From Aron's Forum Posted: Sat Oct 25, 2003 9:12 am Post subject: Green Ringer "Nulling" Mod -------------------------------------------------------------------------------- I wanted to verify what I said in my Impedances post about the Green Ringer - so to try it out I built one up and introduced a 22k pot in series with C2 0.047u ( ... from JD Sleep's schematic) ... well, it works ! Fundamental Nulling is a possibility in the Green Ringer. You can can insert a trimmer and set it at the center of the null range and add a switch to short it out so that you can have both from a switch only. I built my pedal with the pot external so I can turn it with my foot - it can be used to give the octaver effect some animation. You can definitely hear a nulling going on as the pot is turned from short to around 8k (in mine I measured 7.4k at roughly the null center) ... the disappearance of the fundamental, perceived as a drop in volume body, also seems to alter the perceived quality of the octave a little in the process - it is sweeter, more ringy and drier ... the sound seems a little more focused ... I still like the stock sound with its fair bit of fundamental feedthrough so, the pot or switch then can give you both - a Richer or Leaner octave effect ... Here's how my idea works : Most octaver circuits produce their output by adding (mixing) the inverted versions of "harmonically coloured" signals to cancelling out the fundamental (or carrier) and produce a residual harmonic signal that has strong second harmonic content. The Green Ringer produces its harmonic salad by adding the rectified versions of an inverted and non-inverted replica of the input. Because this is tantamount to full-wave rectification the output waveform cannot consist of a pure second alone and so this circuit can be classified as a Dirty-2nd Octaver, as opposed to circuits which are intended to produce a pure 2nd harmonic. Still, the Green Ringer circuit can be tweaked so that the fundamental is nulled out leaving behind only the harmonics - like the circuit is inherently meant to produce. Let's go through the stock Green Ringer circuit. The Input gain stage centered around Q1 acts as a voltage inverter with a gain magnitude of roughly 3v/v ... I used more common values to bias this gain stage in my pedal (R1=470k, R2=120k, R3=15k, R4=4k7) - they preserve the gain and headroom specs to within 10%. The output gain stage based around Q3 is an emitter follower, or voltage buffer, with a voltage gain just below unity. What interests me is the input impedance seen by the circuit at the base of Q3, which is determined by all AC loads in the emitter circuit and the hfe of the device as it sits in the circuit. Assuming the output isn't loaded significantly then the parallel combination of the 10k and 4k7 resistors (ignoring the 0,047u cap) leaves a combination value of around 4k - 4k5. For an average bipolar device with a small-signal current gain of 100 (ballpark) that leaves us with a reflected impedance of 40k to 45k seen looking into the base of Q3. Q2 is wired as a unity-gain heterodyne circuit where the "unloaded" voltage swing at the emitter and collector should be equal in magnitude - that's provided the two 10k loads are well matched. These two signals are passed through equal capacitors and matched diodes to the mixing node at the input of Q3. Notice that the diodes are barely biased "on" by the base current of Q3 - I used 1n60 Germanium diodes and one measured 19.7mV of quiescent voltage across it while the other measured 15.3mV - both barely on. The reason for biasing the diodes, which is what makes this circuit smooth sounding in my opinion, is that it's not up to the signal to supply ALL the current to charge-and-discharge the diode parasitics before it can change states from on to off and to on again. This "trickle bias" makes for faster and more seamless diode turn-on - very smart ! Now, it would seem that if all the components are measured and matched everything should be fine and a clean effect would be produced - but there's one thing that's been overlooked. Namely, the balancing of driving point impedances at the source nodes due to Q2 transistor action. The DPI at the collector looking into it consists of the collector load in parallel with the DPI of the collector itself, which is typically high (100k-1Meg) - so it is dominated by the collector resistor which is 10k-ohm. The DPI at the emitter is dominated by what the base sees devided by the current gain of the device hfe. The base is basically looking at a collector circuit that is dominated by a 15k load and the hfe oif Q2 can be assuned to be 100 (ballpark) - this gives a DPI of 15k/100=150-ohms. So the DPI at the emitter is 150-ohms while at the collector it is very roughly 10k, the DPI at the emitter is much lower and so its signal component will add in a much stronger way than the sigbnal component at the collector. If the mixing doesn't occur in an even manner than the full-wave rectified waveform will have half-cycles of uneven height and a fundamental will register through this pattern. Above I estimated the equivalent mixing node impedance to lie around 45k so it's easy to see that with the 68k bleeders and 10k DPI at the collector source that a DPI of 150ohms doesn't jive - it's way too strong "in relation". If we add resistance (somewhere around 10k) to the emitter circuit then we've made it more compatible with the other drive node, and the mixing circuit impedance levels without changing other aspects of the circuit. Because the Green Ringer produces dirty-Octaving through a process of full-wave rectification it follows from a simple Fourier argument that because of the shape of the FullWave waveform produced a pure second cannot be produced at the output ... so instead, along with a dominant 2nd harmonic other higher harmonic terms will be present at the output. The "Nulling" Mod that I present here allows the tuning out of the fundamental (carrier) so that upper harmonic terms can be heard more on their own. This idea can be adapted to other Octaver units that employ a similar "mixing" principle to achieve production of harmonics. With Octavers I tend to hang around either the 12th fret and play strings against each other there (with lots of dual string vibrato or whammyism) or low strings near the nut with a mutted palm for a keyboard effect - I get a nice strong mutted effect at the bottom of the neck with the Green Ringer so I don't think this octave effect is any less sensitive there or anywhere else ... with the nulling I notice a quality in these two sounds right away - there's more ring bite it seems ... but I agree the "in between" region might sound challenging a little, I think a varied pick attack is the key there - a booster like Charlie said is probably a good idea too ... From talking about this with Mark Hammer this summer we came to the idea that this sensitivity-perception issue in octavers might be related to a "frequency difference" term that is heard when playing up high on the neck but disappears into sub-audio land when playing below a certain point ... It's fairly easy to provide analysis for this in a multiplier based circuit but for a FW-rectifier based octaver I'm not sure how to argue that such a circuit automatically creates spectral side-terms at F+F and F-F like a multiplier circuit does ... the F-F term typically cancels to becomes a residual DC term except when the cancelling is not perfect ... if imperfect cancelling of the difference term occurs then an added low-frequency "component" would be heard as part of the output for notes that are high enough in pitch ... 100% pure conjecture copyright JC Maillet (c) 2003