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Quad 34 MM load

Discussion in 'd.i.y.' started by chartz, Sep 4, 2019.

  1. chartz

    chartz pfm Member

    Hi,

    I want to use my newly acquired QUAD 34 - late grey example with CD input - with an AT VM740 or 150sa, which share a body.

    I normally use a 33 preamp and the sound is great, bright and detailed as I like (405/ESL).
    Same with my other preamps, valve or transistor, there’s little difference.

    Now the 34 makes LPs sounding dull, as if treble was partly erased.

    By default the load is 47k/200 pF (styroflex).

    I tried to reduce the capacitance by just lifting the cap, with the cable from the turntable being about 100 pF.

    A tiny bit better but still veiled and therefore boring. Even bright records are dull.

    CD and radio sound fine, however.

    Any advice welcome :)
     
  2. Tony L

    Tony L Administrator

    I can’t remember how to do it as Rob did mine, but if you either remove or dramatically reduce the always-on rumble filter, which is on the mainboard somewhere, it wakes up no end and actually becomes a very nice phono stage once you match the capacitance and load to your specific cart. When I started with mine all default I had a 2M Black and it sounded terrible; just thin, tizzy with no bass or dynamics. The rumble filter largely hoofed and the load capacitance set to IIRC 35k/39pF and it sounded excellent.
     
  3. chartz

    chartz pfm Member

    Cheers Tony. So it can be tweaked to sound good, which is good news.

    However here I’m not complaining about bass - I have ESL57’s - but about lack of treble energy, of which I’ve usually got plenty.
    The rumble filter does exist on my 33 too and it poses no problem.
    I’ll wait for Robert’s comments on this matter.
     
  4. Tony L

    Tony L Administrator

    For me the rumble filter, or lack thereof, fixed a deadness to the sound. It just sounded a lot more dynamic and powerful.
     
  5. Jim Audiomisc

    Jim Audiomisc pfm Member

    What loading do AT recommend?
     
  6. Tony L

    Tony L Administrator

    IIRC the load is a standard 47k, it is the (total) capacitance that is very hard to achieve being <150pF for many of their better carts. The standard Quad 220pf sounds horrible as with the arm lead it is twice the recommended.
     
  7. Jim Audiomisc

    Jim Audiomisc pfm Member

    OK, so it looks like, in principle, removing the 33's input loading capacitance give the sort of value the maker's suggest. If so, it becomes a case of then experimenting if you want a different response. Hard to advise on that as catridges vary a lot in how the respond to this. Matter of experiment unless you've already worked on the case.
     
  8. chartz

    chartz pfm Member

    I just want my treble back!
     
  9. davidsrsb

    davidsrsb pfm Member

    The MK2 schematic shows C1 and C2 220pF
    You can safely change these to 22pF to give some cellular RF protection and reduce the loading to something more suitable for an AT or modern Ortofon
    There will still be plenty of capacitance from Quads unusual MM input, with a lot of capacitance from the effectively paralleled npn and pnp common emitter input transistors
     
  10. chartz

    chartz pfm Member

    Thanks David. Hadn’t thought of the transistor capacitance at all.
    As I said previously I got rid of the 220 pF caps, and treble is still recessed.
    I was thinking that raising the input resistors to say 56 k or 68 k would perhaps restore this.
    And then again, everything is perfectly balanced using the 33 with the same arms and cartridges (and any of my other preamps for that matter, including the superb SU-A4).
     
  11. davidsrsb

    davidsrsb pfm Member

    Have you tried measuring with a test record? Quad were pretty good at QC, but it is possible that the RIAA was built with wrong value parts. Also is there any way a tone control has got off-centred or a filter permanently switched in?
    The Quad has logic control of the filters, so something may have gone wrong
     
  12. chartz

    chartz pfm Member

    Thanks for the leads David.
    CD sounds fine. I’ll check on component values, but on both channels?
     
  13. chartz

    chartz pfm Member

    Hi,
    I replaced the 47 kΩ resistors with 68 k. Treble is back, at last.
    But that 34 lacks slam, and reverting back to the old 33 is shocking: more dynamic, more bass - essential with ESL 57s - and an altogether far more detailed sound. Yes, the 34 has just been recapped with fresh (genuine) Elna Silmic caps.
    So just what you describe, Tony.
    For example, the vibraphone in Dreams comes really clear with the 33, and is somewhat erased by the 34!
    By the way, where is that b****dy rumble filter? Is it just the 2.2 µF cap?
     
  14. Jim Audiomisc

    Jim Audiomisc pfm Member

    Perhaps worth adding that there is more than one version of the 34, so the LF coupling may vary from one board issue to another. The earliest ones had th simplest circuit. Need to dig out diagrams to say more, but PJW tended to roll off LF.
     
  15. chartz

    chartz pfm Member

    LF is relatively unimportant to me, but I need treble energy and dynamics! I examined the diagrams, but couldn't find anything.
     
  16. davidsrsb

    davidsrsb pfm Member

    Build errors happen when the wrong reel of components was put in a machine or in the wrong bin on an assembly line, so you often get both channels affected.
     
  17. chartz

    chartz pfm Member

    I've found nothing wrong. I've retained the 68 k loading resistors and no capacitors.

    The sound remains duller than that of my other preamps, and I must say that I much prefer the old 33 soundwise. What I find amusing is to read that the old 33 is veiled. It certainly isn't. The 34 is duller, less detailed and undynamic on LP.

    On CD and radio there is little to put them aside.

    What do you think?
     
  18. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    If it sounds even worse than a 33 that's quite something! I find the stock 33 VERY veiled. I couldn't use one personally other than possibly in a vintage/retro system where first class sound quality is not of primary importance.
     
  19. chartz

    chartz pfm Member

    Funny, my 33 is crystal-clear and so musical, perhaps a bit bright but in a nice way!

    I just renewed every electrolytic but that's it.
     
  20. Tony L

    Tony L Administrator

    As I say I felt the 34 was dead until the Rob removed or reduced the rumble filter in the main board. That done and with load and capacitance set for the Ortofon 2M Black I was using I actually preferred it to an EAR 834P I also had at the time, so it can be a capable phono stage for sure (the EAR is well over £1k plus I have some seriously good NOS valves). My 34 is a fully recapped rev. 6 board grey case model. The 34 really shouldn’t sound bad at all if it is working right. It was routinely used with some of the most revealing speakers on the planet (Quad ESL)!

    PS What power amp are you using? The 34 has a very limited output voltage and a very limited number of steps to its volume attenuator. If you are ending up with the volume control wound up very high you may actually be clipping the output as IIRC there is something odd about the gain structure of this preamp and where the vol knob is in the circuit. It was very much designed for a full Quad system driving a 500mV 303, 306 or 405. A standard >1V+ power amp and less than efficient speakers can apparently drive it into mild clipping comparatively easily. I’ve only ever used mine with a 303 or 306.
     

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