AliExpress / eBay JL Hood 1969 Class A Amp

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

  1. Priidik

    Priidik MOT: Estelon

    Friend BWC
    Joined:
    Sep 27, 2015
    Likes Received:
    1,821
    Trophy Points:
    93
    Location:
    Estonia
    Nothing wrong with that. It is not ideal, and with a keen eye you noticed that in my schematic I have damping circuit (snubber) over the diodes, usually 100nF + 100 ohms will do.

    Here, apples-to-apples. The snubbers removed, series inductor removed (1nH is less than straight wire) and current increased to about 4.7 A. Diode dissipation is now about 2 W.
    Note, as Mr @murray hinted about storage capacitors, the snubber as well will practically not affect the diode dissipation, the point of it is to reduce the dU/dt for it to not produce radio frequency noise, thus it would spread out the peak over a wider area.
    upload_2020-10-16_9-24-50.png

    At some point it is best to start building the damn thing, the LTspice can trap you into infinite 'polishing-wank'.
    Besides it is fun to burn some components once in a while, diodes and most transistors are cheap. Use eye protection!
     
    • Like / Agree Like / Agree x 1
    • List
  2. Mihnea

    Mihnea Rando

    Joined:
    Aug 25, 2020
    Likes Received:
    1
    Trophy Points:
    3
    Location:
    Romania
    Thank man! Indeed, I need to start soldering the damn thing. Anyway, I want to place a fuse right on the transformer sec , because that is the most expensive part.
    So you think that the toroidal that I have should be enough to get those clean 18V DC? I need to be sure before I order the second one. I still have the option to return it and exchange for something with higher sec voltage.

    Mihnea
     
  3. Priidik

    Priidik MOT: Estelon

    Friend BWC
    Joined:
    Sep 27, 2015
    Likes Received:
    1,821
    Trophy Points:
    93
    Location:
    Estonia
    Can't really tell what transformer you have, the pic link doesn't work for me.
    From the sim I see about 27 V peak.
    I will assume two secondaries each 19V rms. And 200VA total power.
    To have 5 amps comfortably available at +/-18V in the psu output and you insist on linear regulators (which would drop at least 5V to counter grid and load fluctuations) then this hypothetical tranny would be a bit shy in both voltage and power headroom.
    22..24V per sec and 200VA per sec (400VA total) would be more like it. With more voltage headroom you can play with inductors after the rectifier. (bonus, 24V and 400VA is a common industrial piece, abundant)

    Also, beware when you measure transformer sec voltage to open loop (voltmeter) then you get somewhere around 5..10% higher reading. The stated sec voltage is for full load conditions.

    Regarding fusing, some trannies have bi-metal thermal switch built in to cut the power.

    Bonus tip: when screwing around put a 100W halogen lamp in series with the primary. When you short the secondaries the lamp would just turn on and protect the downstream.
     
  4. Mihnea

    Mihnea Rando

    Joined:
    Aug 25, 2020
    Likes Received:
    1
    Trophy Points:
    3
    Location:
    Romania
    Thank you for the tips! And indeed, the transformer is 2x19V RMS 300 VA (2x7.9 A)! :)
    I will trade it back and get the 24 V version. I think is for the best.
    Thank you again,
    Mihnea

    *I'm absolutely positive that I'll throw some more questions your way at some point. I hope you won't mind.
     
  5. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    • Like / Agree Like / Agree x 1
    • List
  6. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    Finally got it! First impressions are it's very tube-like. Smearing tiny details and not very much in the way of bass extension or quantity but with a great soundstage and tone! I'll take this over the class d amps I've had easily.

    I'm using it with my LG V30 at the moment. I'll get some records to use with my currently dormant Technics SL-10 w/Grado Red2 and MoFi StudioPhono and report back.

    I'll try to remedy the bass issue with bigger output caps once I get them in.
     
  7. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    Alright I popped the cover and I think I have some work to do. Some caps on either side don't even seem to be the same values for each channel, the input caps look ancient, and the input from the power supply has resistors tacked onto them which I don't understand. Should I mess with those?

    Also the transistors look glued in and I don't know if that's going to keep me from getting to the underside of the board.
     
  8. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    There's also a resistor on the iec input? Very weird.
     
  9. dBel84

    dBel84 Friend

    Friend
    Joined:
    Sep 27, 2015
    Likes Received:
    2,199
    Trophy Points:
    93
    If you do not know how to read a circuit and reverse engineer what is going on, I strongly suggest you do nothing yourself and find someone who can and is willing to work on your amplifier. Things are not always what they seem from physical location on a board.. dB
     
    • Like / Agree Like / Agree x 4
    • Epic Epic x 1
    • List
  10. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    That's an idea. I'm a little intimidated.
     
    • Like / Agree Like / Agree x 1
    • List
  11. artur9

    artur9 Rando

    Joined:
    Sep 6, 2016
    Likes Received:
    20
    Trophy Points:
    8
    Is there an aftermarket upgrader for these along the lines of ModWright? That would be intriguing.
     
  12. je2a3

    je2a3 Facebook Friend

    Joined:
    Feb 12, 2019
    Likes Received:
    227
    Trophy Points:
    33
    Location:
    USA
    Home Page:
    I'm not sure how feasible that would be for what was essentially a DIY project published in Wireless World in 1969.
     
    • Like / Agree Like / Agree x 1
    • List
  13. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    @je2a3 It sounds great stock. If I wasn't super curious about tweaks and improvements I could be happy with it stock. Vocals especially are both beautiful and realistic without being overly romantic.
     
  14. je2a3

    je2a3 Facebook Friend

    Joined:
    Feb 12, 2019
    Likes Received:
    227
    Trophy Points:
    33
    Location:
    USA
    Home Page:
    If you track back to page 1 of this thread and go to post #20, the OP @purr1n already did mods to his unit by adding low ESR bypass caps to the the output electrolytics. I did something similar to my amp using a combination of paper in oil + paper caps and gave tips on adjusting bias current + AC balance in my blog.

    Anything beyond that is a redesign, like the use of a bipolar power supply to eliminate the input and output coupling caps which theoretically can be detrimental to the sound. Actually John Linsley-Hood did updates along those lines in 1996.

    Personally, the output caps don't bother me since I'm primarily a DIY/tube/DHT aficionado and use coupling caps a lot in my projects. IMHO, the caps contribute to the original JLH 1969 Class A amp's unique sonic character.
     
    • Like / Agree Like / Agree x 2
    • Epic Epic x 1
    • List
  15. areasonableman

    areasonableman Rando

    Joined:
    Jan 28, 2021
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Busan, South Korea
    Ordered some 4700uf mundorf electrolytics to replace the 3300uf output caps. I've read on other forums that helps out the bass. Also ordered 47uf no-name polypropylene caps for bypassing, and 2.2uf mundorfs to replace the input caps. I'll post more impressions when they get here and I have them installed!
     

Share This Page