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BORBELY AUDIO EB-1102/212 ALL-JFET Professional Microphone Preamp |
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The EB-1102/212 is a high quality, high-gain, low-noise microphone preamp in ALL-JFET technology. It is designed to work with input transformers. Maximum gain of the preamp is 44 dB. Used with a 1:2 input transformer the total gain is 50 dB, with a 1:10 transformer it is 64 dB. The preamp can drive 600-Ohm loads. Fig. 1 shows the schematic of EB-1102/212.
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 |  | | Figure 1. |
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The input stage is using selected and matched JFETs (Q1-Q4) for low-noise operation. They are cascoded by Q5-Q8 for linearity and for low input capacitance. The second stage is also using cascoded high-current JFETs for good linearity. The output stage is a matched complementary pair of JFETs, again cascoded for linearity and for low capacitance modulation. This topology ensures extremely low input noise and exceptional linearity, which allows operation over a wide gain rage without change in sonic character.
The amp is DC-coupled and the offset is controlled by Q9, which is powered by a pair of shunt regulators. Gain is changed by connecting an appropriate resistor between the –INP and ground. The feedback circuit is AC-coupled to avoid DC-thumps when changing gain. Minimum gain is 0dB and maximum gain is 44 dB.
The +INP offers a number of possibilities in accepting different types of input transformers and filters. SW1 can either connect a capacitor in series with the input (C1/C2), or a pad. R1/C3 and R2 are the terminating networks for the input transformers, which have to be selected according to the specs of the transformers. R3/C7 is the high impedance input of the preamp, and can be switched to direct inputs from for example guitars (SW2).
The components R24-R26 and capacitor C25 provide phantom power for the microphones. The primary of the input transformer are connected to the + and – points on the PCB and +48V is applied between the pins marked +48V PHANTOM and ground.
Typical specifications for the EB-1102/212 microphone preamp:
Gain: 0-44dB
Open loop gain: 76dB
Freq. Response: 0dB gain: >1MHz
40dB gain: 650kHz
THD: 0dB gain, 1kHz, 3VRMS: 0.001%
0dB gain, 10kHz, 3VRMS: 0.0015%
40dB gain, 1kHz, 3VRMS: 0.0015%
40dB gain, 10kHz, 3VRMS: 0.0028%
Input capacitance: approx. 2.2pF
Input noise, 30kHz BW: 40dB gain, shorted input better than –130dBu
40dB gain, 200-Ohm source: -127dBu
Max. Output level: +24dBu
Minimum load: 600 Ohm
Power requirements: ±24V @ approx. 35mA. (Low-noise regulator is essential!)
(It is recommended to put the mains transformer in a separate box to avoid hum pickup.)
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 | | | EB-1102/212 All JFET Professional Microphone Preamp (Application specific components not installed). |
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Set-up Procedure.
If you have bought the preamp in kit form install all components on the board, except Q9 and C5/C6. Jumper SW1 and SW2 and connect the +INP and –INP to SGND. Apply ±24V regulated voltage to the board. Connect a DVM, set to 10 or 20V across R17 (or R20). The voltage drop should be 3V, ±5%. Jumper C5 (or C6) and connect FB to SG. Connect the DVM to the output and adjust the offset to zero Volt with P1. (Since the DC gain is 44dB, this requires a very careful adjustment!) Check the voltage across D1 and D2; it should be 10V. Install Q9 and recheck the offset; it should be less than a couple of mV after a minute. Remove the jumper on C5/C6 and install the caps.
Remove the +INP and –INP shorts and install the Application specific components (C1, C2, C3, R1, R2) on the board. Install the board in the case and connect the input (and output) transformer to the board. Also connect the gain control switch with the appropriate resistors to the board. Remove the SW1 and SW2 jumpers and install the extra input function as needed. (Leave the SW1 and SW2 jumpers in, if no input functions are needed.)
If you have audio instrumentation with audio oscillator, THD analyser and mV meter, carry out the normal audio measurements. The mike preamp is now ready.
Application.
A typical application, using Jensen input and output transformers is shown in fig. 2.
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 | | | Figure 2. |
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Gain is changed by switching different values of R from the –INP to ground. For max. gain (44dB) R=0, and for minimum gain R=¥ (open). The switch has to be shorting (make before break) and free of contact-bounce.
Other input and output transformers can also be used (Lundahl, etc), follow the application information on the transformer datasheets.
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