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Transistor equivalent

Discussion in 'General Electronics Chat' started by Nikolai Petrenko, Oct 24, 2015.

  1. spec

    spec Well-Known Member Most Helpful Member

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    Nicolai I have just noticed that the output transistors are configured as complimentary emitter followers. This is bad news from the frequency stability and hifi point of view. Best change to conventional emitter flowers and put a 56 Ohm resistor directly on the base of the driver transistors.
     
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  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I disagree. I have used bootstrapping successfully many times.

    Again I disagree and so does the site where the amplifier came from (Elliot Sound Products) and from this site that shows the Complementary Feedback Pair (Szilaki Pair) produces distortion that is less than 1/20th that of ordinary emitter followers):
    http://www.epanorama.net/sff/Misc/Amplifiers/Complementary_Feedback_Pair.pdf
     
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  3. spec

    spec Well-Known Member Most Helpful Member

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    I'm tired of arguing about fundamental issues- you have your view I have mine.
     
  4. dave

    Dave New Member

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  5. Nikolai Petrenko

    Nikolai Petrenko Member

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    No, no shorted turn because my idea like this: trans.jpg
    In case the bad idea you talked about: the bolt will be heated by current (so it waste lots of electricity and can blow main fuse). I have solution for that bad idea: don't use metal bolt, I will use a bamboo rod then tap the head with nuts. Or use insulator between bolt-wash and chasis or bracket.
    I found a website that talk about problem: http://www.edn.com/Home/PrintView?contentItemId=4204775
     
  6. Nikolai Petrenko

    Nikolai Petrenko Member

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    Thanks. I also think like you! 100uf C+ and C- are not enough and no RL output circuit to balance impedance.
    The sound will be better but amp will be hotter if I adjust trim pot to increase quiescent current, right?
     
  7. Nikolai Petrenko

    Nikolai Petrenko Member

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    OK, OK. I am also tired. But thank both of you any way. I may will try all opinions to fin out which is better to me :D:D:D.
     
  8. spec

    spec Well-Known Member Most Helpful Member

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    You have a handle on the position. A conductive mounting bolt can be and is used provided it does not form a shorted turn.
     
  9. spec

    spec Well-Known Member Most Helpful Member

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    No, the amp will be exactly the same as the quiescent current will be adjusted to be the same with the pot. The quiescent current would be about 40mA. If you go for conventional Darlington output stage it will probably be necessary to adjust a resistor value around the pot. This is no big deal. I will take a look. The quiescent current in the Voltage Amplification Stage (VAS) is the same 6mA for both bootstrap and constant current configurations. With a conventional Darlington output stage you have a defining dominant pole in the VAS so the open loop frequency response will be nice. The output transistors will also turn on and off fast. You could even make the drivers class A if you wanted.
     
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  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Very close:

    The typical Toroid mounting is a Bolt, large metal washer. large rubber washer, torroid, large rubber washer. When mean large I mean HUGE washers.

    See: https://toroid.com/Transformer-Products/Mounting-Hardware Mounting washers with rubber pads. That's what I used.
     
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  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    What? No frying Anchovies, please!

    When you increase quiescent current, the amp goes into thermal run-away.

    The idea is to TRACK the changes the output transistors are doing and bring the difference in base voltages in line.
    1) with the difference between the bases = 0, you have crossover distortion.
    2) With too much difference, that amp heats up a lot.
    3) With just the right amount of bias and the right feedback, there will be no difference in quiescent current as the transistors heat up.
    That's the trick.

    The regulator is often called a Vbe multiplier. It's guessing the Vbe of the output transistors based on their temperature and it wants to keep the NPN and PNP bases very close to n*2*Vbe+(some slop) as things warm up. I used n*2*Vbe because it depends on the driver stage.
     
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  12. Nikolai Petrenko

    Nikolai Petrenko Member

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    OK, I will set Iq around 70mA.
    About transformer mounting, my construction: Bolt, shakeproof washer, washer, large ferro plate, cardboard plate, transformer, cardboard plate, chassis, washer, nut.
    Why don't use free cardboards, steel sheets around instead of buy from store. Of course the cardboard must be thicker than 2mm.
    I will wrap all sides of toroid core with thick cardboard, use masking tape to make it unmovable. Wind primary coil. Wrap cardboard again. Wind secondary coils. Wrap cardboard one more time. Wrap a copper sheet around to prevent magnetism leakage. Wire addition coil to feed preamp, LEDs, fans. Finally cover all transformer with some layers of tape. Done!
     
    Last edited: Jan 23, 2016
  13. spec

    spec Well-Known Member Most Helpful Member

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    Perhaps "may go into thermal run away if not properly designed and laid out"

    The VBE of a BJT typically drops at 2mV deg C. The VBE multiplier does just that and produces a voltage of either 2x VBE for complementary output stages or 4 x VBE for Darlington output stages. It is important to have decent heat sinking for the output trans and drivers and to ensure good thermal tracking and thus reasonably stable quiescent current. The VBE multiplier transistor body should be in thermal contact with one of the output transistor cases. The low value resistors in the output path also help the thermal stabilisation, but on a well designed amp are a pain as far as audio performance is concerned. In fact those resistors are a bit on the high side at 0R33. OR1 would probably be better.

    I have often run AB amps with 200mA quiescent currents and more, without any thermal run away. With the original germanium amps thermal run-away was always a worry and there you may see output resistors as high as 2R2.
     
  14. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Iq will ultimately be a number that with the amplifier idling, the amp should get barely warm. The voltage across the emitter resistor (Iq) is what you generally moniitor.

    In post #732 the voltage between the base of Q5 and the base of Q6 is important. The voltage across R13 and R14 are other important numbers. then there is the DC presented to the speaker terminals.

    You don't want cardboard around the core. Equal spaced windings. You can save time for the primary if you wind it in the Bifilar mode. Hand winding of large toroids can be done with a flat piece of wood with notches. Wind the amount of wire on the stick and then keep passing it through the toroid. Then if you need 240 V, wind as two concentric 120 V windings. i.e.two wires together.

    The secondary is done the same way. So for 35 CT, you wind two 35 V windings. Mark the phase.

    http://www.dupont.com/content/dam/d...ocuments/DEC-Kapton-summary-of-properties.pdf

    tape may prove to be very useful especially between the primary and secondary.

    The flux of a toroid is for the most part confined, but an electrostatic shield may or may not be required.
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    #352. Can't really argue, but in the Leach amp it's about 3.6 V which is neither or your options. Hence my use of n*2 Vbe. That output stage is far from normal.

    The only stressed point is that the bias regulator has to track. This is where thermal and mechanical design are important.
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    The optimum quiescent current for an AB hifi amp is based on only one thing: minimizing x-over distortion. x-over distortion is gross compared to other distortions and is particularly nasty sounding. For this reason it often sets the characteristic sound of an amp. The optimum quiescent current for a typical output transistor pair is 20mA, but below this figure xover distortion incresses rapidly. Above this figure x over distortion increases gradually, so many amps chose 30mA to 40mA as the best compromise. Without dedicated Iq stabilization circuits, the Ig will vary, depending on junction temperatures.
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Your right. I never had the luxury of a distortion analyzer to set Q current. Ultimately it's the best way.

    ==

    I remember once I had nothing to go on, so I adjusted based on a bit over Vbe multiplied. Then monitored the AC current, Q current and heat sink temperature every 10 minutes or so. Got something that was stable and let it idle for 8 hours or so. Then I said done. Probably overkill. Amp never came back for repair.
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    I think you mean #752. I don't see why all amps in the world should comply with two options that I mention for two output configurations of a particular amp.That is illogical.

    The complementary output stage has 2VEBs = 1.2V in the signal path, the Darlington ha 4 VBEs in the siganal path= 2.4V, and the Leach has 6 VBEs = 3.6V in the signal path ( see post 542)
    Note that the Leach has a tripple Darlington output stage.

    Although you have not defined what N is in your equation, if it means the number of VBEs between the VAS and the output, that is exactly what I am saying, and have been saying all along. It is so fundamental to this type of amp design that we do not to discuss it, but the information may be of intrest to Nikolai and others reading this long long thread.

    I have described two output stages which one are you saying is not normal. In any case they are both normal, with the Darligton being the most widely used. I do not know what you are trying to prove here.
     
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  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    You just stated it a LOT better than I did. That's nothing unusual. You understood if just fine which I expected. I apologize for not defining n.

    My version of "Normal" output stage. One complimentary pair. I haven't been to sleep yet. Just lying in bed. Class D is probably the new Normal anyway.
     
  20. spec

    spec Well-Known Member Most Helpful Member

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    The DC presented to the speaker terminal has nothing whatsoever to do with the VBE multiplier or the quiescent current or its setting. The DC offset is controlled by the completely independent overall DC servo loop between the input and output. DC offset is simply not an issue with modern amp designs, some even without the undesirable DC bocking capacitor in the feedback loop of this amp.
     
  21. spec

    spec Well-Known Member Most Helpful Member

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    Now I feel bad. My excuse- I didn't sleep last night with this $%#!! flu.
     
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