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New To Pedal Modding

Discussion in 'Accessories / Connections' started by Guitarfreak, Feb 20, 2010.

  1. Guitarfreak

    Guitarfreak Well-Known Member

    So I am learning the fundamentals of electrical properties and how to manipulate audio signal slowly, but I need some reinforcement especially on some of the finer details. I've got some mods in mind and I am just hoping to clear up some things prior to. The mod is to my trusty SD-1 pedal. I want to increase the pedal's low end response, so I should increase the value of certain capacitors, but which ones? Only the ceramic disc caps, or including the electrolytic as well? Only ones placed in series? Only ones placed in parallel? All of them?

    Could changing the value of a capacitor damage anything assuming that I solder it correctly and it is placed in the correct orientation if it is polar? I am just wondering if there are certain ones that you don't want to touch.
     
  2. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    As well as here, I'd also suggest joining up at Ampage Music Electronics Forum - The AMPAGE Discussion Site, and asking there. A lot of knowledgeable folks there, like Aron Nelson, Mark Hammer, and R.G. Keen. Go to the "Effects" section.

    Some other good research resources (some of which are sites by those guys):

    DIYstompboxes.com - Index

    AMZ Lab Notebook & Guitar Effects

    New Page 1

    The answers are somewhere in the above.

    Have fun,

    Kapt.Krunch
     
  3. Guitarfreak

    Guitarfreak Well-Known Member

    I am reading through the AMZ site and for input impedance it is a bit confusing. For one, I need to review what exactly inductance and capacitance are, but that is something entirely different. In a certain example used to display impedance of a certain circuit it says this:

    Let's compare what happens with a different type of booster circuit, such as the jfet amplifier shown here. Since the gate resistance of the jfet is a very large number, the input impedance (Z2) of the circuit is essentially the value of the R1 resistor, or 1M ohms

    imp3.gif

    My question is, isn't that resistor wired in parallel and running to common instead of being in the circuit signal path? How that resistor would impede or resist the actual forthgoing signal escapes me exactly. What do you gain/lose by running the resistor in parallel as opposed to in series?
     
  4. RemyRAD

    RemyRAD Well-Known Member

    In parallel you are creating an equalizer. In series, an electrolytic capacitor will dictate the low-frequency pass through response and block any DC components or offset. I know folks that don't even want any electrolytic capacitor in series with the input section. I've heard some of these modifications and I don't think they are any kind of improvement. In fact, I've heard some of the weirdest stuff that way. But because we use DC operational amplifiers, their low-frequency response does go down to direct current. You don't necessarily want that. You want a cut off at or below 20 Hz but not down to DC since the amplifier can create a DC component at its output that will look like DC offset. Something to be avoided completely. You don't need to change any of the ceramic capacitors. Those are bypass and filter capacitors . Why the modifications? Why not just buy a piece of equipment you already like the sound of? I really don't believe in reinventing the wheel. Plenty of well educated electrical engineers have already designed circuits far beyond your comprehension. Sure, I could modify my Neve & API preamps to sound " better". But it's really not better it's only different. I want them to sound the way they sound already. Every amplifier design has its own character. You either like the character or you have purchased the wrong equipment. Better isn't always better it's just different.

    My engineering dictates my sound.
    Mx. Remy Ann David
     
  5. Jeemy

    Jeemy Well-Known Member

    I have no idea about the electronics side of it, but my understanding as to the purpose is that a lot of times - especially in a guitar pedal - you have the basic circuitry there but beancounterism has resulted in a few of the 'bottleneck' components not being top-notch, this results in a very well designed circuit (by a well-educated engineer) being compromised (by a decision made by the company's bottom-line thinking). While this thinking is less prevalent in pro audio, its also the case that in, say, Remy's API preamps, the cost is not the overwhelming factor, the quality is. In a pedal made by a well-known Japanese manufacturer, the cost often is the overwhelming factor and a corner is cut that for a relatively small investment, albeit one that would push the original pedal 20% higher and out of its cutthroat market, you can 'spec up' your pedal. Usually from my knowledge this is a much simpler mod than what you seem to be talking about, but modded pedals I own have things like capacitors, film resistors and the IC chip being replaced and are made true bypass or better-bypass.

    A classic example is the Ibanez guitars, when they replaced the locking studs the trem mounted in with non-locking ones, as the decision was made by non-players on the basis that this didn't affect your playing experience. In fact, it lowers tuning accuracy on return by several percent - several critical percent. Its a $20 fix.

    To blame, is the cheaply-made Chinese/Korean market that companies now feel they have to compete against - the same thing people rail about when complaining about prosumer gear reducing studio profitability.

    So better can just be better sometimes.
     
  6. Boswell

    Boswell Moderator Distinguished Member

    Yes, the 1M resistor is across the input, so has signal current running through it. Its function is to provide a d.c. operating point for the gate of the FET, since the input to the circuit is a.c. coupled. The input impedance is the parallel combination of that resistor and the input impedance of the FET. But the FET input impedance is very high (100's of M Ohm), so it can be ignored, and the input impedance is just the resistance of the 1M resistor.

    In the circuit extract shown, all the three capacitors affect the low-frequency response. The C1-R1 combination produces a pole at about 1.6Hz, the C3-R3 combination at about 7.2Hz, and you can't say a lot about C2-R2 until you know what C2 is connected to next in the circuit.
     
  7. Guitarfreak

    Guitarfreak Well-Known Member

    Thanks for clearing that up. I am pacing back and forth in my head trying to understand how cap/res combinations result in only certain frequencies being gated. Like for the high pass the signal goes through a cap, where the bass/treble gets rolled off based on the capacitance value, and then encounters a parallel resistor and.... Or for a low pass the signal goes through a resistor, impeding all frequencies evenly, and then encounters a parallel cap and.... It's like I have most of it, but I fail to understand further.
     
  8. Boswell

    Boswell Moderator Distinguished Member

    Ignoring phase-shifts for the moment, just think of capacitors as resistors whose value decreases with increasing frequency. In a simple R-C or C-R circuit such as the C-R on the gate of the FET in the example you gave, the 3dB point is the frequency at which the ohmic impedance of the capacitor equals the resistance of the resistor. At 1.6Hz, the 100nF input capacitor has an impedance of -10^6j Ohm, which has a modulus value of 1 MOhm and a phase of -90 degrees.
     
  9. Guitarfreak

    Guitarfreak Well-Known Member

    I thought DC didn't have a phase component?
     

    Attached Files:

  10. Boswell

    Boswell Moderator Distinguished Member

    Where does d.c. come into it? The example figures were given for 1.6Hz.
     
  11. Guitarfreak

    Guitarfreak Well-Known Member

    IDK, I thought an FX pedal was DC. Maybe the entire signal path isn't DC?
     
  12. Boswell

    Boswell Moderator Distinguished Member

    Huh? For it to be of any use, the signal path has to be audio: 20 Hz to 20KHz, or whatever numbers you care to choose. This is a.c. in circuit terminology. Pedals pass and modify a.c. signals while running off d.c. power. Condenser (capacitor) microphones receive 48V d.c. in order to operate, but transmit a.c. differential audio signals using the same wires. This is possible because of frequency division multiplexing: the d.c. flows one way at baseband and the audio flows the other way in the audio band. As another multiplex, the same cable can also receive UHF band from your interfering mobile (cell) phone and send pulses to both directions to the microphone and the pre-amp, causing the familiar noises.
     
  13. Guitarfreak

    Guitarfreak Well-Known Member

    oh ok. So inverting phase in a pedal is actually possible then yes? If so then that's ridiculously exciting :D
     
  14. djmukilteo

    djmukilteo Well-Known Member

    GF:
    I think you need to read more about small signal amplifiers ("boosters" as certain countries call them) which is what this circuit is.

    You're also forgetting to put you're guitar pickup (inductor) into the front end of that circuit schematic to fully gather the whole concept here.
    Also you should review Thévenin's theorem - Wikipedia, the free encyclopedia

    So you have a pickup coil connected from the input point to common (GND)
    Start at your pickup. It generates a small amount of voltage and current (your "signal").
    The .1uf cap in series blocks out any DC component that might be in the signal from appearing at the base of the JFET amplifier and "de-couples" the AC component to the input of your amplifier stage. There is always a cap in series between various stages of amplification to "de-couple" any DC and leave only the AC component (you're signal) from getting into the next stage.
    The input impedance at the input base of the JFET with R1 (1megohm) is high (Hi-Z) which basically keeps the signal from being "padded".
    If you were to put a resistor in series with that path you would "pad" the signal making it more difficult for the transistor to provide any gain in it's amplification of an already weak signal defeating the purpose of getting the most signal from the pickup to the base of the amp stage.
    +V is a DC power supply and R2 and R3 act as a power supply voltage divider to "bias" the transistor (you can look that up). Basically bias is how you get gain out of a transistor....
    The electrolytic is across the lower R3 to act as another DC bypass to keep the power supply (+ side is towards the positive DC supply) out of the signal path and the last .1uf cap on the output does the same thing as the input cap did (de-couple and isolate any further DC) in you're signal and you now have a nicely amplified AC signal with some value of gain from input to output ready to go into the next stage of amplification!
    Viola! One stage of simple amplification.
    Hope that helps
     
  15. Guitarfreak

    Guitarfreak Well-Known Member

    WOW. Thank you for taking the time to explain that, that was so much more comprehensive than my instructor's explanation today, who basically labeled the parts of a transistor and that's about it. Bahh, he's a lump. Well that also explains what the lectures said about capacitors being a short to AC and open to DC. That also clears up some questions that I had about resistance, namely why higher resistance sounds good in guitar volume pots and pedal input impedances. It's not the resistance you are hearing... it's the resistance TO GROUND... which results in more signal passed on to subsequent components. I've got some more questions though:

    1. Is it possible to split a single AC signal into two equal halves?

    2. Is it possible to invert the phase of an AC signal 180 degrees using components?
     
  16. dvdhawk

    dvdhawk Well-Known Member

    GF,

    1. Is it possible to split a single AC signal into two equal halves?

    2. Is it possible to invert the phase of an AC signal 180 degrees using components?

    Yes and yes, you'll get to that in your electronics class. Op-amps routinely use the inverted signal, but I don't want to spoil the suspense and you've got much better qualified instructors here than I, in Boswell, Kapt. K, Remy, and DJ. (maybe Kev will chime in too) All geniuses in my book.

    You also seem to keep coming back to this idea of changing the phase of DC. I'm not sure you're completely getting what the phase is. This is a gross over-simplification, (and there are other types of signals in the DC family you'll learn about when you study rectifiers,) but this is where I think the concept needs to start.

    ACDC.jpg

    This would represent the kind of DC your effects pedal 9v battery might have. You can change the polarity of this kind of DC and make the electrons flow the other way, but it's still a flat line.
     
  17. Guitarfreak

    Guitarfreak Well-Known Member

    Oh no I completely get what phase is. At first when I wrongly asked if you could flip the phase of a DC signal I was actually asking if you could flip the phase of the AC signal within a DC powered unit such as an FX box. Case of wrong terminology. I keep asking because it really never gets answered, but if it is possible then I would love to know how because I could use it for some interesting sounds. I have a tone circuit in mind and I could use inverted phase to sculpt the output EQ according to my specs. I believe I mentioned it in another thread, the one about flipping the phase of a DC circuit.

    The idea would be based on the ability to split a single AC signal into two equal halves. Could you use a transistor to do this? So long as you use it as a signal split and not as a signal combiner/booster?

    EDIT: and here that link is
    http://recording.org/cms-comments-c...am-taking-related-questions-3.html#post300102
     
  18. djmukilteo

    djmukilteo Well-Known Member

    If you chop AC in half you have what's called "ripple DC" or "half wave AC". Half the AC wave is above zero and the lower half is cut off or vice versa (below zero)....it sounds really noisy.....buzzzzzz.
    It is the essence of a half wave bridge rectifier circuit which is the basis of making a DC power supply from an AC source.
    DC is always considered any signal or voltage above zero (GND) or below zero but not both at the same.
    AC is a continuous voltage that is moving across zero positive then negative (simple sine wave).

    "Phase inversion" is just reversing the polarity of a circuit or signal...
    Phase inversion - Wikipedia, the free encyclopedia

    I'm pretty sure you wanted to mod your pedal?..Hehe....
    and I completely understand you wanting to get a better understanding of the electronics...that's is what is cool about sound...but again I don't want your "head to explode"....and to be fair I'm sure your instructor understands most of what he's trying to explain to you....but basic electronics is a very difficult topic to comprehend if you don't already know and completely understand the basics of electricity...AC/DC theory, then simple resistance, capacitance and so on....it just gets harder and harder.....and it's not a guessing game, it's really just physics...it behaves in a specific fashion every time depending on how and what force you apply to it....electronics is just a means of controlling electricity....so the more you understand the basics the better off you will be....
    I'm pretty sure you have an idea of what you want your pedal to do or sound like and maybe you could put that into words that make sense to you that might be able to be explained in terms of the electronics needed to do that.....

    Oh and I keep forgetting to put my music up....which is what it's all about!
    See...I can't even figure out how to put that in my profile....gheez

    Google
     
  19. Guitarfreak

    Guitarfreak Well-Known Member

    Oh don't worry about my head exploding, I ask because I want to learn, if you've got it lay it on me. I also had the idea today for a boost/gain circuit which runs at 18v. Once you gate the input and set the impedance, you split the signal into two parallel signals (can you tell that I like this idea?) of 9v and run them through identical low-gain JFET gain stages and then recombine them later. From there you could run it through a tone circuit or straight to the output for a simple boost pedal. Or you could change one of the parallel stages slightly so that when they combine you don't get an even combination, but rather a lower output/smoother sound.

    ...I'd like to think that I've got some good ideas, who knows maybe they're not, but I need you guys to point me in the right direction so that I can learn the basics and get down to protoboard construction this summer. Basically I seem all over the place because well... I am.

    I am...
    1. Trying to get a better grasp on the basics in order to pass this class
    2. Trying to understand my pedal enough to mod it according to my own specs rather than following a mod-by-numbers
    3. Learning with the intent of designing and building my own circuit this summer
    4. Just curious I guess :D

    Thanks for all the help thus far guys. You really are a spectacular bunch :D haha
     
  20. djmukilteo

    djmukilteo Well-Known Member

    OK....I hear ya...it's actually great to hear someone like you who has the excitement and passion you do when it comes to your sound.
    And now diving head on into the electronics....good for you....pathetic perhaps, geeky....ya pretty much.....naw just kidding.....doin things like that reminds of days gone by and I wish I still had you're ambition....so don't ever quit asking questions or stop trying to understand something....I'll bet your intelligent knowledgeable instructors get a big kick out of you! ....sound!....it will change your life!

    BTW I was listening to your "clips" and I want to know how you stop your riffs so dead tight!
    Are you using a gating thing or is that just you grabbing strings?
     

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