AliExpress / eBay JL Hood 1969 Class A Amp

Discussion in 'Power Amps' started by purr1n, Apr 25, 2017.

  1. ruinevil

    ruinevil Acquaintance

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    Mine was grounded to the rear chassis, as you can see in my picture, if Mystic bought from Doukmall-US on Ebay too. But yea, they don't QC their stuff, and I'm sure the MOSFETs they are using aren't checked for function before installation. They did say they were willing to refund some money if I went to a repair shop, but all the local ones are closed due to COVID. On Aliexpress they sent a new module over to some guy in Russia in the review section. I might have taken them up for that offer in a different time.
     
  2. ampguy

    ampguy Rando

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    Have the Nobsound NS-02G it playing now. Using with a Schiit passive SYS preamp, with CD player
    through Schiit Modi multibit, and a Marantz turntable through Parks Audio Puffin through LS-50s.

    Has the sweet sound of tubes. Noticeable with CDs and vinyl of The Nightfly. Also listening
    to Hope Sandoval Until the Hunter, Clairo Immunity. A bit more bass shy than the 60/80W Marantz
    PM7005, and at my listening levels, both the amp volume and preamp volume are past noon.

    Amp is hot to the touch. This model is a Nobsound NS-02G in black. Going to listen to it over
    the weekend, then decide if I want to put the Marantz back in here.

    Definitely not the best amp for loud music with the LS50s. If you like playing your LS50s loud,
    get a good 60-80W 4 ohm stable amp. But for normal / moderate level listening, so far it is pretty good.
    I can turn the gain up on the phono pre to 44db and lets me turn down the amp and/or SYS pre, but
    output from Modi multibit is fixed.

    Also, using a powered 10" sub, but not crossing over, so amp outputs go to sub, sub full range
    goes to LS-50s. The sub itself has a crossover (which I set at around 70Hz), so full range is
    going to LS50s.
     
    Last edited: May 20, 2020
  3. purr1n

    purr1n Finding his inner redneck

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    Nope. Aegir has more power and is more transparent.
     
  4. ampguy

    ampguy Rando

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    Currently am questioning the Schiit Aegir quality. Folks on other forums are having issues. One user bought two Aegirs to run in mono mode with LS50s, but both going into protection mode, possibly an amp/speaker mismatch if user likes loud, but a bothersome aspect is one of the two Aegirs is 20deg hotter than other one. Will wait until the QC resolves, then may try one.

     
  5. dmckean44

    dmckean44 In a Sherwood S6040CP relationship

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    A pair of Aegirs are not going to get along well with this and neither will the Hood.

    [​IMG]
     
  6. purr1n

    purr1n Finding his inner redneck

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    Despite LS50s being small, they need a lot of power and are closer to 4-ohm speakers than 8-ohms. Honestly, I wouldn't hesitate to use pro audio amps that crank out 800W for the LS50s.

    I've already said this in the thread: only consider Aegir if you are running super efficient speakers. If low Z, then they better be super duper efficient.

    Some folks have been able to run regular speakers in small rooms at moderate volume, but this doesn't make up for user error. Audiophiles are dumb, read Stereophile recommended components, and through wishful thinking hope that Aegir will work well when it won't. Aegir is much closer to a flea powered niche amp than a 200W power monster.

    If one is running 20 degrees hotter than another Aegir, confirmed by a FLIR, then one of them is broken. Can't say which because relative temperatures don't tell me anything.
     
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    Last edited: May 20, 2020
  7. ampguy

    ampguy Rando

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    So far, so good at ~70db (phone db meter, not sure if accurate), and that's with the passive sys preamp. Due to the heat in small room, may use Marantz in summer months, and the Class A in the winter.

    I listen louder in the study, esp. with movies, but also have powered sub, and higher efficiency speakers than the LS50s in this room. Have Q150s in another room, which are a couple db more efficient, and actually sound pretty close to the LS50s.

    Am driving old Harmon EPI Tower 120s in yet another room, with a 35W entry Nakamichi TA1 receiver. Have turntable, and Mani phono pre on that setup.

     
  8. purr1n

    purr1n Finding his inner redneck

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    Yes, ease up on the volume. Don't expect 100db unless you have 100db efficient speakers. 70db is fine.

    IMO, Aegir was meant more for the full-rangers, wide-banders, back-loaded horns, front horns with big efficient woofers, and transmission-line speakers. Zu, Omega Voxactiv, Klipsch.

    At least that's what I imagined when I suggested to Jason that he make a flea powered Class A' Continuity version from Vidar.

    I never thought so many would run LS50 because LS50 - probably because was so small, so small amp needed. Oh nooooo!

    KEF grossly and irresponsibly optimistically rated the LS50 at 8-ohms when it's more like nominally 4-ohms. However their sensitivity rating (voltage, so sort of cheating) was at least realistic at 85db for 2.83V. This means they need a lot of current, more power because of its low impedance.

    Basically what I said. The LS50 needs a 800W amp. At least for levels I will occasionally use. But this also means the LS50 cones explode.
     
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    Last edited: May 21, 2020
  9. ampguy

    ampguy Rando

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  10. ruinevil

    ruinevil Acquaintance

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    I bought one from a guy on Ebay for much less.
    Red multimeter probe to one of the contacts and black to chassis (ground). I think it's the medial contact.
     
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  11. ampguy

    ampguy Rando

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    Thanks. Bias was at 24.8. / 25.2, set both to 24.0. sublime so far!!

     
  12. ruinevil

    ruinevil Acquaintance

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    I got a used one that appears similar to purrin's version. It has a little bit of a buzz in the left channel, but not audible from sitting distance, and it's in the bass frequencies, which was different than the Nobsound where it was noticeable high pitched squeal.

    The test voltage doesn't match up to the Nobsound version. When I got it the test voltage was running at 58V and the device was lukewarm. When I brought it below 52V it started to clip in the bass section, so I have it sitting at 52V now. The device is running much warmer now.
     
  13. ampguy

    ampguy Rando

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    Sounds like some kind of variant from the original JLH 1969 design:

    https://jelabs.blogspot.com/2019/03...NX9inEHHGgrIedYD9PU1wcOs8mIFJHiikIjbALkOiJ3Fs

     
  14. Noldir

    Noldir Rando

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    I have had a bad experience with Nobsound as well: I bought a bluetooth apt-x to coax out receiver. The sound was beyond horrible, no highs, bloated bass. Having heard your experiences just confirms I'm not buying anything Nobsound for the foreseeable future. Sounds like they have QC issues or just don't test anything the produce.
     
  15. ruinevil

    ruinevil Acquaintance

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    Give the choice of buying from Aliexpress in China where the cost of shipping back will be equivalent to the price of the amplifier if there are issues, Mystic and I decided buying from Nobsound with a warehouse in California was a better idea. Unfortunately our amplifiers had issues. It's all sketchy Chinese products in the end.
     
  16. ampguy

    ampguy Rando

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    My nobsound ns-02g came from the doukmall us (Walnut, CA) warehouse. It is fine. Had the chassis ground for ground lug, bias was a bit high at 26VDC, which I adjusted to 24VDC. Sounds good with the Tekton Mini Lores, as well as LS50s at moderate levels.
     
  17. Priidik

    Priidik MOT: Estelon

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    Here is a challenge for the brave:

    Conditions, not optional:

    • You have multi way speakers and you use multiple amps and use something more suitable for bass
    • Mids or highs or both have capacitors in series in the speaker XO and nothing before the cap shunting to ground
    • You are comfortably sure what you are doing and understand basic measurements with a DMM
    You act on your on responsibility, I am not forcing anyone to try this!

    To be kind of sure the mids-highs will not be fried by the bias voltage of the amp (around 20 Vdc from my version of JLH) you take resistance measurement from the mid-high speaker taps with a multimeter in ohms range. When it shows increasing resistance reading that eventually reaches 'infinity', then it's likely safe.

    When you are more ocd and want to be actually sure you take a dc source that outputs at least 20 Vdc, add around a kilo ohm resistance in series with the DMM in dc mA range and connect it to the mid-high speaker taps. When the DMM mA reading shows anything other than close to zero (residual and or leakage of 0.1 mA is fine), there is something fishy going on and it will not be good idea to use with those speakers.

    Alright then...

    Take the output cap of from the JLHood, throw it into garbage and solder a bridge in its place. And connect to mids-highs of the speakers.
    (Optionally, the cap can stay and you simply solder a bridge wire across it, same effect)

    The result:
    It's an amp in a different league of what it just was.
    Not even close to trying to juggle around with a low value mkp cap in parallel with the big dirty e-lytic. The XO caps in a quality speaker are simply much better.
    Articulate highs, crystal clarity, carved into stone imaging, welcome: plankton.
     
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    Last edited: Aug 24, 2020
  18. scblock

    scblock Friend

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    For those wondering how hot is hot, I measured a couple temps of my unmodified Nobsound branded version of this amp with a standard (uncalibrated) K-type thermocouple. The amplifier has been left running for about 12 hours in a 75-80 degree F (25 C) room with decent ventilation (nothing above the unit). Temps varied a bit over time, but these were fairly stable averages, and fairly consistent between the two sides of the amplifier.

    Heat sinks: 135 F (57 C)
    Transistor package: 150-160 F (65-70 C)

    That seems pretty hot, but looking at the spec sheets for typical MJ2955 and 2N3055 transistors it seems that they are likely well within the specs given the power dissipation levels. I guess time will tell.

    For comparison, the top surface temperature of my Lyr 3 (which uses the chassis as a heat sink) after being on all day in the same room is about 105 F (40 C), and I think that runs pretty hot.
     
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  19. ampguy

    ampguy Rando

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    Sounds about right. Have you adjusted your DC balance? Mine's currently 25.0 VDC on both sides, but come winter, I might go to 24.0 VDC which will get the heatsinks to ~140ish deg F.

     
  20. Katalyst

    Katalyst Rando

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    Greetings. I have one of these amplifiers and have a question. The one I am using does not have a resistor on the input. I have seen some connected in line with the signal. Others have been across the input rca connection. What is the purpose of the resistor?
     
    Last edited: Jul 30, 2020

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