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EB-804/419 ALL-JFET MM/MC Phono Preamp "Starter Kit" |
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The EB-804/419 is a high-quality, low-cost All-JFET MM/MC Phono Preamp with on-board All-FET regulators. The combination of extremely low input noise and high gain allows you to use it with practically all pickups on the market. For a general introduction to noise in phono preamps please read my article that appeared in the Audio Amateur (1).
The EB-804/419 is using a two-amplifier approach instead of the usual single amplifier. This allows you to optimize it for low noise, RIAA accuracy, High Gain and dynamic range. The block schematic is shown in Figure 1.
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 |  | | Fig 1. |
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There are two possibilities in using the EB-804/419. In Fig.1a the amp on the left is a high-gain amplifier with extremely low input noise and flat frequency response. It can be used with 20 or 40dB gain by changing the feedback resistor. The R-C network between the two amplifiers provides the 75-microsecond roll-off for the RIAA equalization. The second amp provides a further amplification of 24dB at 1 kHz and at the same time boosts the bass according to the RIAA characteristics. For those of you who want to experiment with all-passive equalization I have included the components for the classis Walt Jung network
(3) in the layout as shown in Fig b. Here both amplifiers are providing flat frequency response and all RIAA equalization is done passively between the two amplifiers. MM gain is 44dB and MC gain is 64dB for the circuit in Fig 1a. The gain in Fig 1b. depends on the all-passive equalization network. The Starter Kit is supplied with components for the circuit in Fig 1a. The all-passive network can be ordered separately from Borbely Audio.
Fig 2. shows the schematic of the EB-804/419. The input stage has been developed from the basic 4-JFET complementary amp described in my article: “The All-FET Line Amp” Fig 5. (2), see also copy of the article on www.borbelyaudio.com. I have added one more pairs of complementary JFETs in parallel to the input JFETs to reduce the input noise. Also, a complementary output stage has been added to be able to drive low-impedance feedback network, see Fig 8. of the same article. Since the DC gain of the circuit can be very high (MC application and/or all-passive equalization), I am using a servo circuit instead of capacitive coupling to keep the offset close to zero volt.
R18/C11 provides the 75-microsecond roll-off. Alternatively, R18/C11 and R19/C12 make a complete all passive RIAA equalization (3). As the all-passive equalization network has a high insertion loss (~20dB), the gain of the two amplifiers has to be increased to get the same overall gain. Here, some experimentation is necessary to find the best ratio between noise and dynamic range.
The second stage of the phono preamp uses the same topology as the Starter Kit EB-604/410; see description under Starter Kit EB-604/410. However, due to its high gain (in active eq. Mode the DC gain is 44dB!) a servo has been added. The servo amps are supplied with ±10V from shunt regulators.
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 | | | Fig 2. |
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The schematic of the on-board EB-704/418 All-FET regulators is shown in Fig 3. The topology is very similar to the EB-604/415 regulators used in the EB-604/410 line amp and the EB-604/417 DAC circuits. However, Q7 and Q8 MOSFET source followers have been added to be able to cope with the current demand of the phono preamp (70mA). Output voltages can be changed between 15 and 25V with P1 and P2. If they are used with fixed 24V output, then P1 and P2 can be shorted. Make R5=R6=3k32. Minimum input/output voltage diff. is 3V, but 4V is recommended. Put a small snap-on heatsink on Q7/Q8 if higher unregulated voltage is used. |
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 | | | Fig 3. |
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The All-FET phono preamp is a very sensitive amplifier and is capable of picking up small signals not only through its inputs, but also through vibration. It is therefore very important to make the whole amp as “dead” as possible from vibration. It is recommended to mount the board in a vibration-free way. There are several possibilities here; one is to use rubber standoffs, which are specifically made for this purpose. Further improvement can be achieved by mounting the whole board on a Teflon plate, 6-8mm thick. And a mumetal box will certainly put the icing on the cake… |
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