have been configured in such way that their output voltage is adjustable between ... and ...V. On the pre-regulation boards, two adjustable regulators based on the LM317 and LM337 are placed, so the input voltage of the Super Teddy Regulators can be adjusted as well. Dthe board is fitted with 4700µF Nichicon KG 'Gold Tune' and 100µF Elna Silmic II capacitors. Between the rectifier and the buffer-cap, two 10Ω resistors are inserted, to limit the current peaks and the buffer-cap is also bypassed with a 100nF MKP capacitor.
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