advertisement


Quad 34 MM load

I really can’t explain it. The 34 has a dreadful reputation with ‘internet experts’, I suspect due to the anti-Quad sentiment of many ‘80s ‘flat-earth’ dealers, but has really surprised me, albeit after a little work. You do need to get rid of that HPF on the mainboard though as it just sits on dynamics to my ears. I don’t know which components it is or what to replace them with, but they definitely reside on the mainboard, not the plug-in phono boards.

I’ll just ping @Robert in the hope he sees this thread and can advise. Removing that and setting the phono card loading for my Ortofon cart turned it from something I really couldn’t live with at all on vinyl into a preamp that has surprised everyone who has heard it in my system. It sounds very good with my current MP-500 too, though these days I do prefer my JC Verdier valve pre (which one would sincerely hope given the huge price difference!). The Verdier hated the Ortofons so I used the Quad for several years. It is a good preamp, in fact great for the money.
 
Okay, so let's wait for Robert's perspective. Cheers Tony!

No flat Earth here by the way! ;)
 
On the main board C27 and C29 are limiting, change from 470n to 1u or even 2u2
On the plug in board C12 and C13 could be increased from 1u to 4u7
 
Thanks. So that means an electrolytic for C27 then. Nothing else will fit. Are C12 and C13 part of the active EQ feedback loop?

riaa.jpg
 
Maybe I'm misunderstanding something, but C12/13 look to me to be inside a loop. The diagram also, to me, underscores PJWs tendency to have a high order LF rolloff. FWIW I use 34s and like them. But as general preamps, modified a bit. I've avoided using their 'disc' (LP) input stage.

BTW One of the reasons for tweaking the LF rolloff is that such a filter tends to affect higher frequencies as well. So may sound like an oddity at other frequencies. Similarly, the presence of low level LF affects how we hear higher components.
 
Jim Audiomisc said:
Maybe I'm misunderstanding something, but C12/13 look to me to be inside a loop.
Click to expand...
Apologies, correct Jim. I was working from my phone earlier and did not notice this.
Unusually for old Quad there is a bit of poor design here. Simulation shows those two 1.5uF capacitors turn out to be quite critical and one thing you never did was depend on an electrolytics value.
The ideal design goes off a cliff at ~16Hz, degrade those capacitors to 1uF and it shifts to 25Hz, but boosts 100Hz
Try 2.2uF and the cut drops to about 10Hz, but 100Hz is now down

The 2.2uF C18 also turns out to be sensitive.
Even when new this circuit must have been a bit variable, add a lot of age and who knows...
The mid and top RIAA tracking is fairly good, just a tiny bit of treble boost, which is a common tweak
 
Underlines my earlier feeling, I think. For 'line' inputs the 34 can work very nicely. But I'd tend to either use something else for an RIAA stage or check what the 34 has carefully and consider modding it or ensuring optimum values for my purpose.

In my case this was 'no contest' because I can use the Armstrong 320 preamp design which I'm biassed in favour of anyway. :) I can't help feeling others didn't use that design because of the 'not invented here' factor. (Uses a Hitachi IC and circuit. An IC that impressed me so much I also used it for line stages!)

And the variability of electrolytics is, for me, another reason to make the LF roll-off low down so any variations don't intrude into the audible range. The required value becomes just "big". :)
 
The "correct" way to do this in the analog world if you want a sharp warp filter, is to put the warp filter after the RIAA equaliser and make use of the gain to use higher impedances to allow sane precision capacitor values
 
davidsrsb said:
The "correct" way to do this in the analog world if you want a sharp warp filter, is to put the warp filter after the RIAA equaliser and make use of the gain to use higher impedances to allow sane precision capacitor values
Click to expand...

I'd agree. I used that approach FWIW. The snag is you need to ensure the RIAA can cope with any unwanted large LF wobbles, etc. A bonus is putting any warp/DC filter *after* the RIAA means you can easily make it user-switchable as well.
 
The Quad 34 was launched when circuit simulation was becoming available, but checking for tolerance effects not so thought about. The 44 schematic is harder to find. Does anybody know what that is like
 
Hi!

Very interesting answers.
Still waiting for Robert’s comments.

Right now I can’t live with the 34. It’s just too lacklustre on LP.

The 44 diagram is on HFE. Its original diagram is very different from that of the 34. Then it was revised and modified with the addition of the two transistors.
I had always thought the TDA1034 was a better op-amp...
 
Interesting comments! Dada recommended changing C18 to 3.3 or even 4.7uF. Reading the comments here, I’m assuming it’s not that simple to ‘fix’ this phono stage then with just a couple of cap changes? Using 3.3uF Panasonic ECWF film caps for those positions, and I definitely think they were an improvement, FWIW.
 
Running 3.3 and 4.7u on the simulator, this actually cuts the bass below 100Hz, substantially at 30Hz.
BUT it also cuts the filter Q and therefore reduces group delay
The default Quad filter likely has group delay in the audible range
 
chartz said:
Hi!
...The 44 diagram is on HFE. Its original diagram is very different from that of the 34. Then it was revised and modified with the addition of the two transistors.
I had always thought the TDA1034 was a better op-amp...
Click to expand...
Found it.
So Quad went to a more conventional opamp circuit (with the extra IEC sub bass pole) and the reverted to a circuit similar to the 34.
The 66 is even closer to the 34 design.
he only thing that comes to mind is that ESLs might be extra sensitive to warps, so Quad have used a very aggressive warp filter again and again.
 
Yes I know, ESLs need that cut, but how does this explain the poor quality of the unmodified circuit?
Still waiting for what to do by the way :p
 
OK I've worked out a series of mods to hopefully improve the 34 disc input.... flat down to a couple of Hz and about 1dB down at 1Hz.
 
You must log in or register to reply here.


advertisement


Back
Top