” La Bohème ”

(Phono Preamplifier MK III)

The MK III phono stage ” La Bohème ” evolved from the MK II predecessor. The philosophy and basic configurations of both versions are identical and explained in detail on the page describing the MK II phono stage (click here for that page). However, input stage, RIAA equalization, output stage and power supply underwent major modifications to optimize the sonic performance.

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Figure 1-3: MK III phono stage. Although the phono stage momentarily is without any housing or shielding there is no hum or RF pick-up.

Figure 4: Star-wiring to avoid ground loops. The resistor was glued on the transistors for experiments with different temperatures (click here for details).

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Figure 5: Symmetric XLR inputs

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Figure 6: Connector to external power supply.

Input stage

The MK III input stage was redesigned for higher gain. The 2.5kOhm resistors of the  MK II cascodes were replaced by 4.7kOhm resistors which increased the gain from 40db to 45db. In order to keep the other settings constant, the supply voltage had to be raised from +36V to +48V, which is at the maximum ratings of the 2SK369 (see datasheet) input transistors and BC550C cascode transistors (see data sheet). During operation the voltage at the collector of the BC550C is around 25V. However, damage may occur, if the phono stage is switched on without the 9V batteries inserted (see schematic).

Two of the batteries provide the voltages necessary to control the operating point of the upper cascode transistors BC 550C and the constant current source. These batteries are mounted directly on the PCB to ensure the shortest wiring possible especially for the cascode battery.

Three additional batteries are in parallel to the positive and negative rail capacitors. These batteries are necessary to ensure defined conditions of the input stage when the phono stage is switched off from the external power supply. The two 1N4004 diodes protect these batteries from the power supply voltages during operation. Without these auxiliary voltages the current source cannot be set to zero. Hence the cascode battery discharges via the FETs and the base emitter junctions of the bipolar transistors and damages the electrolyte capacitors of the negative rail by inverting their polarity.

Loading of the batteries is minimal during operation as well as in switched off mode. I use 9V NiMH accumulators and routinely recharge them once a week, independent from the hours of operation.

The BFR 91 of the current source in the MK II version was replaced by a BC550C without any negative consequences.

Figure 7: Schematic of the MK III phono stage (click on the picture to enlarge)

RIAA equalization

RIAA equalization was adapted to the 4.7kOhm resistors and optimized by switching from the cheap MKS capacitors to better KP capacitors. The capacitors were matched to less than 0.1% left / right channel. Also the RIAA response was improved (see Table 1 and RIAA plots). The highest deviation from the theoretical RIAA was 0.16db, measured at 20Hz. The RIAA accuracy is now at the limits of my measuring instruments (AC Millivoltmeter Präcitronic MV21, Voltcraft 7202 Sweep Function Generator). The MK III RIAA network still has a fourth time constant added, but its effect was slightly reduced by the two 10nF capacitors at the inputs of the OPA627 opamps.

Table 1: RIAA response of the MK III phono stage.

Frequency

RIAA measured

RIAA theoretical

RIAA difference

 

db

db

db

 

 

 

 

10

19,82

19,74

0,08

20

19,43

19,27

0,16

50

17,03

16,95

0,08

100

13,03

13,09

-0,06

200

8,30

8,22

0,08

500

2,61

2,65

-0,04

1k

0,00

0,00

0,00

2k

-2,62

-2,59

-0,03

5k

-8,18

-8,21

0,03

10k

-13,64

-13,73

0,09

20k

-18,42

-19,62

1,20

50k

-25,04

-27,54

2,50

100k

-27,54

-33,56

6,02

Output stage

The output stage was optimized for wide bandwidth and low output impedance across the audio band to match the axiom:

“Every point of a circuit that is required to ’move’ must be able to move free and fast.” *

                        * Wright A. Basic Philosophies. In Wright A. The Tube Preamp CookBook. Second Edition, Vacuum State Electronics Maroubra, Australia 1997.

The output stage was implemented with a Lundahl LL7902 audio transformer for low copper resistance and high level capability. OPA627 op amps were combined with current-feedback op amps OPA603 resulting in composite op amps with very high bandwidth, high gain, high output current, excellent DC performance and very low thermal feedback.

The output stage is described in detail on a separate page (click here for that page).

Power supply

The six fully charged 9V NiMH accumulators were capable to run the MK II phono stage for two hours. For longer listening you had to take a break for changing the accumulators. This was OK for me, because if you are attending an opera, you also have a break to enjoy a glass of champagne, but the limited playing time with a single set of accumulators was seen critical by others.

Also in the MK III version I stayed with my paradigm of an accumulator based power supply. However, the MK III output stage needs an quiescent current of 110mA, while the output stage of the MK II version only needs 30mA. So the MK III version powered by eight lead gel accumulators placed in a separate enclosure (fig. 6, 7 and 8).

In operation mode six 1.2Ah lead gel accumulators are connected in series via relais to provide +48V and -24V for the input stage. Two 7.2Ah accumulators provide +/-12V for the output stage (see schematic of power supply). The two 1.5 Ohm resistors limit the current during charging. Hence input stage and output stage have independent power supplies. The power supply ensures a playing time of more than 24 hours.

If the main switch is closed the relais switch from operation mode to charging mode (green). During charging mode each accumulator is charged independently with 13.6V from a LT317 regulator. Without the 1N4004 diodes in series with the regulators the accumulators would feed the relais coils after the main switch is opened and the power supply would not automatically switch back to operation mode.

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Figure 8: Schematic of the MK III power supply.

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Figure 9: External accumulator based power supply

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Figure 10: Control unit of the power supply

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