Quad 606 Upgrades

I have a Quad 606 that I am planning to upgrade the power capacitors. Any Sifu out there that have upgraded their Quad 606? Any recommendations or to share your experience?

Hi mthoi,

If it were up to me,

CAPS
- Don't waste money on 'big name' caps. Just get the best generic stuff Farnell sells. I'll post more when I find the best generic stuff.

RESISTORS
- For audio applications the stability and noise are big factors. No need to waste money on 'big name' resistors with no test data to back them up. Vishay resistors are fine and as high up the price ladder as you need to go!

Vishay Bulk Metal Foil resistors are constructed in such a way as to minimize stray inductances and capacitances beyond the capabilities of metal film resistors. I think that you will find that the long term stability, low TCR and PCR (Changes in resistance corresponding to changes in temperature), consistent frequency response, and low current noise of the foil resistors are the primary benefits for specifying foil resistors in audio applications.

If you look around online, Vishay has a resource guide available on their website titled "7 Technical Reasons to Specify Bulk Metal Foil Resistors" if you are curious about the foil technology. Please also look up the data sheets for their VAR (Vishay audio resistor) as well as their S-series (I believe those are for high voltage requirements if you're also into tube stuff).

Hope you'll be enjoying your hotrodded Quad 606 soon!

Fellow friends,

Does anyone know where to get electronic Staffs (cap, resistor etc) and vacuum tubes in Singapore? Mind to share the location for the most choices and cheapest! Thank you.

hangleng wrote:Fellow friends,

Does anyone know where to get electronic Staffs (cap, resistor etc) and vacuum tubes in Singapore? Mind to share the location for the most choices and cheapest! Thank you.

Sim Lim Tower would be the best choice for caps & resistor. As for tubes never bought them before.

But for audio grade stuffs I still think it best to get from Farnell or RS.

Thank you tycham.

Thank you Scudracer for your feedback.

No prob please share the results on your hotrodded Quad 606. I've got an Eico HF-81 but no progress to date on the rebuild.

I have a Quad 606 too.Wonder if we need to change all the caps and resistors?Or someone may have the schematic for us to share?Where can we get those components in KL?I am very keen to upgrade mine.

Hi Leeck,

I have just ordered Quad 606 upgrade parts from Dada Electronics (via e-bay for the BASIC version of the 606 upgrade-kit) and awaiting delivery. I find it easier to order the parts on-line as opposed to getting around KL/PJ looking for electrical components.

I would update you on the progress and sonic differences.

Thanks very much for your info mthoi.I have just ordered 4 pcs of the 10k uF capacitors from RS Malaysia to replace those at the power input.It was recommended by tycham.Will try that if I can hear any sonic improvement.

I have upgraded my Quad 606 circuit boards using Dada Elecronics' kit. Dada have changed some of the parts - metalised resistors, ceramic caps and electrolyte caps.

I have earlier listened to some of my favourite CDs (mostly jazz, vocal and classical pieces) before the upgrade and (try) to compare the sonic differences.

The results were certainly positive - wider sounstage, clearer vocals, better separation of instrument, and more composed bass on my speakers. It also chaged the caracterisitcs of of my Quad - faster tempo for Jazz pieces. I even got my wife to listen-in and her responses were equally positive.

Quad purist may be agast by the hotrodding, but is an interesting and rewarding project. I am embarking to upgrade the PSU caps on the 606 and Quad 34 pre-amp next.

mthoi wrote:

Quad purist may be agast by the hotrodding, but is an interesting and rewarding project. I am embarking to upgrade the PSU caps on the 606 and Quad 34 pre-amp next.

Aghast! Not really, though we would prefer not to upgrade the more recent models like the 34, 306 & 606 as most of the components rarely give problems.

I have only listen to the upgraded version of the 33 & 303, though not in a proper setup, and I can say that the improvement in the clarity and refinement in the sound quality is remarkable.

Glad to hear that your upgrading have been a success, and have brought very positive results.

what quality of cap, resistor and ceramic changed to ? Is it a straightforward easy job ?

Hi mthoi

Congratulations on your successful 606 service and upgrade! You should hopefully get many many more years joy out of it. From what I understand, this kit brings the Quad 606 up to around the 909 level in terms of sonics. I think the 606 looks better... and so your way ahead! Well done

Not that this has anything to do with you and your Quad.... but for what its worth, I have found over the years of "modifying" circuits... its very hard to be really certain that we have created a "worthwhile" improvement if we only have one unit to base our findings on. Sometimes our efforts and expenditure and hopes and the sweat and concentration and time gone into our "mods" can occasionally cloud our judgment a little...

Ideally we need two units.... thus we can readily compare the "modded" unit against the "standard" unit by going back and forth, repeatedly if necessary. This is something that you would not obviously do just having serviced it!

Having a second unit also enables us to seek the opinions from other skilled listeners to do the same. But sadly not a lot of us can afford to do it this way. Most all of us are constrained by one unit

Please don't misunderstand me. I am not skeptical of the improvements you perceived and described. It is just that the very act of unplugging an amp and plugging it back in cleans the mains connectors.... unplugging and plugging in the interconnects cleans the rca connectors and plugs... and sometimes amplifier boards have many signal and power connectors and the same applies here too! Just these things alone could make a big difference especially as we live in an environment here that is almost corrosive and oxidises just about everything!

What I am getting at mthoi by bringing all this up is that sometimes these "claimed" improvements as claimed by the manufacturer, or the enthusiastic end user, dont really exist. Or if they do exist, the difference is usually to a much MUCH smaller degree then as claimed by them.

Again I say, please dont misunderstand me. My comments above are more towards the general subject of "modifying", not at all aimed at you and your 606 project.

The original Quad company as led by the Walker family had a strict engineering view on things. Most unfashionable today. They were audio objectivists. Sadly we are a dying breed. ( Im one of them too as Im sure you can see!)

I really look forward to you upgrading / servicing your 34 preamp. Perhaps when it is done, we could compare our 34's?

I am currently rebuilding my Quad ESL's and FM3. Perhaps when Im done we could organize a Quad evening? I'm a big big Quad enthusiast!

Regards

P.S. How about some pictures of your serviced 606 boards! That would be awesome!

Chua55 - The replacement parts are Nichicon caps (electrolyte), 1% metal-resistors, ceramic and metal-polyester caps, misc. parts - wires, shielded wires & gold-plated connectors/terminals.

The upgrade from Dada's upgrade kit - change old components with new components. The differences to the original Quad components are the increased value of the Nichicon caps (1,000 mF replacing the original 220mF), 3 ceramic caps replacing the original 3 resistors in the quad board (schematic shows it was supposed to be ceramic caps but the circuit board uses resistors - I preasume resistance also provide capcitance?) and addition of 3 new decoupling caps for the diodes.

I though the upgrade work was easy enough, but I do have some basic electrical/electronic experience from college days. With hindsight, I should probably work with a more experienced person, but it would not be as satisfying. It took me about 5 hours for the upgrade exercise.

Kowtim - I agree with you that it is desirable to have a "stock" Quad to compare the quality of the upgrade. Certainly some of the comparison are "memory" work, what I remember the music "used to sound like" vs "what I am hearing now". And comparison by memory is very subjective. I would look forward to a Quad evening.

Regards.

The replacement parts are Nichicon caps (electrolyte), 1% metal-resistors, ceramic and metal-polyester caps, misc. parts - wires, shielded wires & gold-plated connectors/terminals.

First of all, congrats. Every DIY job comes with some risk.

Nichicon caps are quite neutral. However my personal opinion is that if budget permits, I perform an upgrade with premium parts at signal path. e.g. silver mica, 0.1% resistor/Dale RN/Vishay S102J/Caddock. This is to avoid the future 'itch' of taking it down for another upgrade.

Changing to a bigger cap has modified the design of the original. My belief is bigger cap pool affects the system dynamics. Have you check the zero signal condition before and after.

It is always rewarding after the upgrade. Good job and some photo would help. Do u have the circuit diagram as well.

Some thoughts......not related to the above posts, but certainly inspired by them

The short of it....

Service PROPERLY first. Then modify if still necessary.

The long of it...

What many people don't fully realize is that amplifiers (and other
audio gear) do need to be serviced from time to time. The critical
thing that needs to be done for the purposes of this discussion is to replace some of the capacitors in these amplifiers. Usually the "electrolytic" type of capacitors are the ones that "wear out" in audio gear and are the ones that offer us benefits when replaced.

Now, most amplifiers are usually very reliable devices and go on working for many years. As they go on working continuously, servicing them never crosses the owners minds. But modifying them certainly does!

And so many people get their amps "modified" and have subsequently reported many positive things by replacing their capacitors by designer brand "X" capacitors. Whilst this may be true, I would say that it is not a totally accurate assessment of the situation.

It is my opinion that 80 to 90% of these claimed improvements are really the result of their having been added "FRESH" capacitors into the amplifier, not it being a "designer brand X" capacitor.

It is quite interesting to note what the average life span is of some typical electrolytic capacitors as described by the manufacturer. 1000 Hours if one cares to look at some data sheets. Suprisingly low eh! I put that at a conservative 4 years.

Now... as many would have discovered in practice... many electrolytic capacitors can soldier on up to 40 years or more! But whilst they may continue to work... your amplifier is simply not going to perform at its "factory fresh" level.

And so.... before spending big bucks on those designer caps and mods, do consider getting good general quality caps put into your loved gear. If ventilation is poor due to the particulars of your amplifier enclosure, then do consider high temperature capacitors instead... they do exist!

Here is a shot of another amp of mine, a Luxman L-30, where I changed all the capacitors...a process also known as recapping...







Now, one of the great problems with audio today is that you have an absolute abundance of people wanting to share their success stories. Yet no one wants to share how they went "wrong".... their failures.

All the parts that chua55 mentions are indeed superb top notch parts, and very often adding them into the signal path as he describes can at times bring handsome rewards.

Many people also are very happy to have their gear modified by others using such an approach with various "designer" parts.

Then there are those who simply feel "good" knowing they have these things installed.

Other people have also discovered that modifying audio gear all by themselves is great and rewarding fun.

Not wanting to be a party pooper, but for the benefit of those very few who are interested, I have found that it is also very easy to take things the opposite way. Years ago, I used to modify lots of stuff, and I find no shame in saying that in my past, I had made some things sound worse than original.

It was not that I was technically weak or suffer from poor hearing. The trouble was I was basing the results of my series of modifications on "memory" alone.

Your memory might serve you well for ONE modification that you may make... but if you have done 3 or 4 mods that do many things... how then? You can so easily make 3 steps forwards but along the way accidentally take 4 steps back...

And so many years ago, I realized that you MUST have a stock standard (but serviced) "control" unit or sample available that you can compare your "modified" unit to. Only then can you, (and others) be truly certain that your modification/s are indeed a genuine improvement.

And so..... eliminating or greatly reducing the subjectivity to it all by obtaining a "control" sample is very important if you want to produce and offer genuine, substantial and repeatable improvements to yourself. ( and others)

Now... to accomplish this step alone is so hard and why most just give up and just "wing it" by memory. For those of you who might have noticed, I placed a wanted add for a Quad 405 here for this very reason. I have a Quad 405 and there are a whole lot of improvements I would like to implement and then share with the Quad community. But here is the problem. People here love sticking them in their cupboards in sad non working condition and ask me for stupid prices for them. ( If anyone wants to loan me a 405 long term I would be most grateful!). And this amp is just too heavy and expensive a b**t**d to post over from the UK to use only as an ocassional control sample ... Talk about frustrating!

Lets now say we have a "control unit", what I have also found is that sometimes, fitting in "designer" parts can actually make something sound worse!

To make matters more complex, there is also the belief that if a few of these designer parts are good, then fitting more must be even better. And so, usually, a whole lot of parts are changed shotgun style all in one go.


The end result is, more often than not, best described by the late great Peter walker... "Too much Hifi, Too little Music".

If one reads and listens to what many of the masters of modifying have to say... they say and agree on one same thing... only do ONE modification at a time. And listen to it for some time.

Happy listening to all

Regards

Someone advertised a set of Quad 34 and 405 in echoloft for SGD800 om Sunday. Quite a good price if the transformer doesn't hum, which is the main problem with the 405.

what you mention is quite true from experience.

I do come to roadblock and even backtrack without realising it too.

DIY community is small and limited and sharing of knowledge is very often limited. So we learn along the way.

I see my amp perform the way according to my taste as I DIY it and I compare it in field listening to some gears that are reference series. These reference series design are no hidden secret that one cant get within except a few factors such as quality components (McIntosh OPT) and tuning.

The beauty of DIY is that we can have some juice without big bucks and how to do it matters.

I just wonder what design is inherent in QUAD. Is it a complementary feedback pair or is it a fully symmetrical design and what class is it operated.

Hi chua55

The Quad 606 and some others are what are known as "Current Dumping" amplifiers. It is "sort of" a successor to the Quad 405 which first introduced the current dumping technology.

In audio, ( actually many other fields too ) what distinguishes seasoned professionals from amateurs is an understanding of the Philosophy behind something.

In this case, many audio amateurs focus on the sound quality or lack of it of these amplifiers. And it ends there. They have no interest in them.

Most buy them because they are a genuine Quad product.

Some buy them because they understand the philosophy behind the amplifier. Usually these people can also recognize the absolute genius of this particular design.

In a Quad current dumping amplifier, the amplifier NEVER needs adjustment. ( (Offset and Bias) It self aligns.... As long as the capacitors are performing as they should, you are guaranteed of repeatable and consistent performance.

To answer your specific question chua55, technically, it is, in my opinion, a class B, and a class A amplifier sitting side by side.

The TO3 output stage transistors runs in class B, but, at the crossover region, there is an additional small signal transistor class A amplifier that "fills in the bits". These current dumping amplifiers have feedback, but also have feedforward.

Many people think very highly of the Quad 909. This is a more developed 606.

I think chua55 that you can get hold of a circuit diagram at the hifi engine should you wish to look into them. I myself would love to build a 909 style board to fit into the 405 ( But only one set of output transistors...the 909 using 3 pairs I think, chua55)

These are amps that many mentally healthy people buy to just fit and forget.
They don't suffer from memory loss and loose sight of the fact that there is just so much music to be listened to...

Regards



EDIT: chua55... I just checked and I do have the 909 schematic. If you would like it I would be happy to email it to you mate. You just need to supply me with an address

Hi all,

I was informed by an experienced friend who has many years experience in Quad electronics that the older 606 amps are superior to the new version of 909 since the company was taken over by Verity group after it encountered financial crisis which prompted them to shift all their production to China. The 34 and 66 pre together with the 606Mk1 and 606Mk2 are said to be the best amps in Quad's era that have been produced where the founder's philosophy was still kept intact.

Has anyone auditioned the 606Mk1 and 606Mk2 power amps side by side? What are the differences in sound quality? I was told that the 606Mk1 is a better sounding amp compared to the Mk2.

It was mentioned that since these Quad amps are already very good, modifications to these amps are strongly discouraged unless the old capacitors or resistors need replacement.

Thanks in advance.

kowtim wrote:Hi chua55

In a Quad current dumping amplifier, the amplifier NEVER needs adjustment. ( (Offset and Bias) It self aligns.... As long as the capacitors are performing as they should, you are guaranteed of repeatable and consistent performance.

It self aligns-very true indeed.

The first time I hook a power conditioner and powered my Quad 34 and 306 with a Wireworld Stratus 5 square power chord, the first few minutes it sounded funny and harsh. Thereafter, I guess it must had self aligned and go back to normal and sounded better.

My experience with Quad equipments, even though only for the past 6 months, is like the equipments have a 'brain' of their own.

Hi all,
I been auditioned to 909 twice together with the QC-24 & 99pre, nothing less than the 606MK1 & 2 except China made.
The pinpoint imaging is remarkable and for vocal, QC24 is lusher.

Pity that not many dealers carry Quad products nowadays,
or else we could audition them ourselves without other interference.
Some may influence others due to their own interest.
Its all about our ears experience and not the mythologies.

And Ryder, the Quad + Harbeth is not = paradise as LFD, but still a good match.

'QUAD' is your poblem mr. lavender?

Nope Mr tycham.

Hi Tycham

Quad amplification, as do many others, seem to sound better after a period of warming up.

If after taking this into account, you still find the performance of your Quad amplification lacking noticeably in consistency from day to day, I would suggest that you might consider getting them serviced, and investigate your mains voltage for long term variations.

Anyone who has a Quad pre and power which has seen say greater than ten years service should do what mthoi has done... order in a "service kit" from Dada electronics for their particular Quad model.

Amplifiers have capacitors in them, amongst other things, and these have a limited life span. That is what these kits contain, all the necessary replacement capacitors. They have other things inside as well to provide for a comprehensive service.

By quoting my "self align" comment the way you have Tycham, I gather you have misunderstood me. Most traditional transistor amplifiers require two very important adjustments to be performed. If these go out of adjustment in a major way, either the speakers or amplifier may be mildly or seriously damaged.

These adjustments are not required in a Quad current dumping amplifier. This is a good thing. This is what I mean by self aligning. To simplify things....It prevents "damaging adjustments" from occuring. This self aligning part was not directly intended for better sonics although it helped . But all this only holds true if the amplifier is working correctly. And it will only keep working correctly if you service it in a technically satisfactory manner from time to time.

Read all the above as simply an opinion only. We have with this reached the limitation of a forum...these things are complex things to explain and unfortunately, one can be easily misunderstood.

I have not seen and heard your system and experienced what you have tried to describe. Nothing can replace a physical personal inspection with regards to providing accurate evaluation and diagnosis.

To wrap this up, again my own opinion,... if you have an old solid state Quad pre and power, look into getting a Dada kit for them and having them installed by a competent engineer and not some designer component trigger happy audiophile. Stop at the kit, put the lid back on and just sit back and enjoy the music uninterupted for another ten years. In that ten years time, your Quad amp will still be easily serviced and worth more than most similar "audiof___l" amplifiers bought new today

Enjoy the music guys

Regards

Oh well! I thought it is just like my car suspensions - able to align according to the terrain.

Hi Tycham

What car do you have? Your comment conjures up images of an old Land Rover. Is this what you mean by self align according to terrain?




http://www.imagecontrol.com.au/oldcmp/Cuthbertson.html#Anchor-49575

Regards

Morris Minor 1000

I was informed that the 606Mk1 has the C-core transformer while the 606Mk2(and older 405) has the toroidal transformer. The toroidal transformer was said to exhibit hum. Can anyone verify this?

Hi ryder

As I understand it the 405 and 606 Mk 1 both share a regular laminated transformer whereas the 606 Mk 2 uses a toroidal.

A C core is something different from either of these two transformers types above

Regards

Hi Tycham

The authorities still allow Moris Minors to be used around the isle of Sing?

Your Morris Minor self aligns? Never come across one of those. Tyre dealers wont be happy about that if the secret gets out! How will they ever repay back the loan on them 50 000 dollar Hunter alignment machines?



Since you bring up wheel alignment, the strange person that I am... I actually used to have a Hunter Wheel alignment machine in my living room just for fun! I sold it to a tyre dealer who wanted it as we had the same machine, but mine had the 2 rear alignment heads thus enabling a true 4 wheel alignment.

For those that are old enough to remember, I used to have a really ancient wheel alignment system made by Dunlop. That old system however was, and still is absolutely superb! It worked using optics and mirrors. Extremely accurate.

Ten years ago, I found it very difficult to get good wheel alignments, and so I ended up developing my own mechanical system of doing so.

The internet has since shown me that a few of us around the world had been frustrated with the same problem and we all came up with similar solutions of our own!

Regards

kowtim wrote:


Since you bring up wheel alignment, the strange person that I am... I actually used to have a Hunter Wheel alignment machine in my living room just for fun!

The internet has since shown me that a few of us around the world had been frustrated with the same problem and we all came up with similar solutions of our own!

Thats not strange. Thats BIZZARE!

Your DIY solution?, as long as your steering don't veer into my path if you're alongside or behind me and result into any accident, your solution is good.

ryder wrote:I was informed that the 606Mk1 has the C-core transformer while the 606Mk2(and older 405) has the toroidal transformer. The toroidal transformer was said to exhibit hum. Can anyone verify this?

I believe in torroidal it is less likely to manifest hum unlike the the one in the 405(don't know what it is called, but a squarish one).

Focus! It's suspension, not wheel alignment!

reducing distortion by error add on. This class A at low level and class B at higher output current. Subsequent Quad models have stop to employ current dumping. A plain push pull with class B or class AB tends to be fussy with drift due to aging.

tycham wrote:Focus! It's suspension, not wheel alignment!

Thank you for your ever useful advise. I shall be more mindful of my lack of focus now...

Always more to learn.

Regards

cmboy wrote:

Your DIY solution?, as long as your steering don't veer into my path if you're alongside or behind me and result into any accident, your solution is good.

Hi

Don't worry cmboy. Rest assured I have no wish to be anywhere near you.

Its also been some years since a car has been aligned by me, and so the chances of their being an accident alongside or behind you caused by a precision wheel alignment done by me are effectively zero. You can rest easy now.

At 2500 RM a piece for a Pirelli P Zero, one does wish to maximize tyre
life, and at the rotational speeds these tyres are usually operated at,
initial steering response is a very important factor in helping one in
"feeling" secure, hence the desire for some individuals wishing to obtain a "quality" wheel
alignment.

Regards

mthoi wrote:Hi Leeck,

I have just ordered Quad 606 upgrade parts from Dada Electronics (via e-bay for the BASIC version of the 606 upgrade-kit) and awaiting delivery.

mthoi, can you provide the link to Dada electronics in Ebay that supply the parts for Quad 606 amps? I cannot seem to find it when typing in keywords like "Dada" or "Dada Upgrade Kit". Thanks in advance.

www

You can also find it here:

http://www.quadmodsusa.com/606upgrades.html

I personally think that the Quad 606 basic upgrade from Dada is probably worth it - moving it closer to the Quad 909 (silver mica caps, 1% resistors, etc.). It is difficult to get all these components from different sources. However, if you are looking to replace the PSU caps, look at Farnell or RS. I would probably use higher grade capacitors (as opposed to Nichicon in Dada's kit).

Upgrading the Quad 34 would be another matter. I also bought the upgrade kit from Dada. With hindsight, I would probably take a different path to upgrade the pre-amp as opposed to adopting their Burr Brown 134/2134 op-amp chips. It may sound clearer but it also sounds leaner and "less musical" as Sam from tropical audio put-it. This can be rectified by re-placing some of the signal path capacitors. (Solder IC sockets to the PCB to allow replacement of different op-amp chipsets).

I have hence replaced my Quad 34 op-amp chips with OPA627 (single) and brown-doged+opa627 (dual), plus changes to some of the electrolytic capacitors.

I am currently experimenting with changing the caps (different makes and values). I have since purchased some Black Gate Caps and awaiting delivery as my "final" choice.

I am also experimenting with Burson Opamps (Octave) as well. If I have the budget, I would use these instead of the OAP627.

Reg.

Hi mthoi,
I see you are into opamp replacement for the Quad. I did a change from the original opamp to OPA134PAG4 (Texas Instrument) on a 405-2, but was a bit disappointed, I did not find much change in the sound quality. I hope you don't mind sharing your experience, I have a few questions :- (1) how did you find OPA627 ?, it is better than the OPA134 (2) for OPA627, do you think Burr Brown will be better the Texas Instrument OPA627 ? . Due to the price of these, I had not bought them, instead I got OPA604 + IC-socket which I will try next when I find the time.
Kind regards
Cham TS

Hi Cham,

I purchased my OPA627 via e-bay. Do a search on "OPA627" in electronics/consumer electronics categories. You can either buy them outright for about USD25-30 + PP per pair. Alternatively, you can bid for it in one of the stores. I spend an average of USD 16 per pair (Albert's_Audio_Store) for the single opamp and USD 30 + PP for brown-doged dual opamp.

The BB is manufactired by TI under contract - the OPA134/604/627 are all drop-in replacements. There are several e-stores offering Quad 405 upgrade kits using these opamp chips. My personal preference is OPA627 - clear, fluid and musical. I find the OAP134 a bit laid back. My choice was based on trying to retain the "Quad-like sound" (what is that anyway?) and music choice of classical, jazz and vocals.

My experience in dealing with OPA627 is that it requires decoupling caps at its power supply pins. Connecting a 10-100uf + 100nf at each of the V+ and V- pins to ground helps the dynamics. (The datasheet on on OPA627 recommends 100nf - 10uf). I am using Nichicon caps now but I am waiting for my black gates orders to arrive. I think Elna Silmic II caps are quite good as well.

You may want to consider putting the decoupling capacitors for your existing OPA134 and see if there are any difference. These new chips are quite power hungry compared to the earlier TLC271/TL071 opamp chips used in earlier Quad equipment.

I still think that the opamp and parts need about 50 hours of burn-in before it produces it best.

If you need assistance, PM me.

Reg.

Hi mthoi,
Thanks for your feedback, I am aware of the recommendation for decoupling caps for the newer OPA134, 604, 627. My OPA134 was already with 2 decoupling caps but did not bring any difference compared the original OPA.
Kind regards
Cham