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Leak Stereo 20

Discussion in 'classic' started by Tony L, Jan 31, 2012.

  1. Alex S

    Alex S carbon based lifeform

    I have moved to the 8R taps, bass a little flabby(er) on 16. It took a visitor for me to listen critically enough to care. Also moved the speakers about 2 inches.

    Ground loops were certainly an issue, now resolved thankfully (without doing anything foolish).

    As for valves, Mullards still probably my preference but the EI ECC83 works well as do the 6P14P ERs in place of the EL84s and I can happily live with them as a cheaper alternative.
    paulfromcamden likes this.
  2. Amormusic

    Amormusic pfm Member

    I'll have to reread this all. My Leak is being collected on Sunday (exciting). I've already ordered in some valves ready, including an EI and Mulllard ECC83.
    Bjork67 likes this.
  3. Nagraboy

    Nagraboy Ausculta fili

    Is it sensible to use an ECC82 as the input valve to reduce gain instead of the usual ECC83? I was advised to try this today but wondered if anyone knows definitively?
  4. Tony L

    Tony L Administrator

    No, it changes more than the input gain. IIRC the negative feedback loop is impacted by changes here. This is a key part of the Leak ‘ultra linear’ design where (AIUI) the output is referenced via a transformer tap. Changing valve type is effectively changing the electronic design, not advisable unless you really understand the whole circuit (which I don’t). There are many here who can explain this a lot better than me, but I’d not stray from the stock valve types unless they are a direct equivalent (e.g. E83CC, 6P14P etc).
  5. Nagraboy

    Nagraboy Ausculta fili

    @Tony L

    OK thanks Tony. Actually, I just found a thread from 2012 where @Robert and @Arkless Electronics were advising you on this same question.

    I was just curious after Deco Audio suggested trying it earlier today. I’ve been asking them about doing some mods to my Croft 25 (with cathode follower mod) which I’ve been using with my Leak. They’re going to add a ground lift switch to the Croft so I can put the earth wire back in the Leak mains lead, and will also rewire the mute switch to be a mono switch.

    Perhaps surprisingly, the high-sensitivity Leak 20 and high-sensitivity Klipsch Cornwalls don’t produce too much background noise but I was wondering how to make it even better.
  6. simonpyeman

    simonpyeman pfm Member

  7. toprepairman

    toprepairman pfm Member

    Changing the '83 to a '82 will only reduce the open loop gain.
    However the closed loop gain is mostly defined by the negative feedback so you'll be hard pushed to notice any difference.
    The feedback will be less so the distortion will go up.
    That Lilienthal circuit is very silly as with lower gain you'll end up with poor bass as the NFB will struggle to keep anything there.
    The way to reduce gain is to increase the NFB, but if done alone will cause the amp to become unstable, so you have to reduce the open loop gain also.
  8. Radford Revival

    Radford Revival Trade: Radford Revival

    Those mods look like a recipe for low frequency instability too.
  9. Nagraboy

    Nagraboy Ausculta fili

    What about soldering a higher value resistor across the input to bring down the sensitivity to around 500mV? Would this alter the impedance figure?
  10. kevinrt

    kevinrt pfm Member

    I’m still waiting to hear back from Deco about mine. Did they give you any indication about waiting times?


  11. Amormusic

    Amormusic pfm Member

    I've started reading through this all. I am realising how high gain it is and it reads as if it will be zero to superhero loud!

    Mine arrives on Sun and will be paired with a NVA P50SA MKII passive pre. Should I be plugging in some attenuators before I fire it up the first time? My speakers, whilst I think will be a great match, and pretty efficient at 90db and easy load, are also not cheap and I don't want to damage them (perhaps an unfounded concern).
  12. Nagraboy

    Nagraboy Ausculta fili


    Don’t worry about, it’s not no big deal. Relax man.
    Mike P and montesquieu like this.
  13. simonpyeman

    simonpyeman pfm Member

    ok, so the Audiokarma circuit I messed up loading last time is this:
    No idea if it makes any sense - I can’t find the thread anymore, but there was lots of scoping and stuff going on IIRC. Interesting to see what you fellows make of it.
  14. Alex S

    Alex S carbon based lifeform

    Definitely you don't need or want attenuators with a passive.
  15. Tony L

    Tony L Administrator

    Also remember that the 0.125V sensitivity for full output is the full output of a 10 Watt amp. In theory it is not much different to say the 0.5V of a 100 Watt Quad 405, the latter will just go a lot louder. They likely feel much the same volume-wise. Add high efficiency speakers and anything noisy upstream (e.g. even good valve preamps have some noise) and you have an issue.
  16. Alex S

    Alex S carbon based lifeform

    Can someone please explain the ST20s characteristics - output impedance, hopefully high - damping factor, hopefully low - feedback, hopefully zero.

    The next grand plan will involve TT (I have ideas there) and speakers. I really like what widebanders do and do like the Zu Druids but I’m looking at the Cube Audio Nenuphar Mini that would apparently prefer the above amp characteristics. (Sadly I just can’t accommodate wide speakers).

  17. Tony L

    Tony L Administrator

    Output impedance; fairly low for a valve amp as it has good quality transformers made back when people knew how to make them.

    Damping factor fairly low; its a vintage valve amp so was never designed to fling tiny heavy modern drivers around. In its day speakers were large, low excursion and tended to have paper surrounds. Damping factor only started being a thing to worry about in the ‘70s and ‘80s when small inefficient aggressively ported speakers with heavy large excursion plastic drivers started to appear.

    Negative feedback; fairly high (for a valve amp) as it is an ultra-linear push-pull design. Worth reading the Wikipedia explanation of this design concept.

    I’m sure someone can give you exact figures, though I’d not worry about it. It is a superb sounding amp that still puts much of today’s kit to shame 60 years later.
    Mike P likes this.
  18. Alex S

    Alex S carbon based lifeform

    Thanks Tony, absolutely, the amp’s a keeper, narrow-ish baffle, room filling-ish speakers to go with it is what I’m looking at. Happy with the Druids for now.
  19. Tony L

    Tony L Administrator

    I had a quick look at the Cube Audio speakers and if they can be driven with a 300B SET a Stereo 20 will have no issue at all. They look very Lowther-influenced and that’s home territory for any Leak.
  20. Alex S

    Alex S carbon based lifeform

    Tony, don’t want to derail this into a speaker thread, I’ll drop you a PM at some future point if that’s okay.

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