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ALL-FET Professional Microphone Preamp |
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The EB-103/213 is a high quality, high-gain, low-noise microphone preamp in ALL-FET technology. It is designed to work with input/output transformers, however, due to its extremely low input noise; it can also be used without transformers. Normally the gain of the preamp is 44 dB, but can be increased to maximum 54dB. Used with a 1:2 input transformer the total gain is 50/60 dB, with a 1:10 transformer it is 64/74 dB. The preamp can drive 100-Ohm loads. With terminated 50, 75 or 110-Ohm cables it can drive hundreds of feet without loss, see UPGRADES on our homepage. Input/output transformer(s), application dependent components and gain control are not included in the kit. |
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 |  | | Fig 1. The ALL-FET Microphone Preamp EB-103/213 |
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The input stage is using selected and matched JFETs (Q1-Q4). They are cascoded by Q5-Q8 for linearity and for low input capacitance. The input devices Q1-Q8 and associated resistors are doubled for very low noise. The second stage is also using cascoded high-current JFETs for good linearity. The output stage is a pair of MOSFETs, operating with high bias current in Class-A. 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. Connecting an appropriate resistor between -INP and SGND changes the gain. The feedback circuit is AC-coupled to avoid DC-thumps when changing gain. Minimum gain is 0dB and normally maximum gain is 44 dB. The maximum gain can be increased to 54 dB by changing R12 to 2k21 and R4 to 4R4. Using two 213 boards in balanced mode will give 60dB gain without input transformer, see application notes.
The input 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 R28-R30 and capacitor C24 provide phantom power for the microphones. The primary of the input transformer are connected to the + and ¡V points on the PCB and +48V is applied between the pins marked +48V PHANTOM and ground.
Typical specifications for the EB-103/213 microphone preamp:
Gain: 0-44dB
Open loop gain: 76dB
Freq. Response: 0dB gain: >1MHz
44dB 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: 44dB gain, shorted input -137.8dBu
44dB gain, 50-Ohm source: -132.4 dBu
44dB gain, 200-Ohm source: -127.4dBu
Max. Output level: +24dBu
Minimum load: 100 Ohm
Power requirements: „b24V @ approx. 100mA. (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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 | | | The EB-103/213 ALL-JFET Professional Microphone Preamp (Application Specific Components not Installed). |
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A typical application, using Jensen input and output transformers is shown in fig. 2. |
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 | | | Fig 2. Professional Microphone preamp using Jensen Input and Output Transformers |
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Other input and output transformers can also be used (Lundahl, etc), follow the application information on the transformer datasheets.
Balanced transformerless mic pre using two 213 amps.
Fig. 3 shows two 213 connected as a balanced, transformerless mic pre. Using the 54dB maximum gain option for the 213 will give 60dB gain in balanced mode. It is essential to use a very low-impedance gain-control to take advantage of the low inherent noise of the 213.
The circuit in fig. 3 can of course also be used with input transformer, which will increase the gain to 66 (1:2) or 80 (1:10) dB. This configuration is ideal for Ribbon mics.
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 | | | Fig 3. Transformerless Balanced Microphone Preamp Using Two EB-103/213 |
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