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EB-604/417 ALL-JFET I/V Converter and Filter for DACs "Starter Kit" |
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General Design Information
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Outboard DACs usually outperform the ones built into the CD players. This is normally due to the better design of the I/V converter and the analog filter, that follows the digital-analog converter. The block schematic of the "analog" part of the DAC is shown in Fig 1. |
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 |  | | Figure 1 |
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The amplifier on the left-hand side is a block that converts the current output from the digital-analog-converter to a voltage, hence the name I/V converter. The resistor/capacitor/amplifier combination on the right side represents the filter, which is removing the digital noise that remains from the conversion process. Theoretically everything above 20 kHz should be "removed", and this is what people did in the early CD players, with very steep "brick wall" filters. These filters had a very undesirable transient response and combined with the opamp used the resulting sound was a very far cry from high-end.
With the present over-sampling systems there is no need for such steep filters and we can use ones with much better transient response. The better CD players are using Bessel filters with 2-5 poles. I have chosen a 3-pole one in my design. First of all this is easy to implement and secondly practical tests show that this is adequate for 8 times over-sampling systems.
The gain-blocks shown in Figure 1 are usually IC opamps. There are a number of suitable ones for these functions; Analog Devices and Burr Brown are offering good sounding opamps. Unfortunately a lot of the CD players are using older, less suitable ones, like the 5534 family devices and their derivatives. But even if you use the best one on the market, they cannot compete with discrete designs in my opinion. And certainly not with the ones made with out All-FET technology! Proven through the All-FET lineamps and the new All-FET power amps, the All-FET designs offer unprecedented resolution. |
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 | | | Figure 2 |
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Figure 2 shows the schematic of the discrete All-JFET I/V converter and filter. The I/V converter is a high quality, low noise, discrete opamp (see also the description fo the EB-604/410 lineamp). It can be used in inverting mode (active I/V converter) or in non-inverting (passive I/V converter). 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 can be used for Q1-Q2 and Q3-Q4. P1 is setting up the drain current to 2mA and P2 is used to adjust the output 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.
When used as an active I/V converter the +INP is grounded (R2 is shorted). The current ouput from the digital-analog-converter is connected to the -INP of the converter (R12) is shorted. The feedback resistor R13 determines the conversion factor, i.e. the V out/I in ratio. If the current from the DAC is maximum +/- 1mA, and you select R13=2k21, then the maximum output voltage will be V out=I in x R13=4.42 peak-to-peak or 1.56VRMS.
The 3rd order Bessel filter is built around a complementary JFET follower Q10-Q11. These are cascoded with Q9-Q10 to reduce input capacitance and distortion caused by capacitance modulation at high source impedance. Q10-Q11 is selected for Idss=8-10mA. Idss of Q9-Q12 has to be higher than that of Q10/Q11 to avoid forward biasing. The filter response with the buffer is: -3dB at 55 kHz. The filter components should have 2% or better tolerance. The layout allows using two resistors in series to make up the normally non-standard values for the filters.
On-Board Regulator
The EB-604/417 I/V converter and filter board has on-board regulators. The schematic of the discrete, All-FET regulator is shown in Figure 3. The regulators are discrete FET opamps; the positive regulator is made up of differential JFET pair Q1-Q2 and MOSFET Q5, the negative of Q3, Q4 and Q6. Q1, Q2 and Q3, Q4 are closely matched devices. Due to their relatively low breakdown 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 is 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 noise of the regulators is <5 micro Volt across the audio bandwidth.
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 | | | Figure 3 |
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The configuration in Fig 1. is using the circuit as and active I/V converter. However, the EB-604/417 can also be used with passive resistor I/V converter. In this case the I/V converter is used as a non-inverting amplifier. A resistor is connected from the current output of the DAC to ground. The resistor can be soldered directly between the +INP and the input ground. The current produces a voltage across this resistor, which is basically the audio signal. This is certainly the simplest way to convert the current to a voltage; however, it has its disadvantages. The current output of the DAC is supposed to stay at zero volts, as in the case, when it is connected to the virtual ground input (-INP) of an opamp. To protect the current output the digital-analog-converter chips normally have a pair of diodes connected to ground. If the voltage developed across the resistor gets close to the breakdown voltage of these diodes (approx. 0.7V), it will be distorted. Practical tests show that you have to select the resistor in such a way that the voltage across it is less than +/- 0.4V. With a peak current of say 1mA, the resistor cannot be larger than 400 Ohms. If there are no protection diodes at the current output then the resistor value can be increased, see the datasheet of the DAC you are using.
Due to the limited value of the resistor the output voltage will be lower than the level normally associated with CD players. Assuming a 400 Ohm resistor, the output is about 260m VRMS from a +/-1mA current. In order to bring this up to "normal" output level of 2 VRMS it has to be amplified by 7x times, or 17dB. This can be accomplished by selecting R13 =1k and R12=160R. The -INP of the I/V converter has to be connected to ground. |
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