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Selected Resistor Installation 

Depending on the type of headphones you have, you will set the gain of the headphone 
amp differently. Basically, there are two types of headphones out there: 

 

Modern headphones have low impedance, typically 32 Ohms, but could be as low 
as 18 Ohms. Grado SR-60E’s are typical of modern 32 Ohm headphones. They 
have a sensitivity of 99.8 dB SPL for 1 milliwatt, corresponding to a voltage of 
179 mV RMS. 

 

Some older headphones have an impedance of around 600 Ohms. The Sennheiser 
HD420 is an example of 600 Ohm phones. They have a listed sensitivity of 94 dB 
SPL for 1 milliwatt, corresponding to a voltage of 0.775 Volts RMS. 

Translate this to a common output level of 100 dB SPL, and we find that: 

 

Typical 32 Ohm phones produce 100 dB SPL with 183 mV RMS input 

 

Typical 600 Ohm phones produce 100 dB SPL with 1.55 Volts RMS input 

 
This big difference is a ratio of 1.55/0.0183=8.5 to 1, or 18.5 dB. That’s why we strongly 
recommend setting the gain of the Headphone amp to different values depending upon 
your headphones.  
 

Recommended resistor values for 32 Ohm headphones (most common) 

Designation Value 

Color 

Code 

Done 

 

R20 

30K1 

Orange, Black, Brown, Red, Brown 

 

R19 

30K1 

Orange, Black, Brown, Red, Brown 

 

R11 

NA 

Do not install 

 

R4 

NA 

Do not install 

 

If you install the resistor values for 32 Ohm headphones, you will have 2 spare 10K0 
resistors that are not used. 
 

Recommended resistor values for 600 Ohm headphones (not common) 

Designation Value 

Color 

Code 

Done 

 

R20 

jumper 

Piece of saved resistor lead 

 

R19 

jumper 

Piece of saved resistor lead 

 

R11 

10K0 

Brown, Black, Black, Red, Brown 

 

R4 

10K0 

Brown, Black, Black, Red, Brown 

 

If you install the resistor values for 600 Ohm headphones, you will have 2 spare 30K1 
resistors that are not used. 

Install the diodes 

Install the diodes. 

The polarity is critical. Make sure that the banded end of the diode 

matches the banded end of the silk-screen on the PCB.

  

Polarity is critical! 

Designation  Value 

Rating, Marking 

Done? (

) 

D1 

1N4004 

400V, 1 Amp Rectifier diode 

 

D2 

TVX6V2C 

6.2 V Zener 

 

D3 

BZX79-B10 

10 V Zener 

 

D4 

BZX79-B10 

10 V Zener 

 

Summary of Contents for PR-101

Page 1: ...Page 1 of 21 PR 101 STEREO PREAMPLIFIER Headphone Amp ASSEMBLY MANUAL 2015 2016 AkitikA LLC All rights reserved Revision 1p9 May 6 2016 ...

Page 2: ...grated Circuits 9 Install the Electrolytic polarized Capacitors 10 Install the Relays Transistors and MOSFET 10 Section 4 Install the Wires 10 Power and Ground Wires 10 12 Volt Ground wires 10 12 Volt Power wires 11 Install the Signal Wires 12 Signal Input Wires 12 Signal Output Wires 13 Section 5 Install the Headphone Amp into the PR 101 14 Mechanical Installation 14 Dismount the tone volume boar...

Page 3: ...component locations 6 Figure 3 Locating pin 1 of the IC and pin 1 of the PCB 9 Figure 4 Close up showing installation of 12V GND and 12V power wires 11 Figure 5 Tone Volume dismounted to make room to install Headphone Amp 14 Figure 6 Headphone Jack Wiring 16 Figure 7 Headphone Amp Installed with jack wiring completed 16 ...

Page 4: ...eed You ll need the following tools 1 Phillips screwdriver 1 and 2 2 Pliers or nut drivers suitable for 4 and 6 hardware 3 needle nose pliers helpful but not strictly necessary 4 pencil type soldering iron of 35 to 50 Watts no huge honking soldering guns or blowtorches 5 wire cutters and strippers 6 multi meter to measure power supply voltages and confirm resistor values strongly recommended Helpf...

Page 5: ... or get you to the hospital Read and understand the safety manuals of all the tools you use About Components We reserve the right to make design or component changes at any time without prior notification Recommended Solder The kit must be assembled with 60 40 Rosin Core solder The recommended diameter is 0 032 inches Among many such sources of solder I have used Radio Shack part number 64 009 It ...

Page 6: ...er is an easy way to make sure that you ve picked the right resistor It s a great cross check on the resistor color code Measure twice and solder once A lead bending jig can make for quicker neater assembly It s certainly not necessary Is something in this manual confusing Does something look wrong Send your questions by email to dan akitika com You ll help yourself and everyone who builds the kit...

Page 7: ...as described above 3 Solder the leads on the back of the board 4 Clip the leads Track your progress by placing a check mark in the done column as you install each component Bend resistor leads to 0 45 width Designation Value Color Code Done R21 100 Brown Black Black Black Brown R1 200 Red Black Black Black Brown R2 200 Red Black Black Black Brown R5 200 Red Black Black Black Brown R6 200 Red Black...

Page 8: ...why we strongly recommend setting the gain of the Headphone amp to different values depending upon your headphones Recommended resistor values for 32 Ohm headphones most common Designation Value Color Code Done R20 30K1 Orange Black Brown Red Brown R19 30K1 Orange Black Brown Red Brown R11 NA Do not install R4 NA Do not install If you install the resistor values for 32 Ohm headphones you will have...

Page 9: ...cation Hint a pair of these surrounds each IC Designation Value Rating Marking Done C3 0 1 uF Bypass 104 radial leads C4 0 1 uF Bypass 104 radial leads C7 0 1 uF Bypass 104 radial leads C8 0 1 uF Bypass 104 radial leads C10 0 1 uF Bypass 104 radial leads C12 0 1 uF Bypass 104 radial leads C13 0 1 uF Bypass 104 radial leads C14 0 1 uF Bypass 104 radial leads Figure 3 Locating pin 1 of the IC and pi...

Page 10: ...or 10 µF 50 V radial leads Install the Relays Transistors and MOSFET For Q1 Q3 make sure to match the body shape to the silk screen shape Designation Value Description Done K1 DPDT Double pole double throw 24 volt relay coil Q1 2N3904 NPN transistors Q2 2N3904 NPN transistor Q3 2N7000 MOSFET Section 4 Install the Wires Power and Ground Wires 12 Volt Ground wires Cut a 7 length of 18 AWG Green wire...

Page 11: ... pulling it lengthwise in a direction parallel to the wires Plus and Minus 12V power wire installation Done Twist and tin both bare ends of the red wire Twist and tin both bare ends of the black wire Insert the tinned red wire into the bottom of the Headphone Amp board into the TB5 hole marked POS12V and solder it on the top side The other side of this wire will be connected in a later step Insert...

Page 12: ...onnections This is the thinner gray jacketed cable Prepare the ends as shown in Appendix 1 Reserve the outer jacket removed when you exposed the ends Cut the black wire at both ends of the shielded cable It won t be used Strip twist and tin the remaining red ends 2 places Twist and tin the drain wire at both ends Cut a length of the outer jacket and slip it over the drain wire Insert the red wire ...

Page 13: ... outer jacket covering into the component side of the PCB into the Ground of TB2 Solder it on the solder side Signal Output Wires These wires will connect the output from the headphone amp to the headphone jack Headphone jack wires Task Done Cut a 3 length of black 22 AWG wire Strip of insulation from both ends Twist and tin the ends Insert one end of the black wire into the GND terminal of TB3 fr...

Page 14: ...nt the tone volume board 5 Connect signal wires Dismount the tone volume board Task Done Pull off the front panel knobs volume balance bass and treble Remove and reserve the nuts and washers that hold the volume balance bass and treble controls in place Remove the 10 pin cable from its socket Don t pull the cable but rather wiggle the body of the connector until it comes loose from the socket Remo...

Page 15: ...ls from the component side The standoff will just rest on the chassis floor Install the Headphone Amp board into the chassis using 4 40x1 4 screws inserted through the PCB and into the standoffs installed previously Wiring the Headphone Amp In Headphone Jack Connections Task Done Identify the red black and white 22 AWG wires installed into the Headphone Amp board previously Solder the red white an...

Page 16: ...acent power wires as you solder them in place 2 While soldering keep the wires straight as they approach the board This will keep the insulation from peeling back as you solder Power Wiring Task Done Insert the green wire into the component side of the power supply PCB into the PCB hole marked AGD on TB2 Solder from the component side Route the Red Black pair and the green wire from the Headphone ...

Page 17: ... board There are two ways to do this 1 Lazy way just tack the headphone shielded cable connections on top of the existing shielded cable connections that go to the input selector board 2 Fastidious way Desolder and remove the shielded cables from the input selector board that are connected to TB3 and TB4 of the tone volume board Use a solder sucker to clear the 4 holes in TB3 and TB4 Then follow t...

Page 18: ...wer to the PR 101 and power it up Be careful as potentially lethal voltages are accessible in the PR 101 any time that it is powered with the lid removed Check the plus and minus 12 volt supplies to assure that they are still within tolerance that is within 0 6 Volts of 12 Volts DC The easiest and quickest place to check this may be on the tone volume board at TB5 This is a quick sanity check for ...

Page 19: ...nges from BC547 transistors to 2N3904 transistors As an interesting note these two transistors have the same looking package but the position of collector and emitter leads is swapped between the two The PCB was laid out for 2N3904 pinout but early units were populated with BC547 That means that those units with BC547 were operating with reverse beta It is a testimony to my robust design and a bit...

Page 20: ... its long dimension This creates a scored line through the plastic jacket With a sharp blade not much pressure is needed You may need a bit of practice to get the feel 3 If you ve scored the jacket carefully you can separate the jacket at the score line without using tools Pull the insulating jacket off exposing the cable showing the foil shield the drain wire and the fuzzy string The result is sh...

Page 21: ... the plastic wrap from the red and black wires The drain bare wire red and black wires are exposed now that gray insulating jacket foil shield and plastic over wrap have been removed 7 Save the gray outer jacket as it will be used perhaps cut to half length to insulate the bare drain wire ...

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