Technical OD Pedal Question - OpAmp v Transistor

Discussion in 'Effects, Pedals, Strings & Things' started by Stan Malinowski, Sep 23, 2006.

  1. Stan Malinowski

    Stan Malinowski Silver Supporting Member

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    I'd like to know what are tonal differences in OD pedals which use OpAmps versus those which use transistors? Why do most transistor OD pedals use germanium transistors instead of silicon parts?

    Also which popular pedlas use op aamps as oppossed to transistors? I know my Timmy and Keeley TS-9 use ICs. I THINK my Gaspedal Carb and Skreddy Screwdriver use transistors. I'm curious about my Barber Pedals (LTD, DDSS and Small Fry) and my Fulltone FD-2.

    I've noticed that the pedals that use use transistors tend to use germanium parts as oppossed to silicon transisitors. From my EARLY engineering days (almost 30 years ago) I remember that a) germanium transistors have a lower bias point and 2) most op amps use silicon. Does these play a part in the tonal equation?

    So pedal techies please help me out here!
     
  2. Aj_rocker

    Aj_rocker Member

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    Right im going to chip in here with some stuff i know, PLEASE CORRECT ME IF IM WRONG

    Different op-amps will make the overal sound of the pedal different by a little, not much, as the tone comes from the rest of the circuit,(i.e. caps and resistors).

    Most pedals use op-amps, but some use a op-amp for the gain stage and a transistor for a buffer( making long cable run easier).

    op-amp pedals: DOD 250, MXR distortion+, MXR micro-amp
    op-amp and transistors: TS9 (or anything based on it), Boss sd-1, Boss Ds-1. Toadworks Death rattle
    transistor: most clean booster (but the one above), most fuzz pedals

    Aj
     
  3. Stan Malinowski

    Stan Malinowski Silver Supporting Member

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    I guess I could understand transistors being used in clean boost pedals since you gain get lower gain from a transistor circuit, and hence less distortion, than with a an op amp which typically is multiple stages of transistor gain in a single package.

    Why are fuzz pedals more transistor based than an op amp. Are the transistors biased to be easily driven into distortion?
     
  4. Aj_rocker

    Aj_rocker Member

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    im no expert, but a fuzz is just two clean boost, with one leading to the other. so more gain. op-amps give a smoother sound, whereas transistors can be mean and nasty!

    any more tech question?
    Aj
     
  5. Stan Malinowski

    Stan Malinowski Silver Supporting Member

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    I'm wondering what makes an op-amp smoother compared to an amplifier constructed from transistors? Technically isn't an op-amp really just a series of transistor based amplification stages?
     
  6. Stan Malinowski

    Stan Malinowski Silver Supporting Member

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    Think of a pure sine wave - the signal fluctuates back and forth between it's maximum postive and negative values. If the top or bottom (or both) parts of a sine wave are cliiped harmonics are generated. It's the harmonics which cause the additional warmth added by an OD pedal. Diodes are used in the op amp circuit of a TS design to clip the ends of the signal and cause this desired harmonic distortion.
     
  7. VaughnC

    VaughnC Supporting Member

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    Yup, an opamp is a transistor circuit in a smaller package. You can also build an opamp with regular transistors. However, when you build a circuit with regular transistors the wiring itself will affect the circuit. Which is tonally "better"? Your ears will tell you that....but, either way, the surrounding componants also affect the circuit.
     
  8. VaughnC

    VaughnC Supporting Member

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    The purpose of the diode clipping stage in TS type pedals is to simulate how a guitar amp clips when it is pushed to its limits. However, tubes clip slower/softer than solid state componants, which results in a smoother sound than the faster clipping solid state diodes. However, some diodes clip faster than others....which makes the overall design important.
     
  9. Stan Malinowski

    Stan Malinowski Silver Supporting Member

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    Vaughan,

    I also believe that the individual "transisitors" within an op amp are more balanced for gain and other operating characteristic which, in addition to the effects of wiring in a discrete circuit, allows op amps to be more immune noise and consistent across pedals.

    I ran across an article on the Geofex Website which may have answered my original quesion about op amp v transistor design. I believe my answer is part historical and part technical. When I think back to the early days of pedals (early 70's) there really were no "overdrive" pedals. Pretty much all we had were thinks like the Big Muff and the Fuzz Face (Fuzz type pedals). Back then we also didn't have op amps and the transistor semiconductor industry was still in it's early stages. It leads me to believe that the use of transistors in fuzz type pedals is realy a carry over from the original days of transistir electronics. In addition I found that Germanium transistors were very noisy and gain varaiations were wild between indivdual transistors. Many of the basic characteristics of the early germanium transistors (such as poor frequency response) were those consistent with producing the heavy fuzz-like harmonics of the BM and the FF. I also found that at one point an attempt was made to replace the germanium transistors of the FF with silicon trans - which seemed to be a disaster. Op amps on the other hand, have been perfected over the years to be very noise free and balanced in their gain stages. In my opinion, this opened the door for more refined overdrive pedals which would not be possible with early transistors.

    I think these findings may have answered my question. Please feel free to correct any errors I may have made in my assumptions/conclusions.
     
  10. gaspedals.net

    gaspedals.net Member

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    Hey Stan, rather than go into detail Ill give the short answer. Basically, different build materials will yield different tones.

    I wouldnt say most transistor circuits are germ, probably the other way around. The reason youll see a lot of fuzz circuits utilizing germanium is they tend to have a warmer fuzzier tone. You can substitute SI PNP in a germ fuzzface circuit for example but it will tend to sound less warm and organic, possibly tighter and higher gain. The 70s fuzzfaces used Si and you can hear the difference - some prefer it.

    You can think of dual opamps (as used in the TS) as doing the same thing as a set of Si trannies. Really not much different although again because of the way they are made they will sound a bit different from one to the other. Si tends to have more hfe gain but you can certainly find them in the same value as germ. As I think you mentioned layering stages of low gain trannies has its own advantages in some circuits.

    Hope that helps clarify.

    EDIT: Just read your last post, Geo is a good place to get more info. Yes a lot of the changes have to do with development in technology, which as we all know may seem like advantages but can turn out to be missing something. Si definitely is more reliable and consistent, but theres a reason people are still clamoring for a set of NKT-275s.
     
  11. Stan Malinowski

    Stan Malinowski Silver Supporting Member

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    Jayson (Gaspedals),

    Still LOVIN' my Carb!

    Is the Carb transistor or Op amp based?

    Thnaks for the input!
     
  12. gaspedals.net

    gaspedals.net Member

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    Over the years Ive spent hundreds of hours sitting with hundreds of trannies to find which ones work best with different circuits. With the Carb I just couldnt find one that would yield the tone I had in my head. I knew I needed to go in another direction which called for a totally different approach. Finally after many nights of frustration, I found it! A tiny chinchilla running on a wire wheel encased in paraffin.

    I had previously experimented with other members of the rodent family, but the chinchilla offered something I never thought possible (especially the vintage european ones Im using). I was afraid these suckers would tucker out, but they are little troupers. I kind of feel sorry for the little bastards. Sometimes I even wake up in the middle of the night screaming. But its a small price to pay for stardom.

    ROCK ON! :dude:dude:dude
     
  13. spentron

    spentron Member

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    You all got a bunch of stuff right but miss the big point. Op-amps are very high gain, we're talking about something like stacking 3 cranked distortion pedals in a row, just from one op-amp. Wide-open like that they have poor characteristics, but all that gain can be traded for low distortion etc. by negative feedback. For example run the signal through an op-amp, run the output of that through a couple resistors to divide the level by three, and feed that back into the negative input of the op-amp. It will try to make the negative input exactly the same as the positive input (within maybe .001%) which will make the output of the amp 3 times the input. Hence a linear non-amplifying component controls the result of an amplifying non-linear one.

    An op-amp alone can provide distortion, but this will be almost all simple clipping: an amplifier routes the power supply controllably to the output and this clipping results when there is no more power supply voltage to route. But most op-amp circuits incorporate other devices to add controlled distortion, such as diodes.

    Now transistors ... are much lower gain. You can get rid of the negative feedback and just let it distort. At at low output levels, this distortion is super-soft so even 5-10% distortion is only slightly audible, especially with guitar. Clipping is also softened but not a lot and usually only on one side of the signal swing.

    Transistors also put the effect designer back in control of the amplification stage design, rather than an IC company and designers trying to satisfy many markets. But even the best amplifier circuit designer would be hard-pressed to design with discrete transistors an amplifier that gets all the same things right as an op-amp, and it would cost a lot more etc.

    Clean boosts from discrete transistors are less clean, but add more character and can clip more gracefully.

    As to different types of transistors and tubes, I'll simplify and say they have different characters, but still follow the low-gain building block approach. Tubes do clip softer -- they limit a lot more by just getting near the power supply rail, rather than abruptly ramming it. Still, I think the "smooth" aspect of them tends to be misunderstood. Transistors often produce a treble-clogging effect: the treble of the input signal distorts first and is lost. If you try to bring that back, then you also boost harshness. The ratio just isn't as good. The famous article in the Audio Engineering Society Journal, which first introduced to academia the idea that tubes could be superior, praised tubes for their "naturally bright" distortion characteristics. Distortion makes things seem louder but limits the actual volume signal level. With tubes this can track nicely, even with circuits where this was not anticipated.
     
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  14. gaspedals.net

    gaspedals.net Member

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    Wow, arent we arrogant/condescending. I would say you got a bunch of stuff right, but miss the big point. You make some sweeping statements here that are not always true. Nanananana -Jay
     
  15. Jarick

    Jarick Supporting Member

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    For my ears, transistor designs are more dynamic than op amp. I suspect it's because the transistor amplification/gain scheme is more methodical than an op amp design, but I could be wrong. They usually rely on several stages of subtle clipping to achieve distortion. Op amp designs tend to have a pair of diodes or LED's either in the feedback loop or after the op amp, which ends up being less dynamic but gets more gain. I've also found that transistor pedals are much pickier about the amp than op amp designs, probably because of the more subtle or dynamic distortion which is meant to be a part of the amp's sound. I compare a TS808 to an OCD...both are low- to mid-gain pedals, but the TS808 ends up sounding more compressed but sounds great with almost any amp, whereas the OCD sounds more open but is much more picky about the amp.
     
  16. StompBoxBlues

    StompBoxBlues Member

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    I don't know if anyone went into the "how" of it but...

    Diodes basically saturate after they reach their "turn on" voltage. This voltage can be "biased" to be higher than the nominal .7vdc point...basically they leave the signal alone until it gets past their turn on point, then they go full on...flattening out the signal. They point in two different directions to work on both sides of the AC signal they are clipping. Some designs make one side turn on at a different level for uneven harmonics.
     
  17. n8b

    n8b Member

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    From a design standpoint, Opamps's are more consistent and act predictably in a circuit. Frequency response is more easily controlled when using quality caps, resistors, and Opamp's.

    With Opamps, if you want a mid-band gain of 10, you choose 2 resistors in a ratio of 10:1 (inverting amp) to set the gain. The gain will be 10. In this way, it is very easy to control the amount of clipping of each opamp stage. When combined with clipping diodes, caps and resistors to set the frequency response, it is very easy to control what frequencies get clipped and when they are clipped.

    With transistors, it is much more difficult to achieve consistent performance from device to device unless you test and match them. Transistors are inconsistent and unpredictable. They take testing, time, and a lot more circuit manipulation to implement effectively.

    All of this is crap unless you have a good ear and a lot of time on your hands.
     
  18. kenoflife

    kenoflife Supporting Member

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    Just as I suspected...
     
  19. gaspedals.net

    gaspedals.net Member

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    Ken, Ive always said, you know your vermin!
     
  20. kenoflife

    kenoflife Supporting Member

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    Its because I was born in the year of the Rat...
     

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