|
|
 |
EB-303/406 All-JFET "Super Buffer SB2" |
|
 |
 |
|
| The ALL-JFET “SUPER BUFFER SB2” (1) is a very high quality low-noise, wide-band amplifier. It offers extremely high resolution, combined with natural and transparent sound. It has servo control and can be used with higher gains. With paralleled output it can drive 600-Ohm loads like high impedance headphones. The PCB is 90x145mm and contains two completely independent buffers. The buffers can be supplied from two independent power supplies. The schematic is shown in fig. 1. The SUPER BUFFER SB2 has been developed from the SUPER BUFFER and shares the topology with the professional microphone preamp EB-1102/212. |
|
|
|
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 consists of two pairs of matched complementary JFETs (Q16-Q18), again cascoded for linearity and for low capacitance modulation (Q14-Q15 and Q20-Q21). This topology ensures very low input noise, exceptional linearity and low-impedance drive capability. This 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.
Typical specifications with R17=47K5:
O.L.G: 66dB
O.L. Freq. Response: 22kHz
Unity gain freq. Response: >1MHz
Unity gain rise time: 100nsec.
Unity gain output impedance: <2 Ohm (without R20)
Equivalent short-circuit input noise: 108nV, 20-30kHz
Input capacitance: approx. 2.6pF
Power requirements: +/-24V regulated at approx. 35mA
Recommended PS/regulator: EB-703/259
Min. load: 600 Ohm
|
|
|
|
|
New Page 1
Ch. 1
Ch. 2
| Output |
1kHz |
10kHz |
1kHz |
10kHz |
| 1V |
0.0005 |
0.0013 |
0.0005 |
0.0013 |
|
3V |
0.0007 |
0.0007 |
0.0007 |
0.0007 |
| 5V |
0.0005 |
0.0011 |
0.0005 |
0.0011 |
|
8V |
0.0005 |
0.0012 |
0.0005 |
0.0011 |
Table 1: Typical THD readings for SUPER BUFFER
2, with 6dB gain. (2)
|
|
The SUPER BUFFER SB2 has a number of useful applications like CD buffer, filter buffer, unbalanced-to-balanced converter, balanced-to-unbalanced converter, microphone preamplifier, instrumentation amplifier, line amplifier, etc. Since it can drive 600 Ohm loads, it can also be used as a high quality headphone amplifier for high impedance headphones like the Sennheiser HD600.
Much of older audio equipment like tuners, tape recorders don’t have balanced outputs. Although some of the newer CD players do have balanced outputs, they are using a couple of old opamp for the conversion and they don’t match the quality of the rest of the player. The SB2 can be used as a very high quality unbalanced-to-balanced converter. Fig. 2 shows such a converter with a balanced gain of 12dB. It was developed for Studer tape machines and for Broadcast quality tuners, but can be used for unbalanced CD-players as well.
The upper buffer is working as a non-inverting amplifier with 6dB gain, the lower one as an inverter, also with 6dB gain. If 6dB balanced gain is enough, then make R1=R2=10k for the upper amplifier, and remove C1.
|
|
|
|
|
|
For set-up procedure follow the steps described for the individual buffers. After connecting the two -INP together the offset has to be rechecked at both outputs.
1. Erno Borbely: The All-FET Line Amp, AudioXpress 5/02. See also our homepage under “Articles”
2. Shinichi Komori, Japan: Private communication.
|
|
|
|
|
