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Avondale NCC300 Monos Underway

Discussion in 'd.i.y.' started by Mike Hanson, Jul 5, 2021.

  1. 337alant

    337alant Negatively Biased

    I'm sure what you are saying is good advice, but as I said I have been using different versions of these amps for over 10 years @ 53v rails and all has been well and I have never had one blow up like that?, I'm sure many others will agree with me here
    There will always be failures with DIY builds but there is nothing wrong with these amps and they are not unreliable as you stated ?IMHO.
    I do use speaker protection

  2. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Such a unit is safe until it is asked to produce too much current, such as into low impedance/awkward loads, whilst being powered from excessively high voltage rails. A phenomenon called secondary breakdown can then occur with bipolar transistors (mosfets are basically immune to this) where a hot spot develops on the die and this causes a short circuit. It is precisely why Quad built in the current limiting to the 405 which used very similar output devices and 50V rails.

    The length of time they are used for etc has no bearing on it. It will happen when the conditions are such as to cause it (obviously). Awkward loads with a large phase angle can be especially bad. SOAR of bipolar transistors is only around 0.5 - 1.5 Amps for the usual 15A output devices when at high voltages. The 15A rating is only up to around 10 - 15V often and tails off after that as the voltage rises. In a typical SOAR graph there will be a voltage limit, a current limit and a secondary breakdown limit... giving a safe area under the curve which should not be exceeded. There is also a region with annotation for time which shows for how long certain ratings can safely be exceeded, usually in small fractions of a second. The "woomph" of a stylus lifting or setting down is a long time in electronics terms!!

    It is simple fact that >50V rails with one pair of output devices means this can happen when the conditions are right and there are 5 cases I know about so far of these self immolating in this manner.
    I wouldn't use any more than 40V rails personally.
    Mike Hanson likes this.
  3. laverda

    laverda pfm Member

    [QUOTE="Arkless Electronics, post: 4432753, member: 16103]there are 5 cases I know about so far of these self immolating in this manner.
    I wouldn't use any more than 40V rails personally.[/QUOTE]

    Would these 5 have been on the end of an Arkless Electronics phono amp.?? I have had 6 phono amps (4 of which were DIY) over the years driving various pre-amp/ Avondale power combinations without any issues and never considered there would be an issue.

    Obviously this is now a big issue for consideration for those that have existing Avondale amplification and are looking to implement and or upgrade a phono/head amp.
  4. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    NO. I repeat, and will do so continuously and in block capitals if I need to, there is no issue with my phono stages and I will not stand by and watch my product slagged off because another manufacturer has not followed good engineering practice in omitting current limiting and using too high voltage rails. There is a good reason why Naim use 39-40V rails and Quad use current limiting, as I explained above. Any phono stage, especially with extended bass, can cause this if the volume is left up high when changing records. The likelihood will vary with just how high the rails are and what speaker is in use, plus of course how high the volume is set.
  5. 337alant

    337alant Negatively Biased

    But you will quite willingly SLAG OFF Avondale Amplifiers when the owner and designer is not here to defend himself, thats bad form IMO

    You say you know of 5 cases of one of these amps destruction in this way, there must be well over 1000 of these amps out there
    so if we said 1000 amps and 5 have failed thats 0.5% , thats hardly cause for concern or a major design flaw in my book ?
    divedeepdog and JezmondTutu like this.
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I said up thread that I had kept my opinions on the issues with the Avondales to PM's but there was then the half expected Avondale fan-boi comebacks basically saying that their fave amps must be perfect and my product must be at fault, leaving me with no alternative but to explain the issues with the Avondale power amps.

    Of the blow ups I know of 2 were with my phono stages and when people lifted/lowered the stylus with the vol up high, another was exactly the same but the phono stage was not one of mine, another was when (IIRC!) someone fell asleep with the stylus in the lead out groove, NOT using one of my phono stages, and was awoken by the burning smell, and the last one I don't know the circumstances of but it did not involve one of my units. All resulted in blown bass drivers as speaker protection modules were not fitted.

    Note that "S-man" independently confirmed my views on the Avondale issues up thread.

    As I've said, there are good reasons why other companies don't use a single pair of devices without current limiting and with >50V rails. IIRC I actually warned about just this in relation to these power amps about a year or so ago in another thread.... and got the "well I've used mine with 60V rails and it's fine" type replies... fine until the exact combination of circumstances exceeds the SOAR!
  7. 337alant

    337alant Negatively Biased

    For the record I didnt see anyone having a go at your phono stages

    You harp on about 50V rails but how many of us use 4 ohm speakers? so I don't see a problem, its all a hypothetical smoke screen IMO.
    Yes Naim used 40 v rails and had a SOA but no zobel network
    What is the first mod everyone did to there Naim amp, remove the SOA and it sounded much better
    But how many Naim amps Blew up because somebody didnt use the right length of speaker cable hahaha

    Incidentally I have used my Paradise phono with 65dB of gain for years and no issues at all when dropping the needle down.
  8. S-Man

    S-Man Kinkless Tetrode Admirer

    Just for clarity - I offered my recommendations based on an engineering approach along with my own experiences and methods. I have discussed this with Les and his experience is that they can be run "a bit harder".

    FWIW I would also question Naim's lack of speaker protection and fuses. Once again they have rather more experience of their own amps than I have!

    I don't believe that SOA protection is a significant bottleneck to good sound (based on another amp that has it and still manages to sound wonderful). Lots of people think the opposite and are prepared to take some extra risk for the "benefits".

    Some people think relays in the amp output affect the sound quality, quite possibly including Naim.

    I have no intention of making definitive statements on any of this. I will happily provide my opinion and what I think I know.
    This is DIY - people can make their own choices!

    PS I don't always agree with Les, nor him with me. But he doesn't half know some stuff!
    (The mains transformer he gave me is responsible for the biggest step forward in sound quality I have had for many years - and I haven't even done him the service of using it one of his amps yet :oops:).
    Arkless Electronics likes this.
  9. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner

    That's the salient point....and in this world of restrictions and nanny state mentally...long may it continue!!

    I will temper that sentiment long as people know what they are doing...or are prepared to accept the risks for doing something they don't really understand

    No-one wants a fire in their house and should take every precaution to prevent it
  10. S-Man

    S-Man Kinkless Tetrode Admirer

    Good question!

    I think (thinking about the NCC200 schematic)...
    1) Negative rail fuse fails - the current sources cannot provide any current so there is no current available to drive the output stage bases.
    1a) If it were possible for Tr4 to turn on and provide base current into the output stage then the +ive fuse ought to blow.

    2) Positive rail fuse fails - I am trying to figure out if the VAS transistor could supply current to the output stage. I don't think so but the "a" scenario would apply anyway.

    The fuses are there to prevent a fire, they would probably also save your speakers but probably not save the output stage.
    Or you could use a Naim- style sealed box which is probably fire proof :D.

    The other consideration is that the higher your rail voltages the more chance of speaker damage if something does go wrong. The energy stored in the caps goes up in proportion to V squared.
  11. Chops54

    Chops54 pfm Member

    I thought about using a double pole breaker such that if one side trips it knocks off the other side too. There are two issues with using these, one is that it’s easy to just switch it back on and the other is that they are rated for 230 vac and I don’t know how that would work at 50 vdc ? I can get these as low as 1A at 230 vac.
  12. S-Man

    S-Man Kinkless Tetrode Admirer

    I would have thought that the heating of the bimetallic strip is only affected by RMS (and DC) current and that the voltage rating is the isolation voltage. Therefore the MCB should work at lower voltages.
    Mike Hanson likes this.
  13. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner

    So for those that might be interested...
    120mA bias is equal to 2.178v when measured across (the correct!!) Resistors

    So a lot closer than I expected it to be (as in the two (recommended?) settings of 2.2v or 120mA)

    Amps finished and listening tests ASAP...but got to make a preamp to monos cable up... couple more pictures to follow shortly
    337alant and laverda like this.
  14. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    IF bias is set as usual on these, ie by measuring the voltage across emitter resistors, that is about 10A!!

    I'm not familiar with the NCC300 so if a non typical method is used then ignore this....
  15. 337alant

    337alant Negatively Biased

    Looking good and your measurements are fine
    I found it easier to fit 2 board pins to the inner vacant CComp slots

    Mynamemynaim likes this.
  16. Luca

    Luca pfm Member

    Two weeks ago, I started the listening tests of my just built power amplifier with Quodos boards inside. It is running on 43 +/- 0.5V rails and sounds very good to my ears, better than others Exposure and Naim power amps passed through my listening room. Would five or six volts more be that important to performance or would it just produce more heat?
  17. Mike Hanson

    Mike Hanson Lovely!

    The heat may increase slightly, but it's not a Class A amp, so it's not going be a big deal.

    The main difference is in total power output of the amp, and to some extent linearity (because of the additional headroom). IMO, you're unlikely to notice the difference, thought except perhaps when the amp is really pushed.
    Luca likes this.
  18. Luca

    Luca pfm Member

    Hi Mike and thanks you for the reply. Reading here: ( I find different opinions about the power output.
  19. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner

    a couple of pictures of the completed amps (just the lid to screw down!)

    [​IMG][/url]2021-08-04_09-52-45 by rock solid, on Flickr

    [​IMG][/url]2021-08-04_09-52-28 by rock solid, on Flickr

    [​IMG][/url]2021-08-04_09-52-12 by rock solid, on Flickr

    Also managed a few hours listening last night....and I have to say ...

    what's all the fuss about??

    They sound great..don't get me wrong...but so does my ncc220 amp...probably very similar in fact

    I forget who it was who posted they were not impressed (for a few weeks) then they had a "wow" moment
    I can only hope that happens to me
    Mike Hanson likes this.
  20. laverda

    laverda pfm Member

    Looking, you have the 0 speaker returns to the rectifier end of the PSU?? If so try relocating the NEG 0 speaker wire to the clean end of the PSU used for the outputs...power up and wait 3 days. (may be longer) I know the preferred connection is as close to the transformer 0's as possible (dirty end of the PSU) but IMO the clean end of PSU to be better with the 300's . If its not revert.

    How much F have you on the front end PSU?
    Mike Hanson likes this.

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