'Project:' 12-Watt Amplifier

Discussion in 'The Projects Forum' started by kbyrne, Dec 27, 2011.

  1. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
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    :D Hi I am new to the hobby and need help with a Amplifier Design that I have been working on. I have schematics to post taken with a Internet camera and Jing in jpeg format but don't know about your site and how to upload pictures
    like I have viewed. How do I do That? The box the amplifier is in now is a plastic home constructed but the problem is it has hum. I want to change the box to metal but need info In grounding two power supply boards to the floor near the three prong plug green wire. There are two transformers 24ACV @ 2 Amps each
    going into two power supply boards one power supply board is for one Amplifier board and the other power supply board is for the other amplifier board.
    Question: can a positive and ground supply be hog pointed behind the filter capacitors to close by the (green) wire or on top of the (green) wire for inside of the box Grounding purposes? I am only a home hobbiest and very inexperienced in electronics and computers. Thankyou.
     
    Last edited: Dec 27, 2011
  2. Infinite

    New Member

    Dec 26, 2011
    3
    0
    Hello Kbyrne,
    'hog pointed' , LOL. Generally, an amp or any electrical device has the green wire from the AC plug, which is ground,connected to the case. If it's a plastic case then don't connect it to anything. You have 2 transformers, one for each amp so that's perfect for checking the amps voltages against each other.
    It uses 2 xfmrs so you have enough power for each amp.
    Seems to me that you don't know if the problem is the green ground wire, the power supply, or your amps.
    You could try this-1: temporarily disconnect the green ground wire. You only need the black and white to get 120 volts to the power supply.
    2: Measure power supply output voltages. They should be the same for each board. If you have only one big black capacitor on one of the power supply boards, then you want to find something like plus 16 Volts DC on it. If you have 2 caps, then you want to find plus 16 on one, and minus 16 volts on the other. 16 volts is just a guess, it should be close though.
    You can probably see the power supply output wires and put the meter right onto those without, Black is usually ground. Red is usually positive. I use green for negative.
    If the power supply is good then it sounds like the amp..... but you have 2 and you would rarely, if ever, see both amps go bad.
    Do you have hum out of both amps?
    Is it a loud hum?
    Good luck
     
  3. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    Power supply voltage is under one volt difference measured at terninal blocks at the end of the power supply board. 36.0vdc first 35.9vdc second rough guess. I checked some time ago. Both amplifiers opperational. I suspect Hog Point grounding and changeing from plastic
    box to metal box the answer. Can a positive volts & ground power supply be hog point grounded. I have only seen another type positive ground negative. Do you know the proper way? Thankyou,
     
  4. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    To the super moderator not necessary as I am new to computers and your site. A usefull site thou. Two others are Instructables.com & Utube. Bothe usefull for a variety of reasons
    Thankyou for assistance thou.
     
  5. ampman1952

    New Member

    Dec 28, 2011
    1
    0
    Although I am new to this site, I am not new to electronics. The super moderators post - although a bit abrupt - was very valid, without a schematic to see, it is difficult for others to advise you.

    This much for free though, and very common knowledge : a major source of hum in an amplifier is a ground loop. With all of the A/c radiating around, what seems to be an innocuous pair of connections from your ground wires to a common point will create the loop and act as a receiver for all of that A/C radiation to which your amplifier will include in its desiged purpose, namely to amplify. It is a very common error made by newcomers to the hobby.

    Ground loops are by no means the only cause, where you run the signal carrying components and wires also has a major impact along side general contruction quality.

    The other matter concerning your post which I would put as a word of advice - be extremely careful. Put the wrong wire to your new metal chassis and you will be experiencing some unintended 'excitement' - possibly of a deadly nature. Please note as well the colour of wires, and the standard meaning attached to them, is not necessarily the same in every country. I am not sure where you are from, I am from the UK where the ground wire is Green with a Yellow stripe, Neutral is Blue, Live is Brown.

    Wishing you good luck - but to shorten the odds, read up on the theory.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    In the USA, white is neutral, black is "line" or "hot", and green is ground.

    If no place else, the ground should be connected to the transformers' secondary windings someplace, as if the transformer windings happen to short from primary to secondary, the ground will cause the fuse to blow, or breaker to trip. That's much better than electrocuting someone.

    Along with ground loops which can be difficult to get rid of, failed electrolytic capacitors will cause humming/buzzing. If you used capacitors that were not fresh, they might have lost their dielectric.

    It could also be a bad rectifier in a bridge.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Using shielded (screened) input cables is important to eliminate hum.
     
  8. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    Thanks for advise guys. Is there a proper way you would advise to check electrolitics. I have two on boards both 4700uf @50vdc. The box is plastic now but will be changed to metal. When I figure out computers better I will upload a schematic. But thankyou for advise.
     
  9. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    :D Attn: I finally figured out how your site works and how to attach a gif
    attachment of power supply. If I need to post the power supply gif please feel free to advise me. I do not wish to cause anger thou as I am just new to a hobby ( electronics ) and computers. Thank you Moderator.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The value of 100 ohms for R9 and R10 in your amplifier are far too low. They strain the poor little 2N3904 and 2N3906 driver transistors with a max current of about 330mA but their absolute max allowed current is only 200mA. Change the value to 4.7k ohms.

    The driver and output transistors operate in class-B (completely cutoff) instead of class-AB (always conducting a little) so they produce crossover distortion. Look it up in Google and learn how to fix it.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Oh yeah, it produces hum.
    R1 and R2 bias the input transistor from the raw unregulated supply voltage so they attenuate the raw hum only to half which might still be a lot of hum.
    You should have an RC lowpass filter (1k from 36V then a 100uF capacitor to ground as the filter). R1 can be fed from the 100uF capacitor for much less hum.
     
  12. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    If I was to redesign where would I put the resistor capacitor combination as I have seen that on other designs. I assume at the far end of 36 vdc line other end of line with fuse as that is where I have seen that before on other designs. tanks
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Here is the resistor-capacitor filter:
     
    Last edited: Jan 7, 2012
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  14. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
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    I am redesigning the box & other parts of project so I will look further into all post reply's.
    Thankyou for the advise thou.
     
  15. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    Attn::) The enclosed is the power supply for two 12 - Watt Amplifiers.
    Any feedback as to how to (hog-Point) or central point ground both power
    supplys to the metal box. Where on the schematic should I put a solder pad for this. It will be two wires attached to the green wire of a three prong plug. I assume just at the ground pad of the first 100nf metal film noise capacitor.
    Thankyou
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    You have two 24V/2A transformers that produce 32VDC each. Their total power is 96VA.
    Each amplifier has an output into 8 ohms of about 10.5W so a lot of power is wasted making heat since the transformer voltage is too high.
     
  17. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    Question: Audioguru Could you tell me what type of power supply for two amplifier boards independent of each other in the same metal box you would design. The author of schematic states 36vdc @ 2 Amps. working voltage. Two RCA jacks left and right red & white feed front of amplifier boards. Grounds of each board are sepperate of each other.
    The grounds only go to power supply Gnd. Can one transformer feed two boards. Thankyou.
     
  18. Experimentonomen

    Member

    Feb 16, 2011
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    kbyrne likes this.
  19. Audioguru

    New Member

    Dec 20, 2007
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    You do not need two power supplies for a stereo amplifier, only one power supply is needed.

    A 24VAC transformer produces a rectified and filtered 32VDC, not 36VDC. When the amplifier is just idling and not playing then the 32VDC might rise to 34VDC.

    The output at clipping might be 23V peak-to-peak which is 8.1V RMS which makes an output power of 8.3W into 8 ohms or about 15.1W into 4 ohms.

    The amplifier will be about 60% efficient so the total power per channel is 13.9W when 8 ohm speakers are used or 25W when 4 ohm speakers are used.

    Then the current per channel is 13.9W/32V= 0.43A or 25W/32V= 0.78A.
    A 2A power supply will easily power both channels of a stereo amplifier that uses 4 ohm speakers.
     
    kbyrne likes this.
  20. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    Off to redesign with a metal box One tranformer and one power supply board powering
    both amplifiers. Should I raise the amperage of transformer to 3A or 4A to be sure the transformer runs cooler at full volume?? Thankyou for feed back. First attempt at amplifier box in stero. Next attempt two 45Watt Hex-Fets in stero bet need to get rid of hum
    first to gain knoledge.
     
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