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Home Theater Amplifier Kit |
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The DC-50 is a high quality, DC-coupled Class-A/AB power amplifier, nominally rated at over 50W into 8 Ohm per channel. With 3 pairs of output devices and proper power supply and heat sinking it can deliver over 100W into a 4 Ohm load.
The EB-693/129 driver circuit uses the same topology as the Class-A lineamp, described in the 3/90 issue of The Audio Amateur. Cascode JFETs have been added to the monolithic input FETs to improve linearity and to reduce the input capacitance. It is also using special, high frequency second stage transistors with very low collector-base capacitance. This results in very good linearity across the audio range.
The EB-693/114 output stage consists of two or three pairs of TO-3P power MOSFETS, driven by TO-220 MOSFET drivers. It is recommended to use three pairs of output devices when driving 4 Ohm loads. The output devices are connected directly to the PC board, which simplifies assembly. |
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Transistors Q1 through Q10 form the double cascode input stage. Q1 and Q2 are monolithic JFETs of the type 2SK389 and 2SJ109. These are low-noise, medium gm devices. They have relatively high internal capacitances, which result in high input capacitance. This, in turn, might cause stability problems, especially with high impedance feedback network. Q1 and Q2 are first cascoded with JFETs which are referenced to the sources of the input FETs. Then, the cascode JFETs themselves are cascoded with bipolar transistors, which are also referenced to the sources of the input FETS. This results in very good linearity and very low input capacitance.
The input JFETs are operating at 2mA each. The input stage current it set up by trimpot P2. Trimpot P3 is used to adjust the offset of the amplifier. Constant current diodes D1 and D4 are supplying a 1mA current to the 5 Volt reference diodes D2 and D3. Resistors R7/R18 and capacitors C4/C9 are pre-filtering the supply voltage to the current diodes and are reducing the voltage across them at the same time. If the total supply voltage of the amplifier is less than ±45 Volt, there is no need for the series resistors.
The second stage, which is a folded cascode circuit, consists of Q11 through Q14. Q11 and Q13 are TO-92 devices, they are operating at approx. 2mA, and they provide the bias for the Q12/Q14 TO-126 pre-drivers. Q12 and Q14 are operated at approx. 10mA to drive the input capacitance of the output stage. Q12 and Q14 need to be heatsinked. A suitable heatsink can be made of a 120mmx25mmx2mm Aluminum. The devices need to be insulated with mica and should be mounted with a generous amount of silicone grease. Alternatively you can use a silicon rubber insulator, which do not require silicon grease. The quiescent current is adjusted with trimpot P4. Diodes D5 through D8 provide a current limit with very low impedance loads or short circuit.
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 |  | | The DC-50 Driver Stage |
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The EB-693/114 output stage is a source follower circuit, consisting of TO-220 drivers and TO-3P plastic output devices. The advantage of the TO-3P package is that it is bolted with single-hole mounting, directly to the heatsink. The three legs are connected directly to the PCB. Naturally, TO-3P packages need proper heatsinking, just like metal cans. The heatsink has to have a 0.5-0.6 K/W thermal resistance per channel. The output devices have to be insulated from the heatsink with mica or silicon rubber. It is recommended to install a thermal breaker with a cutout temperature of approx. 70 degree C on the heatsink.
A word of caution concerning shorting the output of the amplifier. Although it can survive a short-term short, it can't take a long term one, because of thermal overload. If you are likely to run into frequent and long shorts we recommend that you install a more sophisticated protection for your amplifier. |
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 | | | The DC-50 Output Stage |
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It is recommended to use a 200VA transformer per channel, or 400VA for two channels for this amplifier. It is also recommended to have separate windings for the driver stage: 2x36VRMS/100mA, giving approx. +50V DC. The output stage needs 2x30-32V RMS at 2.5/3A per channel. The filter capacitors should be minimum 10000 uF/50V for the output stage (22000uF is preferred) and 1000 uF/63V for the driver stage. Larger transformer(s) and capacitor(s) improve bass significantly. Both supplies have to be grounded at the same point, for example at the PGND of the driver board, or a star ground point elsewhere. Connect a 0.luF/100V film capacitor between the two supply pins on the boards (plus-to-plus and minus-to-minus), to establish a common high frequency reference between the two. You can also use a single supply for the driver and the output stage. You have several possibilities in running such a system. You can for example run both from an unregulated supply of ±40-45V, or a common, high current regulator of the same output voltage.
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 | | | Proposed wiring diagram for the DC-50 Home Theater amplifier (one channel shown) |
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