Quad 33 Revision - Illustrated Guidelines

These are the step-by-step guidelines for upgrading the Quad 33 amplifier to modern high-end standards.

You can order the components from your local dealer or you can use the 33 upgrade-kit available via www.dadaelectronics.eu or via the Ebay webshop - Objectnumber 200014330586.

All resistances are Metalfilm low-noise 1% and all capacitors are Audio-grade (except in the power-supply) and 25 Volt or more.

Watch the polarity of all diodes & electrolyt capacitors!

If you need a (free) copy of the schematics or the service-manual, send an e-mail to info@dadaelectronics.eu. You can also get 7/7 technical support about upgrading the 33 via this e-mail.

Step 1 - Modifying the power-supply from 11 to 16 Volt

Increasing the power-supply voltage to 16V makes the pre-amp more stable. The reason why Quad used an 11 Volt power-supply was the size of the capacitors at the time the 33 was designed.

We will also remove the 5V line on pin 4 of the output that was intended for switching on the power-amplifier but that was never used with the Quad power-amps.

• Desolder the wire-beam and the mains-connections on the power-supply board. Remove the power-supply board (2 screws holding the transformer flanges). Remove all components except the transformer from the board.
• Replace R500 and R501 with 27R resistances
• Replace MR500 with a 1,3W 16 Zener
  
• Replace MR501 & MR502 with 1N4004
• Replace C500 & C501 with 4700MF. Mount them pretty flush so you can still access the screw hole in the transformer flange
  
• Replace C502 with 1000MF axial
• Remove R502, MR503, C503 & C504

Watch the polarities. The 3 diodes have the Katode (the black band) to the center of the board. C500 and C501 have the - on the topside and C502 has the + on the topside.

Step 2 - Changing the Balance-caps on the motherboard

Those 2 electrolyt caps are old and have dried out.

• Replace C5 & C6 on the motherboard with 100MF

Step 3 - Upgrading the amplifier boards

We will reduce the sensitivity of the Radio 1 & Radio 2 inputs from 100mV to 250mV. This will allow for "modern" sources like CD-players to be connected without distortion and this will also improve the signal-to-noise ratio with another 6dB.

This is optional, if you only use Quad input-sources (and no CD-players ore sources with line-voltage) you could skip this step by not replacing R411/412.

Of course we will also replace all the old electrolyt-capacitors.

Repeat these steps for both amplifier-boards.

• Replace C401 with 2,2MF
• Replace C405 with 47MF
• Replace C406 with 22MF
• Replace R411 with 1K (optional)
  
• Replace R412 with 1K2 (optional)
  

Step 4 - Upgrading the Phono preamplifier board

We have to change R300 to adapt the phono-preamp to the new power-supply voltage of 16V & we'll replace all electrolyt capacitors. R305 & R308 are replaced with 1% metalfilm for better stability.

• Replace R300 with 1K5 to adapt the board to 16V power-supply)
  
• Replace R305 & R308 with 82K
• Replace C300,C303,C307,C308,C311 & C312 with 47MF
• Replace C301 & C302 with 100MF
• Replace C313 with 220MF (or 470MF)

Mount the capacitors fairly tight otherwise the board won't fit in the amplifier.

Step 5 - Adapting the Disc Adaptor Board

As we decreased the overall amplification we should increase the sensitivity of the Phono-input to stay around the same level.

Other combinations or the 4 resistances are possible to adapt the Disc-input to the PU-element.

• Replace R105 & R106 with 560R
• Replace R107 & R108 with 100R

Step 6 - Replacing resistances on the Tape Adaptor board

4 resistances on the Tape-adaptor board are replaced with Metalfilm resistances.

C202/203 are replaced with 2,2MF to improve the bass response and the phase coherence.

• Replace R203, R204, R215 & R216 with 220K
• Replace C202 and C203 with 2,2MF, the + side to the end of the board.
  

And... That's it!

Stefaan