Hi Just getting started could do with some help.

Discussion in 'General Electronics Chat' started by dodger74, Jan 5, 2015.

  1. dodger74

    Thread Starter New Member

    Jan 5, 2015
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    Hi

    I am trying to learn about guitar effects pedals and amps.

    I am looking to build a recreation of a 1960s Vox AC 30, point to point construction. The chances of me ever been able to afford an original 1960s top boost AC 30 are dependant on the wife letting me re-mortgage the house, (never gonna happen):eek:. I have decided that with no knowledge of electronics messing about with some which could kill me is probably not a good place to start.

    So I have started to build effects pedals, 9v a much safer starting point than 240v me thinks.

    I was hoping that one of you good folk could help me to understand what is happing in this basic boost circuit and point me in the right direction if I have anything wrong.

    The circuit...

    [​IMG]
    My understanding is..

    .002uf cap is a filtering cap....and is there to filter any dc from the guitars pick up...

    the 430k and 43k are a voltage divider and set the impendence level for the transistor..( Is impedance level the power to activate the transistor?)

    I am un sure what the 390 resistor does

    and with position of the 10k resistor do electrons flow from the power input into it as well as out of the transistor? I cant see this happening with every thing that I have read and its confusing me.....Also I don't understand its function in the circuit.

    Lastly the .002 out cap is also a filter cap to filter out dc...

    I have built this guitar effect and it works great..but to be honest it was a solder by numbers exercise rather than converting the schematic to a layout as I found a great tag board lay out which was idiot proof. (good job really):p

    Thanks for any information in advance.
     
  2. Brevor

    Active Member

    Apr 9, 2011
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    54
    Ok the .002uF is called a coupling cap, it blocks the DC from the transistors bias network (the 430K and the 43K) from flowing back into the guitar. At the same time it couples (hence the name) the AC signal from the guitar to the transistor.

    The 43K and 430K resistors are a voltage divider that sets the bias voltage on the transistor which determines it"s DC operating point. You would need to study transistor amplifier design to fully understand that, too much to go into here. (they do also affect the input impedance of the circuit)

    The 10K resistor is called the load resistor the output signal is developed across it. and the 390 ohm resistor works with the bias resistors to stabilize the circuit.

    The .002 cap is also a coupling cap.

    That is the basic idea very simplified, you would need to do some serious study to go deeper.

    PS. My son is an avid guitar player, a lot of my spare time goes into building effects pedals for him.
     
    Last edited: Jan 5, 2015
  3. #12

    Expert

    Nov 30, 2010
    16,284
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    You are in the right place, but your questions are primitive...so you're going to get basic answers. When you understand enough to ask more difficult questions, you will get more comprehensive answers. Not to be insulting, just afraid of going right over your head. We try to adjust our answers to suit the questioner.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    What is the typical source impedance for a guitar pickup? What is the voltage output level? What is the typical frequency response? Is it flat or does it have a +6db or -6db per octave frequency response?

    Nevermind. I took my own advice and read the Wiki...
     
    Last edited: Jan 5, 2015
  5. dodger74

    Thread Starter New Member

    Jan 5, 2015
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    Hi all Thanks for getting back to me.

    Mike I have enjoyed my homework, not done any for a few years now.

    Ok so answer time.

    A guitar pick up is basically fine copper wire wound round a magnet. This creates a magnetic field when the strings are strummed or picked the vibrating string disturbs the magnetic field. This disturbances induces a voltage.

    So in my layman’s terms…
    When you strum the string it creates the force (volts) which sets electrons flowing.

    The typical voltage that is produced by a pick up largely depends on the pick up ie. Type of magnet number of copper wire winds another factor is the strength of the strum but an average Is 100mV to over 1 V.

    The same wire that carries the electron flow has the ability to oppose the flow this is what is called resistance. (OHMS LAW, I need to learn the equation) Yet impedance is different from resistance as impedance is the resistance of AC current. Ac current changes direction, ( I am gonna look at the equation for that also). This is different from DC Current as DC current is delivered straight and does not surge back and forth.

    I am thinking that the impedance could in effect cancel out or effect the flow of electrons if the inductor (pick up ) can not provide enough voltage.

    I have no idea at this point how to work out the impedance of a pick up. All I can say is my beloved Strat bridge pick up as 6.8 ohms of resistance.

    As for frequencies response I am guessing as 6db+

    Thanks once again guys for the response and any help given on today's essay.
     
  6. #12

    Expert

    Nov 30, 2010
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  7. dodger74

    Thread Starter New Member

    Jan 5, 2015
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    Wow 6v Bet that Paul is screaming....
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    For quick results, the OP could go for a basic JFET circuit that gives much higher input impedance - best for that is a source follower, but it has no voltage gain, the common source JFET amp doesn't lose all that much of the high input impedance, and gives some voltage gain.

    High input impedance improves sustain - but you're still at the mercy of the volume pots across the pickups, behind the guitar finger plate - the JFET booster is good if you're going for active electronics in the guitar.
     
  9. #12

    Expert

    Nov 30, 2010
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    I don't generally abuse it like that. I was only testing for an amplifier design. The real screaming comes from a volume control knob that goes to eleven. :D
     
    wayneh likes this.
  10. #12

    Expert

    Nov 30, 2010
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    Here's a high impedance input with gain. Probably too much gain. You will have to turn it down some.

    Especially notice the 3.3k and the output capacitor. They work a lot better than your stomp box circuit while the volume control has nothing to do with the output impedance.
     
    Last edited: Jan 6, 2015
  11. dodger74

    Thread Starter New Member

    Jan 5, 2015
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    Those immortal words "Yer But it goes all the way to eleven":p
    [​IMG]


    Spinal tap as to be one of the greatest Movies of all time.

    And the Pic is living proof there are a small number of amps pit there that go to 11....
     
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