A little help with preamp board problems please!

Discussion in 'General Electronics Chat' started by Larry4911, Mar 24, 2012.

  1. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    I bought a couple of preamp boards from Ebay. I have posted about this set-up I am trying to build before. The problem I now have is with a 7812 power regulator circuit that I am trying to add to power other parts of the unit.

    Basically, the problem seems to be that the preamp boards neutral lines are held to ground. This seems to be causing a diode to blow in my power regulator circuit or have I missed something that is obviously stupid to you guys?

    What do you guys suggest to get me out of this corner I am in? Is there another power reg circuit I can use that is workable with the neutral held to ground or can I just clip the output and input grounds to the preamp boards?

    I will try to upload my schematic and PCB express files to aid you in helping me with this problem.

    Thanks

    Larry

    Addendum: OK I cannot send a pcb file for some reason but I have managed to attach the sch file.
     
  2. jimkeith

    Active Member

    Oct 26, 2011
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    I hope you are using transformer isolation (step-down transformer) because the AC mains produce simply too much voltage for your regulator (Vin Max = 35V), and of course also causes said grounding problem. Schem otherwise looks ok.

    On the pcb layout, simply take a screen shot via the "print screen" key function and past it onto paintbrush--then edit and save it to a *.jpg file format. ExpressPCB is a bit stupid about attempting to cripple this function.
     
  3. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    Of course I am using an isolation transformer. The preamp boards are still working after a test. If I wasn't using a transformer they would have gone up in smoke before now. I have a 15-0-15 transformer attached to the preamp boards and am wanting to use one of these windings to power a 12v regulator. However as I said it appears to be the grounding issue that is giving me a problem.

    I have attached a print screen image of my expressPCB circuit.

    Thanks

    Larry
     
  4. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    I've just had a thought. Both preamp boards have a pair of 78/7915's on board. Would it be acceptable to take a link off the preamp boards 7815's output to supply my variable 317(for 3v LED driving) and a simplified 7812 circuit (for LED chip driving) to supply my led bar graphs and other devices or would this create an unacceptable imbalance in the preamp boards power?

    I don't know the drain on the preamp but was told that it is only about 130mA MAX so shouldn't be a problem with heat in the 78/7915s.

    What do you think?

    Thanks

    Larry
     
  5. #12

    Expert

    Nov 30, 2010
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    Here's the deal: That power supply board expects a single winding of a transformer from which it makes positive and negative peak voltage, stores them in a capacitor and makes the regulated voltage. Then it delivers the negative peak voltage at the output and pretends that is ground.

    Not true.

    To tame this beast you have to remove D1, D2, and D4 and replace D1 with a jumper.
    The AC supply wire closest to the edge of the board becomes ground and the output connection closest to the edge of the board is the same place.

    With this method you only get half wave rectification. That means you will have a higher ripple voltage per amp...and so on. If you can see what I've done and go, "Duh! I so should have thought of that." then procede. If you think I'm telling you how to paint by the numbers, you don't belong modifying boards.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Please can somebody translate the SCH schematic into a PNG schematic that can be seen by everyone?
     
  7. #12

    Expert

    Nov 30, 2010
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    There might be a way to skin this cat from the pre-amp board. I just worked with what I had.

    You go, guru!
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Where is a normal PNG schematic?
     
  9. jimkeith

    Active Member

    Oct 26, 2011
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    Potential power-up race problem:

    If one regulator in a ±regulated voltage system (7812 /7912) delivers significant voltage first, it tends to put reverse voltage on the slower regulator and thus causes it to latch--this may be destructive, but cannot remember. The best way to prevent this is to put a 1N4004 back diode across each output to gnd so that if it gets hit by reverse voltage, the diode conducts rather than the regulator until power-up is complete.

    Do not know if this may be an issue in this case, but it is a simple fix.
     
  10. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    OK yes. I have added a JPG of the SCH file for those that wanted an image file. I have also added other detail for a more complete explanation of what I have completed so far. Any criticisms and advice would be welcome?

    I suppose I could get away with a half-wave 12v rectified power supply if I only use that one for the LED bar-graph displays drivers. Would it be best to re-engineer the PCB to accommodate two 4700µF smoothing caps just to make sure I don't get any unstable operation from the 3915s in the bar-graphs?

    I guess I will have to use 79/7812s for the mixer stages because I need a really smooth ± supply for that bit and 12s are what I have in my box. But as I have said before I haven't started designing that bit yet. Any tips or design considerations you have would be greatly appreciated?

    Thanks for your advice.

    Larry
     
  11. Spence

    Member

    Apr 23, 2010
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  12. jimkeith

    Active Member

    Oct 26, 2011
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    Check out this basic full-wave CT configuration
     
  13. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    OK. So, I have amended it for half-wave and uploaded an image of the sch and pcb files for this supply. It makes it cleaner if I keep the ground as pin 2 of the 2 pin connectors which I am using and I have already got my head around using pin 2 so I will continue to use it. Let me know if there are any mistakes or amendments you think I should make?

    What about a method of discharging the caps on switch off? I seem to remember my first every power supply that stayed on for a full minute after switching it off, with a very small load, because I added to much smoothing. How can I avoid this happening with this one? Bearing in mind I need a smooth supply.

    Yes, that circuit looks almost as if it is straight out of the 78XX datasheet. That is basically what I will be using for my mixer circuit when I build it.

    Thanks all.

    Larry
     
  14. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    By the way, I meant design tips and considerations around mixer stages not power. Sorry for that misunderstanding. I want to make a mixer with talk over and headphone monitoring with two inputs from the preamp boards. Basically the sort of thing that you would normally get in a twin deck DJ unit.

    I have in my box some NE5532ns which will do nicely for the mixer outputs and a hand full of LM386N which I have used before and should work fine for a headphone amp. Can I use the LM386s for a mike amp also?

    Thanks again for your input.

    Larry
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    An LM386 is a little low cost power amplifier. It has too much hiss to be used as a microphone preamp, the NE5532 is much better as a preamp but it has two opamps inside.

    The LM3915 has a regulated voltage reference inside so its supply does not need to be regulated. 12V is too high for the LM3915 because it will get too hot. I have an 8V supply for my LM3915 and use a 10 ohm power resistor in series with it to share the heat.
     
  16. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    So, the 386 will be sufficient for the headphone amp and I'll have to find a schematic for a mic amp using a 5532. No problem there then.

    Surely that cannot be? The datasheet says 3 - 25Volts for the 3915. I am using low power LEDs and have a separate variable supply (tested outside the case with 12v in and working fine) to feed them. I wanted to give myself more versatility on the type and number of LEDs. So, I figured a variable supply was a must for the bar-graphs.

    I think I should just go ahead and make a double pole supply, with the 79/7812s, for the mixer et. al. and just take what I need from that.

    Yes, that is what I shall do. I already have the variable one for the LEDs and the heat and current from the preamp boards. So, one more supply is better than two more. Especially when I take heat and current into account. I don't know why I didn't think of that before. DUH!

    Thanks again for all your comments.

    Larry
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    My LM3915 has two 1.8V red wide-angle LEDs in series on each output. Their current is set at 26mA.
    With an 10V supply (an 8.4V Ni-MH battery charging all the time) and all LEDs lighted in the BAR mode, the heating is (10V - 3.6V) x (26mA x 10)= 1.66W. Its maximum allowed heating when it is not enclosed is 1.365W. The heating of the series 10 ohms resistor is 0.68W so my IC heats with only 0.98W.
     
  18. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    So, your Vled is 10volts? There is your problem. The very first page of the 3915 datasheet says if Vled is above 7Volts then you need a dropping resistor to help dissipate the heat.

    In my opinion, having to use an expensive and bulky power resistor just to dissipate heat is not only costly in components but wasteful in energy and we should take every effort in design to eliminate this waste. I know having an extra power supply is an extra expense but in my case I am more concerned about reducing heat generation than expense.

    My Vled is variable and set at 3Volts and current is variable. So Power dissipation should not be a problem as far as I can make out from the datasheet.

    Thanks again.

    Larry
     
  19. #12

    Expert

    Nov 30, 2010
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    Just one suggestion...instead of having all that copper not attached to anything, connect it to ground and simplify the path from the input ground to the capacitor negatives to the output ground.
     
  20. Larry4911

    Thread Starter New Member

    Mar 15, 2012
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    Wouldn't doing that produce a big opportunity for interference?

    I don't know how to do that in expressPCB anyway. I haven't been using it for long.

    Is there any reason why I shouldn't power it up and test the power supply with no load or should I give it a load first? If so what load would you suggest?

    Thanks

    Larry
     
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