Uber Quad rebuild project - input please

Made for a damn fine headphone amplifier, but it's in a cupboard somewhere and hasn't been used in about 8yrs. I think I have a project from teh seasonal break there...

 

Well yes, bothers me as it was quieter before - but it isn't noisy in normal use.
Previously you could wind up the volume to max on line and just get a gentle hiss.
Now its a loud rushing sound with some faint buzz/hum. Only near maximum volume though and typical of many other active pre amps I've heard.

There is also more bleed at HF - again not much and only with volume wound up but it wasn't there before. I's suspected flux on the board (it is very tightly packed) for this but even with it scrupulously clean it made no difference.

PSU has not been touched this time. I simply recapped it a year ago. No point replacing the TL071 there and it remains in place. The rails however are now wrong.
Overall about 1-1.5v down and even more asymetric than usual. The rails also 'walk' by around 0.5v over several minutes.
BTW, performance is worse with a 100nf film cap on each op amp - those were thew first things I removed.

So it works, sound ok but is clearly technically inferior to previously.

Your comments on the 44 are interesting but note this uses higher supply rails than the 34 and the PSU is different.

My conclusion is that the 34 needs a more robust PSU to cope with this degree of modification. The standard unit is nicely in balance and works well as a result. As usual, the engineers at Quad knew what they were doing!

 

I was just reminiscing. I'd almost forgotten I'd done that job.

The shift in DC level bothers me. If I were you I would simulate the supply circuit
with the old and the new load and check if circuit analysis predicts the shift.
If not you've got a real problem somewhere. I would be a shame to have a 34 in that condition. I still feel the pain of letting my 34/306 go, so this is getting almost personal.


--

Due to a defective ESL-63 I now have my old ESL-57s in the living room.

 

The answer is simple Rob, it's not that the mods that people typically do are wrong- it's just that you aren't competent to select and implement a subset of them without messing it up.

It's not that modding is wrong or can't bring about improvements, just that you aren't up to the task. I would have thought the logic of that is inescapable to any one viewing what has taken place.

 

Typo.

Please read "it would be a shame".

 

Why the personal attack? I'd like to think of myself as reasonably competent when it comes to electronics, but I wouldn't dream of tackling a complex device such as a 34 without a lengthy trial and error process. I don't think too many people are capable of modifying such without prior experience and full predictability.


Then again, I do think that many of the mods people present on popular fora are wrong, misguided, misguiding.

 

Surely it's easy enough to put the 34 back the way it was anyway? If someone has the skills to mod something they should be able to un-mod it again afterwards! The main thing here is Rob is honest enough to admit that he didn't like the result. I don't understand why he's receiving criticism and even insult for this at all.

Tony.

 

Because he's just using the forum as a springboard to support his commercial and objectivist viewpoint that modding doesn't bring benefits. it's no worse or better than we've seen from small speaker and plastic amp manufacturers.

 

The company Rob is involved with modifies a Beringer DAC / room correcting thingy!

Tony.

 

Bollox.

 

Not so.

The mods are sold by at least two commercial outfits as a kit - I've done nothing different here.

And it works well enough - but is worse than stock IMO.

You have spectacularly failed to grasp the ethos of the thread Simon.
It is precisely intended to make the point that modifications should be properly targeted, and that some testing should follow to back up the claims made.

 

Yes and it works fine - just the switch upgrade, re-instaling the coupling capacitors and performing the phono stage upgrade make for a something resembling stock but with a better phono stage.

Replace the other 10 op amps and bypass the caps and it falls over - relatively speaking.

The phono upgrade is the key area, it is the one area where an old 70s op amp can be found wanting as noise is clearly reduced with a modern replacement.

And we'll gladly provide anyone with enough technical data and test results to demonstrate the effectiveness. No guesses or assumptions and all very transparent to the end user.
I guess we could also suggest replacing every cap and resistor in the unit for something boutique and charge a lot more for it. Money down the drain though and bordering on deception IMO.

Absolutely right, along with some sold commercially that are of questionable/no benefit.

Modifications need to be targeted and address an identified issue IMO. There is far too much 'lets bypass this' and lets swap this for that' or 'lets just rip out this part of the circuit' and it often leads nowhere and can sometimes give a worse result. The post at the start of the thread makes clear that this 34 was offered up as donor for all sorts of changes. So far, some work and some don't. Surprise surprise

 

Simon, Rob is scaring everyone away with his 'my way or the highway' viewpoint. He's not developing a USP he's alienating a large proportion of what remains of ZG's long term posters. This isn't Diyaudio.com and he's not an electronics designer.

His inability to correctly apply a set of third party modifications to an amp tells us nothing more than that he was incapable of applying them- it says nothing of modding in the wider field.

It says nothing of these particular mods or how well they would perform when correctly implemented, after all he didn't buy a kit. It's just FUD, oooh stay away from DIY modding that way lies failure.

So in this respect I think it's fair to say it aligns with your service of offering 'commercially approved' mods to a limited range of low price kit. Just because he's not worth paying doesn't make the link between your business and him any less apparent.

Now I'm just stating my opinion on the matter, but the thread reads as a bullshit attempt to steer people away from 'general' modding and I'm far from the only ZG member to hold that opinion, even if others can't be arsed to voice it on the forum as they don't wish to upset any perceived status quo.

You could shorten the entire thread to this.

"Rob Holt makes an amp measure worse by attempting to copy the mods of a third party."

it's meaningless.

 

Of course I'm not suggesting all mods are an improvement- but then i didn't start a thread titled 'Uber quad rebuild project' when it would have more aptly had a somewhat less grandiose name.

 

Complete claptrap. You've had the arse ever since I said that sending a state of he art dac to a third party to have its guts extensively reworked was a compete waste of time and money. Get over it. Perhaps you'd care to share the before and after performance tests with us sometime. Then perhaps you can lecture others on ability.

As for scaring people away from DIY, you clearly have some reading difficulties of late.
Which part about a 7db improvement in phono stage s/n ratio and similar benefits to the 405 input stage did you not understand?



Back to the topic at hand, for those interested in the Quad mods there are some developments on the power supply wobble and erratic led issues.
The upgraded cmos switches are the likely cause as reverting to the originals (thankfully I installed DIL sockets) does three things - lowers noise, cures the led issue and the rail stability returns. To complicate maters, if you return to the older TL07 op amps the switch issues recede other than some led flicker.

The later HC 4066 cmos family are pin compatible but there is variation between the brands, and it seems variation between the manufacturer specification sheets for the same part number!
The differences are in the important area of supply voltage. So far for the same part number I've located three sheets all quoting a different maximum voltage for precisely the same part number (not the low voltage 'T' version). That is the likely cause of the issues experienced and I'll do a little more digging.

So far I'd say the upgrade kits are fine. They consist of just the op amps and a few replacement like for like caps. Just do that and you get a line stage that performs pretty much as before but a better phono section - well at least in terms of noise. There is nothing else in the standard kits (contrary to the ill informed comments of our friend earlier).
Rail decoupling actually seems to make this pre amp worse - or at least it does with these chips, so don't go there. The 134/2134/TL07 don't need it for stability in this circuit.
Be careful with the switches - jury is out until I can clarify the voltage issue, but it looks like 'Maxim' versions might be correct for this job.

That will do for now.

 

No need to worry Werner, playing fine at the moment and into the power amp section of a lovely, nearly mint Pioneer SA9800. That needed a little attention to a regulator which is why its in use. Been meaning to fix it for eight years and just got round to it!
Sounds very nice indeed but you wouldn't pair those two for the looks

 

You'd have to use ear protectors to enter the room if the volume knob of my 34 was in that position on the radio input!

Tony.

 

Rob, that you think i should 'have the arse' about your view on a piece of gear you aren't really qualified to comment on, lacking both any experience with the sound of the modified device and the electronics knowledge to do the work in the first place surprises me.

That would be like me getting upset at the opinion of a retard on nuclear physics- pointless.

Should you have a full electronics workshop capable of making the required measurements to ascertain if it is indeed improved I'd be more than happy to bring it round for you to test and in return if you'd like you could round to my house and hear how it sounds through a decent hifi.