TL071-based bass preamp

Discussion in 'The Projects Forum' started by darkroom, Dec 26, 2014.

  1. darkroom

    Thread Starter New Member

    Oct 30, 2014
    18
    1
    Hi! I'm trying to build a very simple (step by step) electric bass preamp based on the TL071 opamp. I choose that opamp since it's cheap and very easy to find, and seems that's a standard for this type of applications.

    I'm not copying any design, I'm just learning through the process, so I would want to hear some recommendations up to this point.

    Here's my current crude and basic circuit in LTSpice (already mounted on breadboard):

    upload_2014-12-26_20-2-48.png

    My rationale is:

    * 100K resistor pair to bias the single-supply to VCC/2.
    * 4k7 resistor pair to form "virtual ground" to VCC/2 ( I also tried 1K / 1K in breadboard to maximize waveform distortion)
    * Pot_Gain: a potentiometer to control gain/drive level.

    Of course, I expect my first approach (I'm a beginner, remember :) ) to be very crude.

    I want some recommendation on my following thoughts:

    - Does the 100K divider form a 50K ohm input impedance? I've seen audio circuits for electric guitar with 1M or 2M impedance at input. For electric bass, it's probably at the same league.
    - Does the gain resistor (and the resistor network) affect the input impedance due to the feedback circuit? I tried fairly large values (470K or 1M) and the op-amp clipping seems very distorted (which is what I expected) but signal loss was evident.

    Just for completion, here's the output waveform (after the C2 cap) for several pot_gain values from 5K to 100Kohm:

    upload_2014-12-26_20-10-36.png


    Thank you very much for your patience.
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    R6 and R7 are going to fight with R1 and R2. Throw them away and connect R5 to ground with a capacitor in series to eliminate the DC considerations.

    Edit: Post #8 Crutschow did the math for us. 16 uf for a bottom limit of 10 Hz. 4 uf for a bottom limit of 40 Hz, but that's the -3db point. I would say 8 uf or larger so this capacitor has about zero effect at 40 Hz (bottom note on a typical bass guitar).

    Yes, your R1 R2 are making the input impedance too low. This is where you add something like a meg to the input line from the junction of R1 R2. They make the voltage, the meg keeps the signal from leaking out through the voltage divider.

    The limitation on how many megs depends on which op-amp you're using. The TL071 is a very good amplifier for creating a high input impedance. Some are better, but this one is good, and low noise, too.:)

    The feedback loop does not affect input impedance. It is being driven by the amplifier, not the input signal.
     
    Last edited: Dec 27, 2014
  3. darkroom

    Thread Starter New Member

    Oct 30, 2014
    18
    1
    Thank you very much AACFanatic.

    My first design included R5 to ground but it didn't work; I connected R5 to "virtual ground" circuit instead. Now thanks to your recommendation, amazingly, R5+capacitor to ground works!. How does it play with the VCC/2 bias and/or the "virtual ground" concept?

    RC circuits are a kind of "replacement" for resistor-based voltage divider circuits like these?

    Again, thank you for your patience.
     
    Last edited: Dec 26, 2014
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Actually, my name is Number Twelve...#12

    You only need one "virtual ground". Make 2 of them and they are guaranteed to not be exactly the same, so you eliminate one with a capacitor to block DC flow and allow the AC gain determined by pot and R5 to continue.

    The capacitors are frequency determining components. They pass nothing at DC and pass some current at higher frequencies. Capacitors are not resistors because they change their impedance according to frequency. Resistors are the same at all frequencies. Voltage dividers are made of resistors. Frequency limits are made with capacitors.
     
    DickCappels and darkroom like this.
  5. darkroom

    Thread Starter New Member

    Oct 30, 2014
    18
    1
    Awesome, thank you #12, i'll report my progress soon!
     
  6. JWHassler

    Member

    Sep 25, 2013
    201
    33
    Keep in mind that a capacitor to ground in your feedback network will make a high-pass filter (gain = 1 @ DC and (pot/R5+1) at high-frequencies.)
    This will be important in a bass preamp.
     
    #12 likes this.
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,991
    3,737
    If you were under 30, I think it would be read as, "hashtag twelve"!
     
    #12 likes this.
  8. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Yes, for a value of 1kΩ for R5, the value of the capacitor to ground should be at least 16μF (electrolytic) for a high-pass corner frequency below 10Hz.
     
    #12 likes this.
  9. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    The typical input impedance of a Fender Bassman amplifier is between 50K and 68K ohms. I think you should keep your input impedance in this range.
     
  10. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    That's 68k to the first grid and 68k to ground (or the second instrument plugged into that channel). So, 136k if only one input.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Guitar electronics is a mix of folk lore, cut and try, and who did it first. All, with little regard to us engineers and our ways of doing things.

    The guitar input has a 68K resistor in series with it and the grid of the input tube, with a 68K resistor to ground, which acts as an attenuator and filter. The grid capacitance is in parallel with the 68K resistor connected to ground. This is a very common input configuration for guitar amps. The DC resistance is 2 X 68K = 136K, (which is meaningless at AC), but the AC impedance is lower than that.

    So, what I am saying is that an preamp for a guitar is not a simple (flat) op-amp amplifier. It has to have a specific impedance verses frequency curve on the input.
     
    darkroom likes this.
  12. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Amen to that!

    However...I have designed amplifiers for a guitar player with an amazing sense of sound. One of them was an in-guitar pre-amp that was dead flat from 21Hz to 200 KHz, reduced the impedance to 150 ohms, and let him plug that into his amplifier. He loved it. This is in direct opposition to the idea that you have to have a specific impedance or frequency curve.

    Your guitarists mileage may vary.
     
  13. darkroom

    Thread Starter New Member

    Oct 30, 2014
    18
    1
    Yes, totally correct. I would expand the "guitar electronics" concept to the "audio" world. There is not only folklore, but myths, mysticysm and pseudoscience. I care more about musicianship instead of endless cults to some "vintage" (or "NOS") component or miraculous machine.

    Thank you.
     
  14. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    The grid capacitance is 1.6 pf according to Tung-Sol
    That's 4.9 meg at 20KHz.
    In my estimation, that is not very important compared to the 18 pf PER FOOT of guitar cord.
     
  15. darkroom

    Thread Starter New Member

    Oct 30, 2014
    18
    1
    I've finally settled on a first design, with a decent sounding overdrive (at least in my equipment on my bass). I modified the input cap to 2.2uF and the capacitor-to-ground in the opamp feedback to 47uF (i got some 100uF on my toolbox but seems unnecessarily high).

    Also the R1||R2 divider resistors were changed to 180K for a higher input impedance.
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Good point.
     
Loading...