Uber Quad rebuild project - input please

No Simon, the chap you got to bodge the thing should be providing you with such information. I've read the list of changes made and they don't require the knowledge you seem to indicate necessary. In fact a knowledgable and 'qualified' person would realise that the modifications to that machine are quite unnecessary!

 

LOL - yes I can imagine!

The Pioneer power amp is less sensitive than the Quad amps - the 34 plays pretty quietly at that setting.

I can't believe it has been 8 years since i bought the SA9800 and had it shipped from the US. It had an intermittent fault in the pre amp with a regulator that looked perfectly well soldered onto the board but one pin was making poor contact. As the the amp and reg heated up so the the fault condition developed. Such faults are infuriating as they aren't obvious, but they need time to isolate - and this thing has rather more pcbs and components than a typical brit amp.

Whats interesting is that this thing is now 30 years old and performs amazingly well.
There are 14 internal pre sets to set the circuit operating condition and every single test point gave reading well within spec - nothing more than 5% out IIRC which is quite something given the age of the amp.

They really did throw everything at this amp. The MC and MM phono stage specs are the best i've seen (or heard) and the thing is whisper quiet in operation.
Going back to bodging, this would be cap swappers dream come true. But its 30 years old and sounds stunning. A good example of when to leave well alone.

On the 34, I said i'd give you some recordings of it against straight wire (not with gain ) and if you want a few just say. I've now sourced some alternative cmos switches which should better those in place and work more reliably.

 

But I'm not interested in whether it measures better or not Rob. I'm not interested in some idealised notion of perfect sound because I want it to sound the way i want it to sound in my room.

That's why i'm not in the least bit upset by your viewpoint on it. Had you heard the unit, or if you held a suitable electronics qualification then I might value your opinion in some way, but as it is you are just some guy spouting rhetoric on a website.

You are no different to someone like Steven Toy, both of your viewpoints are supported by what each of you believes to be important, but sadly limited by your closed world views which exclude each others position.

I'm not surprised that you aren't willing to put your viewpoint to the test, and check to see if perhaps Colin has chanced upon some mods that take a price point bit of kit beyond its normal performance envelope. Why on earth would you wish to upset your own world view and make yourself look stupid.

Carry on.

 

LOL - I put my ears on the line all the time Simon which is way I advocate blind tests.
Nowhere to hide.

But if you happen to be passing London way do pop in and we can asses your 840 against say a Dacmagic - blind. Unless you have two 840Cs of course and one in stock condition. Then again, you know this isn't likely to happen which is why you suggested it.

 

My present CEO is the bloke who designed the 9800 tuner to go along with that integrated. He's also the bloke who bought my FM4/34/306 when I needed cash for a big DIY project.

 

Not hearing any difference doesn't prove anything Rob, remember you can't prove a negative. I'm 100% with you on blind tests though, you know that. i just differ in that i think there are things we don't measure or don't know how to measure to correlate to what we hear and i doubt the validity of using unfamiliar systems.

At no point did I make any claim to being able to identify my cd player in an unfamiliar set-up. I know i simply couldn't it would take me hours of listening to become attuned to the particular nuance, or otherwise, of someone else's hifi, that is if their system was even resolving enough in the first place. I only know mine and YNWANS well enough to make what I consider valid critical judgements for any EUT.

You're welcome to bring your dacmagic up to mine anytime though to do the same test. There's no need for me to hear another dacmagic in my set-up. Done that already, juts like trying my cd840 against somebody elses, Scott from this forum in fact.

 

I must admit that I was surprised to read that component upgrade kits are available for the Quad 306. As I recall Quad were very much into their 'straight wire with gain' - 'all amplifiers sound the same' phase when the 306 was produced. I wouldn't have thought many owners would be interested in altering it.

 

These kits are not coming from Quad, but from third parties. The main reason being there are a hell of a lot of Quad amps around, IIRC they made 94,000 303s! They made them bloody well too so a fair number still look half decent and work. The one I have in my TV system was bought by it's original owner in 1970, it still looks pretty much as new and has been serviced by Quad twice in it's 40 years. I like the sound it makes as is, though apparently you can take them to a far higher level. Worth having a read around Net Audio, the 303 being the favoured amp for modding. There will be less 306s knocking around, but again it looks like a nicely made and solid amp that's not expensive second hand and I suspect some will want to fiddle about with them. I've a lot more time for Quad power amps than preamps to be honest, they were in most studios I've visited and never seemed to get in the way at all. Just good honest kit.

Tony.

 

Umm...I did realise that Quad don't make the kits, but thanks anyway. As you say though, well made and lots of them still around (cheap too).

 

Mark, the 306 is essentially a 606 (and therefore 707 and 909) with only one pair of output transistors per side and a smaller PSU. Otherwise it is the same amp.
They differ from the the 405 by omitting the op amp input stage and applying gain in the stages that follow. So simpler than a 405 though still with the current dumping circuit at the core.

Hardly any elctrolytic caps inside other than the main PSU and the topology means there is nothing to adjust. Upgrade kits are pretty much a fresh set of caps and flashy looking speaker posts. Not much else to 'upgrade' really.
Very sensitive at 375mv for full output so good for those with passive pre amps.

 

Excellent tuner. Used to have one but sold it for an Accuphase which I have to say was a little better.
If another comes along with wood sleeve in decent condition I'll snap that up to go with the amp.

 

The final round up of mods is as follows.
Comments on sonics are on sighted listening only at this stage.
Some detailed technical testing with results will follow in the new year when I have time.

Quad 34

New PSU caps - made no difference so the old ones were probably perfectly fine.
New op amps (OPA134/2134) throughout - no sonic upgrade on the TL071/2 except in the phono section where noise is clearly reduced.
Low noise transistors on the phono card, solid aluminium caps in place of the tantalums and a reduction in collector current through the transistors. Worth doing - a few dB less noise - but no real sonic gain.
Maxim Cmos 4066 switches - sounds a tad cleaner and operationally these work perfectly. Be very careful with other 4066 replacements - some don't work well and upset the operation and PSU voltages.
Remove all electrolytic signal coupling caps. Lots in a 34 and all at 100uf. I'd already replaced these with Silmics but removing them all made no difference to the sound. A bit of a surprise given the quantity but there you go.
Increase output to 1.5v. Well worth doing as this allows a reduction in power amplifier sensitivity. Net noise drop of about 10dB. Easy to do and probably the one must do for anyone running this series of electronics.
Move the RIAA high pass filter down to 30Hz. One cap swap and worth doing. TTs are better behaved these days and tend not to rumble, and we don't use those daft very high compliance cartridges. Quad were right to be careful in the 70s but this can be relaxed today.

The following are a complete waste of time and money:

Expensive high speed op amps. I've put AD797s in with appropriate decoupling. They work and sound superb. Unfortunately they sound identical to the much cheaper 134/2134 chips so don't waste £10 a go on these.
Substantially larger PSU caps. Go a little larger in capacitance and certainly up the voltage rating but don't go mad and start sticking 5kuf caps in the PSU.
Rail decoupling on the op amps. Makes things worse (noise) and the 134/2134 doesn't require it in this design.

Quad 306

Super little amp and this plus the 606 are essentially a 405 with a further decade knowledge and experience thrown at them.

The 306 has a tiny case and this limited the capacity of the PSU caps. Caps have both shrunk and improved in the 20 years since the 306 was born. Increase the caps to 6.8kuf (from 4.7k) and go to 63v rating. Panasonics are excellent and good value.

Reduce the input sensitivity to 1.25v (from 375mv!)

Thats it - leave the rest alone.

The 306 uses a TL071 as a dc servo and you can decouple the rails on this if you wish.
Go ahead, but it does nothing to the performance. The 306 doesn't use the op amp front end as in the 405.

Quad 405

Lots you can do to this one but I've done the following:

As with the 306, the 405 has a small case and can benefit from larger capacity caps of the sort not available in the mid 70s. So fit some nice 15kuf BHCs (original 10k).
Recap the main boards. Kits are available to replace all the electrolytics. The 405 runs hot and is unventilated so definitely do this.
Reduce the input sensitivity to 1.5v. Do it - 405s are a bit hissy but not after this mod.
OP amp upgrade. An OPA134 in place of the TL071 reduced overall noise by a couple of dB but doesn't alter the sound. Given the op amps are cheap you should definitely do it, just don't expect a sonic lift as the TL is more than capable in this circuit. Johnson noise dominates in this circuit so without the sensitivity reduction, a couple of dB is the best you'll get. Don't fit ultra low noise or high speed devices as you will gain absolutely nothing.
OP amp decoupling. Worth doing though I confess to struggling to hear the benefit. Compulsory if you ignore the above and fit super-fast devices.
Should be better in theory so nothing to lose in view of the cost.

Both the 405 and the 306 would benefit from some case ventilation IMO. This is something I'll work on in the new year when I can source some spare case parts.

Happy to give part numbers and more detailed instruction but little here is new - its all out there so have a browse.

 

Nice summary.

IME there is more that can be done to the 405, but it's well into the realms of modification, not 'servicing with modest updates', and therefore requries a 'scope and test loads and time etc to be sure of.

One thing that isn't, and is pretty basic to the way the 405 operates: replace C5 on both boards, it's a 100uf/50v cap. More doesn't harm, but neither does it significantly help (so - pretty much any modern 100uF /min 50v part will work fine, 2 needed total). This part fundamentally affects PSRR of the Class A stage, and when it ages the amp is susuceptible to hum break-through. Well worth replacing every 5yrs or so.

 

Hi Martin,

I think that with this type of kit, it was fundamentally right when it left the Quad factory and modification falls into two clear camps - service/update or more adventurous stuff that makes these things no longer Quad at heart. Nothing wrong with the latter but by the time you've done the necessary rebuild you might as well buy something else IMO.

Quad didn't stand still and Walker embraced improvements as and when they came along. Giving these already competent amplifiers a freshen-up' does more than expected IMO.
Just adjusting the pre out level and power amp sensitivity alone brings rewards and makes both units useable with modern kit.

The Maxim 4066 devices when running at the voltages found in the 34 should have 'on' resistance of around 30R compared to 200R for the original Philips chips.
Given the high impedance circuits inside the 34 and the tiny current flows through the switches this should make the Maxim devices transparent - with the original not far off.

 

you may have missed a trick ...or I may not have spotted it ....one of the main causes of noise is the diodes in the psu ...swap out for equavalent hexafreds or schottky's ...if the units using the ubiquitous 1n00x then you'll be in for a treat ...

I'll agree with some of your conclusions ....but have found that in my modding of gear ..thats about 50 different units totaling over 200 units now ...lots of mf stuff [talk about just make do components ...or designed down to reduce there sonic capabilities?]

if your kit uses decent caps that are in good condition ..you need to get the very best to get any obvious benift from a change ...often this is not cost effective .

one area that can provide a step forward is to get rid of the cheap base metal rca sockets ....if fitted with dins don't swap as these tend to be quite good ...just limits you to the types of ic commonly available..

if you can jump from pcb to output socket with silver or good copper either cut the tracks or beef them up ...you can only do this by trail and error ..one ought to be better than the stock solution .

If you can eliminate the dc blocking caps ......this my improve sound

if you can't then try paper in oil for these positions ....though these tend to be physically large ...I've not found any film cap to get close and as for electrolytics they emasculte any music signal pased through them .

 

Any measurements for that 'noise' Pete?

 

I'll run a set of measurements for the usual stuff in the new year, but DADA Electronics recently measured a stock Quad pre with some interesting results.
Even running the elderly 071 op amps and Philips switches the THD was down at 0.0055%. With BB op amps and switch upgrade this falls to 0.0007.

The cmos and op amps replacements are not expensive so they are worth doing IMO - though stripping the board is quite time consuming given it is pretty cramped and there are lots of ICs.

The only tangible benefits are the phono stage upgrade and changing the pre output levels and input sensitivity of the power amplifiers.
Both give very real measurable improvements that are not subtle.
My 24/96 ADC has a numeric dB level readout so can do quick and easy indications of noise when driven by the pre or power amps (via padding for the latter). It'll be interesting to see the full set of results in more detail.

I have one more mod to perform - some LME49710HA op amps for the phono section to see if an ultra low noise op amp brings any further benefit.
There are one or two with even lower noise but the op amp in this circuit must be stable at unity gain which rules out some candidates such as the AD797.

Wire and socket upgrades were not considered as IME they make no difference at all.

All good fun.

 

Are you sure Rob it just seems so feasible that replacing one piece of wire that is perfectly able to conduct electricity with another would bring about larger changes than replacing active components. cont on pg 2001

 

Just to follow from post 57, LME49710HA now in place and working very well.
You will need to use rail decoupling on these unlike the 071 or 134.
Also, it makes sense to mount such op amps as close to the pcb track as possible, so DIL sockets abandoned for the phono section.

That's your lot for a while.

Tin hats on the left: