The Pioneer M22 as a head amp? (and recapping guide)

Discussion in 'Headphone Amplifiers and Combo (DAC/Amp) Units' started by skem, Aug 5, 2018.

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  1. JeffYoung

    JeffYoung Friend

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    (4) is going to put the outputs at a different bias. If the output transistors have a much flatter spot in their gain curves right at the bias then that /could/ explain the difference. (It's a stretch though.)

    Anyway, either way you should re-bias the modified channel. Short the inputs. Measure the voltage between Q8's emitter and Q9's emitter on the unmodified channel. Now measure the voltage on the modified side and adjust the trimpot below D4 till it matches the unmodified voltage.
     
  2. skem

    skem Friend

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    @JeffYoung

    Normally I re-adjust everything after any modification—but for some reason both channels were out of whack on DC offset and bias. I must've been twiddling something and then forgot during the weeks between replacement and measurement. Putting adjustments back in place made the channels behave symmetrically again. Thanks for the tip off. I feel like a moron for not having checked that right away.

    New measurement
    M22 IMD bias-and-offset-fixed.png

    But this reminds me of an oddity that pre-dates all of this. Maybe you can shed some light on it. I cannot get the output-transistor biases anywhere close to where they should be when the DC offset is near zero. The manual specifies 400mV/output-transistor emitter. That is obtainable only by upping the DC offset at the output terminals to 0.3V (by changing the offset to the input diff pair). When the DC offset is nicely adjusted to around the -0.02V spec, the maximum obtainable bias is only 80-100mV/emitter. Nevertheless, measurements look OK at this low bias (and the distortion is almost unchanged by the bias over a very wide range). My amp came with a modification of unknown origin that I thought might be responsible for this. The base resistors on the pre-drivers (Q5 & Q7) have been increased from 100R to 139R. Does it make sense that these larger resistors may be the reason I can’t reach full bias? What is the significance of this low bias when the numbers look fine? (Eg., maybe poor open-loop performance and thus slower settling time?)

    Alas... I also now have a new problem. The self-protect circuit kicks in every few seconds for random periods of time. *sigh* Seems to get worse as it gets warmer. This will take some debugging. Maybe the varistor or thermistor is going bad... unsure. This used to happen after long periods of use.. now it happens after short periods of use. Another delay. ;-)
     
    Last edited: Dec 17, 2019
  3. JeffYoung

    JeffYoung Friend

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    Excellent news!

    The change to the driver base resistors was likely done for stability. This topology has a tendency to want to oscillate. The base resistors will slow that stage down a bit, reducing the instability. In any case, it will have no effect on the bias.

    The low bias means that you will have a smaller class A envelope (before it goes into class B). Try adding another 100 ohms of resistance between the bias trimmer and the diode string.

    As for the self-protect circuit, I know what I'd do with it. :D
     
  4. PTS

    PTS Friend

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    Thanks for the info, skem. I'm a tech newbie, so it's important to get suggestions from people in the know. I wouldn't be opposed to @JeffYoung input also.

    Good to hear that a decent coupling cap is an important upgrade. I just went back and read about installing Audyn Reference caps under the RCA jacks, using lengths of wire. What are the specific specs? Seems there are a few types of Audyn Reference caps?

    The Mundorf M-Cap Supreme cap installation has been a dilemma. I wasn't sure what their purpose was exactly, only that Scott Frankland (the tech who's doing the mods) installed them on my previous M-22, explaining "The Mundorf caps will be connected in parallel with the big filter caps up top. Large caps are less able to bypass high frequencies than smaller caps, so it has been customary in modern times to add a small, high-quality film cap to large electrolytics to shore up their high frequency performance. This will make the top end faster, clearer, airier, and more detailed. It’s fairly subtle though"

    This is what it looked like (crappy iPhone photo) from the underside:


    [​IMG]


    With my current M-22, he's suggesting something a little different - "locating the bypass caps after the rail fuses (I'll ask to find out what fuses he's referencing) on the PC boards rather than across the big filter caps."

    The ELNA Silmic II caps were just a suggestion for the best type he has in stock. I don't think they're expensive, so if old caps need replacing and it makes little difference to the sound, these will work as good as any quality cap?

    I've only heard the M-22 through speakers, and the noise floor was almost non-existent (near black). I assume it's more noticeable when using the amp for headphones. Does reducing the switching energy of rectifier diodes noticeably effect the sound? I don't understand the HF reference.
     
    Last edited: Jan 30, 2020
  5. JeffYoung

    JeffYoung Friend

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    First off let me state my bias: I'm not a huge believer in cap upgrades.

    There are places where polypropylene (or styrene) is appropriate, and the input coupling cap would be one of them. But keep in mind that large caps can pick up RF, and the problem is exacerbated by putting them on long wires. Personally I think the best compromise between quality and size is the (lowly) Mundorf MCap. I used them in my cost-no-object Pass HPA-1 clone.

    finished2.JPG

    finished3.JPG

    Local power rail caps can be useful. PSU bypass caps less so (and in fact they can set up parasitic oscillations if not chosen carefully). In either case polypropylene is not required. PET (polyester) will do fine. Something like a small Wima or Epcos/TDK (the little blue boxes in the above pictures).

    Finally, it's worth considering that the Pioneer engineers were very skilled, and didn't skimp a lot to save costs when it came to the M22. So I wouldn't count on anything more than very marginal improvements from any upgrades.

    I do like the Elna SILMICs when it comes to electrolytics. But they don't come in large sizes.

    These are just my opinions. I encourage others to form their own. ;)
     
  6. skem

    skem Friend

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    Agree with @JeffYoung

    The RF noise from the big input caps is real. We’re still talking very low (-90dB ish), but it’s there. I used coaxial-shielded wire to attempt to help keep it low. I’m going to try the small ITT PMT/2R 3.3uF that @legarem likes and report back, but not until mid May.

    @legarem’s finding that his amp has “magic” at 800mA bias and my inability to discern or measure any difference with bias change has me wondering what’s up. I also want to do the square wave test @legarem did. Last time I did that they looked flawless but it was ages ago and I don’t recall the frequency.
     
  7. JK47

    JK47 Guest

    Maybe somehwhere along the way of massive mods you lost it's mojo?
     
  8. skem

    skem Friend

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    Very likely. Most probable place for mojo loss seems to be on miller caps. I have experimented with multiple caps there and haven’t detected any difference. But mine has always had something wrong because the bias does not get where it should be without modifying the design. Still sounds great. But ...yeah... nervosa.
     
  9. PTS

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    If I'm reading this correctly, are you suggesting the Mundorf MCap for coupling caps?

    These two statements calm my desire to (perhaps unnecessarily) sink lots of money into the M-22. Thanks!
     
  10. JeffYoung

    JeffYoung Friend

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    Yep. It's the one upgrade I'd be tempted to make.
     
  11. sp33ls

    sp33ls Friend

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    I'm looking at picking up an M-22.

    Skem, would you now change any components you've listed in your Digikey parts list? From what I've heard, this amp is very musical in nature (and slightly warm), and I'm curious how restoring all of the components impacts its voicing overall.

    Edit: Did you also modify anything to enable 115-120VAC, or are you using a stepdown transformer?
     
    Last edited: Apr 3, 2020
  12. skem

    skem Friend

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    Not really. I’d skip the resistors. I prefer my Matched zetex Transistors over the original (a five pin long-tail pair In one package). I’ll probably wind up selling mine in due course. Not using it these days. Input coupling cap is a very personal thing. I think I would try something different than what I have now. Will try some suggested by others here in a month or so.
     
  13. sp33ls

    sp33ls Friend

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    Matched Zetex Transistors over original -- are you referring to the output transistors (replacing the 2SC1402/2SA744)?
    I'm planning on replacing the input transistors (2SA798) with a matched pair of KSA922s, but hoping to initially leave the original transistors -- that may change lol.

    For the relays, I'm assuming you replaced all three relays with that same part (PB770-ND)? Do you recall which diode you used on the relays, was it 1N4004s?

    Appreciate the tips! Mine is on its way and should arrive middle of this week. Heatsinks will need some attention, but the chassis appears to be in otherwise pretty decent shape.

    I'll be getting some use out of a dim bulb tester with a 150W bulb over the next while... :)

    [​IMG] [​IMG]
     
  14. skem

    skem Friend

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    The zetex are in place of 2SA798. Instead of the ksa
    And to answer your other question, I still use a step down transformer.
     
    Last edited: Apr 5, 2020
  15. sp33ls

    sp33ls Friend

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    Hey skem (and others), when you were checking your DC offset, did you notice any oscillation at all?

    I've uploaded a video showing the behavior I'm seeing. I just re-capped (excluding the large filter caps), and swapped the input transistors to a pair of KSA992's. I tried to match as closely as possible, but I'm not sure how much I trust the hFE meter on my multimeter. I placed some high quality thermal paste between the two transistors to aid in thermal contact.



    Also, pointing to your earlier posts, do you think that the US version of the manual isn't optimal for the Japanese version of this unit regarding the 80mV DC offset (and 850mV bias)? I'm curious what your offset (at the outputs) and bias (TP+ / TP-) settled into after you were done.
     
  16. dBel84

    dBel84 Friend

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    You might have some micro leakage - how well did you clean the flux off the boards?

    I typically see gradual changes as things heat up ( which is expected ) - I do not think you are seeing true oscillation in the electrical device sense but rather very small changes in flux which is causing your output to wander marginally ( remember this is mV ) - I would have to look at the design again but I would also check the gate resistors.

    edit - assuming this is both channels behaving the same ?

    ..dB
     
  17. sp33ls

    sp33ls Friend

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    Hey skem,

    I hope you've been well! I was browsing filter caps for an M4 and was looking at the Nichicon LNT series again, since it appears the M4 shares the same filter cap size as the M22.

    I then came across this discussion pointing out that in the LNT datasheet, Nichicon advises to not install with the screw terminals facing down, in the case of a vent.

    Someone reported talking to a Chemi-con rep and stated:
    “I've spoken to a UCC rep, who gave me a pretty detailed answer.

    He said that older wax-based aluminum electrolytics could have safety issues with being mounted upside-down. If the wax warmed up, it could run to the bottom, cover the vent and resolidify. This would effectively prevent the vent from operating, potentially making a bigger boom in a failure condition.

    He also said that more modern waxless caps still had potential issues with electrolyte leaking out through the vent. This is especially true if the cap isn't being run with high ripple currents. If the cap is warm, the high temperatures put pressure on the vent, creating a better seal."

    Someone else on the electronics stack exchange commented:
    "Actually even the larger electrolytics with a polymer vent can be mounted in any position as long as the vent can operate. The manufacturers recommend mounting them with the vent up so that if the vent does operate you lose the least amount of electrolyte"

    It's probably a low risk that the vent would need to operate in the first place, but thought this to be an interesting tidbit that I hadn't really considered.
     
    Last edited: Dec 3, 2020
  18. Tachikoma

    Tachikoma Almost "Made"

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    What kind of resistor would you recommend for this application?

    To be honest, I couldn't hear any difference at all with load resistors on the M22. They make a fairly obvious difference on my transformer coupled tube amp though.
     
  19. skem

    skem Friend

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    The M22 is surprisingly immune to high-impedance loads. My XA25 really needs the resistor. I don’t have specific recommendations. I use a commercial ohmmite pair designed for braking in electric cars.
     
  20. JeffYoung

    JeffYoung Friend

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    The XA25 is the "hockey puck" amp, right? How's it compare to the M22?
     

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