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2A3 20W膽機電路圖設計

admin @ 2014-03-15 , reply:0

概述

AmplifierConfiguration1.Overallconfiguration6BM8:triodeunitforvoltageamplificationstage--6BM8:pentod……
Amplifier Configuration
1.Overall configuration
6BM8: triode unit for voltage amplification stage -- 6BM8: pentode unit in triode connection driver -- interstage transformer -- 2A3 fixed bias --
push-pull output transformer 5Kohms.
The voltage amplification stage and driver use 6BM8 that is well-known as triode-pentode multiple-unit small power tube.
2A3膽機電路圖(點擊看原圖)
2. Output stage
The tube operation is class A2 push-pull that is driven up to grid + field in order to obtain the power efficiently out of 2A3 whose plate dissipation is 15W. Plus, fixed bias system is employed for efficient use of the supply power voltage.
The operating condition of the output stage is as follows.

Plate voltage
320V (at 0 input), 290V (at max. output)
Plate current
80mA (for 2 tubes)
Grid voltage
-65V
Load
5Kohms(between P-P)

Up to about 12W, the operation is class A1 and from 12W and above class A2 that is driven into grid + field. Since the grid current flows in grid + field, a powerful driver stage is necessary not to suffer the grid current and to supply high voltage.
Output transformer is
RX-40-5.


3. Fixed bias circuit using MOS-FET

The fixed bias circuit must absorb the grid current flowing in the grid of 2A3 not only supplies the negative voltage for biasing the grid of 2A3. When the grid is swung into grid + field, the signal is rectified by the grid and thus generating DC component. By this, the grid voltage goes further to negative side, which reduces the plate current and thus suppresses the output increase. In order to increase efficiently the output power, the fixed bias circuit must retain the bias voltage constant even if the grid current flows. In this amplifier, an MOS-FET source follower circuit is installed after the normal fixed bias circuit to absorb the grid current.
The reason of lay-outing Nch MOS-FET on the opposite side to the general voltage regulator source follower is that the grid current works to increase the bias voltage not to decrease it. Nch MOS-FET consumes the current from the grid that works to increase the bias voltage and thus keeps the voltage on the grid constant.

4. Driver stage
The combination of 6BM8 pentode unit in triode connection and
RC-20 interstage transformer obtains high power without being affected by the grid current. The connection of RC-20 interstage transformer is primary in series and secondary in push-pull connection. Phase inversion is made by 1: 0.9+0.9 and amplification operation by 1:1.8. The secondary of RC-20 is open (without load). This enables both to obtain the natural and unconstrained sound quality proper to open operation and to feed entire supply current from the interstage transformer to the grid of power tube.
The operating condition of 6BM8 pentode unit is effective plate voltage 190V, plate current 10mA.
5. Voltage amplification stage (first stage)
The voltage amplification stage uses 6BM8 triode unit. Plate load resistance is 100Kohms, putting the unit on a standard operating condition. This unit is directly coupled with the next driver stage. High-cut filter made up with 10Kohms and 50pF and provided at the top of input is for suppressing incoming high-frequency noise or leakage noise from SACD.

6. Negative feedback
The gain of this amplifier without negative feedback is 65, being considerably high for use. Besides, the damping factor is about 1.5 because of the combination of the plate resistance of 2A3 being sufficiently low but with the output transformer of 5Kohms load between P-P. Thus, the damping at low frequency would be poor.
So, we employ negative feedback to reduce the gain and increase the damping factor. After providing 12dB of NFB, the gain will become 16, quite appropriate to use. However, it is practically difficult to provide an overall negative feedback 12dB to the circuit including two transformers: interstage and output transformer.
Therefore, for this amplifier, we have employed both 6dB minor negative feed back from the plate of driver tube 6BM8 pentode in triode connection, and 6dB major negative feedback from the speaker terminal of the output transformer.
With 6dB major negative feedback, the damping factor is appropriately set about 3.8.
Major and minor negative feedback presents a very stable
Square wave response in spite of the circuitry including two transformers.

7. Power supply
Power transformer is a reasonable multipurpose transformer PCM-170M of
Noguchi. DC 325V is obtained by diode center tap rectification of AC 290V. Choke coil is a multipurpose PCM-1520 of Noguchi. Electrolytic capacitor 400V 100uF is a small-sized general purpose radial type made by Unicon that has once been distributed by Softone.
Negative voltage supply for fixed bias is –100V that is made from AC 70V half-wave rectification.
Power supply 2.5V 2.5A for 2A3 filament is DC to reduce hum noise level. Two lines of DC2.5V are obtained each by bridge rectification of two windings of 3.8V 3A that are found between the terminals 6.3V-2.5V of PMC-170M. After the rectification, the ripple components are reduced by 10,000uF.

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