K272X - STEREO TUBE HEADPHONE AMPLIFIER
Note that this is an improved version of the original K272:
FIRST DEVELOPED BY OATLEY ELECTRONICS IN 2009!
REVIEWS for this unit can be found by searching for “K272 headphone”. Comments relating to this amplifier include “Excellent sound”, “blew me out of my favorite listening chair”, “stunning” etc. Also search for reviews by Mark Houston, and the audioXpress magazine: “Oatley K272A Head Amp audioXpress Review”
http://diyaudioprojects.blogspot.com.au/2010/11/oatley-k272a-head-amp-audioxpress.html
http://diyaudioprojects.com/Tubes/6418-Tube-Preamp-Headphone-Kit/
SO WHAT IS THE IMPROVEMENT? :
The original version (K272) used relatively expensive 9V batteries, so the filaments were connected in series to conserve battery power. This meant that the channels had a slight unbalance, as they effectively had a slightly different supply voltage. The upgraded version (K272A) corrected this unbalance, but to conserve power it used two separate AA batteries to supply the filaments. This unit (K272X) uses 8 relatively low cost, and high capacity AA batteries, so the extra current drawn by the paralleled filaments is unimportant. Also the raised supply to the tube and the CMOS amplifier makes for extra “headroom”. Instead of being 9V and 5V these are now 12V and 6.8V.
MORE NOTES:
This low cost tube based Headphone Amplifier lets you experience “tube sound”. It is based on two low power consumption US Made Raytheon JAN6418 subminiature pentodes. The amplifier also employs an low distortion, and low noise HEADPHONE POWER AMPLIFIER IC (NOT AN OPERALTIONAL AMPLIFIER) incorporating CMOS technology. This results in excellent overall performance, over the whole audio spectrum.
The tubes that are used (JAN6418) were made in USA, very late in the tube-making era: Around 1980. It is interesting to know that Audio Technica used one of these tubes in each of their model AT3060 condenser microphone, which sold for around $600US. Professionals describe this microphone as having a “Warm Classic Tube Sound”.
Even when used in conjunction with solid state power amplifiers, professional musicians have described tube preamplifiers as adding a real “richness”, and “warmth” to the sound.
The warmth in this preamplifier can certainly not be attributed to the heat given off by the filament: The 1.2V filament draws just 10mA! You can barely see it glow in a dark room. This whole amplifier draws just 35mA from the 12V-battery pack.
The gain of this preamplifier is adjustable from 0 to 7 times, and it can be easily increased. The circuitry also includes an LED battery voltage monitor.
HOW IT WORKS:
AMPLIFIERS: There are two separate amplifiers, being for the Left and Right channels. The Right channel employs tube V1 and a CMOS operational amplifier IC1:B, and the Left employs V2 and IC1:A.
Since the L + R channels are almost identical the Right channel only is described here…The input signal is applied via coupling capacitor C2 to a trim-potentiometer VR1, and it output is connected to the grid of tube V1.
The amplified signal (7X) at the anode of V2 is applied via coupling capacitor C4 to an inverting buffer amplifier stage based on IC1:B. The gain of this stage is set by the resistors R10 and R6: Gain=R10/R6, and in this case it is 1. For example changing the value of R7 to 2M would result with a stage gain of 2X. In this case the overall gain of the preamplifier would 14X: (7X2).
Voltage divider made up by R7 and R6 biases both the non inverting inputs of IC2 to +3.4, and thus the quiescent DC voltage at the outputs of both the OP-AMPS is set to ½ of 6.8V rail voltage: 3.4V. Capacitor C8 couples the output signal from IC1:B to the load.
6.8V REGULATOR: The output voltage of the 7805 5V regulator IC (IC2), is boosted by the 1.8V drop across the Red LED in Common terminal. The output voltage is therefore 5+1.8V=6.8V. The regulated 6.8V supply is used to power the PT2308 Amplifier IC. It also supplies a constant current of 10mA to each of the filaments via LED3 and LED4, and the current limiting resistors R2 and R4.
BATTERY VOLTAGE INDICATOR: The series combination of LED L1 and resistor R12 is connected between the Battery positive and the 6.8V regulator output. The LED requires 1.8V across it to illuminate and will become very dim when the battery voltage falls to near 9V.
NOTE THAT THE VALUE OF RESISTOR R12 WAS CHANGED TO 390R
ONLY THE PCB AND ALL THE ON-BOARD COMPONENTS ARE SUPPLIED WITH THIS KIT, AS PICTURED ON THE FIRST PAGE, AND LISTED IN THE FOLLOWING PARTS LIST:
MISCELLANEOUS:
1 X K272X PCB:
2 X 6418 Valves/Tubes
4 X Rubber Grommets
2 X 100Kohm Trim-potentiometers
1/4W RESISTORS:
1 X 0ohm ( Link), 3 X 390ohm, 3X 12Kohm, 2 X 180Kohm, 4 X 1Mohm
CAPACITORS:
4 X 0.47uF 200V Metalyzed Polyester
5 X 100uF/25V Electrolytic
2 X 1000uF/16V Electrolytic
SEMICONDUCTORS:
4 X HLMP 1340 Red LEDS
1 X 7805 5V Regulator IC
1 X PT2308 Dual CMOS Amplifier IC
CONSTRUCTION NOTES
The Anode pin (1) of the tube is at the end which is marked with a red dot. This pin is inserted into the pad, which is square in shape.
To eliminate possible Microphony, two of the supplied Rubber Grommets should be inserted around each of the tubes. The Grommets will easily slide around the tubes after first being damped in water, It also helps to leave the leads of the tubes quite long (Around 10-15mm) so they act like springs. The tubes can also be mounted horizontally, and spaced away from the PCB.
THE GOLD PLATED PCB HAS LAGE CONNECTING PADS
Connecting wires can be soldered to the underside of the PCB
To save on the cost of shipping these notes will not be included with the kit, they will be available on our Website. Search for K272X at:
oatleyelectronics.com
PACKING TO SAVE COST ON SHIPPING: AUSRTALIA POST HAS A SPECIAL RATE FOR SHIPPING LETTERS WITH A MAXIMUM WIDTH OF UNDER 20mm...We based the shipping on this rate and include the TUBES!! which will be carefully packed!
NOTE AL:SO that we will soon offer a package of 3 kits that includes shipping to anywhere in our world for $100AU!!!