Saturday, December 18, 2010

Quad 66 Revision Kit Design considerations

We get more and more questions about how the sound quality of the Quad 66 can be improved.

The 66 is often used together with the 606 which may be one of the best Quad amplifiers ever made. But there certainly is room for improving the 66 by replacing some of the components with better quality.
  • The bipolar capacitors can be replaced with Nichicon polar electrolytes
  • Some TL072 opamps can be replaced by Burr Brown OPA2604
  • The switch-IC's van be replaced by better quality with lower THD

It is not clear why Quad used cheap bipolar 100µF electrolytes between the stages. We switched on the 66 and measured the voltage over each electrolyte to determine the polarity. This way we could replace them all with high-quality polarised Nichicons which are a lot better for audio. Bipolar caps are in fact two capacitors of twice the capacitance in series and this doesn't correspont with the "less is more" High-end Hifi philosophy.


The TL072 can be easily replaced with the pin-compatible Burr Brown OPA2604 which offers a much better sound. On the Disc-board we will use OPA2134 because of it's good signal/noise figures and excellent behaviour at low signal levels.

The TL074 quadruple opamps are still a problem as there is no Burr Brown alternative in a DIP-package. We are looking into the possibility to replace them with OPA4134 with a SOIC to DIP adaptor but maybe we will leave them in place.

Finally we are testing Fairchild and Maxim alternatives for the switching IC's

The first 66 we revised had a much more transparent and dynamic sound after replacing the critical components. We are still making decisions about some other components and writing the revision manual and we will offer the kit in the Webshop in the beginning of 2011. We can already do the revision for you in our workshop.

See also the forum-discussion Quad 66 Hybrid Elements on this subject.

Stefaan

5 Comments:

Anonymous Anonymous said...

Hello
I am considering a revision of my Quad 66 pre (mk2). What exactly is done in your revision? The bulbs on my 66 need replacement - is this part of the revision?

Kind regards
Henrik

10:32 AM  
Anonymous Stuart Adamson said...

You state, "The bipolar capacitors can be replaced with Nichicon polar electrolytes" however in the Capacitor Sound C. Bateman, at the bottom of the download section, it states on page 69 "In every distortion test the bi-polar capacitor produced much lower distortions than were measured on similar value and voltage polar capacitors. Have you done any distortion measurments to support the use of polar capacitors in the 66

Stuart

11:43 PM  
Blogger Stefaan said...

Hello Stuart,

There is more than distortion that effects the sound of a capacitor. Two capacitors in serie will have a higher ESR. What we will try to do, is ommitting the coupling capacitors, as is done in the 34 kit. We keep you updated.

11:38 AM  
Anonymous Peter Wierinckx said...

The TL074 quadruple opamps are still a problem as there is no Burr Brown alternative in a DIP-package : why not try the Quad High Performance Audio OpAmp LME49740NA DIL-14 ?

9:18 PM  
Blogger Eoin said...

I am looking forward to this kit. I hope it's out soon.

9:13 PM  

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