AliExpress / eBay JL Hood 1969 Class A Amp

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

  1. philipmorgan

    philipmorgan Member of the month

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    > Are the cap values too large to be replaced with the big soup can film caps?

    No
     
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  2. purr1n

    purr1n Finding his inner redneck

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    The issue with big caps is real estate. You can always cut holes in the chassis to make smoke stakes.
     
  3. ohshitgorillas

    ohshitgorillas Friend

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    I can kinda see what you mean looking at the pictures more closely. If I took the pot out and used my Saga as a preamp, it looks like there might be a little bit of room in the front. I'm sure I'd figure something out in any case. I'll snag one next payday, still sounds like a good time to me
     
  4. Priidik

    Priidik MOT: Estelon

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    You'd need another chassis for this.
    Given the effort and the potential payoff it's very likely not worth it.
     
  5. Wushuliu

    Wushuliu Acquaintance

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    For the curious, there is also a much smaller Chi-fi JLH-based headphone amplifier, the L1969Phone SE, on ebay for $38 here. $18 without the case. It's what I use (albeit modded). As I am (relatively) new to the headamp side of things and diy-adjacent I can only compare it to a Tangentsoft MINT, to which I find it superior. Less power than the larger amps of course but IIRC from a diyaudio thread, having a single rail power supply minimizes the hum the larger dual rail versions tend to suffer from.

    I may get a Vali 2 or Magni 3 so I can have another reference point.
     
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  6. ohshitgorillas

    ohshitgorillas Friend

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    Thanks to @dmckean44 I now have a Nobsound version of one of these sitting on my shelf. It's warming up nicely, tracking the heatsink temp with an IR thermometer, it's at about 121F and has been on for about an hour and a half. I think it really hit its stride around 40 minutes. I'm upgrading from a Topping TP60 which was actually surprisingly good imho, but this thing has a meatier sound, or perhaps is just letting the meatiness of the Bifrost Multibit shine through more. I will keep listening and return with impressions once I feel I've really gotten to know the thing. I will say, even with 98 dB speakers, I was a little worried that this thing wouldn't be enough for my TV's relatively low gain digital output, but it's perfectly sufficient to get absurdly loud.
     
  7. ohshitgorillas

    ohshitgorillas Friend

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    So, in terms of the JLH '69 vs the Topping TP60, I don't think there's a huge amount of difference. They have about the same level of detail retrieval and dynamics, but the JLH definitely has better soundstage/imaging and gives a more solid sense of a performance in the room. I think there also might be a slight veil, but I'm sure that will be reduced with the addition of some film cap bypasses.

    One small concern, are my speakers supposed to make a thump after the thing turns off?
     
  8. crazychile

    crazychile Eastern Iowa's Spiciest Pepper

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    Turn off thump may not be so common these days, but it wasn’t uncommon 30 years ago. Some of the big Adcom and NAD amps did it. As long as it isn’t too loud it isn’t an issue.
     
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  9. ohshitgorillas

    ohshitgorillas Friend

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    [​IMG]

    Finally got around to putting in two 30uF paper in oil caps, NOS Russian military. They fit really well, although I haven't bothered to mount them in there properly. I also replaced the output wiring.

    Sonically we're still in the first hour of playtime, but it's definitely painting with a finer brush, so to speak. Faster, more precise, more dynamic. Very nice.

    Eventually planning on replacing all the signal wiring with Mogami two-core shielded wire (input) and Neotech solid core or Cardas litz 16 or 18 awg UPOCC. This amp is really nice, I'm really glad I came across this thread.
     
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  10. shabta

    shabta Facebook Friend

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  11. Zampotech

    Zampotech Friend

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    I want to note at once that I am not familiar with this model of the amplifier.

    Mathematical calculation shows that 15 watts at a supply voltage of 22 voltscan theoretically be obtained at a load of 4 ohms. But probably at the cost of huge distortions. The real power with comfortable sound quality is probably about 11 watts.

    My conclusions are based on mathematical calculation and little experience with JLH amplifiers

    PS: The body of this amplifier is excellent.
     
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  12. Noldir

    Noldir Rando

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    Good find! Just wondering though, the "original" version had 3 transistors on each side. The "new" one only two per side? Or am I missing something? Like: a complete lack of knowledge of amplifier design? :p
     
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  13. fraggler

    fraggler A Happy & Busy Life

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    This version has 3 per side and I think looks better than either of the previous two. Probably harder to tweak, though with the layout.
    https://a.aliexpress.com/_dS5oczU
     
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  14. Noldir

    Noldir Rando

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    Oh wow! That does look nice! Shipping is killing it however for me, almost as much as the amp itself
     
  15. dmckean44

    dmckean44 In a Sherwood S6040CP relationship

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    It's still cheap even with the shipping for what it is.
     
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  16. ohshitgorillas

    ohshitgorillas Friend

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    Today I am considering opening this thing up again and replacing the RCA jacks and signal wire with higher quality components. This is a pretty straightforward operation, but I'm also curious about cutting the volume pot out of the equation and running it like a power amplifier, since I use a Saga for volume anyway and always just leave the amp itself turned up all the way (maybe one of these days I could replace the pot/knob itself with a VU meter?). I'm no expert but I think that bypassing the volume pot would change the input load. There is already a resistor across the two input jacks... am I correct in thinking that, to maintain the same input impedance, I should instead use a resistor that would be the sum of the current resistor plus the pot value, or (less ideally) use two resistors in series? Looks like a 100k resistor and a 50k pot, so replace it with a 150k resistor?
     
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    Last edited: Apr 8, 2020
  17. Noldir

    Noldir Rando

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  18. ohshitgorillas

    ohshitgorillas Friend

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    Just a quick update, I did try bypassing the volume pot and ended up with some DC on the output. I'm thinking it's because I forgot to restore a common ground between the two channels. I ended up opening it up and adding two upgrades: I replaced the input capacitors with MKP film caps (they fit VERY nicely) and bypassed the power caps with 0.1uF film caps. Lastly, I connected the chassis to the earth ground on the IEC inlet.

    I'll include pictures once everything is done and put back together, but I still need to re-add the signal paths and make sure they are mutually grounded. I hope this solves the DC on output issue, but I have found myself relatively busy with remote work and will be putting it back together this afternoon. One thing that has been holding me up is the 10k ohm resistors across the original inputs. I don't have a spare pair of 10kohm resistors, so I'm wondering if 8.3kohm or 13kohm would work just as well? I have some matched 10ks incoming but in the meantime it would be easy enough to wire up with something close in value.
     
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  19. ruinevil

    ruinevil Acquaintance

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    I was watching this guy build what appears to be the output portion of the 1969 and he was running it through his analyzer. He noticed clipping below 35 Hz and marked increased harmonic distortion once the signal hit 13kHz, but its pretty clean otherwise.
     
    Last edited: Apr 18, 2020
  20. ohshitgorillas

    ohshitgorillas Friend

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    Okay, that took longer than it should have but I'm still learning. When I cut out the pot from the signal path, I also incidentally removed the signal ground from the chassis ground. This resulted in some nasty noise. I thought DC on the outputs was also an issue, but as it turns out, the DC went away when

    I eventually just wired the pot (incl. chassis ground wire) back in with some two-core Mogami stuff, as I didn't have anything good on hand to use as a chassis ground point. The other things I did:

    • Overpriced WBT NextGen RCA sockets (bought for another project and then didn't fit)
    • Replaced input caps with French gray and black MKP film caps, 2.2uF 400V. Brand name escapes me.
    • Bypassed power capacitors with 0.1uF film caps. Something I got on clearance. I worry that 0.1uF isn't enough to make a large enough impact.
    • Added wire from one of the trafo shield screws to previously unused earth ground tab on IEC inlet, to ground chassis to earth.
    • Used 13k ohm resistors across inputs rather than 10k ohm because I didn't have any spare matching 10k ohm resistors
    • Bypassed output capacitors with vintage Soviet 30uF paper-in-oil capacitors (did this a while back).
    It's only been on for a few hours and these caps need time to fully break in, but the depth, soundstage precision, and 'textural' detail retrieval are much improved. I'm actually quite impressed. I don't have a lot of experience with speaker amps, but it sounds very very good so far.

    [​IMG]
    [​IMG]
    [​IMG]
     
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