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Post by JohnH on Apr 30, 2016 15:29:17 GMT -5
Short of building one, there is really nothing more that I can think of to explain and demonstrate how I think this circuit would operate. I think it would work. Sumgai disagrees. That's all cool with me. I am curious enough about it that I probably will build one, though it is not top of my list. If I do, I will post the results either way. I've built it. I've used a 25k potmeter and a 12k resistor and a 1u cap to ground (asymmetric 12V power supply so there are some extra components to bias DC). Past the 12k resistance of the potmeter I get oscillation. Perhaps gain = 2 is the maximum. The sound gets brighter when increasing the gain but it doesn't sound as a shift of the resonance peak but more like boosting the ultra highs. I'm listening clean into a headphone amp. Any suggestions? And what did spice say about oscillation? I will increase the 12k resistor for a less nerve-wrecking experience. Cool! That sounds like it could be doing what it's expected to do (by me anyway). I'm not surprised by the instability. Once the gain gets too high there is positive feedback. On the sim, the max gain is about 2 but how far it needs to go to approach full cancellation depends on the ratio of C2 (the real capacitor) to C3 (the capacitance that is being compensated for). Ideally there would be a preset incorporated around R1 and R5 to set the max gain that is still useful, just before where the instability starts. It would be the ultra highs that are being affected, and the difference in sound would be similar to that between the basic cable, and if you plugged in through a very short cable such as a short patch cord. Audibly, you may not hear a peak as such, just less of the usual drop-off of highs that guitars have due to many electrical and non-electrical effects. There is another version of this circuit in the form of a tone control. You might like to try it. Everything is the same except both C2 and C3 are real capacitors, of size suitable for tone controls, say both the same 0.01uF or 0.022uF. at gain =1, you'd hear the tonal effect of the full value of C3, then as the gain advances towards 2, you hear the effect of smaller capacitance as C2 is driven in reverse to cancel the effects of C3. As a test rig, that version may be a clearer demo of what is happening. Thanks for trying it.
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Post by perfboardpatcher on May 15, 2016 3:04:55 GMT -5
After a lot more testing I have to admit that I didn't manage to get it to work the way I wanted it to. The potmeter works well for shifting the 2nd order low pass but the height of the resonance peak isn't constant. The graph plot produced by SSpice is definitely incorrect. My observation is that initially when turning up the potmeter the resonance peak decreases but at higher frequencies the resonance peak increases way above what it was at the very beginning. Strangely enough when the thing starts to oscillate it's not at that high frequency but at a lower frequency where there wasn't much of a resonance peak.
What would work is to use the potmeter as trimpot and have pairs of C2/C3 in a fixed ratio, say 4:1. If for instance C3 is 1nF then C3 + guitar cable capacitance is a bit more constant than when relying on cable capacitance alone. C2 can be 470pF. I managed to shift the resonance peak 2 octaves up.
Another idea for a circuit would be to load the guitar's output with a variable inductor and compensate the signal loss with make-up gain.
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Post by ashcatlt on May 15, 2016 11:05:12 GMT -5
There is another version of this circuit in the form of a tone control. You might like to try it. Everything is the same except both C2 and C3 are real capacitors, of size suitable for tone controls, say both the same 0.01uF or 0.022uF. at gain =1, you'd hear the tonal effect of the full value of C3, then as the gain advances towards 2, you hear the effect of smaller capacitance as C2 is driven in reverse to cancel the effects of C3. As a test rig, that version may be a clearer demo of what is happening. Sounds like this could actually do what tonefiend's experimental tone control tries to fake. Course, that one is all passive... Edit - BTW, can we please not be one of those boards where people quote entire long posts full of pictures and stuff several times in the same thread so that we have to scroll through a whole page of the same thing over and over again? I hate that! The Pictures thread over on DIYStompboxes is just stupid with it, but I always thought we were more polite than that. Speaking of DIYSB, I think I'm going to see what people over there think about this. There are some real big E engineers over there, and a whole lot of tinkerers, and if it works, or even comes close, this could be kind of huge. Edit again - I did it. Very interested to see where that goes.
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Post by reTrEaD on May 15, 2016 12:04:20 GMT -5
BTW, can we please not be one of those boards where people quote entire long posts full of pictures and stuff several times in the same thread so that we have to scroll through a whole page of the same thing over and over again? I hate that! The Pictures thread over on DIYStompboxes is just stupid with it, but I always thought we were more polite than that. I'm not sure that actually indicates a lack of politeness. Maybe just a lack of savvy? But I do agree that stripping out the clutter would improve the way a thread reads. In a few cases, quoting a diagram does make sense. Any references made to what happens in a particular part of the diagram or necessary changes might be easier to understand with the diagram in the same post. But in general, quotes of pictures and long posts leads to unnecessary clutter. Especially when one quotes a post that already has a quote within it. Yikes.
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Post by reTrEaD on May 15, 2016 13:27:44 GMT -5
What would work is to use the potmeter as trimpot and have pairs of C2/C3 in a fixed ratio, say 4:1. If for instance C3 is 1nF then C3 + guitar cable capacitance is a bit more constant than when relying on cable capacitance alone. C2 can be 470pF. Will that accomplish anything useful? In my view, the object of the game here is to mitigate the effect of cable capacitance. And the beauty is, we can insert this anywhere outside the guitar, even as late in the chain as immediately before the amplifier (or the first pedal if you use pedals) BUT, if we're intending to use this as a tone control, it would seem best to place it before the guitar's volume control. I took John's diagram and twisted it about, hoping to create an easier way to grasp the concept. The box on the left represents the guitar pickup and volume control. The box on the right represents the cable capacitance and load. When the guitar volume control is at maximum, the cable capacitance is essentially in parallel with the internal capacitance of the pickup. This changes (lowers) the frequency of the resonant peak. But when the guitar's volume control is rotated counter-clockwise, things look very different. The series resistance provided by the upper section of the volume pot and the cable capacitance (C3) form a high-cut filter. The capacitive reactance of 1nF at 10kHz is roughly 16k ohms. It doesn't take much series resistance provided by the volume control to create a strong filtering effect. The 'point', in my humble opinion, of this application of the circuit is to provide the current necessary to charge C3 (the cable capacitance) through C2 rather than through the series resistance of the volume control or the internal resistance and inductance of the pickup. We're 'negating' the effect of the cable capacitance. Doing so makes the volume control act only as an attenuator, without the high-cut filter effect. If C2 = C3 then the gain required to negate the effect of C3 is exactly 2. in this case, the voltage across C2 = (V Out X 2) - V Out = V Out Since the voltage drop across both capacitors is equal and the capacitive reactance is equal, the current at any given time is equal. If C2 > C3 then the gain required to negate the effect of C3 will be less than 2 but greater than 1. If C2 < C3, the gain necessary to negate the effect of C3 will be greater than 2. I'm not certain but I believe the gain necessary to negate the effect of C3 would be: C3/C2 + 1 --------------------------------------- Use as a tone control, before the guitar volume control: C2 could be rather large. The gain would be variable from 0 to C1/C2 + 1
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Post by JohnH on May 15, 2016 15:23:31 GMT -5
Hi ReT, Ash and PBP, thanks for giving this another push, all really good input. Lets see what the DIY SB's have to say.
This circuit is clearly very dependent on exactly how its set, and not to go too close to the abyss. So in a practical application, there has to be a preset to keep it on the stable side of it's range.
I reckon as a tone control, it would be quite predictable since it would be working with two known larger capacitors, and the unknowns such as cable capacitance and pickup winding capacitance are less significant.
In the cable capacitance negation regime, it would be important to set for the specific cable, and I think this is how the 'Zerocap' product works ie the circuitry is integral to the specific cable in order to create a stable and reliable product. As experimenters, we can devise more versatile settings, and the expense of occasionally deafening perfboardpatcher when it goes outside of the range.
The pickup capacitance is a part of the total that is likely to be contrary relative to the sim that I made. That simple three part pickup model with R, L and C is useful in many applications, but the truth is the pickup is far more complex than that and any capacitance is distributed, and so not directly in parallel with the cable even at max volume. So that may limit the extent that total negation can be achieved, but should still allow a significant part of the actual cable capacitance to be mitigated.
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Post by reTrEaD on May 16, 2016 0:15:08 GMT -5
but the truth is the pickup is far more complex than that and any capacitance is distributed, and so not directly in parallel with the cable even at max volume. That, and the fact that in most cases the pickup capacitance doesn't stay the same. If we switch to a different pickup, put pickups in parallel or series, or split a HB, the capacitance we might try to negate isn't the same value. I reckon as a tone control, it would be quite predictable since it would be working with two known larger capacitors, and the unknowns such as cable capacitance and pickup winding capacitance are less significant. I can see two different methods to achieve the same goal. One would be to have a single large capacitor for C2 and omit C3. The gain would be varied from 0 to 1. A gain of zero would put the full capacitance in the circuit. As we approach a gain of 1, the effective capacitance becomes smaller until finally when the gain is equal to 1, the capacitance becomes nil. The other method would be to use both C2 and install the same size capacitor for C3. Vary the gain from 1 to 2. If one were to go to this length to have a 'variable capacitor' as a tone control, it would probably make sense to use a buffer after the volume control. That would render the cable capacitance insignificant and the volume control wouldn't suffer from treble loss at reduced volume settings. Since we already need a power source for the opamp, we'll use that to power the buffer.
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