Filtered Amplifying

Discussion in 'The Projects Forum' started by GFTMC, Apr 9, 2008.

  1. GFTMC

    Thread Starter Member

    Jan 20, 2008
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    Hi. My friend wanted to build a simple circuit for his electric guitar which will increase, once a switch is set on, the volume of only the lowest sounds the guitar makes (A bass Amplifier)

    I have thought up of a simple circuit which is ought to do the job:
    [​IMG]
    [​IMG]

    This circuit should accept input and output from the AC in/out (There are two plugs involved: one from the circuit to the amplifier, and one from the guitar to the circuit). It should, in theory, pass the same amount of signal always through the right most wire all the time. When the switch is on, the signal should pass through the inductor, which will allow only to the lower bands to pass, after which they will be amplified in the transistor.

    Once the lower bands will be amplified, they will rejoin the rest of the signal and get to the output, where they will run through a speaker.

    So, my main question is: will such a thing even work? I don't have much experience in these things, but I do feel I am close.

    My second question, is how much gain should the preamp have, provided it is supposed to approximately double the lower band.


    Thanks in advance.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    What your friend needs is what is known as a "low-pass filter". You're close. ;)
    There is an entire chapter devoted to filters in the E-books on this site.
    http://www.allaboutcircuits.com/vol_2/chpt_8/index.html

    If the filter is designed carefully, it's likely that no amplification will be necessary beyond what he already has.

    But more information is needed - at what frequency does he want the attenuation to start, and how rapidly does he want the attenuation to increase? There are many possible designs for filters; selection depends upon what is required in the output.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    A resistor in series then a capacitor to ground is a simple filter that reduces high frequencies. It is a treble tone control that is turned down.
    You don't know anything about transistors so don't use one, it is not necessary anyway.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You really surprised me with this comment. You're normally quite helpful; and this is quite out-of-character, and uncalled-for. Perhaps you're having an off day.

    GFTMC, our OP, is obviously a newcomer to not only these forums, but to electronics as well. He's also interested in learning, and generous - as he isn't doing this for himself, but for a friend, and hoping to learn something in the process.

    While the circuit he originally posted has problems obvious to the experienced, it does show that he's put some thought and effort into it.

    I certainly believe that GFTMC deserves a great deal more courtesy than you've shown by your post.

    Educate, don't denigrate.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    I concur with SgtWookie's assessment. Encouragement and respect are always in vogue.
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    To be in between, sometimes I get frustrated with people too, which is why I didn't get into teaching. I guess you take the good with the bad, since he has given some really good advice with a little bit of sass once in a while.

    Wookie, you're definitely a model poster who never acts out of line, which is the welcomed product of a lifetime of dealing with people and being in the military :D

    Steve
     
  7. GFTMC

    Thread Starter Member

    Jan 20, 2008
    18
    0
    Anyway, according to what I understood from your posts, I shouldn't use a transistor, simply because it is an overkill.

    Thank you for your help.

    Let me just see that I got this straight: I could use, instead of the inductor, a resistor and a grounded capacitor. Than, All I'll need is a potentiometer to be used as a resistor, and a small capacitor of about 1uf (So that the output frequency could be changed and fitted to the player's taste)
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    The capacitor and variable resistor should work fine. You might need to experiment with component values.
     
  9. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
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    Most guitars, not all, come with tone control pots which are simple treble-cut circuits. They consist of a pot with a capacitor which creates a single-pole, low-pass, RC filter.

    There are many older amplifiers and other audio equipment that have a pot called "bass" that supposedly increases the bass response, but it's merely the above circuit that cuts treble instead of boosting bass (and are typically looked down upon as cheap or "phoney" bass circuits).

    To actually boost/cut bass and treble so that it sounds good, you need an active (opamp, xstr) circuit. These battery operated circuits have been installed in guitars for many years now, both by OEM's and as after market devices you can purchase. But, don't confuse these with the pre-amp types that only amplify all signals from the pick-ups.

    Attached is an old and often used circuit for cut/boost of both bass and treble (you could play with values of C23 & C25 for guitar or bass frequencies).

    On a standard guitar the open bottom E-string is ~82Hz, while the upper E-string is ~330Hz (two octaves higher). On a bass, the lower point is ~41Hz (an octave lower, and half that of the guitar).

    For the simple RC network, you need to determine the frequency point where the higher tones begin to diminish (the "cutoff" point).

    Fc = 1 / (2*pi*R*C) : where "R" is a pot at minimum value, and "C" is some fixed capacitor of chosen value.

    When R is at maximum value, then it is the lowest frequency (and everything above) that will be excluded to some extent. This passive filter will have a response of -6dB per octave, which means that the higher the frequency or tone, the more it is diminshed.

    Example for a guitar, if the Fc is 165Hz (pot's min setting), then a 660Hz high "E" (12th fret, two octaves higher) would be ~12dB "quieter" than the middle "E" of 165Hz. Now, let's say you turn the pot to it's max setting and the Fc becomes 82Hz, that same 660Hz signal will now be ~18dB "quieter" than the 82 Hz signal (but still only ~12dB quiter than the 165Hz signal). As you may notice, this type of single-pole response is not very agressive, and is why multi-pole boost/cut "active" filters are used (opamp, xstr). There are "passive" multi-pole filters done without "active" devices, but because the signal can only be cut (not boosted) the loss of volume of the signals/tones you do want to get thru can become an issue.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Sorry.
    You posted your schematic over at Imageshack who takes forever to wake up.
    Today I see only half the schematic. With a transistor that shorts a battery.

    I have opened this post about 5 times and have the same problem each time.
    If you attached your schematic here to your post then there won't be this problem with Imageshack.

    A guitar pickup is supposed to feed a preamp that has a very high impedance. Some experts say at least 3M ohms. Your inductor's impedance is far lower than 3M ohms.

    The bass and treble tone control posted has an inverting opamp at its input so its input impedance is too low for a guitar. Its filter has a very gradual slope.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Ok, since I haven't heard any specifications for frequency cutoff except for Nomurphy's helpful post, I plugged some numbers into TI's Filter Pro software; the result is attached.

    It's an active filter (means that it will require power) that you could use a commonly available operational amplifier like a TL071 - that will require a dual power supply, but you can use a pair of 9v "transistor" batteries to power it. (A double-pole ON/OFF switch will be your friend here.)

    If you need more help, or want the cutoff frequency changed, just let us know.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The requirement is for amplified low frequencies to be mixed with the original signal.
    So the high input impedance lowpass filter must be followed by an amplifier circuit then a mixer circuit.

    The filter might cause phase-shift cancellation of some frequencies when it is mixed with the original signal.

    So maybe just the bass control part of a tone control circuit will be OK.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  14. SgtWookie

    Expert

    Jul 17, 2007
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    Interesting, but it isn't phase-coherent (as per the disclaimer in the intro).

    As Audioguru alluded to earlier, and as you can see in the chart accompanying the earlier circuit I posted, phase gets skewed as you near the "corner" of the filter, or the -3dB point. Attempting to re-mix from such phase-shifted outputs will result in very annoying non-linearity frequency response, where one frequency is nearly cancelled out, and an ajacent frequency is nearly doubled in amplitude.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    I think a Linkwitz-Riley crossover avoids phase cancellation and adding in a crossover network.
     
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