Monday, August 07, 2006

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 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
. 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!



Anonymous Anonymous said...

HI Stefaan

Just to make some of my comments public:

1. Are the H/M/L input & output levels on the tape board affected by the sensitivity decrease? and if so how?

2. I have used the 4th side of the disc adapter board to provide a CD input as per the Quad information sheet. Do I need to adjust anything here for the decreased sensitivity?

3. Can you provide pictures with the polarity of all electros and diodes marked? Once you've stripped the board I think it's a bit dangerous to intuit which way round things go from first principles.

4. Duncan Harbeth used to recommend changing C202/203 on the tape adapter board from .68uF to 3.3uF to improve the bass response & phase, but my modelling suggests that you would have to *reduce* it to .01uF before you got even 1dB down at 20Hz. YMMV.

8:55 AM  
Anonymous Anonymous said...

Follow up to point 4: this is indeed a worthwhile mod. My model was wrong (forgot the effect of the volume pot).

With C202/203=.33uF you are 3dB down at 50Hz, not good at all.

With C202/203=.68uF you are 2dB down at 25Hz (this is the mod Quad introduced in production). Still not great unless you like rumble filters.

With C202/203=3.3uF you are 0.5dB down at 20Hz. Much nicer.

10:08 AM  
Anonymous Anonymous said...

Oops again! With C202/203=.68uF you are 2dB down at *35Hz*, not great at all.

10:11 AM  
Anonymous Anonymous said...

and yet another oops, it was Duncan Harwood and it was 2.2uF.


.33uF: -3dB at 50Hz
.68uF: -3dB at 25Hz
2.2uF: -3dB at 10Hz

You have to wonder whether the original .33uF wasn't just a mistake, the same one I made.

5:45 AM  
Blogger Stefaan said...

Thanks, Esmond, for your usefull tips.

I will adapt the revision guidelines and add 2,2MF replacement caps for C202/C203 from now on in the kits.

In the same philosophy I will also add 1MF replacement caps for C200/C201.


4:08 PM  
Anonymous Anonymous said...

After upgrading the PSU board, and before replacing anything else, you should test it by refitting it into the 33, removing all the other boards, and applying power. The worst that can happen with all the other boards out is that you blow the PSU caps and diodes you've just replaced (incorrectly) and the 2 balance caps on the motherboard which you were going to replace anyway. You should measure +16VDC at the business end of the zener diode (the one that doesn't look like the other two).

9:49 AM  
Anonymous Anonymous said...

While you have the two amplifier boards out, check the large green capacitors for dry joints by gently pulling one end at a time and observing the joint on the track side of the board. I've seen this fault in all the 33s I've serviced so far, including my own! It produces crackles and bumps and eventually loss of that channel. If you find a dry joint, remove all the existing solder and re-solder, applying the heat and the solder to the stub lead coming out of the capacitor and letting it run down onto the PCB. Don't heat the PCB. You should end up with a nice rounded solder joint that you can't pull. Make this check on the phono preamp board and the phono and tape adapter boards too. Quad built these things very well but 40 years is 40 years!

6:09 AM  
Anonymous Anonymous said...

Hi Stefan,

I spent ages replacing the electrolytic capacitors on the 33 circuit boards including balance capacitors. Only recently did I change those in the power supply and this is what made the biggest difference to the sound. Big lesson is that the power supply capacitors also need changing.
because they are also part of the audio path.


11:23 AM  
Anonymous Anonymous said...

Yes, but by how much really all these mods does make the 33 sound like a high quality preamp, if at all ??

The difference in sound quality between my 33 and 303/FM3 pair is just huge... Does it worth the effort anyway ?

5:18 PM  
Anonymous Anonymous said...

It's a good idea to measure PSU voltage after installing parts. On zende diode end, I should expect 16VDC and how many Amp? Thanks! Jeff

3:06 AM  
Anonymous Anonymous said...

Hi, it's a great idea to measure the voltage after installing PSU, I should expect 16VDC and how many Amps? Thank you!

3:36 AM  

Post a Comment

<< Home