Audio Circuit Denmark


LYNX Power Amplifier v3.0

Quick Assembly Guide v.1.0

Last updated 08.06.2008


1. Introduction and Specifications

2. Circuit Description

3. Component List

4. SOA protection calculation

5. PCB assembly

6. Pre-Test

7. Final assembly and test

8. Schematic and PCB layout

9. Basic wiring

10. Power Supply suggestion


The LYNX Power Amplifier is intended as a non-commercial diy-project, and it is therefore builder’s responsibility to

assure that assembly and wiring are made correct, and that suitable components are used.

Audio Circuit Denmark can therefore under no circumstances be held responsible for any incorrect information or

documentation, damages to persons and material, loss of money or what so ever.

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 1

1. Introductions and Specifications


This project had only one goal, and that was to upgrade the old v2.2 layout and create a rock-solid and powerful

sonic Power Amplifier like those used in recording studios. However, if you are fond of the deceased mighty and

"muscular" Power Amps from Phase Linear, SAE, GAS and Dynaco etc. you may be intrigued by this project as the

result is very close to those.


Components shown in the schematics are only suggestions. The circuit had shown good characteristics and had

produced a clear sound, when build with low cost components. So with this circuit you have the possibility to either

build a low cost dynamic Power Amplifier or a Studio Class Power Amplifier.

Technical Specifications:

With the use of a heavy Power Supply combined with a large Capacitor Bank, the LYNX Power Amplifier are able

to deliver loud and clear sound, and have shown capable of handling loads down to 2 Ohms without any instability.

Specifications rated at +/- 55 Volt DC Supply Voltage with 2x22.000 uF capacitor bank:

Output Power:

150 Watts pr. Channel into 8 Ohms

275 Watts pr. Channel into 4 Ohms

400 Watts pr. Channel into 2 Ohms

The Amplifier can run with supply voltages from +/-30 to +/-60 Volts DC with minor changes of some resistor


2. Circuit Description

The input signal from a Preamp or a Volume Control is applied to the Inverting Input (Pin 2) of the U101 Opamp

through a low-pass filter (R101 and C101), while the feedback signal (determent by R120 and R121) is applied to

the Non-Inverting Input (Pin 3) of the U101 Opamp.

U101 makes the first low Voltage Gain, and provide low output impedance to the Voltage Amplifier stage Q101 and

Q103 (Positive Voltage Amplification) and Q102 and Q104 (Negative Voltage Amplification).

The signal is then feed to the Current Gain Stages, which are build-up as “Three Deep Darlington” (TDD) circuits,

each consisting of one Pre-Driver (Q106 and Q107), one High Power Driver (Q110 and Q111) and the Output

Transistors (Q112 to Q119).

The Bias circuit consists of Q105, C104 (frequency compensation), R113, R115 and a Potentiometer R114, which

allows the Bias to be adjusted.

The +/-15 VDC Supply to U101 are provided by R122, ZD101 and C107 (Positive Supply) and R123, ZD102 and

C108 (Negative Supply).

A Current Limiting Circuit (R124 to R127, C105 and C106, D103 to D106 and Q108 to Q109) is fitted to protect the

output stage from delivering higher Currents than the SOA specify. The use of C105 and C106 allows high Current

peaks (dynamic peaks) on the output without limiting the output Current. Only if the current flow continues above

"Safety Level" the capacitors are charged and Q108/Q109 limits the output current by lowering the signal to the


If you are afraid of DC-Voltages on the input (from the outputs of your other equipment) you can add a 1uF to

2.2uF film capacitor to the input in series with R101.

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 2

3. Component List

Resistors: 0.6W: Size 7.5x2.5 mm, Pad distance on PCB: 10 mm

2.0W: Size 12x4 mm, Pad distance on PCB: 16 mm

5.0W: Size 17x6 mm, Pad distance on PCB: 22 mm

Item Description Mfr./Type Alternatives Notes

R101 1K, 0.6W 1%

R102 100K, 0.6W 1%

R103, 104, 107, 108 1K, 0.6W 1%

R105, 106 13K, 0.6W 1%

R109, 110 3.3K, 0.6W 1%

R111, 112 330R, 2W

R113 1K8, 0.6W 1-5%

R115 210R, 0.6W 1-5%

R116, 117 180R, 0.6W 1-5%

R118, 119 33R, 2W

R120 10K, 0.6W 1%

R121 390R, 0.6W 1-5%

R122, 123 (see next page) 2W

R124, 125 (see chap. 4) 0.6W 1-5%

R126, 127 100R, 0.6W

R128 to 135 0R22 to 0R47, 5W

R136, 137 10R, 5W

R138 to 145 0 to 10R, 0.6W 1%

R114 5K potentiometer Cermet, Bourns Equivalent 10-24 turns

C101 330pF WIMA FKP 2 WxL: 2.5x7.2 mm 5mm pin spacing

C102, 103 33pF WIMA FKP 2 WxL: 2.5x7.2 mm 5mm pin spacing

C105, 106 22uF, 50V Dia:8mm, P:3.5mm Non-Polarity

C104, 107, 108, 112, 113, 100nF, 63/100VDC VISHAY MKT WxL: 2.5x7.2 mm 5mm pin spacing

114, 115


C109 100pF WIMA FKP 2 WxL: 2.5x7.2 mm 5mm pin spacing

C110 470uF, 16V Panasonic EHG Equivalent 5mm pin spacing

C111 100nF, 150V Epcos WxL: 5x14 mm 10mm pin spacing

U101 OPA627 OPA134 Low DC-Offset


Q101, 108 MPSA42* OnSemi Equivalent

Q102, 109 MPSA92* OnSemi Equivalent

Q103, 107 MJE350 OnSemi Equivalent

Q104, 105, 106 MJE340 OnSemi Equivalent

Q110 MJE15032** OnSemi Equivalent MJE15030

Q111 MJE15033** OnSemi Equivalent MJE15031

Q112, 114, 116, 118 MJL21194 OnSemi Equivalent

Q113, 115, 117, 119 MJL21193 OnSemi Equivalent

D101 to 106 1N4148 Equivalent

D107, 108 BAV21 Equivalent

L101 2uH, 20A 16mm pad spacing

ZD101, ZD102 15V Zener, 1W 8mm pad distance

J101, 102 2-pin row 2.5mm pin spacing


FS101, FS102 See pics next page For fuses 5x20mm

Fuses See chapter 4 5x20mm

8-pin DIL socket for U101 1

Spacers M3 (malefemale)

4 Length 8-10mm

* Watch out. Some old versions (not On Semi) had a different pin layout. Check the manufacturer datasheet

** Beware of fakes (I had to scrap more than 100 transistors bought from a cheap supplier as the blew up in this amp)

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 3

R122/R123 calculation

This is how you calculate R122 and R123 (2W Zener resistors) @ Iz=20mA

(Rail Voltage–ZV)/0.02A=R (example with +/-55VDC Rail Voltage: (55-15)/0.02 = 40/0.02=2000R = 2K

How to make L101

Take app. 15 to 20 cm. of solid 1.5-2 mm thick insulated copper wire.

Carefully wrap the wire 10 times around a round object app. 8-10 mm in

diameter (e.g. a pencil). Make sure the windings are tight and pressed together.

Then carefully bend the ends to fit the 16mm hole distance on the PCB.

Cut the ends, and carefully remove 5mm of the insulating at each end.

Now carefully pull the coil of the object.

PCB pin-layout for transistors





1=E 2=B 3=C

1=B 2=C 3=E

1=B 2=C 3=E

Q101, 102, 108, 109

Q103 to 107

Q110, 111

Q112 to 119

PCB Pin-layout for R114 Bias trimmer (Bourns are shown)

Pin configuration for Opamp

Pin 1, 5 and 8 has no connection on PCB

Fuse holder for 5x20mm fuse (both types can be fitted on the PCB)

5mm pin spacing (RS Components)

22.5mm pin spacing (Farnell)

Important correction to PCB silk screen!


This should have




This should have



No that this is only a correction to the silk screen. PCB tracks are correct and match the MJE340/350 transistors.

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 4

4. SOA protection calculation

How to calculate R124 and R125

Below is shown the SOA (Safe Operating Area) for the MJL21193/94.

If you are using other output devices, please use the SOA protection curve for those in the calculations.

Below you see the protection circuit and the R124 and 125 to be calculated

To calculate the protection circuit you need to decide what Rail Voltages you will use for the output stage.

Also you have to decide the value of resistors R128 to R136 (0.22R to 0.47R).

Following example is the calculation for +/-50VDC Rail Voltage and the value 0.22R for R128 to R136.

In the SOA curve we can see that at 50V the MJL’s can handle 4A for 1 sec (red lines) @25C o . Knowing the value

of R128 to R136 (0.22R) we can calculate the Voltage across these at 4A. Using Ohms Law (IxR=V) 4Ax0.22R =


As Q108 will turn on at Vbe at app. 0.6V, the current across R126 at 0.6V will always be 6mA.

Now the Voltage across R124 can be calculated: 0.88V-0.6V=0.28V. Knowing that the Current through R124 and

R126 must be the same (not taking Base Current of Q108 into consideration), the Current through R124 is 0.006A

(6mA) at 0.28V. Using V/I=R we get 0.28V/0.006A= 46.7R (to be on the safe side and to take into mind that

Voltage and Current can be up to 45 degrees out of phase, select a resistor value lower than 46.7R e.g. 39R).

As the MJL’s are rough devices, you may be extra careful when using other output devices.

Select fuses according to the above calculation. In the above shown case the suitable value would be 16AT (app.

3.5A for each of the four output devices = 14A and then a little extra)

Read more on SOA on Elliott Sound Products great site: http://sound.westhost.com/soa.htm

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 5

5. PCB assembly


Components should easily be soldered in if you use following order:

1. Start with all 0.6W resistors according to the Component List

Always measure the resistance of each resistor before attaching it to the PCB!

2. Zener diodes ZD101 and ZD102 and all diodes D101 to D108

Raise Zeners app. 5 mm from the PCB, as they get warm

4. Opamp U101. Be careful to inset the opamp correctly.

Note that pin 1 is marked on the silk screen and by a square pad on the PCB

5. Transistors Q101, Q102, Q108 and Q109.

6. Capacitors (all)

7. 2W and 5W resistors (all) and potentiometer R114

Raise resistors app. 5-10 mm from the PCB as they can get warm

8. Transistors Q103, Q104, Q106 and Q107

9. L1, fuse holders and Terminal blocks (if used)

10. Now fit the four M3 spacers with the flat end down at each corner of the pcb.

Now you are ready to fit the Bias transistor Q105 and the Drivers Q110 and Q111.

Bend the legs 90 degrees as shown below and fit the transistors from the solder side.

Now place the pcb on the four spacers on a flat clean surface (or use the heatsink you intend to use), and adjust

the transistor placement so that the back of the transistors are laying flat on the surface.

Start by only soldering the middle legs (on component side), and then carefully align the transistors before

soldering the remaining legs (also on component side).

Q105 Bias transistor

Q110/111 Driver transistors

Check and double-check all solderings!

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 6

6. Pre-test

You can now make a pre-test without the output devices attached.

Note! Depending on Rail Voltages the driver transistors Q110 and Q111

can get very hot or even blow up if not fitted with heat sinks !

Connect a regulated +/-30 to 60 Volt DC-Supply to the +DC and –DC

terminals labelled “Supply to Voltage Amp”.

Positive Supply to the +DC terminal and Negative Supply to the –DC

terminal, using a 0.5 to 1A fuse in each supply line.

Connect the Power Supply GND to both the Supply GND and the Input GND


Connect a DC Volt Meter between “tp A” (at R118) and “tp C” (at R119)

– Positive input to “tp A”.

Power up the supply and watch the reading of the Meter.

Carefully adjust the R114 Bias potentiometer.

If the Voltage reading changes and you can adjust the reading to app. 1.4V,

then everything seems to be in order.

Measure between “tp B (at R118) and Gnd.

The reading should be lower than 50mV (DC Offset).

If not, make quick measurement across each Zener diode to confirm that the

+/-15V to the Opamp are present.

Then disconnect power and check for bad soldering etc.

7. Final assembly and test

If your pre-test result was ok, then fit the output devices Q112 to Q119 the same way as described above.

Now attach the complete assembled PCB to the heat sink.



Image here

After fitting the PCB, use a Ohm Meter to check that none of the Bias, Driver and Output transistors has connection

to the heat sink.

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 7

Final test

Connect Power wires to both the “Supply to Voltage

Amp” and the “Supply to Output Stage”.

Remember to connect the Input Gnd to Power Gnd.

Connect the Voltage Meter Gnd to Power Gnd, and

power up the amp.

Measure the Voltage across R128 (“tp E” and “tp F”) and adjust the Bias

trimmer R114 according following to set Bias to 50mA:

If R128=0.22R: 10mV, if R128=0.33R: 16mV and if R128=0.47R: 23mV

Then measure across R129 (“tp G” and “tp H”). The readings should be

app. the same.

Check the readings and adjust the Bias trimmer to the above for every 5

min. for 30 minutes. After 30 min. the Bias should have stabilized and

only measure small variations.

If you have an oscilloscope and a function generator, then apply a low

level signal (e.g. 100 mV/1kHz sine wave) across the Input terminals and

observe the output at either “tp F” or “tp H”.

As the amplifier has a gain of app. 25, you should now see a clear 2.5V

sine wave. If you observe any crossover distortion, then adjust the Bias

trimmer R114 until distortion disappears.

Slowly raise the Input Signal level until the amplifier clips, while checking

for abnormal signal handling or oscillation on the output.

Image here

Image here

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 8

8. Schematic and PCB layout

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 9

Silk Screen

Top Copper

Bottom Copper

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 10

9. Basic wiring

Here shown with one common Ground point (Start Ground)







For those in countries using Safety Ground, I think a 10R/5W resistor from Common Ground point to Safety Ground

should do it, however as we do not use this in Denmark I’m not sure………

You can read more on Elliott Sound Products: http://sound.westhost.com/psu-wiring.htm

10. Power Supply suggestion

Recommended Specification

Rail Voltages:

Transformer rating:

Capacitor Bank:

+/-30 to 60 Volt DC with common ground

300 to 500 Watts per channel

min. 2x15.000uF with 2x100nF parallel

Below is shown an example of a simple Power Supply for the LYNX Power Amp





Read more on Elliott Sound Products: http://sound.westhost.com/power-supplies.htm

Thanks to Rod Elliott for his permission to link to his very informative site: http://sound.westhost.com

LYNX v3.0 Power Amplifier by Jan Dupont, Audio Circuit Denmark


Page 11

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