guitar preamp circuit - theory

Discussion in 'General Electronics Chat' started by fez, Sep 24, 2012.

  1. fez

    Thread Starter Member

    Dec 6, 2009
    47
    1
    Hello. I have a question pertaining to analog circuit THEORY/sense, using a guitar preamp circuits just as an excuse. I will use the MXR MicroAmp schematic found here: http://www.generalguitargadgets.com/index.php/projects/15-boostersrouters/70-mxr-microamp-project. I'm doing things on these lines and plan to continue and evolve on them, however I'd like to know the why's along with that what's as well.

    1. In engineering, compromise is always present. I can see R7 and R8 being used to bias the input to half the DC voltage, but why do they have the values that they do? Why not higher, say 1M? Wouldn't that result in even LESS power consumption? Potential dividers are used a lot so this a very important question. In its crux, the question becomes: in circuits dependent on RATIOS of impedances, what is the drawback to having higher impedances? What is the advantage and disadvantage to having them lower (aside from higher power consumption)?

    2. Most the caps here are coupling or decoupling. But my question is similar to the previous one - why do they have the values that they do? Why not higher, or lower? Also, what purpose does C3 serve?

    3. What purpose does R1 serve? Wouldn't it be better to leave it out? It only ends up loading the guitar (tonal degradation). Also, what is the point to R9 and R10? They, too, only end up loading the opamp (current/power consumption). What is the purpose to R3?
     
  2. liquidair

    Active Member

    Oct 1, 2009
    89
    5
    1. TBH, I'm not 100% on the actual values used there but it likely has to do with filtering the half supply node, thus keeping noise off of it. In general, the larger the resistance values, the larger the amount of noise generated by the resistors.

    2. The cap values are chosen for the desired low frequency response. For instance, C5 sees R9 + R10 || Rin of the next device (which is assumed to be high enough to ignore in a guitar circuit), so the low frequency of the output stage is below 1Hz. C3 is there to make sure the opamp does not amplify DC. C2 is there to limit the op amp's ability to amplify high frequencies.

    3. R1 sets the input impedance of the pedal and does load the guitar, but this is ok. Guitars like high impedances, 1M is pretty much standard in tube amps. You can't leave it out because it is providing a discharge path for C1, although it's value could be lowered for slightly lower noise, but 1M is as low as I'm go. Keep in mind it is in parallel with R2 so it's value appears to be ≈ 7 Meg. Tonal degradation happens when the guitar is loaded too much, i.e. lower than 1 Meg, higher impedances generally sound brighter and snappier. R9 and R10 are also critical, as R9 sets the output impedance to 470 ohm. R10 provides a discharge path for C5 and is therefore a must.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    R1 and R10 set the input and output impedances,

    C3 is to prevent pin 2 from going to 0v, so as to keep the output at half supply (as set by R7/R8). and to be used for the cut off frequency/gain.

    C1 is to block dc, and with R3 also used as the frequency /gain.
     
  4. dataman19

    Member

    Dec 26, 2009
    136
    29
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Keep away from that website because the transistor in the first simple circuit is UPSIDE-DOWN! No wonder its "plenty of" voltage gain is only 4 instead of about 20.
    Also its base bias resistor values are wrong and are too high.

    The second circuit is OK.
     
  6. fez

    Thread Starter Member

    Dec 6, 2009
    47
    1
    Thanks for the replies! I have a problem with the preamp now though - when I attach headphones to the output, I hear a somewhat distorted sound. However, when I remove the output coupling capacitor, replacing it with a short, the sound is better! (Somehow my headphones didn't burst or break at absorbing a DC voltage...). What sorcery is THIS, now?
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The opamp preamp can drive an amplifier input that has an input impedance of 10k ohms or more, not low impedance headphones.
    Headphones are driven from a headphones power amplifier circuit that is fed from this preamp.

    If you remove the output coupling capacitor then the opamp gets hot and the headphones might be destroyed by the resulting DC voltage.
     
  8. dataman19

    Member

    Dec 26, 2009
    136
    29
    AudioGuru...
    Maybe you should actually read the circuit description instead of making a rash statement.
    ...
    Yes the transistor is upside down - the text describes why. It also presents a view that prototype designers overlook.
    ...
    The circuits are "in-fact" working Guitar/Instrument preamps. I use them, and so do a few of "my" clients - who all swear by them.
    ...
    Yes the 2N5088 is an older "low noise" transistor, but it has infinite possibilities. I also only happen to have about 10,000 of them in a bin. So they are in fact an economical choice for me at about $0.09 each.
    ...
    The discussion is about amplifier gain and preserving frequency response and noise figure. The examples in the whole paper outline seven circuits with seven different characteristics, all by orientation and biasing. The idea was given to me by a colleague, and I wrote a paper on the flipping of transistors - earned me a high mark at CalTech.
    ..
    Yes the gain is only about 4 - "That's the point of the discussion"
    ..
    There are hundreds of variations on the web of these exact preamps, and hundreds of production guitar boosters that employ these as well. So if the individual looking for a preamp wants more gain, select a higher gain design - or actually read the description to get a better understanding of how to obtain the gain you want.
    ...
    My point was to point the person to a "working" circuit design and increase his options.
    ...
    Since this is a "non" profit web site and the info is offered at face value. I saw no harm in offering another idea. No harm intended.
    ...
    Dave R. Mason
    Phoenix, AZ
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    The text DOES NOT describe why the transistor is upside-down.
    The text on page 1 says that it is a basic common emitter circuit (the circuit with the upside-down transistor) and it has has plenty of gain but it has hardly any gain.
    The text on page 3 says it is a common collector amplifier. No it is not! A common collector amplifier is an emitter-follower that has no gain, not an upside-down transistor.

    The text says that the second transistor is flipped but it is actually a basic common emitter amplifier with nothing flipped.

    I simulated the circuit with the upside-down transistor. Its performance is awful. It is almost cutoff so it rectifies most electric guitar signals. Don't you call it "fuzz"?
     
  10. dataman19

    Member

    Dec 26, 2009
    136
    29
    AudioGuru - thanks for the critique. I looked at the preamp page - noticed that when it was posted it wasn't configured correctly. The associate that did the posts for me 1) swapped the schematics - so the text didn't match up right) 2) sumarized and totally messed up the text. I take the fall though - it is "my site" - I should have been on top and verified the info.
    ..
    Funny thing is - the site page has been up for over two years and no one caught it????
    ..
    Anyway the Inverse Amp is not intended to be driven by a higher input signal than 2-10mV (I think we were using it for a telemetry sensor that ran at about 14mV). But the "original intent" was and still is a guitar/music application. I noticed that you were driving it with 200mV (which in reality with todays' modern pickups is not that far out of line - I plan for a 45mV to 85mV range in design criteria. I also find many that actually put out as much as 100mV+ initially). The main problem with this inverse design is that it is easy to drive into clipping with a high input signal level. Also the circuit has been around for some time, and "new" guitar pickups are a lot stronger now than in the 1970's/1960's.
    ..
    We originally used this design in a guitar fuzzbox circuit. Then we used it in a low level line buffer application. Since it isn't a high gain circuit, and most high signal sources don't need it - we never though of using it as a regulated preamp (other than as already stated).
    ..
    Incidentally, the actual frequency response is better than good (if not over driven). The preamp I like is the high gain common emitter with the 17X gain factor. Still, this is a basic circuit but has real world uses without a lot of modifications needed.
    ...
    Incidentally - I like your signal analysis pics - may I post them with the inverse amp circuit on the web page? The two analysis show exactly what the original caution note that was left out of the page(who knows why?).
    ..
    Thanks for the heads up - I seldom get back to reread the numerous content on all of m websites - the info ones are especially neglected - "My bad". Once I am finished with this African project I plan to spend a few weeks reading through all the content to make sure the research fellows have all done their tasks correctly.
    ..
    Dave
    Phoenix, AZ
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A preamp for a magnetic guitar pickup usually uses a high input impedance vacuum tube, Jfet or opamp with Jfet inputs like shown on the first post of this thread. The high input impedance allows the inductance of the pickup coil to resonate at about 4kHz with the capacitance of the cable giving a "twang" to the sound.

    Here is a graph of the frequency response of a typical magnetic guitar pickup with various amounts of loading impedance. Also here is a typical preamp for an electric guitar.
     
  12. dataman19

    Member

    Dec 26, 2009
    136
    29
    I like your FET Preamp. But there are trade offs, FET preamps can be popped by a lazy guitarist plugging in things and unplugging things. I once met a rocker (who will remain nameless because of what I called stupid brilliance),,, anyway, he used phantom wiring to power a preamp in his acoustic guitar. In an upstate New York Rock fest he was nearly electrocuted when the clouds opened up during his on-stage performance. the crowd loved it (thought all the jerking, etc was part of the act). I have seen stranger incidents over the years, a lot of professionals have made some really dumb mistakes (even me - I had a few real doozies - but prefer to keep them in the past).
    ...
    The original idea of the project a few years back was to get a series of "transistor" preamps that would provide building blocks so that non technical hobby guitarists could play around and do things for themselves.
    ..
    We got bogged down in lengthy discussions about FET preamps, but no one liked the sensitivity to voltage spikes. And absolutely no one wanted to include additional protection. But FETs have come a long way since the 70's. I see that they are less prone to failure as their earlier cousins.
    ..
    When it came to circuit isolation ideas, every one has a favorite, and not all work at a universal level (the single cap always worked for me - and since it was so elemental and basic, it was only mentioned lightly).
    ...
    In the 80's there were more books than web pages - so a lot of info was not readily available on BBS boards and early Internet Boards. (yes I am dating myself by mentioning BBS's - but hey, I go all the way back to ARPA and NSFIS Networks - the "original Internet networks".
    ...
    Since I am going to have to redo the entire Preamps section we might as well add a few FET preamps. Like I said earlier, the web page isn't a cure all, it is simply a suggestion. That's why we included the transposed transistor/flipped transistor circuit. Besides in my early days fuzz was in.....
    ..
    I have disabled page two and three (mainly because they were never completed and were sitting there with cloned page one info - not good.). There were ten more preamp ideas that were supposed to be incorporated, oh well, spilt milk.
    ...
    Once we initially posted the site, we found out that some of the info was not released - and had to chop the site up to keep the legal beagles happy. (a lesson learned about academics and research release paperwork....).
    ..
    We had an excellent Tube Guitar preamp, but the McIntosh family claimed in infringed on one of "their" tube preamp designs. It went away like a lot of other stuff. But in defense, the undergrad who designed the tube guitar preamp actually went to work for McIntosh Audio Labs (which explained how it wound up infringing upon "their" tube preamp design).
    ...
    Ten years + and we are still fighting some of the legal battles.
    ..
    Still, What doesn't kill ya will make ya stronger (or at least a little wiser).
    ..
    Dave
    Phoenix, AZ
     
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