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A review of the 33 and 303 upgrades is available here.

The classic QUAD 303 although very good for its vintage is now behind the times as far as sound quality and technology are concerned. The sound quality of the 303 can now be brought up to very high performance levels with a new Power Supply circuit board (PCB) which has been specifically designed to fit inside the chassis just as the original did, but with much improved performance.

The Power Supply

As with all audio equipment the power supply plays a crucial role in overall performance. A weakness in the power supply will always produce deterioration in sound quality. Important aspects of the power supply are how the PSU voltages are produced and also their return path from the audio circuitry back to the power supply (the ground). The original power supply board has been redesigned using modern components and circuit techniques. There is no longer a need for the supply voltage to be adjusted to the required value as in the previous design. A stable voltage reference is used and then amplified using a single sided power amplifier with a set gain. This then provides a stable drift free voltage which is capable of sourcing many amps without 'sagging' more than a few mili Volts. Close decoupling is used with low impedance components to provide an additional path to ground for any noise which may still be present on the supply rails. This provides the best possible Power Supply for the active circuitry. As for the ground return, this is done using very low impedance ground planes which have been specifically 'split' to direct the ground currents back to the regulator ground with minimal interference of the sensitive signal grounds.

This easy to fit upgrade module provides a far cleaner, solid transparent sound with great sound staging, stereo focus and bass depth. All that is required is to de-solder the four wires on the original PCB and then solder the wire onto the upgrade PCB in the same place.

Now in improved MK1.2 form with additional circuit changes and the replacement of the original opamp with the OPA604 from Burr Brown have produced additional transparency, imaging and detail giving a better insight to the music and performers.

Schottky Rectifier

The rectifier in an amplifier is used to convert the AC waveform from the transformer to DC pulses. These pulses are then fed to the reservoir capacitors which store electrical charge and fill in the gaps between the pulses to provide a DC voltage which can now be used to feed the power supply board.

With standard rectifier diodes there is a slight delay when the diode has finished supplying its pulse of current to the PSU capacitors and is switching off. This causes a spike to appear and produces noise which is then superimposed on the  power supply line. Schottky diodes do not have this turn off characteristic and therefore produce a cleaner, quieter power supply.

The NET Audio 303 Schottky Rectifier uses 10A TO220 diodes in a bridge configuration with additional filtering on the incoming AC line and outgoing feed to the PSU capacitors to reduce any mains born noise and remove any rectifier switching noise. This will produce a very clean rectified DC voltage on the main PSU and therefore create increased insight, detail, depth and clarity to your upgraded Quad 303. 


NET Audio 303 PSU Upgrade Board MK1.2

NET Audio Schottky Bridge Rectifier and Filter

Click Here for the PSU wiring instructions and Here for the PCB layout in PDF format.

Below is the wiring information for the 303 MBA V1.0 PSU Board which is now being shipped for both stereo and mono 303 amps as it supersedes the standard 303 PSU MK1.2.


303 PSU Upgrade Board Connections

NET Audio Schottky Rectifier Wiring

Note:- The one penny sized capacitor on the transformer is removed when fitting the Schottky Rectifier. It is normally positioned across the transformer output windings and would be across the yellow wires in the photograph above. The red wire on the Rectifier board is the +ve output and should be connected to the positive on the power supply capacitors. The +ve is the connection on the far left hand side of the rectifier board in the middle photograph for all current boards. If your board is a mirror image of the one shown then the +ve connection is the one nearest the diodes. The latest rectifiers have + and - marked on the terminals on the underside of the board and AC on the terminals which should be connected to the transformer.

The bias can be set as per HERE but be carefull - the bias adjustment is very sensitive. A good digital voltmeter with a 200mV range is recommended.

303 PSU Upgrade Board Fitted - Original Development PCB


Additional Modifications for the experienced.

Improving the amplifiers PSU rejection.

Further improvements can be made to the 303, one of which is to increase the PSU rejection of the amplifier. The main gain stage transistor TR102 is biased directly from the supply through R112 - this has the effect of feeding small amounts of PSU noise into the amplifier which the feedback loop has now got to remove. An improved method is to replace R112 with a constant current diode to improve the dynamic impedance of the biasing component to substantially reduce any PSU noise which may be fed into the gain stage. A J507 constant current diode and a 22K resistor is used to replace R112, raising its dynamic impedance from 39k to over 5 million ohms. These are measured values. The J507 should be connected one way only - with the left hand pin looking from the flat side of the package being connected to the +ve supply.

Improving the Bootstrap on the output stage.

For 303 units after serial number 11500.

A bootstrap circuit is used to provide a constant current drive to the output stage of an amplifier under dynamic conditions. The power supply is fed through R116 and R117 (4K7 and 4K7) to produce a current of approximately the supply minus the voltage at TR103 base divided by the total resistance of R116 and R117 combined, i.e. 67-34/9400 = 3.51mA. As long as the voltage at TR103 base stays constant then the current thorough the two resistors stays constant - hence a constant current source. As soon as an audio signal is applied then the voltage at TR103 base will vary relative to the supply and therefore so does the current through the resistors which now produces distortion in TR102 which now has to be corrected for by the feedback loop. If a capacitor of large enough value is connected from the output of the amplifier to the junction of R116 and R117 (C106) then under dynamic conditions as the output voltage goes up and down then it also pushes the R116/R117 junction up and down by the same amount. This keep the voltage across R117 constant and therefore the current through it constant, reducing distortion in TR102 as this is the source of its collector current. If the output standing voltage set by RV100 is changed then the TR103 base to PSU voltage changes, so does the current through the resistors, which also now changes the voltage across TR107 and therefore the bias changes, hence the correlation between setting the standing voltage which also alters the bias slightly. This situation can be rectified by replacing R117 with a 3.6mA constant current diode making the current flowing through this path constant irrespective of the output standing voltage and also maintains a constant current under dynamic conditions as the bootstrap would do. The outcome is improved long term stability and an increase in sound quality.

J507 Diode with 22K Resistor plus J510 in place of R117.

Click on image for larger view.


Quad 303 amplifier schematic can be found here.

Extra Main Board Modification

Remove R115 (2.2 ohm resistor) on both channels and replace with the 2.2R resistor supplied in the upgrade kit, colour code Red, Red, Black, Silver, Brown, Red.

R115 is on the top side of the board and links across the two tracks as shown by the red line.


Upgrading the output stage.

The 303 can also be upgraded further by modifying the output stage to be fully complementary instead of the original quasi complementary design. Quasi complementary designs use two NPN output devices with one NPN and one PNP driver to produce a push pull output stage. Amplifiers were normally designed this way as PNP power transistors in the past tended to be much more fragile than their rugged NPN counterparts and therefore a quasi output stage would be more reliable. There is no need for this configuration now as modern PNP power transistors are the equal of their NPN brothers and they are also sonically superior to the older transistor types. The fully complementary output stage now has equal driving characteristics in both directions.

Recommended transistors would be MJ15003 for the NPN types and MJ15004 for the PNP types. These are more than adequately fast without being too fast which would cause instability. They are also very rugged high power transistors and should be very reliable in operation if correctly mounted on the heatsink with mica washers and good quality heatsink compound.

The output triple on the left is the standard Quad design - the one on the right is the fully complementary modification. Notice that the 10 ohm and 68 ohm resistors positions have been reversed and that the base drive current for the output PNP device is now taken from the collector of the driver, not the emitter as in the standard circuit. Both output Darlington triples are now exact complements of each other.


         Quad 303 heatsink showing location of new output transistors - bracket has been removed.


To take the drive for the new PNP output device from the driver collector, a small modification has to be made to the amplifier board. Cut the track carefully and then solder the wire link in as shown in the photograph. This takes the drive for the output transistor from the collector instead of from the emitter. Then remove the R126 (10 ohm) and R127 (68 ohm) resistors and reverse their positions on the board and re-solder them in place.


When wiring the new NPN and PNP transistors please take into account when connecting the PNP transistors that the collector and emitter connections wires are not connected to the same transistor terminals as when there were NPNs on the heatsink i.e. the original collector wire for the NPN now goes to the emitter of PNP when and the same for the original emitter wire - it now goes to the collector of the new PNP transistor.



                              Quad 303 Standard Wiring                                     Quad 303 Wired for complementary output stage. 

Replacing the Output Inductor

The standard output inductor on the 303 is adequate for normal use but has quite a high series resistance which increases the output impedance of the 303 and reduces the 303s ability to control the loudspeaker it is driving. Fitting a new inductor which has twice the current capability of the standard part reduces distortion on the output stage at high current levels and improves the sound quality.

The inductor can be found in the middle of the two output resistors.

Increasing the Current Limit on the 303 Output Stage    


Standard output stage bias and current limit components.

Increased current limit by the addition of an extra  4 1N4148 diodes.

The 303 current limit is set to around 4.3 amps which prevents excessive output stage current damaging the power transistors. It has been suggested in Hi Fi circles that an amplifier with a higher current capability always sound better at driving real world reactive and dynamically changing loads such as loudspeakers than an amp whose current limit is set to a theoretical maximum which should be enough to drive an 8 or 4 ohm resistor to the claimed power of the amplifier. Loudspeakers are not simply resistors but are a resistive load which also has inductance and or capacitance at different frequencies and does not behave like a resistor at all and can take larger currents than expected at a rated power. 

To resolve this problem a simple mod can be done to the 303 to increase the current limit. The 1N4148 diodes  (1S920s in the original 303) in conjunction with the bias circuit are used as a clamp to prevent more than 1.3V to appear across the 0.3 ohm output resistors, so providing a current limit of 1.3/0.3 or 4.3 amps. This current limit can quite easily be increased by adding more 1N4148 diodes in series with the original - two more adding another 1.3V to the clamp circuit, increasing the current to 2.6/0.3 = 8.6 amps. The output transistors are rated at 15 amps so it would be safe to drive 8 or 4 ohm loudspeakers, but the 303 is now no longer short circuit proof as in the original as excessive power would now be produced in the output transistors possibly causing damage to them.

The alternative to the diodes would be to reduce the 0.3 ohm output resistors to 0.15 ohms to double the current limit but this has the negative side effect of making the output stage more sensitive to temperature than the standard design and producing an amp with less stable bias currents on the output stage with temperature - decreasing the stability from the original design is not a good idea. Increasing the clamp voltage to provide the increased current limit does not change the bias stability of the 303 at all.

It is also a good idea to increase the output inductors current capability so that it does not saturate during high current which would add distortion to the output signal. If the high current modification is fitted then new inductors are suggested. These will be chosen and added to the website shortly.

    Adjusting the Bias.

The bias and offset voltage pots in the 303 tend to be open frame types which collect dust on the tracks making them jumpy and tricky to adjust correctly.

These can be replaced with more modern enclosed types as shown below. Prices are at the end of this page and they are supplied as pair of 1K and 4K7 pots. These are used to replace the original values for the bias adjustment (1K) and DC standing voltage adjustment (4K7).

The bias pot is now 1K as the original 2K2 was very tricky to adjust and had too much range. If the original pot was turned to the wrong end excessive bias would result with sometimes permanent damage to the output stage. R132 pictured below is replaced with a 2K4 (was 2K2 or 1K8) which provides a smaller adjustment range than the original and should be carried out in conjunction with the bootstrap modification for best stability. These modifications make the bias adjustment far easier than on the original design with finer resolution when setting the bias.



The bias can now be set as per HERE.

For those that want the Best.

The 303 once upgraded is a great amp capable of taking on very expensive modern equipment but it still lacks in certain areas, mainly the input DIN plug cannot accept modern high quality phono leads, the output connectors are standard off the shelf banana sockets and the mains input connector cannot be used with higher quality mains cables. 

NET Audio has designed a new panel to accommodate owners who would like to connect their 303 to their preamp and speaker with high quality cables or who just prefer the convenience that the new panel provides.

The DIN socket is replaced with gold plated, PTFE insulated phono sockets, the speaker terminals are now high quality gold plated binding posts which will accept banana plugs, bare cables or spade connectors and the mains inlet is now replaced with a fused IEC inlet plug which can now be used with standard high quality mains cables. 

In place of the fuse which is now included in the IEC inlet plug, a switch has been fitted which lights up when the amplifier is on. The original voltage selector fits in the same place as before.

NET Audio 303SE Front Panel.

The panel is manufactured from high grade 2mm thick aluminium plate which has been anodized dark bronze to match the original 303 casing. The lettering is engraved into the panel and then dyed pure white. 

The panel will be supplied with the correct fuse and spare fitted, the switch and all the connectors plus a length of twisted silver plated PTFE insulated wire which is used to rewire the input connectors to the amplifier boards. 

NET Audio Front Panel Wiring.

When wiring the NET Audio front panel in the two phono socket grounds should be soldered together and then connected to the chassis ground. This is the input ground and must be wired as shown to provide a route to the 0V rail of the amplifier for the input circuitry as in the photograph below. (Green wire supplied)

The two 2K2 resistors should be soldered across each channels speaker output sockets as per the original back panel.

The ground terminal of the IEC socket should also be soldered to one of the screws on the front panel to provide a ground for the chassis for safety reasons as in the photo below.


The purple wire from the original 303 wiring loom is then soldered to the lower of the two left hand terminals on the IEC socket and the black wire is connected to the upper terminal marked N which is to the left hand side of the ground terminal which is marked E.

The remainder of the wiring is shown below.

Remove the outer sheath from the original neon bulb and connect the black wire as shown in the above photograph to the 100K resistor. This black wire is the original wire which led to the neon bulb and it connects to the black wire which comes from the IEC inlet socket - the neutral line. The other end of the 100K resistor is then soldered to the gold terminal of the power switch. The purple wire which is connected to the voltage selector switch is then soldered to the middle switch terminal and the purple wire from the mains IEC inlet socket is soldered to the outer switch terminal.

All front panel switches supplied after January 2008 have internal resistors fitted instead of the external 100K resistor  - the gold terminal of the switch should be connected directly to the black wire on the voltage selector. (Orange wire supplied)

The amplifier input wiring grounds are connected together and then soldered to the two joined ground tags on the phono sockets. The left channel input wires are soldered to the amplifier board nearest the front panel and the right channel input wiring are soldered to the the amplifier board nearest the transformer. 

Quad 303 Amplifier PSU Upgrade Board MBA MK1.0

The 303 PSU has been updated slightly so it can be used in the stereo and mono 303 PSU and is now the 303 MBA V1.0

Upgrade includes:-

  • One 303 MBA MK1.0 PSU (supersedes previous V1.2) upgrade Board - Aligned and Tested

  • Full instructions on how to fit the upgrade module are here (PDF Format)

Total Price excluding P+P = £55 


NET Audio Shottky Diode Rectifier and Filter Board

303 Schottky Rectifier upgrade includes:-

  • One NET Audio Shottky Diode Rectifier and Filter Board

  • Full instructions on how to fit the upgrade module (PDF Format)

Total Price excluding P+P = £24.00


Quad 303 Replacement Capacitors

303 Replacement Capacitors for both channels of a Quad 303 Include:-

  • Two Panasonic 10000uF 63V Speaker Coupling Capacitors

  • Two Bypass Capacitors Capacitors

  • Two Panasonic 4700uF 100V PSU Capacitors

  • Six Panasonic Audio Grade Capacitors (3 per channel)

  • Two Input Capacitors

  • Four capacitor clips (optional, please add separately if you require them)

Total Price excluding P+P = £48


£5.00 for four optional capacitor mounting clips.


NET Audio 303SE Front Panel

303SE Front Panel Kit Includes:-

  • One Anodized 2mm Thick Aluminium Panel

  • Four Gold Plated Speaker Binding Posts

  • Two Gold Plated Phono Sockets

  • One Fused IEC Inlet Plug

  • One Illuminated Mains Switch

  • One Length of Silver Plated Input Wire 

Please allow up to 3 weeks for manufacture if not in stock.

Total Price excluding P+P = £65.00  - Out of Stock



Miscellaneous 303 Parts

£3.20 for a pair of J507 CC Diodes and a pair of 22K resistors. - No Stock



£4.50 for a pair of J510 CC Diodes. - No Stock



£4.80 for a pair of high power output inductors.


�2.50 for two Input Capacitors




Bias and Offset Modification Parts

£2.00 for two 1K, two 4K7 enclosed pots manufactured by Piher, plus two and two 2K4 1% resistors. These are for 303s from serial number 11501 onwards.


Labour Charges

All the modules and parts can be fitted for you - the charges are as follows:-

Power Supply module fitting              £10
Rectifier Module fitting                      £15
Transistor Replacement                     £20
Capacitor Replacement                     £20
Test  and Alignment                          £10
Front Panel Removal and Fittinng      £12

Return post and packing £12 standard for 303 Amp.

Quad 303 Full Upgrade

A full set of parts except for the output transistors to completely upgrade your 303. 

  • One NET Audio 303 Power Supply

  • One NET Audio Schottky Rectifier

  • Two  J507s + 22K resistors

  • Two J510 CC Diode

  •  Two 1K and two 4K7 enclosed pots manufactured by Piher and two 2K4 1% resistors.

  • Two BC Components Audio Grade 10000uF 63V Speaker Coupling Capacitors

  • Two BC Components Audio Grade 4700uF 100V PSU Capacitors

  • Four capacitor mounting clips

  • Two Speaker Bypass Capacitors

  • Six Elna Audio Grade Capacitors (3 per channel)

  • Two SMR Input Capacitors

  • Two high power inductors

  • Heat shrink

Total Price excluding P+P = £135.00. This item is sent Recorded Delivery in the UK.


Output Stage Transistors

Full set of output transistors and new power supply transistor to convert the 303 to complementary output stage. This consists of 3 x MJ15003 and 2 x MJ15004.

3 x MJ15003 and 2 x MJ15004 Power Transistors = £25


A set of 5 Silicon greaseless washers. These are used to mount the transistors to the heatsink without using messy silicon grease. When fitting the transistor mounting screws to secure the transistors to the heatsink using these washers, the screws should be turned untill it can be felt to be tightening on the transistor but no more so that it is finger tight and not hand tight. This should be done on all 10 screws. Once this has been done each screw should then be turned another 90 degrees to ensure correct torque but not be over tightened as it can cause the transistor to cut through the washer. Please check that all transistors are insulated from the heatsink (greater than 10M ohms) using a ohm meter before re-assembly. 

Five greaseless TO3 washers = £2.50


A set of 5 mica washers. These are used to mount the transistors to the heatsink and should be used in conjunction with the heatsink compound paste. When fitting the transistor mounting screws to secure the transistors to the heatsink using these washers, the screws should be turned untill it can be felt to be hand tight so the transistor is firmly mounted on the heatsink but not too tight as to bow the transistor base. Please check that all transistors are insulated from the heatsink (greater than 10M ohms) using a ohm meter before re-assembly of the wiring loom.

Five Mica TO3 washers = £1.00


Tube of Heatsink Compound Paste. This paste is applied on either side of the mica washer to aid heat conduction to the heatsink and is the most reliable method but is also a little messy. 

25g Tube of Heatsink Compound = £3.20


Full Upgrade

A full upgrade to the 303 can be completed at NET Audio as below

  • Fitment of a NET Audio 303 Power Supply

  • Fitment of a NET Audio Schottky Rectifier

  • Replacement of the output and main power supply transistors

  • Conversion of the output stage to complementary as above

  • Addition of the J507 + 22K resistor modification

  • Addition of J510 Bootstrap Modification

  • Two high power inductors

  • Two BC Components Audio Grade 10000uF 63V Speaker Coupling Capacitors

  • Two BC Components Audio Grade 4700uF 100V PSU Capacitors

  • Four capacitor mounting clips

  • Two Speaker Bypass Capacitors

  • Six Elna Audio Grade Capacitors (3 per channel)

  • Two SMR Input Capacitors

  • Five output stage Power Transistors

  • Five greaseless TO3 mounting washers

  • Heat shrink

  • Full alignment and bench test

  • Return postage and packing

The upgrade requires prior notification and the turn round time is on average 4 to 5 working days. The new NET Audio front panel is not included in the full upgrade but is an optional extra.

Total Price not including return P+P = £220 - Postage and Packing is charged at £15 plus the remainder will be invoiced when the 303 is ready to be returned if the customer is outside of the UK.


Please see the 303 MK3 page for upgrades of higher performance.

To Special Order - Your 303 can be converted to NET Audio 303MBA Monobloc Standard

  • Fitment of a NET Audio 303 Power Supply

  • Fitment of a NET Audio Schottky Rectifier

  • Replacement of the output and main power supply transistors

  • Conversion of the output stage to complementary

  • Additional board modifications

  • Addition of the J507  modification

  • Addition of J510 Bootstrap Modification

  • One high power inductor

  • One Panasonic 10000uF 63V Speaker Coupling Capacitor

  • Two Panasonic 4700uF 100V PSU Capacitors

  • Three capacitor mounting clips

  • One Speaker Bypass Capacitors

  • Three Elna Audio Grade Capacitors

  • One SMR Input Capacitor

  • Three output stage Power Transistors

  • Three greaseless TO3 mounting washers

  • New Rubber Feet

  • Full alignment and bench test

  • Full matching of both left and right channel amp boards

The upgrade requires prior notification and the turn round time is on average 1 to 2 weeks.

Total Price not including  return P+P = £210 each.


 Please see the 303 MK3 page for upgrades of higher performance.