Having trouble undersanding something please help

Discussion in 'The Projects Forum' started by damien83, Mar 6, 2016.

  1. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1
    Hey there...
    Building a guitar amp and having a little trouble getting my head around how to obtain more clean volume. I've been at this for a few months now.

    I have a ne5532 op amp which has a max current output of 16ma and max output voltage of 22V peak to peak. I have this op amp feeding a darlington pair push pull classab power amp stage with a current gain of 200.
    Since the ne5532 maxes out at 22V peak to peak does this mean that if I am running it at 22v that
    Volts X A = W so.....12V (half waveform) X 16ma = 38W ?

    At a guess - I figure I can get a transformer with a larger output voltage swing but this means my op amp won't be able to handle the voltage...or I get a high current op amp. Is this right? In order to get more volume/power ? I have tried paralelling 3 of these ne5532 op amp's and I don't get more volume but a much cleaner sound...I would have thought that this would give me more power as there's more current but I'm obviously missunderstanding something, or..everything. Any help appreciated, thanks. Any past posters that have dissed me in the past, don't bother commenting - you will be ignored.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Your circuit won' t last very long as you are feeding the NE5532 with a to high voltage:

    NE5532_maximum_ratings.png

    Bertus
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    High power, op-amp free, solid-state amplifiers have been around for decades; there must be thousands of designs out there.

    Here is a typical example:

    http://sound.westhost.com/project3a.htm

    May I, constructively, suggest that you study these designs instead of struggling with this op-amp mash-up that you keep returning to every few weeks and then repeating the same question but in different guises.

    More volume = more power which means you need to apply a higher voltage across the loudspeaker terminals and, of course, the current to back it up.
     
  4. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1
    Hey Blocco yeah I'm considering going down that route, would a transistor long tailed pair collector current mirror be a good choice or am I better to go down the route of building a few common emitter/common collector stages?

    I don't want to build the one you posted as I simply can't make sense of it.

    I have the power amp part sorted out and it works well, so I just need something to go in front of it...maybe a class A, maybe a long tailed pair, maybe an op amp - I don't really care but I don't have much money. I'm on a disability pension with a fucked back so my options are fairly limited. I don't mean to ask the same questions over and over again but if you look at the content of my questions you'll see that I'm asking now if the current output of the first part of the circuit is critical or not, this is something I have not asked before at all. I guess I am attracted to my "op amp mashup" as you call it due to the high voltage gain and easy construction. I'm not going to be able to use it anyway with my new transformer as it's 37+/-V so will have to either get a high voltage op amp or build a pre amp from discrete components like you suggested. Just trying to get my head around all this, I go through periods of giving up and then comming back to it & having another go, asking some more questions etc. If it's a neauscience to you, then you don't have to reply.
     
  5. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    In addition to what Bertus said, the opamp has no negative feedback at Dc. This is a problem. Also, why the 470uF capacitor on the output of the opamp?
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    It's not a nuisance to me, but if you want to design a high power audio amplifier you must first understand the basics of the generic audio power amplifier. Once you understand and can recognise these stages (and it isn't that difficult) much of what you are trying to do will become clear. It sounds like you're stuck on this op-amp thing and it just isn't necessary, move on. The voltage amplifier stage of the amplifier I linked to consists of just four transistors and a few passive components which amount to far fewer than are contained in any op-amp.

    Most power amplifiers consist of just four simple types of electronic building block so if you can understand these you are most of the way there to, at least conceptually, appreciating what's going on:
    • Emitter follower
    • Common emitter
    • Constant current source
    • Differential/ long-tailed pair
    Slightly less common being:
    • Current mirrors
    • Cascode amplifiers
    As I said way back in an earlier post, you can't guess something like this into existence. By the way, if I were building a guitar amp now, I'd just use one of the many the high-power amplifiers chips. But, if you're saying that you want to learn how to design an amplifier then that's what you must do, stop guessing, there are no short cuts.
     
  7. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1
    I'm not trying to guess, and not looking for a short cut. Just asking questions so I can learn.
    Thanks for the info.

    I understand for the most part the operation of emitter follower (voltage amp) , common collector (current amp) ..long tailed Pair (differential amplifier that works as a voltage amp I think?)

    It's ok, I'll figure it out..eventually lol.

    I just need to find out - how much current my pickups produce so I know how much current gain I need.

    I already calculated that I need an amp with a voltage gain of 470 100mv X 370 = 37 (37 being the ceiling of my power supply)
    Now I just need to find out 1. How much current I need to ampify and 2. How to calculate that in a stage that is common emitter.

    I realize that in common collector mode it's just the Hfe of the transistor but not so sure about common emitter mode.
     
  8. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1

    I found this bit of info from -
    http://www.electronics-tutorials.ws/amplifier/amp_2.html

    I think this answers half my question regarding a common emitter stage.
    So the fixed current gain of a transistor still applies in common emitter mode?

    If this is true then why does it say in the tutorials that the current gain of a common emitter stage is less than a common collector stage?

    Beta Value

    [​IMG]

    Beta is sometimes referred to as hFE which is the transistors forward current gain in the common emitter configuration. Beta has no units as it is a fixed ratio of the two currents, Ic and Ib so a small change in the Base current will cause a large change in the Collector current.

    One final point about Beta. Transistors of the same type and part number will have large variations in their Beta value for example, the BC107 NPN Bipolar transistor has a DC current gain Beta value of between 110 and 450 (data sheet value) this is because Beta is a characteristic of their construction and not their operation.

    As the Base/Emitter junction is forward-biased, the Emitter voltage, Ve will be one junction voltage drop different to the Base voltage. If the voltage across the Emitter resistor is known then the Emitter current, Ie can be easily calculated using Ohm’s Law. The Collec
     
  9. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    A common collector amp (also called emitter follower) can usually drive more current than a common emitter. Maybe that's what they meant. The output impedance of a common emitter amp is about equal to the collector resistor, while the output impedance of an emitter follower is about 26mV/Ie.
     
Loading...