The quad 33 saga continues:

The quad 33 saga continues:

Well; having acquired a second Quad33 control unit in poor condition I was presented with a few possibilities.

I could go for an all out modification, In my experience this type of bodging generally results in a product that is very different from the original and you would have been better off beginning with a clean slate. In my opinion you are better off starting from fresh with new parts and a good clean case, you end up with much better results.

The second option is to restore the original unit back to its original condition, but I already have a good condition Quad33 so that seemed a bit pointless. As an added disappointment, the filter selector switches had failed on the second 33 control unit and is pretty much (but not entirely) beyond repair so it will probably become my spare parts source.

I am keen to keep my first good Quad33 control unit in original condition, keep the original sound and wherever possible the original componentry. What to do? Well I was curious to see what improvements replacing the 30 year old electrolytic caps could do and just how much they had deteriorated over time. The original electrolytic caps used in my good Quad33 (Serial number 79889) are all Roederstein EK series (My earlier Quad33, serial number 30006 used Callins sourced capacitors). These “ROE Red” capacitors seemed to be quite popular in English 70’s and early 80’s audio equipment and seemed to enjoy a good reputation as an audio cap in coupling, DC blocking in feedback loops and bypassing applications. Unfortunately the red phenolic casings (hence “ROE Red” nick name) can sometimes breakdown over time and the capacitors electrolyte can leak out. I regularly see these “ROE Red” capacitors in older gear with bulging tops cracked sides and the tell tale sign of yellow residue seeping out of the cracks. None of the ROE reds in my good Quad 33 showed these symptoms but the doubt was still there in the back of my mind.

With a second set of boards donated by the less than operational Quad 33 Control unit I was now able to rebuild a set of boards with newer modern low ESR series Electrolytics of the same value while leaving the original Quad 33 boards unmolested. As well as replacing the Electrolytic capacitors I was also able to populate the boards with modern metal film resistors. The older carbon resistors can sometime drift in value over time as well as generate higher levels of noise, particularly in low signal and high gain applications such as phono cartridge preamplifiers. I decided to leave the original film capacitors in place since these are fairly robust and don’t suffer the same affects of aging as carbon resistors and older electrolytic capacitors.

Original QUAD 33 plug in boards.

Rebuilt QUAD 33 plug in boards using modern components.

A few other minor changes worth performing on top of the above component replacements are changing the input loading of the Phono stage from Quads unusual choice of 68K back to a more standard 47K. This is an easy change and one that you could do to any Quad 33. Just change the resistors R101 and R102 for the M1 position, and the same for R103 and R104 for the M2 setting; job done. The other change worth doing is reducing the gain of the output stage by about 6 dB. Modern source components put out a bit more signal level than they did in the 70,s so pulling back the overall gain by about 6dB is going to give better a volume control action and a little less noise from the output stage as well. This is also an easy tweak that can be performed by changing R411 on the output board from 1K8 to 1K and R412 from 470 ohms to 1K2.

One more modification that is worth considering is the removal of the low frequency 30 Hz filter. The capacitor C400 on the output board is a 0.068 uf film capacitor running into a 47K load provided by R402. Swap this cap for a 1uf film cap and you should move the inbuilt low frequency filter to a point way below the audio band.

The other electrolytic capacitor in the audio circuit that is quite often overlooked on the Quad 33 is the DC blocking capacitors in the balance control, C2 and C5 on the filter board. I replaced these with the same low ESR series capacitors I used elsewhere and this was the only concession to keeping the Quad33 original that I had to make (although I did store away rather than toss out the original 100 uf ROE reds). I also checked the main PSU Caps but these appeared to be in good physical condition with no tell tale sign of swelling or electrolyte leakage. To set my mind as ease I also removed one of the caps and measured its static value. This came up very close to its original marked value; it also held its charge overnight. Based on these observations I figured that all three PSU capacitors are OK so I put it back and left it in its original condition.

The Quad33 waiting to receive her newly rebuilt boards

So was the change worth the money or the effort. Money wise, the cost was almost nothing, less than the cost of a bottle of my favorite RED. The effort was another thing, removing and cleaning old PC boards is not an easy job, a good clean soldering station, a good solder “sucker,” and a roll of de-soldering wick is a pre-requisite. Patience and ingenuity is also required when dealing with older PC boards. The bond between the Copper track and the PC board base on older PC boards will deteriorate over time and re-heating solder joints will often cause the copper track separate from the board. This can present its own challenges and should be considered before undertaking such work on older and more valuable equipment. Fortunately I had only a few minor incidents that were easily repaired. Eventually I got all the boards re-populated, visually checked, tested, and ready for the final listening session.

Well how did it sound? In truth it isn’t dramatically different at all. The top end was a little more open and detailed while at the same time a little smoother with a little less “graininess.” Bright instruments such as brass or close miked voices appear a little less harsh and edgy and early historic orchestral recordings that had me reaching for the slope filter now seem a little smoother. Some of that shut in character that has been attributed to the standard Quad33 has also been removed but the same basic presentation was still there. I am guessing that the newer electrolytic capacitors are the main reason for this. At the end of the day I’m pretty happy with that, the Quad is about music rather than HiFi.

The reduced gain in the output stage does make the volume control a lot less “twitchy” and as the volume control now tracks a little further up the scale and over a wider range of knob movement. I did notice a significant drop in broadband “hiss” type noise from the phono stage, however; the one thing you do notice from the phono section is more hum when you advance the knob up to the maximum setting. The noise is not noticeable at normal volume levels and you don’t hear it at all. At normal volume levels the hum never intrudes on the music but the reduced hiss from the phono stages improved components means that although the mains bourn noise hasn’t changed, it is no longer masked by noise generated within the Phono stage. I suppose it the analogue equivalent of dither.

I am still very happy with the Quad33 as a control unit / Pre-amp and the changes I have made didn’t detract or take away from any of the Quads character. The gain change has made the Quad33 more user friendly in a modern context and the newer electrolytic capacitors should last a considerable more amount of time before they start to degenerate. At the other end of the spectrum the Bass does seem a little firmer and more substantial, It also appears go down deeper now that the 30Hz low frequency limit has been moved to a much lower value. Many could argue with total credibility that most recordings don’t contain information this low. Most record labels actually filter these frequencies out because subsonic frequencies can play havoc with their recording and mastering process and 90 percent of domestic equipment doesn’t go this low anyway. Quite often it’s a psychological thing (like a lot of other things in audio), if you think that it is making an effect than you will hear it whether it actually is making a difference or not.

With better materials and manufacturing processes, modern electrolytic capacitors should have a considerably better service life than the 70’s electrolytic capacitors. The metal film resistors were perhaps a little bit of overkill but they are dirt cheap to buy and it’s only the time spent rebuilding the boards that you have to find. As a project to kill a few hours it’s probably a worth while exercise, if you have to pay a technician to do it then its going to cost you money for his time, if you can get him to agree to do it. So unless you can do it yourself its best to leave the resistors well alone and enjoy it as it is.

The best thing about this setup though is that although I have modified a set of boards, I can still easily go back to the original specification and componentry with nothing more than opening the case and swapping back the original boards.

This should just about rap up my Quad observations as I think I have come to the end of this road. All that is left to do now is enjoy the Quad 33 / FM3 / 405 trio for what it is. As I said previously this little Quad setup is a cheap way of getting together a good little classic vintage HiFi sound. Vintage HiFi is the new high end and some of the prices being asked for classic pieces of kit are becoming quite silly. Fortunately this setup is still quite reasonably priced and can provide many hours of fun and enjoyment for those who want something a little different. The also make a perfect partner for a set Quad ELS57 speakers, unfortunately these have been hit by the Silly price big and you are going to have to pay for these …. Ah well I suppose I can think about these while I start saving for the money.

Happy listening.