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EB-604/410 ALL-JFET Lineamp "Starter Kit" |
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The "Starter Kit" EB-604/410 line amplifier is a high-quality, low-noise ALL-JFET circuit, with on board regulators. It is designed to work with low gain (6, or 12 dB) hence no servo is used.
The PC Board contains two independent circuits. The amps can be supplied from one common or two independent power supplies.
The schematic is shown in Fig 1.
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 |  | | Figure 1 |
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The input stage is a complementary differential circuit, consisting of Q1-Q4. Each of these JFETs is working with 2mA drain current. Q1-Q2 and Q3-Q4 are closely matched devices. Alternatively dual monolithic JFETs acn be used for Q1-Q2 and Q3-Q4. P1 is setting up the drain current to 2mA and P2 is used to adjust offset to zero volts. The second stage Q5-Q6 is a folded cascode, coupled to the input stage through resistors R3/R4 and R10/R11. It is using matched high Idss JFETs and is operating at close to 10mA. The output stage, consisting of Q7-Q8, is a complementary JFET follower circuit. Again, these are closely matched devices, operating at 8-10mA.
Only high quality components are used in the amplifiers. The resistors are non-magnetic DALE CMF-55 (RN-55), the caps are Nichicon FG-type "Fine Gold" MUSE electrolytics, and polypropylene film caps. The PCB is FR-4, with the component side printed.
The schematic of the discrete, on-board ALL-FET regulator is shown in Fig 2. |
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 | | | Figure 2 |
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The regulators are discrete FET opamps; the positive regulator is made up of differential JFET pair Q1-Q2 ad MOSFET Q5, the negative of Q3-Q4 and Q6. Q1, Q2 and Q3, Q4 are closely matched devices. Due to their relatively low dreakdown voltage, the maximum input voltage to the regulators is limited to 31V. If the input voltage is >31V then the higher-voltage dual JFETs K389/J109 have to be used. The differential pair JFETs are working at 2mA drain current each, which is set up through the 6.9V reference diodes D5 and D6 and source resistors R3 and R4. The reference diodes are fed from constant current diodes D1 and D2. The output MOSFETs Q5 and Q6 are TO-92 devices and are capable of supplying maximum 35mA. The minimum input-output voltage difference is 2V, but 4V is recommended. If the output voltage is fixed to 24V, then adjustment is not necessary, and P1 and P2 can be left out (R5=R6=3k32, P1 and P2 are shorted). The output nosie of the regulators is <5 micro Volt across the audio bandwidth. |
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