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50w rms into 8 ohm load amplifier circuit using single dc power supply

Discussion in 'Audio' started by Jemz143, Oct 17, 2018.

  1. Jemz143

    Jemz143

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    Oct 17, 2018
    Am looking for an amplifier with 50w rms output into 8 ohm load (speaker).. Which IC is suitable for this project.. Am using single dc power supply..
    Please help me with necessary details (circuit diagram)...
    Thanks in advance....
     
  2. davenn

    davenn Moderator

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    what googling have you done so far ?
     
  3. Jemz143

    Jemz143

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    Oct 17, 2018
    I found circuits for dual power supply
    And also for below 50w rms circuits with 4 ohm load....my problem is that my subwoofer is rated at 50w rms at 8 ohm...currently am using 20w (using la4440) amplifier which is not make me satisfied... When i googled, i found that topic matching "amplifiers with speakers" which make me realize the poor sound quality from my subwoofer.... Thats why i am trying to make an amplifier that suits for my speaker...
     
  4. BobK

    BobK

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    Using an amplifier with lower power does not affect the quality of the sound as long as you don't overdrive it. It only effects the loudness. Because hearing is logarithmic, going from 20W to 50W will give you a perceived change in volume of less than double.

    Bob
     
  5. Jemz143

    Jemz143

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    Oct 17, 2018
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Why?

    When you are designing an audio amplifier, specifying a single dc power supply severely limits your circuit design options. The sub-woofer is accepting low-frequency alternating current audio signals and producing sound by moving a voice-coil back and forth about a centered position in an axial magnetic field. The centered position occurs when no signal is applied to the sub-woofer.

    To produce 50 watts into an 8 ohm impedance requires a 20 volt (rms) AC voltage across the speaker terminals. That's about 28 volts peak amplitude on either side of zero, or about 56.6 volts peak-to-peak. To get this from a single power supply will (1) require about 60 volts DC and (2) an electrolytic capacitor coupling between the amplifier output and the sub-woofer speaker terminals connected between the amplifier output and power supply common.

    The amplifier output from a single 60 VDC supply will be at about 30 VDC with respect to power supply common when the audio input signal is zero. If you apply this DC voltage to the sub-woofer voice coil, it will either cause draw excessive current and burn up, or the voltage will cause the voice coil to move far away from it's normally centered position and create distortion and clipping of the audio sound output. Hence the need for the dc-blocking/ac-coupling electrolytic capacitor.

    You could avoid item (2) by using bi-polar ±30 VDC supplies, or a slightly greater pair of opposite voltages, say ±35 VDC. See schematic below. You cannot avoid item (1) if you need to provide 50 watts into an 8 ohm load.

    The reactance of the electrolytic dc-blocking/ac-coupling capacitor should be small compared to the 8 ohm speaker impedance, say 0.8 ohms at the lowest frequency of interest, but 0.08 ohms would be even better. Ideally, the dc-blocking capacitor would have zero ac-coupling reactance, but that is impossible.

    If the sub-woofer response goes down to, say, 10 Hz then XC = 1/(2πFC) = 0.08 yields a value for C = 1/[2π(10)(0.08)] = 0.1989 F = 198,900 μF or about 200,000 μF. Good luck finding, or being able to afford, one of those puppies with a voltage rating of 100 WVDC or thereabouts.

    Even if you allow for a capacitive reactance of "only" 0.8 ohms instead of 0.08 ohms, that still requires a humongous 20,000 μF electrolytic capacitor. And all because you refuse to consider using a bi-polar power supply to power your sub-woofer amplifier. OTOH, if you change your mind, maybe you could try building this circuit:

    [​IMG]

    Also, maybe you could learn a little more electronics before imposing arbitrary limits on your circuit designs. Having a single power supply is a WANT, but there is good reason to believe it is NOT a NEED for this application and causes needless extra expense.
     
    davenn likes this.
  7. Jemz143

    Jemz143

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    Oct 17, 2018
    Ok... I got it... Thanks for your information
    ..
     
  8. Audioguru

    Audioguru

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    The datasheet of an LA4440 shows 19W at a horrible sounding 10% distortion. A graph shows 14W at 1% distortion that can be heard when its speaker is 8 ohms, its supply is 18V, it has a huge heatsink and both channels are bridged. Its output is 10W with low distortion when bridged.
    An LM3886 IC can produce 50W intro 8 ohms with low distortion when the supply is +/-35V or 70V.
     
    Last edited: Oct 20, 2018
    hevans1944 likes this.
  9. hevans1944

    hevans1944 Hop - AC8NS

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    I must admit that @Audioguru's suggestion to consider using the LM3886 IC is an excellent way to get on with your life. Make sure you provide an adequate heat sink for the IC.

    OTOH, building from scratch with discrete transistors, resistors, capacitors, and diodes is a great way to learn about analog electronics. Just have plenty of spare parts on hand when you "let the smoke out" of some of them.:D
     
    darren adcock likes this.
  10. Jemz143

    Jemz143

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    Oct 17, 2018
    Thanks for your information...
     
  11. WHONOES

    WHONOES

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    No problem with the design philosophy though there are some improvements that could be made but, this particular version would not be very reliable as the BC108 input transistors are only rated at 25V and on a ±35V supply would have 35V across them causing them to fail. Replacing them with KSC1458's which are rated at 100V.
     
    hevans1944 likes this.
  12. hevans1944

    hevans1944 Hop - AC8NS

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    @WHONOES:
    The above quote was extracted from a rather lengthy series of comments here, most of which I have not read. Most of the comments I did read exhibited a vast lack of electronics knowledge, but this one was in response to
    so maybe Pascual noticed the same thing you did. I have no idea what the voltage rating of any of the BCxxx series of transistors is and no interest in pursuing the matter any further. Maybe someone who has actually built one of these amplifiers can shed some light on how well it works. I kinda lost interest in home-brew hi-fi amps when they started offering class D amplifiers in IC packages for dirt-cheap prices.

    Today I listen to music and such on a five-channel surround sound Yamaha receiver with more features (and a sub-woofer add-on) than I know what to do with. Relatively inexpensive at Best Buy a few years ago, and a huge improvement over the Samsung hi-def flat-screenTV with built-in TV speakers.
     
  13. Audioguru

    Audioguru

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    Do not try making an audio amplifier posted on a website (Circuits Today) in India that uses antique BC108 transistors that have a supply voltage that is much too high for them. I did not check if all the other transistors will work in that old circuit and if they are also antique. The value of R2 appears to be too low so that the differential amplifier has loss instead of gain.

    I used BC108 transistors when I was in University in Canada 54 years ago. I have never been to India.
    Look in Google for 50W Audio Amplifier Circuit and most are on websites in India with errors and using antique parts.
     
    hevans1944 likes this.
  14. WHONOES

    WHONOES

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    The value of R2 in the power amp above is about right. This because: at a supply of ± 35V, the tail resistors of Q1 and Q2 set the collector currents of Q1 and Q2 at about 1mA. The voltage drop across R2 with 1mA passing through it is 680mV which is just enough to turn on Q3 taking into account R7.
    As an aside, the output stage biasing comprising D1, D2 and R9, R10 is less than ideal as it takes no account of Vbe variations in the output transistors.
     
  15. Audioguru

    Audioguru

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    The amplifier circuit was probably designed on a simulator where "typical" Vbe spec's are used without having an adjustment for minimum to maximum variations in the diodes. The simulation software ignores over-voltage and over-heating of the BC108 transistors and maybe the other transistors.
     
  16. WHONOES

    WHONOES

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    I'm sure you are right or the designer did not understand the implications of what he was doing.
     
  17. Audioguru

    Audioguru

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    Websites "over there" have many circuits designed wrong. Maybe they use trial and error with many errors until it suddenly works"?
    Maybe good luck allows a circuit using parts that were selected at random instead of researched and of designing using the range of spec's? Maybe one circuit was made and it happened to work but the parts were thrown together without using a detailed design then that circuit probably will not work if parts have normal lower or higher spec's.
     
  18. hevans1944

    hevans1944 Hop - AC8NS

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    Ya hafta wonder if they even know about Monte Carlo variation of parameters to create a design tolerant of parts variations. Since a lot of the folks "over there" do circuit modeling in software, you would think it would a natural thing to perform a Monte Carlo analysis before committing a "design" to hardware...
     
  19. WHONOES

    WHONOES

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    Absolutely. It always makes good sense to perform a MONTE' especially when designing a totally discreet circuit for you never know what may be lurking to catch you out.
    But I suspect as AG and Hevans have alluded they stick some bits together with little regard for the vagaries of the components and a vague idea from something they have seen somewhere and when they manage to get something to work eureka, suddenly they are an engineering god and their circuit is the best thing on the planet and must be disseminated universally.
     
  20. hevans1944

    hevans1944 Hop - AC8NS

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    Back in the late 1960s, when I was taking an electrical engineering course in circuit analysis and design, we were introduced to the concept of Monte Carlo Analysis. Our software tools were limited then. I had time-shared access, as well as punched-card batch-program access, to an RCA Spectra 70 mainframe, but only a very little programming skill in BASIC and FORTRAN. So the Monte Carlo Analysis had to be performed "by hand" using a desk calculator, a very tedious procedure.

    In any event, my "experience" was limited to the classroom exercise of designing the base bias network for a Class A common-emitter transistor amplifier, given parameters of resistance tolerance, temperature coefficient of resistance, and the expected range of hfe for the transistor over the temperature range the circuit had to operate. IIRC, we didn't Monte Carlo for effects of changing the power supply voltage, which for a real application you would certainly do.

    The experience provided a whole new perspective on the perils awaiting the prospective analog circuit designer. Up until then my approach had always been to "try something similar to whatever worked before" without actually really understanding the circuitry involved. I suspect this paradigm is what some of the folks "over there" follow to this day. It may be a good way to get started in electronics, what with its "hands on" approach and immediate feedback on whether something works or not... heck, I still sometimes use it even today when approaching new (to me) technology... but it is certainly no replacement for certain knowledge of how things work. For that you need to study and understand.
     
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