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johnm 9th May 2008 09:58 PM

Quad 34 - parts clarification
Hi folks!

Could any Quad 34 experts out there please tell me what the following components do, AND if they can be omitted/bypassed:

N1 - switch noise supression?

C77 and C78 (am aware of the orientation issue ;) )

IC23 (in PSU compartment) - is it worth changing this for something more modern, and if so.... any recommendations?

IC21 (PSU compartment again) - is it worth replacing this regulator? Again any tried and tested suggestions for a cleaner PSU?

Should add at this point all the usual Quad 34 mods (most are mentioned here on DIY Audio) have already been implimented to great effect. It's just the above I need to address for the last nth percent of improvement (well... a hopeful improvement ;) ).

Many thanks.

- John

P.S. The circuit diagram may be found here:

Mooly 10th May 2008 07:43 AM

If it were me I would look at replacing all electrolytics on something this age. Use low E.S.R. 105 degree types, they are cheap enough, and make sure they are correct phsyical size.
IC 21 and 23, I don't think you will find any difference whatsoever swapping these. You need to use a scope and measure the rails to see how clean they are. A couple of inches away from the regs and it's the quality of the decoupling and the layout that matters most. N1 could not see it ! :)
The OpAmps in the audio side of things, very definite yes here, I have my own favorites (OPA604 and OPA2604) they do sound good. Remember to check the current drawn by any OpAmps you swap, some types are very power hungry and it soon adds up.
Have fun

johnm 10th May 2008 11:04 AM

Thanks for your reply Carl.

Yep the electrolytics have been replaced by low ESR Panasonic FM units. Input cap for CD/Aux is also changed to Black Gate N type. new op-amps are OPA627's and OPA227s. They really improved the sound. Th ebiggest upgrade came from taking out ALL the poorly matched carbon film resistors, and replacing with hand-matched metal film types. I wouldn't want to do all this work ever again, but the results were well worth it. I also by-passed the balance control and this improved things also.


- John

Mooly 10th May 2008 11:12 AM

Thats good, interesting about the resistors as well. The only tweak I can think of is adding a small value cap soldered directly across the OpAmp supply pins (not each pin and ground). It's something I do, but as ever, does it really help ? nice to think so though.

johnm 10th May 2008 11:21 AM

I added 0.1uF ceramic bypasses on the OPA627s only - the OPA227s seem fine without them.

Replacing the other ICs is beneficial as well (the ones that deal with switching).

Mooly 10th May 2008 11:27 AM

Think you have covered most items really. I am not a Quad owner by the way :) . A chap down the road had a Quad set up , it would have been the 34 and the 405 together with Gale 901's I think. He has Sugden gear now.
How do find the Quad musically speaking ?

johnm 10th May 2008 11:45 AM

Before the mods it was best described as 'warm and woolly' as the saying goes. Nice liquid midrange, but not much in the way of accurate bass or treble reproduction. Stock it really isn't much to write home about and I can see why some people dismiss it.

The mods have completely transformed it into a modern device which has bested my KingRex Pre-Amp and - to my ears at least - sounds better than a passive ALPS Blue on a box. But there's never any solid state harshness - it just sounds correct.

Mooly 10th May 2008 04:55 PM

Thats interesting, I think I would agree with that from what I remember of it.
Happy listening,
Regards Karl

johnm 10th May 2008 07:28 PM

And to you Sir :D

VivaVee 10th May 2008 10:03 PM

>>N1 - switch noise suppression?

Can you give a clue as to where this is on the schematic?

>>C77 and C78 (am aware of the orientation issue )

As suggested, replace with a good modern 105 degree low ESR electrolytic. Or use a RC network ( 100uF electrolytic in series with, say, 100 Ohms) paralleled with a film cap (say, 100nF polypropylene). I suggest these values as the RC network means that the electrolytic is 'working' in the bass region (below ~ 66 Hz) and the film cap covers the frequencies above that. Also the suggested values are likely to fit i the case - always a concern when modifying an existing piece of kit.

>>IC23 (in PSU compartment) - is it worth changing this for something more modern, and if so.... any recommendations?

This opamp sets the 'ground' reference because Quad was too cheap to install a transformer with multiple secondary windings. If you think about it, ALL signals must pass through this opamp. So it is as important any other in the preamp. Think of it like a dc servo. So an opamp with good dc precision is important (low Vos in particular - R123 1nd R124 are too low for bias currents to have any impact. Certainly for any reasonable opamp choice) - so select your favourite FET input opamp.

>>IC21 (PSU compartment again) - is it worth replacing this regulator? Again any tried and tested suggestions for a cleaner PSU?

The 7918 is certainly ancient but capable of excellent performance if implemented in an exemplary manner. You could replace this with an LM317 but this is NOT a drop in replacement as you would need to add extra feedback resistors and bypass caps. And the pinout is different which makes life awkward. Or you could one of the many 'super' regulator options that are available hereabouts - it depends on how much you wish to expend in terms of time and effort.

If you are looking for a less intrusive but still effective improvement, then add a series RC network ahead of the 7918 to improve its ripple rejection. C74 (1000uF) is already there so all you need to do is

1. Break the connection between the positive terminal of D34 and the positive terminal of C74 and insert a 22 Ohm resistor.

2. Break the connection between the negative terminal of D34 and the negative terminal of C74 and insert a 22 Ohm resistors.

This will increase the ripple rejection by > 20dB which is pretty much the performance gain you would get by replacing the 7918 with an LM317. I would mark up the schematic for you but the pdf is protected.

Hope this is of some help.

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