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NET Audio 303 MK3 Amplifier Boards

The NET Audio Quad 303 MK3 circuit boards are now in their final testing stage and are now in production.

The Quad 303 can be upgrade to a very high level of sonic ability but to go further by simply modifying the original Quad boards has its limitations. For higher levels of performance to be reached new NET Audio boards have been designed to replace the originals in the same way as the PSU and rectifier boards were designed to improve on the original items. The NET Audio 303 MK3 amplifier boards have been designed which still uses the Darlington Triple output transistor stage to keep with the traditions of the original 303 but extensive circuit redesign has been made to increase the performance of the amplifier as a whole. The boards are the same size as the Quad boards and the mounting holes are identical so the NET Audio 303 MK3 boards will easily slot straight into a 303 MK1 (up to serial number 11499) or 303 MK2  chassis without any mechanical modification required to the chassis. The simply solder straight into the original wiring loom.

The new NET Audio 303 MK3 boards have bee designed to obtain increased levels of detail, tighter and deeper bass response, cleaner treble and better imaging with increased load driving ability than the best 303 MK2 full upgrade that NET Audio has previously achieved. The new MK3 had to beat a fully upgraded NET Audio 303 MK2 substantially to be worthy of sale as a NET Audio 303 MK3 and to guarantee a genuine leap in 303 performance. This goal has easily been achieved with the new NET Audio 303 MK3 amplifier boards.

The output stage has a link on board which allows the NET Audio 303 MK3 boards to be used with the Quad output transistors in their original quasi complementary circuit or to be used with the new MJ155003/MJ15004 output transistors in a complementary NPN/PNP configuration as described on this page Quad 303 PSU. If you have previously upgraded the output stage on your Quad 303 then these boards will simply wire straight in.  On Semiconductor MJE15031 and MJE15032 driver transistors are used to replace the original BC441/BC661 type transistors as the originals are older lower performance devices. The current limiting circuit on the output is adjustable between the standard 303 current limit and an increased current limit so that the NET Audio 303 MK3 can drive difficult loads down to 4 ohms and still be within the safe operating area of the output transistors. The design, however,  is not intended for continued high power use (disco etc) into low impedance loads (few Hi-Fi amps ever are), as the heat sinking on the 303 is not sufficient for this, but has been designed for normal domestic use as an amplifier of very high quality used to drive speakers which may dip to 4 ohms instead of the usual 8 to 16 ohms that the original design was intended for.

Installation and transistor checking notes can be found here.

Customer review A/B'd against a standard 303 Full Upgrade:-

"Simply not possible from a 303! The soundstage is huge with great space and definition between all instruments and voices. When I was sitting in the "sweet spot", my Pulse loudspeakers simply disappeared! Instruments represented at either extreme left or right were placed well beyond the traditional listening triangle. The vocal textures have an excellent level of detail, sounding tonally accurate and natural. Treble is smoother and the bass is more controlled (whilst still retaining impressive weight). Bass definition is also clearer and more defined with subtle changes in tone competently represented."

Specifications:-

Power Output 50W RMS into 8 Ohms
Input impedance 33K Ohms
Input sensitivity 575mV for 50W output into 8 Ohms
Frequency Response (-3dB) 5Hz to 90Khz 
Current limit 7.5 amp peak - 3.55 Amps peak are required for 50W into 8 Ohms
Stability - Unconditionally stable into any load.

Distortion into 8 ohms:-

                 1W           10W         45W

200Hz   0.0025%    0.0045%    0.011%

1KHz    0.003%    0.0024%    0.0062%

10KHz  0.013%    0.018%  

NET Audio 303 MK3 Circuit Board


NET Audio 303 MK3 Boards

The NET Audio 303 MK3 amplifier boards can be fitted to any Quad 303 by simply removing the original Quad 303 boards and then soldering in the new NET Audio 303 MK3 boards using the same wiring sequence on the wiring loom without any other alterations. Please see photographs above. Alignment will be required in the 303 once fitted - all that is needed is a digital voltmeter and a small screwdriver to adjust the trim pots.

NET Audio 303 MK3 Boards consist of

  • Two NET Audio 303 MK3 amplifier boards
  • Fitting instructions in downloadable PDF format

Total Price  = £160

NET Audio 303 MK3 Full Upgrade Kit

The NET Audio 303 MK3 Full Upgrade Kit contains all the parts to convert a standard 303 to a complete MK3.  Instructions on fitting of parts can be found on the other 303 upgrade pages.

NET Audio 303 MK3 Full Upgrade Kit contains

  • Two NET Audio 303 MK3 amplifier boards
  • One NET Audio 303 Power Supply Board
  • One NET Audio 303 Schottky Rectifier Board
  • 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
  • Five output stage Power Transistors
  • Five greaseless TO3 mounting washers

Total Price  = £290


NET Audio 303 MK3 Full Upgrade

The full upgrade consists of two NET Audio 303 MK3 boards, full assembled and tested and fitted into your 303 MK1 or MK2 chassis, PSU and rectifier boards as well as the output stage transistor replacement to suit the MK3 boards. Your 303 will be completely stripped down and the full upgrade fitted at NET Audio and fully bench tested once complete and will effectively be a totally new amplifier from front to back - only the wiring loom, transformer and chassis remain from the original 303. If a new front panel is required, then this can be bought separately from this page and will be fitted free of charge when your 303 is upgraded.

NET Audio 303 MK3 Full Upgrade consists of

  • Two NET Audio 303 MK3 amplifier boards
  • One NET Audio 303 MBA Power Supply Board
  • One NET Audio 303 Schottky Rectifier Board
  • Replacement of the output transistors with high performance devices
  • Replacement of the main PSU and Speaker Coupling Capacitors
  • Full bench test once the upgrade has been completed
Total Price  = £350

NET Audio 303 MK3 Monobloc

The NET Audio 303 MK3 Monobloc conversion is the ultimate 303 upgrade. It converts two standard 303 amplifiers to mono units for total left to right separation. As each Monobloc has its own power supply which is not shared with the other channel then it will produce a improved, more dynamic sound, closer to reality than its stereo cousin. Ideal to get the best out of you ESL 57s.

NET Audio 303 MK3 Monobloc Conversion consists of

  • One NET Audio 303 MK3 amplifier boards
  • One NET Audio 303 MBA Power Supply Board
  • One NET Audio 303 Schottky Rectifier Board
  • Replacement of the output transistors with high performance devices
  • Replacement of the main PSU and Speaker Coupling Capacitors Full bench test once the upgrade has been completed
Total Price  = £275 per channel

 

 NET Audio 303 MK3 MonoBloc Full Upgrade Kit

The NET Audio 303 MK3 MonoBloc Full Upgrade Kit contains all the parts to convert a standard 303 to a complete MK3 monobloc.

NET Audio 303 MK3 Full Upgrade Kit contains

  • One NET Audio 303 MK3 amplifier boards
  • One NET Audio 303 Power Supply Board
  • One NET Audio 303 Schottky Rectifier Board
  • One BC Components Audio Grade 10000uF 63V Speaker Coupling Capacitor
  • Two BC Components Audio Grade 4700uF 100V PSU Capacitors
  • Three capacitor mounting clips
  • One Speaker Bypass Capacitor
  • Three output stage Power Transistors
  • Three greaseless TO3 mounting washers

Total Price  = £200