Arduino-Controlled EQ for a Tube Guitar Amp

Discussion in 'General Electronics Chat' started by visorakmark, Jul 16, 2013.

  1. visorakmark

    Thread Starter New Member

    Jul 11, 2013
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    Hey guys! First post here. Hopefully this is the right place.

    I am studying electrical engineering and am building a guitar amp for a summer project class. I have decided to buy this kit. http://www.ax84.com/p1.html

    My professor and I want to modify this amp, specifically the tone stack. We'd like to replace the current tone stage with an arduino controlled filter.

    So, thinking theoretically. We could build a simple RC flter and replace the resistor with some voltage controlled arduino resistance. My professor has recommend I look into FET's. Considering FET's are voltage sensitive devices, it might be able to accomplish what I want.

    I've never done anything like this...so, I guess I am asking for some help! Where should I start? I plan to design and test the circuit on a breadboard with a function generator. Just gotta design something...

    Link to the ax84 P1 schematic: http://www.ax84.com/static/p1/AX84_P1_101004.pdf
     
  2. poopscoop

    Member

    Dec 12, 2012
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    16
    Not sure how you're going to get arduino to control this. While a FET is voltage controlled, arduino only does LOW/HIGH for output. With only one pin, your option is to turn the filter on or off. You can get creative on what the definition of on/off is, IE one of 2 various resistance values, but without incorporating additional output pins into the design you're limited to two options.

    I would suggest using multiple pins (I think there are 14 digital pins on Arduino), each connected to a FET or transistor which then controls one of several parallel resistors in the RC circuit. This will allow you to combine resistors, giving you several dozen unique values when you vary between multiple resistors in parallel, single, no resistor, etc.

    I'd suggest looking into an IC transistor array to save space and trouble.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    You might look into the LM13700. You can use it to make a voltage controlled resistor. It's described in the data sheet.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    This is an amp made by tubes is it not? Due to the high voltage used. This task may be quite complicated.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    But I don't think there are high voltages in the tone stack? DC voltage is blocked by C7 thru C9.


    [​IMG]
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    But is not C7 connected to 140 volt?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Or more. But does that mean there is 140V on the tone controls?
     
  8. visorakmark

    Thread Starter New Member

    Jul 11, 2013
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    Hey guys, thank you for the comments!

    About the 140V on the tone controls: My understanding is that voltage will not affect the tone controls. Tone controls only affect AC signals, correct? Considering the signal is read on the right side of the circuit, the 140V on the left can be ignored. I am sure these comments are exposing my ignorance. Please enlighten me if I am mistaken. =^)

    I ordered a few of these under the recommendation of a professional recording studio technician and tube guitar expert: http://www.alldatasheet.com/datasheet-pdf/pdf/197304/MICROCHIP/MCP4012T-103E/CH.html They have told me this digital pot can easily fulfill what I need. What do you guys think? Could I just replace the resistor in a simple RC filter with this and connect the Arduino to it?

    Also about the Arduino, my professor assured me the Arduino Uno is capable of supplying voltages in between 0 and 5 volts. PWM is apparently able to accomplish this. Shouldn't I be able to fade the resistor just as the LED in this example? http://arduino.cc/en/Tutorial/PWM
     
  9. #12

    Expert

    Nov 30, 2010
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    The amplitude of the signal will be way over 5 volts at the tone stack. The digital pot seems to be unable to survive the AC amplitude and it has 120pf on its wiper. That will interfere. I even measured a guitar at +/-3 volts with no amplifier. Then, there are pre-amps and effects pedals that will boost the input amplitude even higher than 6V p-p.

    You have some designing to do if you're going to get the signal amplitude low enough for the digital pots. You can't use AGC, it wrecks the dynamics. You will have to both attenuate the input signal and use a clipper circuit to protect the pots. Probably best to do the tone controlling before any tubes. The technology of tubes is about getting a lot of amplitude, losing a lot in the tone stack, and using a gain recovery stage. Can you do that before the first tube stage and not wreck the signal to noise ratio? I'm thinking, j-fet input stage to have high input impedance and low impedance to drive the tone controls. Cut the signal amplitude down with a resistor-only voltage divider, then adjust the tone, use an op-amp with a +/- 15 volt supply to boost the amplitude, then feed it to the first tube. Then, leave out the original tone stack and consider whether you don't need a tone recovery stage to get the amplitude high enough to drive the output stage.

    That digital pot has no memory. I don't know if you can buy non-volatile pots or if you will just tell the Arduino to reset all pots to previous values at start-up. People tend to get their adjustments right and leave them alone.

    One of my hobbies used to be designing stuff for a vacuum tube only, nut-case guitarist, with incredible ears, and see how many transistors I could use without him knowing there were any transistors in it. Fun stuff.:D
    As long as I kept my bandpass up to at least 23KHz, he couldn't find the transistors by listening.:rolleyes:
     
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  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    As #12 said, the signal voltage on the tone stack will be much higher than the digital pots can tolerate. I've been researching and have found digital pots that can withstand 30v p-p, but that's not enough.

    You can get digital pots that are non-volatile if you want. Another issue is that that the pots you are considering seem to have a max resistance of 50k, so you can't duplicate the tone stack pot values using those particular chips. An interesting exercise might be to recalculate component values to use the lower pot values. Or, other mfrs do have higher values available.

    I wonder if you couldn't use the HI-Z Jfet input stage suggested, followed by a re-calculated tone stack using the digi-pots, followed by the tube stages.
     
    Last edited: Jul 18, 2013
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How does it go?

    "There's nothing you can do with one vacuum tube that you can't do with 5 or 6 transistors" :cool:
     
  12. #12

    Expert

    Nov 30, 2010
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    Yep. That's the quote. Problem is, 5 or 6 transistors don't use anywhere near the power of even one tube heater filament. That's why you sneak in transistors when you want to do some processing.

    and, yes, you can scale the tone stack, but it seems stupid to do that after the signal is up to near a hundred volts peak to peak and try to cut that to 5V then reconstitute it with a recovery stage. Repeating myself, try to do it before any tubes. You will have something you can package separately as an "effects" box or you can build it into the amp. I've done both.
     
  13. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I was also talking about the tone stack before the tubes. :rolleyes:
     
  14. #12

    Expert

    Nov 30, 2010
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    I know. I try to address the OP. Sometimes the OH (other helpers) think I'm talking to them. No biggie.
     
  15. visorakmark

    Thread Starter New Member

    Jul 11, 2013
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    0
    This was the first impression of several folks I talked to. I wasn't to sure about hooking up an arduino in the middle of an amp circuit...

    Well it doesn't seem as nifty but the effects box approach makes way more sense. My Prof sent me this page as a starting point for using an FET. http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm

    What do you guys think? Where would the signal come in? I'll need an op amp at the end of this effect box so to prepare the signal for the input stage of the amp like #12 was saying.

    I guess I need to break open PSPICE and start drawing.
     
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