Saturday, January 26, 2013

High Quality Vacuum Tube Headphone Amplifier

Theres no transformer in output, it only need coupling capacitor in the signal path is the output cap. A relay circuit was used to provide muting and a time delay was used to keep the output muted during warm-up in order to protect the headphones from the high voltages that would otherwise be present at the output of the amplifier.

A separate chassis was used for the power supply to keep noise levels as low as possible. Selection of input tubes (6SN7s) for low noise is important in order to realize the full capability of this circuit. Since I already have a regulated source of 12 volts DC, I intend to modify this amplifier in the future to use 12SX7s.

The power supply is a fairly conventional design using a 6550a as a series pass regulator and a zener string to set the operating voltage.

The amplifier circuit is a 6SN7 run in grounded cathode mode and is self-biased using an un-bypassed cathode resistor. The input stage is direct coupled to a 6CA7 that is used as a cathode follower. The output from the 6CA7 is coupled to the headphones using a 47uF capacitor, providing extended bass response. The output capacitors are bypassed with 1uf and .01uF polystyrene capacitors. An Alps 'Black Beauty' volume control is used to help preserver sonics and easily allows adjustment at low gain levels with adequate tracking between the two channels.

A switchable cross-feed circuit is present at the input of the amplifier to be used to help correct imaging problems, if desired.

Schematic of Hadphone Amplifier

Power Supply of Headphone Amplifier

Thursday, January 24, 2013

Class A Single end 2SK1058 Amplifier

Sure there has to be single active component in an amp in order to get the gain to be called an AMPLIFIER. That's a simple amplifier as a single end tube amplifier. A single end vacuum tube Amp need a resistor or two and an output transformer. So I design a FET amp around the same simplicity of design. One audio grade MOSFET, a couple of resistors and capacitors and a heavy duty well filtered power supply. The schematic for the amplifier section is shown in Figure 1.

The amplifier is built around the Hitachi 2SK1058 N-Channel MOSFET. A schematic and pin diagram for the 2SK1058 is shown in Figure 2. I used Sprague capacitors for the input coupling and a large electrolytic on the output with a 10µF polyester bypass capacitor. The four 10W non-inductive wire wound resistors act as the load. The four resistors are 15ohm and 10W each which I wired two in series for 30 ohms and then the two sets of 30 ohms are wired in parallel to give a total resistance of 15 ohms. These get extremely hot and burn about 30W at idle. Yes, Class-A amplification is very inefficient. I burn over 20 Watts to get only about 4.8 audio Watts per channel! The heatsink I used is rated at 0.784 °C/W.

2SK1058 Pin Out

Schematic 2SK1058 Amplifier

Power Supply of  2SK1058 Amplifier

Stereo 2SK1058 Amplifier Module

Thursday, January 10, 2013

300B Single End Vacuum Tube Amplifier

Current (new) production 300B tubes like the Electro-Harmonix 300B and the JJ 300B sound wonderful at a fraction of the Western Electric 300B price. A direct coupled driver stage and a low mu triode array are the specific reasons why this design works so well. Good output transformers are the key to good performance. Magnequest FS-030 or XE-60S are good choices.

The schematic for the 300B Single-Ended Vacuum Tube Amplifier is shown below. Using audio output transformers with a primary impedance of about 3000 to 3500 ohms you can expect about 8W of output with at about 2.5% THD. 

Saturday, January 5, 2013

Single End Tube Power Amplifier

  • This a simple Tube Amplifier with singgle end Tube.
  • The power supply use 4xIN4007 Silicon Diode.
  • The filament supply is regulated at 6.3V using a LT1085 voltage regulator.
The schematic taken from the book "Build your own Audio Valve Amplifiers" by Rainer zur Linde. 

Wednesday, January 2, 2013

Harmon Kardon Tone Control Circuit

Tube : 12AU7/12AX7
Frequency Response :

The vertical scale is linear in dB from minus 9 to plus 18 with marks every 3 dB. 
Frequency is from 10 cycles to 100 kc on a log scale. 
There are 5 curves making the shape of an hourglass turned on its side. 
  • The top curve starts at 10 cycles and plus 15 dB. At 50 cycles it is at 13 dB and falls until it reaches a minimum at 850 cycles and 1.5 dB. It rises again reaching 8 dB at 10 kc and topping out at 9 dB at 25 kc. It falls to 7.5 dB at 100 kc. 
  • The second curve starts out at 10 cycles and 6.5 dB, 50 cycles, 5 dB, 100, 3 dB, 200 cycles, 1 dB 400 cycles 0 dB. At 1 kc it is at 1 dB, at 2 kc, 2 dB and 10 kc 2.5 dB. Past 20 kc it starts to decline and is at 0 dB at 100 kc. 
  • The third curve is a straight line at just a scoash over 0 dB. 
  • The fourth curve starts at minus 4.5 dB at 10 cycles. It is minus 4 at 50 cycles, and has risen to 0 dB at 300 cycles. It rises about half a dB above zero and crosses zero again at 1 kc. It is at minus 1.5 dB at 10 kc and minus 2 dB at 100 kc. 
  • The fifth curve starts at minus 8.5 dB at 10 cycles and is at minus 8 dB at 50 cycles. It is at minus 3 dB at about 250 cycles and rises to a maximum of minus 0.5 dB at 850 cycles. It falls to minus 3.5 dB at 10 kc and minus 4 dB at 100 kc. End verbal description.