Pioneer M-22 Power Amp - Epic Japanese Gear From the Past

Discussion in 'Power Amps' started by Armaegis, Mar 30, 2017.

  1. dBel84

    dBel84 Friend

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    Thanks for clarifying.
    :) been lulled into viewing everything as single device ala zen simple, and took the 0v as output but it is ground. Will look more closely at the circuit for my own education but regardless, 4 devices each biased up will produce plenty of heat.

    Would like to hear one some day, pity the old Japanese mosfets are dwindling. My M2x is very special and I am sure the Toshiba FETs play a large role in that.
     
  2. legarem

    legarem New

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    I listened the amp today. it seems what was magical disappeared. When I listen to a piano, I have the impression it has less harmonics texture. Sound seems more front with less air. The note attack is perhaps better with bass tighter but... Temperature with 500 mv reach 60*C at idle and it lower to 52*C when the amp plays. I'll rebias it to 800 mv to be sure about I heard.
     
  3. legarem

    legarem New

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    I rebiased the amp at 800mv. It takes less than 10 seconds to hear the BIG difference. With 500 mv this amp plays like most good normal amps. With the specified bias recommendation, the amp is magic. I wouldn't keep this amp with the sound it delivers with 500mv bias. It was made with over 800mv bias as recommended and it plays correctly with this specified bias. Don't loose your time to try it. It is night and day difference. I used 800 mv instead of 850 mv as recommended because my amp has 100 volts transformers for Japanese market. As the DC power supply voltage is a little higher, I lowered 50 mv from the specified recommendation. Now, how much time the output transistors will tolerate all this heat ? This amp is the only thing in my system I turn off after listening music. Ideally, this amp should have bigger heatsinks so it could be safe for output transistors.
     
    Last edited: Jan 25, 2020
  4. legarem

    legarem New

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    The amp stopped working this afternoon. I listened music and it suddently shut off. I let it cool and even the pilot light doesn't work. Some job for tomorrow.
     
  5. skem

    skem Friend

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    Legarem, since you are going to take it apart, would you indulge me with some close up photos of one of the boards. And in particular the transistors. I’d like to see what you have installed to see why mine might not behave like yours.
     
  6. legarem

    legarem New

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    Last night I opened the bottom and the main fuse was burned. I replaced it and it burned rapidly at startup. I looked at the DC resistance at the 33 000 uF caps and there's near 0 ohms on one. Something is shorted in the amp. The output transistors don't appeared to be shorted. More on that later.
     
  7. dBel84

    dBel84 Friend

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    Glad to see that they might be OK.
     
  8. legarem

    legarem New

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  9. skem

    skem Friend

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    Hey, thank you!
     
  10. legarem

    legarem New

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    With 120 VAC on 100 VAC power transformers, the V+ and V- is 34 VDC instead of + - 28VDC. With this higher voltage, the bias should be set at 650 mv to have the same power dissipation at idle on output transistors. More tryings to do about sound quality at this setting. The diode fail in the power supply makes me fearful.
     
  11. Tachikoma

    Tachikoma Almost "Made"

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    Why don't you just buy a 110V - 100V stepdown?
     
  12. legarem

    legarem New

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    This is probably the worst thing you can do for the sound quality unless you have a super heavy beefy oversize step down transformer. As already explained, the power supply voltage in the M22 is rather low so from 28 to 34 VDC, this is not a huge difference. As the standard bias recommended for the amp is already on the high side, this is risky to go with higher transistor dissipation. That's why I lowered the recommended setting.
     
  13. Tachikoma

    Tachikoma Almost "Made"

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    Well, that's the thing, an oversized stepdown isn't expensive. Given that the M22 draws about ~200W, a 500W stepdown should be more than enough.
     
  14. legarem

    legarem New

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    If the amp biased at 650mv plays correctly, I don't consider the step down transformer useful. More on the sound later.
     
  15. JK47

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  16. JeffYoung

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    A push-pull amp will deliver twice its bias current before leaving class A. So at 350mV the 4 output pairs will drive up to 2.8A in class A (giving 20Wrms into 4ohms). After that it will be in class AB until it runs out of voltage swing (or bumps into some kind of limiter circuit).

    At 800mV bias it could source 6.4A, which would be 160Wrms into 4 ohms with enough voltage. Since it doesn't have that kind of voltage swing it will stay in class A until it clips or hits its limiters.
     
  17. legarem

    legarem New

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    This afternoon, I listened the amp with 650ma bias. With this bias, the amp is tighter in the bass and seems to have a more treble presence (balance tilt ?) I can live with the amp as is because my speakers have a switch to tame 3 db over 12 000 hz. Tomorrow, I'll look at the 10 kHz and over square waves of this amp so it can perhaps explain why this amp is so lively in the treble.
     
  18. JeffYoung

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    If you're getting overshoot you could increase the value of C10 (on the feedback side of the second differential) to tame it. But it's already quite large -- so I'd be surprised if it's overshooting.

    [Edit: it occurs to me that there might be multiple versions of the schematic with different component numbering so I've attached the one I'm looking at.]
     

    Attached Files:

  19. legarem

    legarem New

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    Hi Jeff. Finally, there's no overshoot at all. What goes in goes out. It's amazing to see such square waves. You can see here in Dropbox, 50 hz, 20 khz and 100 khz square waves. 20 khz and 100 khz are perfect. The best I have seen on any amp tested. https://www.dropbox.com/sh/211w10zud9az4ut/AABoOty9tyfu7NC-DstUNSysa?dl=0
    I also measured the power with a 5 ohm load on only one channel. At 1 khz, it delivers 87 watts before clipping.
     
  20. legarem

    legarem New

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