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Homework Help: Building a pre- amp circuit

  1. May 8, 2012 #1
    I have to build an audio pre-amp for an important test next week.

    Attached is the circuit schematic and deliverables wanted.

    I would appreciate anyone who could draw out the actual component placements
    on the attached stripboard print.

    R10 is a variable resistor in particular the placement of this component i find difficult.

    Thank you

    Attached Files:

  2. jcsd
  3. May 8, 2012 #2
    But this is largely your assignment. We should only give you pointers, here - not free homework, yes? (/;

    Your pdf indicates that the straight line which is part of the two-line icon for, "capacitor," corresponds with its positive leg (provided that we are dealing with polarized caps). This leg should be made to connect with what is known as the "wiper" of your variable resistor (or, potentiometer, or, attenuator, etc...). That arrow pointing at the center of the resistor (zig-zag) icon is normally meant to symbolize (the wiper of) a variable resistor (not "varistor"), or pot, in America I had thought (confirmed by Wackypackia, here:

    (imagine link - need 10 posts or a bag of spells for this power))

    . I should imagine that this is how variable amounts of gain are obtained from the preamp.

    One bit of lore you might enjoy is that the term, "passive preamp," is used sometimes by those who are referring only to attenuation without even voltage buffering amps to isolate the attenuator's impedance (AC effects) on the transfer of voltage (communication). Purely passive volume controls (less has got to be more blameless?).
    So, why not call these "passive preamps," since they are passive and they are in front of the amp?
    The description I liked best was when my colleague pointed out that the preamplifier is an amplifier, itself, which is previous to the power amplifiers, downstream. Specifically, if it doesn't contain an amp, it can't be a preamp. It is not that it is merely a gain-change-capable device which is previous to amplification downstream, as it were. It must be, also, an amp. It taketh away - or it restoreth what [the signal] may lack [in signal, always with noise at no extra charge].

    - Impov Thupperverz
  4. May 8, 2012 #3

    I see what seems to be a major problem with the preamp circuit. The block diagram shows only a single connection between the microphone and the preamp. However the microphone really has two connections, signal and ground. The preamp circuit has no ground. To where are you to connect the microphone ground? There are solutions of course, but I suspect you are not allowed to change the circuit.

    Before computers, the way I laid out circuit boards was to take quadruled paper (graph paper) and draw the pads of the components as seen from above leaving a lot of space between components. I would draw the connections between pads on one side of the board with red pencil and the traces on the other side of the board with a blue pencil. In subsequent passes, I shrank the space between the components and sometimes rearrange them to make the layout tighter.

    There are some free student packages such as Orcad and PCExpress that will allow you to layout small circuits.

    http://ptf.com/orcad/orcad+student+version/index4.html [Broken]
    Last edited by a moderator: May 6, 2017
  5. May 9, 2012 #4
    Thank you very much for the help very much appreciated....I will look into everything again today and get back to you if thats ok with you? I believe the microphone diagram you are talking about is only a pictured example of the function of preamp but does not correspond to the actual schematic given.

    In regards to the test my actual test is to get this circuit working and as you can see show gain and phase shift to my lecturer using signal generator,power supply,oscilliscope etc..

    I am going to look into it more.I do have a photograph of this circuit that i had original built a few months ago yet did not work hopefully more light could be shed.

    Thank you very much for your help so far and your quick reply.

  6. May 9, 2012 #5
    Also there is a small part attached to this test regarding the use of P.Spice....unfortunatly i am using a mac and there is no such way for me to get P.spice as it is PC based.I do have access to computers with P.Spice and my actual understanding of how to build circuits on this is not how it is meant to be done and therefore didnt work for me.

    I have attached this document with this reply.

    Also the picture of my original pre amp some months back.

    Thanks again....unfortunately tied up today in other matters so will not get a chance to sit down properly until later.

    Very much appreciated.


    Attached Files:

  7. May 9, 2012 #6
    A good way to troubleshoot your circuit is to program your circuit into SPICE and compare voltages at the nodes between what SPICE calculates and what you measure. If you don't know SPICE, I strongly suggest you learn it.

    http://www.filerex.com/get/windows/education/science/ltspice_iv.html [Broken]
    Last edited by a moderator: May 6, 2017
  8. May 9, 2012 #7


    User Avatar

    Staff: Mentor

    I don't have a mac, but I've heard that LTSpice will run under something called "wine" on a mac. Google: LTspice for mac.
  9. May 10, 2012 #8
    Thank you very much again Mesmer8,Skeptic 2 and gneill for your help all of it is sheding light on this circuit.In particular thanks to mesmer8 for clearing up the 'wiper' connection and related schematic symbol problem i had.very helpful.I will look into LTspice today as it is part of my project to build the circuit in P.spice and test relevant voltages.also to give me a better idea of the working of the circuit.

    I will have a few more questions im sure and thanks again for everything so far.

  10. May 11, 2012 #9
    I have re-drawn the circuit onto the actual size of stripboard I will be using,The middle pin of R10 is the 'wiper'.

    I would appreciate any feedback on this and if mistakes have been made.

    The drawing is attached.

    Thank you


    Attached Files:

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