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Old 18th November 2003, 10:46 AM   #1
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Question help me figure this out please

ok so i have been staring at this schematic for a really long time and i still dont get it. can someone please hold my hand and walk me through WHY it does what it does? its from a popular guitar distortion pedal/preamp which achieves a constant level of clipping over the entire frequency range of the guitar by boosting the mids and highs which would either be only slightly clipped or not clipped at all because of the lower voltage as the frequency increases. later on down the line, the preamp boosts the lows effectively relowering the mids and highs to their normal level so its not overpoweringly shrilly. the following is the schematic for the preemphasis section that boosts the highs. im told it starts boosting around 900hz but i have no idea where this number comes from or how it even affects the feedback loop in the opamp so its even boosted. can someone walk me through what the transistor does to actually boost the levels and possibly provide an explanation of the frequency determining factors. i cant seem to figure it out and no one i've asked has ever seen the type of circuit before.

thanks!
greg
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Old 18th November 2003, 12:13 PM   #2
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the transistor is operating as a gyrator - simulating an inductor. The transistor effectively reverses the phase of the capacitor, creating an inductor. Not sure of the maths for this circuit, however this technique is common in graphic equalisers (although most now use ICs instead of transistors).

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Old 20th November 2003, 03:08 AM   #3
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oh i think i see now... assuming its target freq is 900hz, its simulating an inductor to ground which when paired with the .027uF cap shunts signal to ground around 900hz, like a bandpass, effectively eliminating some of the negative feedback from the output of the opamp, which essentially raises the output level of the opamp? am i getting it now or totally off track?
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Old 20th November 2003, 05:48 AM   #4
GregGC is offline GregGC  Canada
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Quote:
Originally posted by xdissent
oh i think i see now... assuming its target freq is 900hz, its simulating an inductor to ground which when paired with the .027uF cap shunts signal to ground around 900hz, like a bandpass, effectively eliminating some of the negative feedback from the output of the opamp, which essentially raises the output level of the opamp? am i getting it now or totally off track?
I think you are correct.
Looks like that thing has a very high gain for the res. frequency determined by the L/0.27uF so it clips, and outside that resonanse F the gain decreases.
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Old 22nd November 2003, 05:19 AM   #5
Dj Dfa is offline Dj Dfa  United States
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instead of starting my own thread i thought someone in here might be able to answer my question...what is clipping in its technical deffination? can you please explain everything you can about clipping that you can do quickly? I appreciate ANY help.
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Old 22nd November 2003, 07:35 AM   #6
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Quote:
Originally posted by Dj Dfa
instead of starting my own thread i thought someone in here might be able to answer my question...what is clipping in its technical deffination? can you please explain everything you can about clipping that you can do quickly? I appreciate ANY help.
here, clipping is when the op amp runs out of supply voltage. for instance, in this example, with the signal source at 250mV set at 1kHz, the op amp would want to amplify it to a little over 15V. but because the + and - supplies to the op amp are only at +/-5V, it will start to amplify, and when it reaches 5V, it "runs out" of voltage and the top and bottom of the wave will be flat. in reality though, it wont even make it all the way to 5V because of the diode drops in the op amp (because its made of transistors.) different op amps will provide different available voltage swings though. ive attached a similation with a typical u741 op amp. hope that helps.
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Old 23rd November 2003, 04:22 AM   #7
Dj Dfa is offline Dj Dfa  United States
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thank you so much for the help. i really appreciate it. i feel kinda stupid asking some of these questions, but i gotta ask em. So excessive amounts of clipping can ruin an amp right? It means your pushing your amp too hard right? Like it cant supply the amount of power that your asking to supply? Thanks again,

Max
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Old 23rd November 2003, 09:57 AM   #8
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Quote:
Originally posted by Dj Dfa
thank you so much for the help. i really appreciate it. i feel kinda stupid asking some of these questions, but i gotta ask em. So excessive amounts of clipping can ruin an amp right? It means your pushing your amp too hard right? Like it cant supply the amount of power that your asking to supply? Thanks again,

Max

well, "ruin" is pretty subjective.... in this case its definitely desired because the circuit is DESIGNED to produce distortion in this fashion. but in a normal amp that you're actually wanting to sound decent, yeah... this should be avoided at all cost. it doesnt necessarily HURT the amp in this case though its not like its driving it over the limit... it just reaches the limit and stops. anyway. its always a good idea to ask something if you're confused, its the only way to learn. good luck
greg
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