Crackle OK ??? - No Crackle far better!!!

    Hi folks
    I never did like the dc noise on potentiometers used as gain controls (XEVZ) - crackle OK!!). Most guitarists view this as a fault!!
    Here is a solution for those of you who may be interested - I'm sure it has been done before though!!
    Simply replace the 5k reverse log ( or the 5k log that works backwards!!) with a 10 k resistor. Then add a 10uf electrolytic, positive terminal to the junction of the source and 10k resistor. Connect a 10k reverse log (or 10k log pot backwards) to the negative terminal of the electro and connect the wiper of the poti to earth. There you go - no more crackle!!
    Cheers
    Steve

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    FWIW, the photos I have of a SHO clearly show a linear taper potentiometer. It also shows shockingly cheap components and it's built on a stripboard.
    You can find very detailed pics of the the SHO and Fuzz Factory on this interesting Japanese site: http://www.effector-repair.com

    Re: Crackle OK ??? - No Crackle far better!!!

    Hi Markm
    No need to apologise :)
    I may need to eat humble pie :-[ - I didn't take into account that the gain pot is actually lowering the dc voltage as well as the AC gain :-[ - it varies between 1.4 volts and 0 volts!!
    My suggestion will not allow as much AC gain as the unmentionable (SHO etc), but stay tuned - I am sure this problem can be solved by one of us :)
    Oh yeah - nice to see you made the switch, - you are most welcome here 8) :D

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    Unfortunately, the only way I can see to do it would be to change the circuit. It's funny, I put that same kind of control on a Rangemaster clone once to be able to mess with the bias, but I ditched the idea because of the crackling (it makes the stupid thing sound like it's broken). I can't see why ZVex thought that was a good idea. Sure, the overall sound of the box might be good, but that crackling is annoying. We should build a stock crackling one and compare it to a "better" one that doesn't crackle and see if there's much sound difference.

    Re: Crackle OK ??? - No Crackle far better!!!

    Zitat

    A conductive plastic pot should significantly reduce the crackle.

    Yes and no....
    The original Rangemaster used a wire-wound-poti to reduce the crackle.

    The problem here is the misuse of the BS170 which was originally designed for switching applications like also the VN10KM or similar.

    In the region between 2k5 and 5 k there is only minimal crackle, the crackle occurs only between 0k and 1k5, why ?

    If you measure the voltages you willl find out, that it is not the gain setting, the reason is the DC-bias.

    Lets ignore the AC-chatacteristics of the circuit. The gate of the FET is always powered by half of the drain-voltage. so we get the following results (measured with 1M resistors):

    Poti-value Drain Gate Source GS-difference
    5k 7 V 3,5V 2V 1,5V
    2k5 5,9V 2,95V 1,5V 1,45V
    100 Ohm 3,3V 1,65V 0,1V 1,55V

    If you look at the data sheet you willl see, that the BS170 is working near its turn-off point. With higher drain voltages this is not such a big problem as with lower drain voltages. If there is only a minimal DC-change which will below the turn-on voltage of the FET it will switch of the FET which causes the crackle. When the Fet is off, the voltage at the drain moves toward 9V, giving 4,5V at the gate and this will be enough to switch the FET on again.

    So with a better DC-bias the crackle should get minimal. I would suggest to try 1M5 from the gate to ground or use a 500k trim-poti in series with the (1M)-resistor to ground to keep the FET in the safe-operation area.

    I can´t see a reason to use 10M at the gate.

    analogguru

    Man sollte sich selbst im Leben nur diejenigen Versagen zum Vorwurf machen, bei denen man es gar nicht einmal probiert hat.

    Re: Crackle OK ??? - No Crackle far better!!!

    I'd say it's there for the increase in high end, or sparkle, if you will. You need an extremely high input impedance for this.

    Aber wem sag ich das? ;D

    Re: Crackle OK ??? - No Crackle far better!!!

    Hi AG
    The 10M is so you can plug a piezo pickup in to it - ha ha :P

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    Hi again AG

    My simulations suggest increasing the drain gate resistor to 2M2, and leaving the gate to ground resitor at 1M is a better idea, and the AC gain is virtually unchanged. I won't know how this affects the crackle - I will try it out tomorrow and let you know - got to feed my dogs and off to bed ( it's 11:10pm at night here, middle of Winter and very damp, cold outside ).

    Anyway the simulated drain voltages are considerably higher than the 3 volt pinch off of the Mosfet.

    Poti-value Drain Gate Source GS-difference
    5k 8.25 V 2.58V 0.7V 1.88V
    2k5 7.83V 2.44V 0.56V 1.88V
    0 Ohm 6.12V 1.9V 0V 1.9V

    Guten Tag - und Gute Nacht von mir :)

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    Zitat von soulsonic


    Unfortunately, the only way I can see to do it would be to change the circuit. It's funny, I put that same kind of control on a Rangemaster clone once to be able to mess with the bias

    I tried doing almost the same thing myself and pretty much gave up on it.

    Re: Crackle OK ??? - No Crackle far better!!!

    The crackling pot is part of the circuit layout and this special layout is responsible for the sound of the SHO.

    When you increase the gain, the pot also modifies the BIAS-setting of the MOSFET.

    Jürgen

    Re: Crackle OK ??? - No Crackle far better!!!

    Hello markm
    I still think we can improve on the Super hard On and Box Of Rock etc.,though ;)
    Juergen is right - the gain does affect the bias and adds considerably to the assymetrical 2nd harmonic distortion created by this very high gain ( 30dB ) circuit.
    keep those thinking caps on :D

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    Hi AG
    I just tested the SHO with the 2m2 and 1M combination - still plenty of crackle though ;( ;(

    measured results similar to simulation :D


    Poti-value Drain Source
    low gain 5k 7.5 V 0.5V
    full gain 0 Ohm 6.1V 0V

    certainly much higher than the 3v pinch off on the drain :)

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    Did you also try 1M from D to G and 1M5 from G to gnd ?

    checked my email....and working

    analogguru

    P.S.: and how is it about connecting the gate to gnd resistor from gate to source ?

    Man sollte sich selbst im Leben nur diejenigen Versagen zum Vorwurf machen, bei denen man es gar nicht einmal probiert hat.

    Re: Crackle OK ??? - No Crackle far better!!!

    Hi AG
    Yes, I tried the values you suggested, but the simulation shows severe clipping on the bottom half of the waveform. :(

    here are the simulated results:

    Poti-value Drain Gate Source GS-difference
    5k 6.61 V 4.06V 2.15V 1.91V
    2k5 5.78V 3.47V 1.54V 1.93V
    0 Ohm 3.26V 1.96V 0V 1.96V

    with 1M5 from drain to gate and 1M from gate to source I get:

    Poti-value Drain Gate Source GS-difference
    5k 7.06 V 3.98V 2.08V 1.9V
    2k5 6.17V 3.29V 1.37V 1.92V
    0 Ohm 4.84V 1.93V 0V 1.93V

    I can't see how this will affect the crackle though ::)

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

    Re: Crackle OK ??? - No Crackle far better!!!

    Maybe you could try a multipostion rotaryswitch to dial in different Source resistor values from 5K to 0. It will likely make a little pop every time it's switched, but a pop is much less annoying than a crackle. Besides, most guitarist tend to expect a little pop when the turn a switch. It would maybe pop less if the switch is make-before-break or if the values went down incrementally by having the switched resistors in parallel to a "starting position" 5K fixed resistor - the last position would be a short that would give the 0 ohm value.

    Another idea could be to just settle on the "sweet spot" of the crackle control and just have a fixed resistor of that value with some kind of volume control added at the end of the circuit and maybe a "gain" pot that controls a cap bypassing the fixed Source resistor.

    Re: Crackle OK ??? - No Crackle far better!!!

    Hi soulsonic
    Good idea with the switch.
    Unfortunately the second idea only allows one position for the mosfet bias.
    The varying bias control alters the symmetry of the wave form - at low gains it still looks like a clean sine wave with plenty of headroom to spare. As the crackle - sorry gain - control is rotated to 0 ohms the assymetry or 2nd harmonic distortion is progressively increased.

    In other words it is not just a gain control but a harmonic distortion level control as well - quite effective as many have noticed ;)
    Cheers
    Steve

    Ich liebe dieses Forum, und mein Name ist Steve von Neuseeland

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