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Sunn Amplifier Power Amp Puzzle

Discussion in 'Repairing Electronics' started by Bud_J, Jul 9, 2017.

  1. Bud_J

    Bud_J Member

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    Okay, I've checked into this and I realized I made a major mistake. Because I have the bulb limiter after the Variac, the voltage measured at my Kill-A-Watt, which is plugged into my Variac, is not the voltage at my amp. I put my multimeter across the AC side of the amp's rectifier and measured 51 VAC. That means all the values I posted previously were not at 90VAC, but 51VAC.

    Sorry about that. Wasn't thinking. I'm not sure what that means for the current reading measured by the Kill-A-Watt, as the 40W bulb in the limiter must have been responsible for part of that current.

    I think I have to reverse things . . . plug the bulb limiter into the mains outlet, then plug the variac into the limiter. Then (I think) the Kill-A-Watt will give me the right voltage and current. NOPE. Just tried it. I can't get the Variac, Kill-A-Watt and multimeter to agree. Maybe it's time to ditch the bulb limiter?

    Still . . . I wonder if there should have been half of that 90mV idle current, even at 50 VAC? Or do you think it needs more than 50 VAC before we start to see idle current?

    Sorry for the goof-up . . . I'm learning.

    Thanks for all the help . . . you're helping me learn (as are my mistakes).

    Peace,
    Bud
     
    Last edited: Aug 3, 2017
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You need to plug the Kill-A-Watt into the mains ahead of the variac.
    At that point it will accurately measure the power used (the variac takes negligible power. You can see that when you turn the variac voltage to zero).
    It's designed to operate at the mains voltage and may not give an accurate power readings at voltages significantly below that.

    The bulb limiter doesn't (and can't) add to the current since it is in series with the amp.
    It will subtract some from the amp power, depending upon how much it is lit.
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I had a similar work problem because "they" really wanted to measure power and they had this wonky variac phase angle fired SCR controller. So, like $900 and use a 3 phase real power gizmo to measure three ~0-30 V circuits and it worked when you pick the right stuff.

    The point is, you at last got 15V for the + supply, so you had ENOUGH voltage to make that tick.

    If the bias regualtor is disabled, it's kinda a good thing during the power up process, but not a good thing for music.
    Not sure if it should be around 2.4 V or 2*3*0.6V = 3.8V. No voltage across the emitter resistors means it's not operating in class AB. It may be operating in class B with "notch distortion", because that's what class B does.

    The emitter resistor of 90 mV is very critical. Guess what, it's controlled by that POT you don't want to move. If you stay below 90 mV the amp should not "BLOW UP" on you. When you go significantly higher than 90 mV, thermal runaway starts.
     
  4. dave

    Dave New Member

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  5. Bud_J

    Bud_J Member

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    Thank you, Crutschow. I've yet to try that configuration. It was kind of a bummer yesterday to realize my amp was only operating at 50VAC. I had been excited to think it was operating at 90VAC. I think I'll back up a few steps and try to bring it up to a higher voltage while monitoring TP6. If that goes well, then I'll check the regulated +/-15vdc supplies to see if they come closer to being in balance. Then I'll look for the idle current per KeepItSimple's suggestion -- by checking the Vdrop across R21 and R50.

    I'll post my findings later.

    Peace,
    Bud
     
  6. Bud_J

    Bud_J Member

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    I've been studying Class A and Class AB amplifiers online, so I actually am able to to follow that ;-)

    When I was peeling the RTV off the big caps to replace them, a little voice in my head said, "You know, Bud, sooner or later you're gonna have to peel the RTV off that bias pot, too."

    I know I'm being skittish -- maybe too skittish because of the previous meltdown. See my prior post to Crutschow--I think I'll get things back up to a respectable voltage and look for that idle current again. Is this a case where I should see 45mV if I have the AC up to 60VAC? Or does it not work that way? If it *does* work that way, then do you think I can reliably set the idle current at half voltage by setting it to 45mV? Or is it better to get the amp up to 120VAC and do it then?

    I'd feel better about messing with the idle current once I solve the -15vdc supply problem. Unless you think the two things could be intertwined?

    Thanks for all the help. I should have some new findings posted later!
     
  7. Bud_J

    Bud_J Member

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    Alright, with this configuration I figured something out. With a 40W bulb in the limiter, even with the variac cranked to 130VAC, the most voltage the amp can see is 60VAC. So I measured 60VAC at the rectifier with my probably-not-true-rms meter and took some measurements. If nothing else, I'm getting good at taking measurements ;-)

    @ 60VAC:
    The current reading on the Kill-a-Watt, now at the top of the chain (plugged directly into the mains), is 270mA
    Rails are nicely matched at +/- 37.2
    Regulated supply still has the disparity: -13.07 and +15.03

    TP6: 2.5mV

    R21 .2mV
    R50 .1mV

    This leads me to some newbie questions . . . why is idle "current" measured in mV? Are they just giving us the voltage drop to save us from performing the calculation to milliamps?

    Is the 270mA measured at the Kill-a-Watt indicative of anything? Since we're using a bulb limiter, I imagine this current is meaningful in some way, but there's nothing on the schematic to indicate how much is too much? I know if the bulb glows bright for more than a second, then that's bad -- but I only know that because you guys taught me that.

    I used the Kill-a-Watt meter on my 100W Marshall and it draws 220mA. And I tested my Laney HC50 practice amp the same way and it draws 50mA. I realize this isn't the "idle current" of the power amp, but of the entire amp. But I still wonder what it might tell me.

    Is there a rule of thumb for how much an idling guitar amplifier should draw from the mains? The Sunn we're working on seems like it'd be drawing 540mA -- half an amp -- at full voltage.

    Thanks for keeping me steered in the right direction!
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Schematic -linked from page #1: http://www.electro-tech-online.com/attachments/sunn-alpha-slave-pa-same-as-beta-lead-gif.106993/

    Short C13, please. This will disable the bias regulator. Really, it's a good thing to do. With the bias regulator working, this means that some of the output transistors are partially on. It compensates the thermal bias change and different transistor gains. Right now, we want them OFF.

    So, if stuff is behaving properly with the bias regulator OFF, there should be viirtually no voltage across the emitter resistors. You could put your meter so it sees both of them.

    With a direct-coupled amp, everything has to work. Cross-conduction is what blows things up. When one rail is stuck, it might blow up a speaker.
    There is always a little cross conduction provided by the bias regulator. When you replace a transistor without re-adjusting the bias current the bias is disturbed.

    The normal way is to use whatever the factory gives you. The best way is to use a distortion analyzer. Not having either, well too little alters the sound and too much creates thermal run-away. So, it can be brought up in increments while monitoring the current (voltage across the emitter resistor).

    So, shut it off by placing a short across C13 while looking at input current and the voltage across the emitter resistor. One should be sufficient unless they are popped.

    So, most of the amp failures that I've seen generally will have blown output transistors. BUT all of those pesky, low value, little resistors nearby need to be checked. Sometimes the emitter resistor fails. The last few stages of the amp are voltage gain and power gain and sometimes a VI limiter.

    The servo thing should still work, so you'll get ~0V across the speaker terminals. Monitoring input current is still a good idea bringing it up n the variac.

    Any transistors that are paralleled, really should be matched for Hfe. They will likley fail down the road under abuse.

    With the bias regulator disabled you can inject a signal. It will likely have cross-over distortion, but you can check the symmetry.

    In that really nasty (mirror imaged board), I did "cut out" the bias regulator and got it working independently and by itself with an external supply

    ==

    So, if things check out with no bias regulator, then it's time to check that. It is a sort of chicken and egg problem. They both have to work, but disabling it is EASY.

    When it's finally enabled, your pretty confident that the rest of the amp is working. e.g. symmetrical signal, maybe some speaker offset, no bias current.

    So this time when you bring it up, you want R42 to be at it's minimum while monitoring the voltage across C13.
     
  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Apparently.
    The idle amps is just the current that the circuit is taking with no load signal.
    It varies with the amp design and the amp power rating.
    It would be difficult to readily infer its value from the schematic.
    Normally it's a small percentage of the full-load (with maximum audio output into a speaker).
    If you Sunn draws a half amp at idle, that's about 60W, high but probably normal.
     
  10. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Apparently.
    The idle amps is just the current that the circuit is taking with no load signal.
    It varies with the amp design and the amp power rating.
    It would be difficult to readily infer its value from the schematic.
    Normally it's a small percentage of the full-load (with maximum audio output into a speaker).
    If you Sun draws a half amp at idle, that's about 60W, high but probably normal.
     
  11. Bud_J

    Bud_J Member

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    Okay, you win! I shorted it, brought the variac up till I saw 60VAC across the Sunn's rectifier and I re-did the measurements from before:

    CR5 Vdrop: 13.63vdc
    R25 Vdrop: 4.28vdc
    C13 Vdrop: n/a
    R23 Vdrop: -24.8vdc
    R1 Vdrop: Grrrr. just realized I didn't write this down, but whatever it was looked in keeping with prior readings.
    R3 Vdrop: 22.8vdc
    Rails read good again.
    Regulated supplies were still off by 2 volts: -13 and +15

    Pretty much the same readings for the emitter resistors, too: .1mV on each.

    I take this to mean that the amp is behaving the same with or without the bias regulator, and so either the bias needs to be adjusted because of the new output transistors and/or that the bias regulator needs to be swapped out, even though it read good out of circuit?

    I still wonder: Could this be responsible for the disparity in the +/- 15v supply?

    Thanks for all detailed instruction!
     
  12. Bud_J

    Bud_J Member

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    Thank you, Crutschow. That puts me more at ease.
     
  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I'll take a stab at what crutshow responded to.

    It was listed on the schematic, but nit where to measure it, right? You have to set it in volts, so why not use volts. It really is the "Idle current ADJUSTMENT". e.g. Adjust to 90 mV

    "270 mA" - not too big. Look at it in terms of power (V*I) or % power out/

    The A/B info here http://www.audioholics.com/audio-amplifier/amplifier-classes basically suggests that it's a design issue.

    1/2 an amp at 120 V is 60 W. Yes, I think it's too much.

    OK, so I need to look at your last most while I was writing and falling asleep writing this one.
     
  14. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    OK! See if you can bring the amp completely up with or without the lightbulb, just using the Variac. Keep an eye on the kill-a-watt meter. Keep the capacitor shorted.

    100 W, I think would be too much. I really think 60W is too much.

    You can try this way:

    While bringing up the amp, give it a signal from your functton generator: guess 0.5V p-p. Observe the output while bringing it up.

    I believe the output should appear as an initially clipped sine wave gradually getting better with some amount of crossover distortion.
    Look primarily at symmetry.

    There is still no load on the amp and I think the current drawn will actually be less than at idle. Take a look at the DC out measured at the speaker terminals.

    ==

    I know shorting a capacitor just doesn't seem right.

    I'm still not happy with the -15V supply being -13 V. I m happy that the overall ripple of the main supplies are down.
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    ==

    From what I recall, the bias regulator might be dead or not working based on your voltage measurements across C13 when it wasn;t shorted. A signal test, I hope, should reveal amplification issues.

    The Op amp acting as a servo may actually complicate things when trying to do static tests. With a signal, the servo action should really be disabled. You can't have zero output at the speaker terminals when you have music, can you?

    So, this amp wants to make very close to zero volts at the speaker terminals when there is no signal.

    I'm hoping you'll see some asymmetry supporting the IDEA that the -15 supply may be overloaded/
     
  16. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The servo op amp has a very low frequency response and so does not affect the output in the music frequency range.
     
    • Agree Agree x 1
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    That's what I was trying to say. You said it better. I should have worded it as the servo amp is disabled during non-static tests.

    Thanks crutschow
     
  18. Bud_J

    Bud_J Member

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    I was having trouble understanding why. Then I remembered when I first built my bulb limiter, I tested it by putting a 60W bulb in it and plugging a lamp into it that also had a 60W bulb. I remember that both bulbs were half-lit. Ah ha! Then the 60W bulb in my head began to glow all that much brighter. I'm starting to get it. Hence "limiter."

    Thanks for the tip!
     
  19. Bud_J

    Bud_J Member

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    Well, I got my assignment and look forward to getting to it today. Maybe when the rain hits.

    At a couple points during prior testing, I had lifted the -15vdc wire on the connector that goes to the preamp and it made no difference. That should narrow down the possibilities as to what could be loading down the -15vdc supply. I saw IC1 as a possibility (esp. since I had changed one or two 4558s on the preamp board). That's when I lifted it to measure and caused all the trouble that started this thread. I'm still too much of a newbie to know what else on the power amp board might be suspect, though this doesn't stop me from conjecturing wildly ;-)

    Anyway, we'll see what the signal test turns up. Finally, I get to use my bargain function generator from eBay!

    Will report back later.

    Thanks for all the help,
    Bud
     
  20. Bud_J

    Bud_J Member

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    Well, some of this riddle may be solved. I've been assuming I'm supposed to see 120VAC at the AC side of the rectifier on the Sunn because my brain was thinking this was the line, but it's not. It's the secondary of the transformer. Looking at the schematic, I'm only *supposed* to be seeing 66VAC. The GOOD news is that I *have* been running the amp at pretty much full voltage! Which means it only draws 220mA at the outlet.

    That out of the way, here are my findings with a 1KHz 240mVp-p sine wave injected directly into the power amp (bypassing the preamp board for now, which has its own problems), and the master volume turned up to about 3 or 4 (hard to tell as it's not mounted) we get a 5Vp-p sine wave at the output at full power. Actually, we get that even at around half power, where it starts to drop off as I turn the variac down.

    At full power, there is detectable crossover distortion, but pretty tiny--might not see it if you're not looking for it. Around half power the output wave begins to lose amplitude and the crossover distortion grows more severe as we get down to 1/4 power, 20% power, etc.

    At 10% power the amp starts singing Elvis Presley tunes. Not sure what that's about.

    Other finding: Sometimes I had trouble getting the signal to appear at the output. I had to reset the function generator a couple times and/or disconnect the signal wire and reconnect. I remembered you had said to the inject the signal as I was turning up the variac, so maybe this is normal? Next time I run tests I'll have a 2nd probe at the input to see where the signal might be getting clamped down, if it is.

    No other distortion of the sine wave as I was turning it up.

    Thoughts?

    This part was kind of fun ;-)
     
  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The incandescent light bulb is basically a current dependent resistor and it's really based on the resistance temperature curve of Tungsten. Tungsten has a resistance about 12x lower at room temperature.
     

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