Please help - I'm building a guitar amplifier from an op amp.

Discussion in 'The Projects Forum' started by damien83, Sep 20, 2015.

  1. damien83

    Thread Starter New Member

    Sep 17, 2015
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    Hey forum

    I'm building a guitar amplifier from an op amp (Or perhaps multiple op-amps in series to give me more gain if I need). The op amp/opamps will then feed a class ab push pull complementary pair bank of high power transistors mounted on a common heat sink (with insulation pads and thermal paste)

    I've had a problem where I just can't get enough volume from my guitar. If I turn the gain up on the op amp I get terrible distortion before it gets loud enough for me to get my groove on so to speak. I do not own an oscilloscope and only guessing what is happening with the wave form. I first believed that I am not forming enough current gain and so I put a common collector buffer stage in between two common emitter stages with minimal improvement which made me think perhaps the amplifier is constricted by the voltage rail figure and current handling capacity of my transformer which just happens to be 37 volts rail to rail with a 50w capacity. The transistors are small 5 watt tip 41c and tip42c complementary pair which do get hot but not hot enough to warrant a heat sink even which makes me think that perhaps they are not being driven 100 %. Either it is this, or is it a case of my op amp which is a n5532. Since these things only have a current handling capacity of 53mw I thought maybe I'm not getting enough current gain there so I swapped it out for one that can handle 500mw (havn't go the chip number off hand but can supply it if needed) Other than that I'm going to check to see that the cheap Chinese guitar cord I got off ebay is not causing a signal loss due to it's skimpiness on the width of copper cable inside the insulation.

    I'm just a beginner so these are all questions more than statements or perhaps just like theories I have with my limited knowledge. Please don't hesitate to correct any of my statements as I am a fan of constructive critisism. ;-)


    Regards -

    Knifey
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    To make it loud enough, I suspect you will need a min of ~20Watts into your speaker impedance. If using a 8Ω speaker, your push-pull power amp will need power supply voltages as follows:

    P=E^2/R or E^2 = P*R = 20*8 = 160, or E = sqrt(160) = 12.7V.

    Now that is 12.7Vrms, so the peak-to peak voltage across the speaker is 2.828*12.7 = 36V, which means that the amp has to be powered either with a single 36V supply, or a +-18V split supply, or a bit higher...

    If I guessed wrong on the power required, or the speaker impedance, you will have to repeat the calculations for your specific case. I suspect your power supply voltage is much less... It is not lack of gain, but clipping in the power stage...
     
  3. #12

    Expert

    Nov 30, 2010
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    The 5532 is a good audio chip but I stretch to imagine an op-amp that can drive the output stage you need.
    Filling in the middle by posting a schematic for us would be nice.
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Most modern solid-state guitar amps use an integrated-circuit for the power amplifier, perhaps you should consider doing the same; it sounds like you're trying to guess a power-amplifier into existence and that's unlikely to be a successful strategy.
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    You should figure out if you are trying to build a preamp (line level amplifier meant to drive a power amp), or a power amp.

    Typical opamps like ne5532 are meant to drive a high impedance load (in the Kohm+ range). To drive a speaker, you will need a power amp.

    A good place to start would be those audio power IC chips - essentially high powered opamps. LM3886, TDA2030, ... There are plenty of inexpensive modules out there so it is a minimum risk route.
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Here is a decent amp with low enough distortion to pass as a guitar amp. Use your NE5532 as a preamp circuit.


    image.jpg
    image.jpg


    24VAC x 1.5 amp transformer secondary (approx 36 volt rectified and filtered DC), with 3300uF or more power supply filtering.
     
    absf likes this.
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If the power transistors are not hot enough to need a heatsink, that's a clue. But without posting your schematic, we have no clue.

    ak
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    A few years ago, one of the hobby magazines (possibly Elektor) published a design for a power amp with massively parallel op-amps.

    There were follow up instalments on using 2 boards together in BTL mode for a lot more power.

    There a various power op-amps on the market with output current somewhere in the region of 1A. The TDA2002 (and bigger) are pretty much power op-amps, and NS produced several high power buffers that over the years have proven popular for audio work.
     
  9. damien83

    Thread Starter New Member

    Sep 17, 2015
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    1
    Cool that's awesome!

    I put together an lm386 which puts out 350ma I think instead of the 32ma that the one I was using previously..
    ne5532 I think it was. Anyhow, no improvement.

    I then tried throwing my amplifier onto a switch mode power supply I had available that I could turn up to 100v and bingo I got more volume! Until my amp went bang (blowing a cap and one side of the transistor bank)

    The transistors I am using currently are only tiny to-220 package bjt's (tip41 and tip42 complementary pair)

    Would they have more likely gone bang due to too much current or too much voltage? A by-pass cap blew at the same time. This by-pass cap was only rated for 16v but I figured since it is passing a.c the voltage rating would not matter.

    Thanks for your input everyone.

    Ian - I am hoping to stick with the preamp/power amp configuration as I put a lot of work into understanding how push pull amplifiers work. =)

    I am thinking perhaps of re-winding a microwave oven transformer to give me 100+ volts. Is there any way of getting more volume with a lower voltage besides using lower impedance speakers and a lower value for the output resistors.

    Thanks in advance

    Oh p.s havn't had time to scan a schematic but I will soon for you guys to sus out.

    REgards -

    Knifey
     
  10. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1
    .....
     
  11. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1

    Hey mate

    Yeah the transistors weren't hot at 30v but as I got close to 100 they really started to heat up even with a heat sink. If I use bigger transistors/bigger heat sinks can I go even higher for more volume/power or is there a better way to kinda tap into that available current?
     
  12. ISB123

    Well-Known Member

    May 21, 2014
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    You need to heat sink them properly which means you have to use thermal paste. More voltage will always mean more current unless you use some sort of current limiting!

    If you have a 1KΩ resistor on base of transistor which has gain of 100β it would mean that at 50V you would have 50mA at base of transistor and 25A at Collector.

    tip41C has 6A max. output now I have no idea how your circuit looks but if you go over those values its going to overheat and fail.
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    There is a MUCH better way.

    ak
     
  14. #12

    Expert

    Nov 30, 2010
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    This makes as much sense and plugging your car battery into a wall outlet. (ps, Don't do that!) The, "better way" includes knowing the voltage rating on your capacitors and transistors, doing the math, etc. I designed and built a small tube amp, and never blew up a part. It CAN be done, but you, sir, are frankly dangerous. I hope you have the good sense to not let any other people be near when you crank the voltage up and parts explode.

    Which brings me to the relevant part: What do you expect to gain by getting advice here?
    This crew can do anything (within reason) with audio amps, but they will still explode if you just crank up the voltage. You still haven't posted a schematic, and that is necessary, even if it's crap. By posting here, you will find out whether it has weak points or just needs to be abandoned. That's the procedure. Show us where you are and tell us where you want to go. Be prepared to follow instructions, pay for good parts, make mistakes, and figure out how to fix them. Otherwise, you are like a monkey with a pistol. You don't even know why it goes, "bang".
     
    blocco a spirale likes this.
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Does this question mean Ohm's Law doesn't apply where you live?
    What is wrong with our schools?
     
  16. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The OP doesn't waste time concerning himself with Ohm's law, or design, or thought, or learning the basics...etc...etc...

    He's more of a; just keep guessing and cranking up the voltage, that's bound to fix it, kind of guy o_O
     
    GopherT likes this.
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    On your question how to get more output for lower Vcc; The BTL configuration gives 4x the output power per given Vcc and speaker impedance.

    The TDA7052 BTL amplifier gives decent output right down to 5V.

    The TDA7052A has built in DC volume control.
     
  18. damien83

    Thread Starter New Member

    Sep 17, 2015
    14
    1
    Using more voltage was a last resort as I've tried adding current gain stages (Common emitter) and voltage gain stages (common collector) with no luck as to what I was trying to achieve.

    Since my op amp (Using this as the voltage gain stage) was distorting at low volume I considered I just needed voltage rails that were further apart from 0v to get more volume without distortion.

    Everything I've learnt so far has been from reading articles.

    WIthout an oscilloscope I can only estimate how close the wave form is to the rails so that doesn't help.

    I didn't post a schematic as there are piles of class AB amplifier schematics on google that could have been utilized.

    I do understand ohms law and understand that voltage overcomes resistance and allows more current to flow and this brought me to the conclusion that I just needed more voltage to get more volume.

    Despite knowing that my tip41c & tip42c transistors could handle 100v my caps weren't rated for this and expected stuff to go bang & used a perspex shield for this reason. It nothing more than an experiment.

    So yeah, I'm really dangerous for blowing up a capacitor - Lucky the huge explosion it caused didn't take out 4 city blocks. I should just choose a different hobby as I am clearly a monkey with a pistol. .

    Everyones input has been very helpful as I love being made fun of and insulted. I hope my perceived stupidity and carelessness made you guys feel better about yourselves. =)
     
    AnalogKid likes this.
  19. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Why would this prevent you from posting the schematic for your amplifier?
    No one can help you without it.
     
  20. ISB123

    Well-Known Member

    May 21, 2014
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