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Post by ashcatlt on Apr 20, 2016 14:29:25 GMT -5
In case you're into playing around with cable crapacitance, but can't be bothered to keep a variety of lengths around, and have an extra Benjamin laying around, there's now the UnderTone Audio Vari-Cap cable. We could build this in a box for like $15, though of course the actual cables we use will impact the results. I can't watch this video about the "science" right now, maybe it's sound? Course, it's not exactly a new or original idea, just a relatively novel implementation. |
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Post by JohnH on Apr 20, 2016 15:28:52 GMT -5
I've heard of these before, and its an interesting concept. the tech video above only talks about how changing capacitance of the cable interacts with inductance to change the high-end peak, no news there!
But his gadget is more interesting than that. I think it is not just a switched arrangement of capacitors, but has a variable control with a pot, with 15 steps. And it can go down to a very low capacitance level of 150pF, probably less than the natural capacitance of the cable. To me that suggests an active circuit. I thought about this before and this is how I think it could work:
The key point is that at a given frequency, the current flow through a capacitor depends on the signal voltage across it and the value of the cap. Double either, and the current flow doubles. So instead of varrying the physical cap value, if we can vary the voltage across it, then the effect is the same.
So, possible concept:
The signal in is directly connected to the signal out, straight through like any normal cable with no resistance.
The signal is sampled by a high-impedance opamp circuit, who's output gain can vary using the pot from positive x the signal to negative x the signal.
A cap goes from signal to the output of this opamp.
The effective extra capacitance that is added changes: If the opamp output is zero (like its a virtual ground), we get the actual cap value added across the signal.
If the output of the opamp matched 1x the signal, we get no effect from the cap, since it is connected between two points that are at the same voltage.
If the opamp output is higher and positive though, we effectively subtract capacitance. Or, if the opamp has negative output, we add more than the cap value. Variable settings of the pot provide all points in between.
I did test all that with a Spice sim. It seems to work in principle. But I havent built one since, as interesting as it may be, I couldnt see anything particularly useful that could not also be achieved by a very simple buffer circuit on the guitar output, which has other benefits as well as negating cable capacitance.
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Post by ashcatlt on Apr 20, 2016 19:24:00 GMT -5
Yeah, you can get what looks like variable capacitance from a pot and transistor too. That's how the classic wah pedals work. But how can this be active? Where is the power coming from? Like I said, I haven't watched the vids, but unless it's got a battery in there... I'm pretty sure it's just some SMD caps and a switch. Sourcing a 15 position mini switch like that is probably not easy, I suppose, but is it still worth that much? |
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Post by JohnH on Apr 20, 2016 22:13:21 GMT -5
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Post by ashcatlt on Apr 21, 2016 10:24:16 GMT -5
Wait! What? I guess I had in mind a variable capacitance across a relatively short run of cable and then a buffer so that the rest of the cable didn't matter, but this thing says that it doesn't buffer the signal*!?! That means that they are somehow counteracting the natural capacitance of the cable itself. Is that what you were talking about? I might need you to run that by me one more time. *Their nifty little thing with the light bulb really only proves that the thing is DC coupled through. A buffer with a bipolar supply and no coupling caps could light that bulb about as bright as its own power supply would allow. |
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Post by JohnH on Apr 21, 2016 16:52:55 GMT -5
Yes thats it...straight through wire from in to out.
Just as a passive cable capacitance, or extra capacitance connects from input-output to ground, so does the active 'variable capacitor', which unlike a real one, can have a negative value and so negates the real cable capacitance. Will post a diagram..
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Post by sumgai on Apr 21, 2016 21:02:50 GMT -5
Make no mistake, I'm paying attention here!
To put it another way, you had me at "negative capacitance". Oh boy, this I gotta see.  |
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col
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Post by col on Apr 21, 2016 22:33:49 GMT -5
Make no mistake, I'm paying attention here!
To put it another way, you had me at "negative capacitance". Oh boy, this I gotta see. No kidding! Makes me think of exotic particles and (highly) theoretical physics (such as 'negative energy'). I'm pretty sure I won't understand the explanation, but I too am looking forward to it anyway. |
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Post by gumbo on Apr 22, 2016 5:58:07 GMT -5
"Negative Capacitance" sounds like our refrigerator most of the time....
g-f-b
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Post by sumgai on Apr 22, 2016 15:20:27 GMT -5
"Negative Capacitance" sounds like our refrigerator most of the time.... g-f-b
No, you're thinking of "blivet" (or blivit, same idea) - stuffing in more than it can hold. Not the same thing as nullifying a naturally-occurring electrical charge in such a way that you can be said to have effectively created an anti-electrical charge.
When this is all over, I'll present the little man behind the curtain, whom I'm fairly sure ashcatlt, col, yew, b4anj0, perfy and John already suspect is the real culprit here.
Afterwards, I'll really have something to say about uneducated people passing themselves off as Engineers... but I digress.
sumgai
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Post by JohnH on Apr 22, 2016 17:17:24 GMT -5
OK, so Id better try to explain what I was on about! This relates to the effect of a capacitor, in the application where we use it to 'bleed off' treble from an output to ground, such as in a guitar tone control, or, relating to the capacitance of a guitar cable. Consider the following, starting with Case A, which is a passive capacitor, as usual:  Case A, passive capacitor to GroundThe effect that the capacitor has on the output is dependent on the current flowing from the output Vo, through the capacitor, and in this case, to ground. This current is dependent on frequency, capacitance value and the amplitude of voltage across the capacitor, in this case Vo. Case B - introduce an amplifier stage, with zero gain. In this case, an active stage (with a low output impedance) starts to be introduced, but it has no gain, so its output voltage V1 is zero. The cap sees no difference to Case A, and our signal still feels the same effect from the cap. Case C - amp with gain x1In this case, the amp gain is increased to x1. V1 now equals V0. There is now no voltage across the capacitor, no current can flow through it. The effect of the capacitor is zero. In moving continuously from Case B to Case C we have changed the amplitude of the voltage across the cap. Changing the voltage across the cap has the same effect on capacitor current as changing the cap value. The current going from Vo into the cap is the only thing htat affects the output. That point is the key thought here. Case D - gain >1Now it get more interesting. Consider amp gain x2 for example, so V1 = 2V0. Now the voltage across the cap is not 1xVo but -1xVo. Current into the cap is exactly reversed. If this was a guitar circuit in which usually a cap will be bleeding off treble to ground, now it is feeding some extra back in. As a separate freestanding model, the diagram in Case D may be unstable, since it is feeding back to its input with a gain >1. But if this was part of a larger circuit, with an aditional passive capacitance to ground at least just a little more than C, the effects of our electronic 'negative capacitor', added in parallel to the slightly greater real cap, would leave just a very small capacitance effectively remaining. See Spice sim below. Case E - negative amp gainWe can also use this electronic module, with negative gain, to increase the signal voltage across cap C. This increases current through the cap, making it behave in the circuit like a larger cap. SPICE simHere is a sim, set up to track the range from no change to passive capacitance, up to as far as possible negating the capacitance of say, a guitar cable. Pickup and guitar is modelled by the box on the left, and the cable and amp input by the box on the right.  The gain varies, using pot R1, and the null point where overall capacitance is almost completely negated is finely turned by R5, with R1 at max. The resulting sweep of frequency response plots are exactly the same as stepping the range of cap values, from the cable capacitance (represented by c3), down to a very small value. That is how I reckon the 'Zerocap' system must work. |
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Post by ashcatlt on Apr 22, 2016 17:40:32 GMT -5
Holy frijoles that's freaky cool!
Seems to me like we could build this into the front of whatever is the first active stage after the guitar, even at the end of a super long cable, and... An extra knob on your amp to compensate for (or accentuate!) cable capacitance. Add this to a DI. Oh the possibilities!
Thanks John.
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Post by sumgai on Apr 23, 2016 2:05:40 GMT -5
OMG!  O.M.F.G. ..... I am so speechless. I mean, I don't know where to begin, so I think it'd be best if I don't say anything for awhile yet....  |
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Post by gumbo on Apr 23, 2016 3:01:17 GMT -5
OMG! O.M.F.G. ..... I am so speechless. I mean, I don't know where to begin, so I think it'd be best if I don't say anything for awhile yet.... I too, am speechless.... ....but I'll reserve this space in case I think of something later... |
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Post by JohnH on Apr 23, 2016 5:17:24 GMT -5
OMG! O.M.F.G. ..... I am so speechless. I mean, I don't know where to begin, so I think it'd be best if I don't say anything for awhile yet.... I too, am speechless.... ....but I'll reserve this space in case I think of something later... It seems I might have let off a bit of thought grenade here. Or was it a brain fart? Places to use this idea could include various locations where a cap or capacitance is directed with one end towards ground, eg: As discussed, to reduce cable capacitance effects, which could be at either end of the cable A variable tone capacitor inside a guitar changing gain vs frequency characteristics in overdriven amp and distortion circuits |
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Post by ashcatlt on Apr 23, 2016 10:26:14 GMT -5
OMG! O.M.F.G. ..... I am so speechless. I mean, I don't know where to begin, so I think it'd be best if I don't say anything for awhile yet.... If you've got something to say, then say it! It sounds crazy, but John's explanation seems to make sense to me, the Spice model seems to corroborate, and like I said it's very much like the way a Crybaby works, so it seems feasible. Then again, it kind of looks a lot like splitting the signal, high-passing and amplifying one side, and then mixing them back together... |
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Post by col on Apr 23, 2016 15:31:07 GMT -5
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Post by sumgai on Apr 23, 2016 20:54:51 GMT -5
col,
Thanks for that patent reference, I would not have thought to look for such - who would think that a patent could be issued for this thread's stated title topic, that of "negative capacitance"? Let me read that thoroughly before I set out to bring all of the colors back inside of the lines.
Should be back later tonight, but it might be tomorrow, we're having guests over for the evening....
sumgai
p.s. By and large, I'd say that your understanding of most things electronical is more than sufficient for our purposes here in The NutzHouse. 
p.p.s. I'm thinking that b4nj0 must not be coming around lately. As a (fellow) HAM, he's certainly qualified to put the kibosh on much of what's been stated, and to clarify the real story behind that patent. (In case you might be wondering.... if you think we guitarists have a problem with cable capacitance, try transmitting a couple hundred watts at 30MHz over some RG-58A to your 140-feet-away antenna!!) |
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Post by col on Apr 24, 2016 19:55:36 GMT -5
Unless I'm missing something, the patent is different; it is a triaxial cable. Hopefully, John will explain the differences.
I also note that (according the patent holder's claims) a large proportion of cable capacitance occurs within the jack plugs. For this reason, his patent includes a third conductor through most of the length of the plugs.
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Post by JohnH on Apr 24, 2016 20:19:16 GMT -5
Unless I'm missing something, the patent is different; it is a triaxial cable. Hopefully, John will explain the differences. I also note that (according the patent holder's claims) a large proportion of cable capacitance occurs within the jack plugs. For this reason, his patent includes a third conductor through most of the length of the plugs. I agree, I think that sounds like a different though related way of dealing with cable capacitance, in which the shield is driven by a copy of the signal, so there is no capacitive transfer from shield to hot conductor, but the signal ground runs through separately. I'm guessing admittedly. BTW - I'm still sitting in the middle of the tracks, waiting for sum train from the town of Kibosh to come along. What do you think Sumgai? Its all speculative on my part, not having tested these things. But soon the statute of limitations will run out on crimes against electronics that may have been committed on this thread, and a verdict of 'tacit agreement' will be entered. |
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Post by sumgai on Apr 24, 2016 22:23:39 GMT -5
John,
Sorry, but the conductor of my particular train hasn't waved the lantern yet, nor shouted "All Aboard!".
In essence, I've taken a close look at the patent, and yes, it's for a non-standard cable, along with non-standard connectors. Fine, good for him. Too bad he still gets the theory wrong, but if he can float it past the patent examiner, who am I to pee in his Cheerios? Still and all, for guitarists who presumably don't want to spend more than necessary to get the job done, this ain't gonna fly. IMHO, of course.
Reading further, this particular patent application does a good job of quoting prior art. Too bad he missed one of more salient points. Namely, you can drive a shield all you want, but in effect, you've just loaded a dual-conductor cable, which if I correctly remember my days of low-impedance cabling setups for singers on a stage, loading/driving a second conductor ain't particularly necessary. What with the mixer having so much control over the tone, worrying about the miniscule amount of capacitance leakage between the two conductors was (and still is) a waste of time.
Look folks, I'm in the old fogies home when it comes to modern electrical doo-dads. But even so, I still know a thing or two. Such as, in a two pole circuit like John's diagram (and chart) above, the implied capacitance is in parallel with the alleged capacitance-cancelling circuit. Uh uh, no can do. In support of my bald statement, see Kirchhoff's Laws, here. The gist is, current will flow through each and every branch of a parallel circuit, nothing can make that "unhappen" (short of disconnecting that branch entirely). Bottom line: if current is flowing, then various electrical properties are gonna be observable, end of story.
Second, when you short the output of an op-amp to either input terminal, you no longer have anything except possibly a buffer. Can it be designed in such a way as to affect some other parameter, like for instance capacitance? Sure, why not... a lot of esoteric things are possible for a good designer. But the real question should be "why would you?". In this part of the word (audio frequency spectrum), a lot of laymen are paying attention to the damnedest things, all in the name of that Mighty Mojo. As I said a few paragraphs ago, most musicians don't want to spend any more money than necessary to get the job done.
(Now that said (about spending money), I do have to admit that most likely, something on the order of 98% of all the effects pedals out there are redundant, and overkill, and way too expensive for what they deliver. But that's just me, and I'm in the vast minority, so take that for what it's worth - 'nuff said.)
Third, I'm surprised at you John. You just showed us a two-pole diagram, where the lower leg of the capacitor is implied to be going to ground, as it would in the real world of guitar players who are using more-or-less standard cords or cables. Yet here's a circuit with some components that are inserted in between that cap's lower leg and ground... howzthatagain? In a standard cable, the capacitor is implied, due to the dielectric properties of the materials used. If we're gonna install this circuit into a box, how is one supposed to break that implied connection, and insert your various components? 
And now for the real nitty-gritty. It ain't the capacitance, folks. Nothing to it, for one who's actually looked at (and possibly studied) how capacitance works. Long story short, I'm gonna send you all off again on yet another World Wide Wait chase, this time to learn about reactance. This is the property that affects tonal response, and in summary, it explains how a capacitor affects frequency response. But when all the shouting is over and done, you can modify a circuit's tonal response by tweaking the overall reactance value, but you can not directly negate a capacitive electrical charge. Physics is.
Yes, I'm being literal here, but think on our premise - we Nutz are here to help, not to mislead others. The very title of this thread, and the opening post (thankfully espoused by a non-Nut), claim to negate capacitance in a guitar cable. Like Cosby's Noah said to Gawd, "Riiiiiiight." Neither I nor anyone else can negate capacitance directly, we can only use additional components to modify the effects of that capacitance, by changing the reactance value.
But here's a question for ya, and this is gonna count for 100% of your final grade:
If you have to start installing live componentry (drive the shield means that a current is actively applied), then why not just put in a simple low-parts-count buffer, and be done with it?
Holy smokes, Sparky, we can still use our regular (and affordable) guitar cables - what a concept! It's the K.I.S.S. principle all over again!
And I should say something about how the innerwebs has enabled anyone with an IQ greater than a raw carrot to post crap as if they were Gawd's gift to the unwashed masses.... but I'm gonna hold my tongue. In fact, the Conductor just said that I have to get back off of the train now, someone's waiting for me inside the station house. Wonder who that could be..... 
HTH
sumgai
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Post by newey on Apr 24, 2016 22:25:51 GMT -5
Well, the theory here is pretty much beyond my present understanding. But it seems that someone needs to get out the ol' soldering iron and demonstrate such a module. After all, as Reagan said: "Trust, but verify". And, as ChrisK said: "Empirical is". |
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Post by JohnH on Apr 24, 2016 23:32:36 GMT -5
Excellent!
I am now much more convinced that this type of circuit might work! That is, I see all of those good points have proper answers.
However, there is one thing that I do agree with, and since this was to contribute 100% to my final grade, I'll accept the A++. As I said in my first reply on this thread:
Though Ash pointed out an interesting application that perhaps you could incorporate this at the amp end, which would keep guitar and cable as standard.
I may have to just build one just to see what it does, (though myself, I like the sound of a decent 10' cord best). Unfortunately however, for the rest of the afternoon, I promised to work on painting a wardrobe for my wife.
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Post by sumgai on Apr 25, 2016 1:23:22 GMT -5
John,
Here's your Gold Star, but you'll have to paste it on your forehead yourself.
And I should've recalled that you had earlier said the same thing about "why not a buffer?", and given you credit - mea culpa.
I did receive one PM about my being too literal, whereas the same overall end result comes about, regardless of one's terminology. I must rebut that sentiment.
In The NutzHouse, we strive to be correct for the simple reason that we all need to be on the same page. In most cases, what constitutes "correct" is common agreement, and 99.9999% of the time, in comportment with the world's standards. Which leads me to....
As stated in my last post, one can not negate an electrical charge present within a capacitor. However, one can indeed "route around" that electrical charge and negate it's effect. What I did, and most particularly, what I saw in the posted video (and on their website) was an attempt to keep the record straight. When you speak of capacitance in a guitar cord robbing tone, that's technically not correct... it's the reactance at fault. However, we all know what we really mean, so it's just common sense to not upset that particular apple cart, and let the world-and-dog call it by the term they all accept, proper or not.
But when it comes to "doing something about it", then I have to take note, and put things back in order where needed. In this case, most of the guitar playing world will recognize exactly what the guy is trying to do, and by using familiar terms, the video star and the viewer are both on the same page. However, when someone who's reading more material on the matter comes across other terminology, then that engenders confusion. My job: to prevent that confusion when-and wherever possible. Call it my calling, if you please.
Thanks for listening, and have a good 'un. 
sumgai
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Post by perfboardpatcher on Apr 28, 2016 13:32:23 GMT -5
SPICE simHere is a sim, set up to track the range from no change to passive capacitance, up to as far as possible negating the capacitance of say, a guitar cable. Pickup and guitar is modelled by the box on the left, and the cable and amp input by the box on the right. The gain varies, using pot R1, and the null point where overall capacitance is almost completely negated is finely turned by R5, with R1 at max. The resulting sweep of frequency response plots are exactly the same as stepping the range of cap values, from the cable capacitance (represented by c3), down to a very small value. That is how I reckon the 'Zerocap' system must work. My take: The innovation seems to be an extra (middle-) shield in between the core (signal) and outer shield (ground). The middle shield is connected to the opamp's output. C2 is the cable capacity between core and middle shield. C3 is connected incorrectly, the cable capacity between middle and outer shield is located between the opamp's output and ground. |
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Post by JohnH on Apr 28, 2016 15:28:45 GMT -5
SPICE simHere is a sim, set up to track the range from no change to passive capacitance, up to as far as possible negating the capacitance of say, a guitar cable. Pickup and guitar is modelled by the box on the left, and the cable and amp input by the box on the right. The gain varies, using pot R1, and the null point where overall capacitance is almost completely negated is finely turned by R5, with R1 at max. The resulting sweep of frequency response plots are exactly the same as stepping the range of cap values, from the cable capacitance (represented by c3), down to a very small value. That is how I reckon the 'Zerocap' system must work. My take: The innovation seems to be an extra (middle-) shield in between the core (signal) and outer shield (ground). The middle shield is connected to the opamp's output. C2 is the cable capacity between core and middle shield. C3 is connected incorrectly, the cable capacity between middle and outer shield is located between the opamp's output and ground. Thanks for looking at this. The intention in the Spice sim is: The guitar is normal and is represented by everything within the blue box on the left. Out of the guitar comes just a hot connection and a ground, into a standard guitar cable with one core. The capacitance of the guitar cord is the fixed capacitor C3, within the blue box on the right. Next to it is R4 which is the amp input impedance. Everything else is the magic 'black box' that is designed to defy as many laws of physics as possible. Actually, none are violated! With the values in the diagram, the usual capacitances as expected in a guitar circuit are C1 (100pF pickup self capacitance) and C3 (1nF, approximates a long guitar cord). C2, also shown as 1nF, part of the extra electronics, is driven by the opamp. In order to approach a null where the effects of capacitance are negated, the pot R1 is set to '0' per the diagram, and the gain of the opamp is now (100+92) / 92 = 2.09. The voltage across C2 is then 2.09 - 1 =1.09 x the guitar output voltage, but in antiphase with it. At a given instant of time, current flowing from the guitar output into C1 and C3 is very close to that coming out of C2, and they balance with very little net effect on the signal. In practice, the extent to which this balance can be achieved would depend on how closely the guitar and cord match this kind of model, and also the perfection of the components (particularly the opamp) and how accurately they can be set. My hunch is that a system like this might in practice be able to negate say 80% or 90% of the capacitance effects, after which other spurious effects not modelled would start to dominate. Hopefully still useful though, or if not, at least interesting. |
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Post by sumgai on Apr 30, 2016 0:09:55 GMT -5
John,
Belatedly, I realize that you are on a mission. In that case, your idealized circuit can't work, for the reasons stated in my post above.
Quite simply, your circuit will not cause a substantial change in the amount of charge across the "capacitor" between the cable's core and outer conductor - that's a physical constraint. What you can do however, is model the reactance of the cable, and then counter that with your circuit. I'm sure you already have all the data (formulas and such) at hand to enact this kind of model.
HTH
sumgai
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Post by perfboardpatcher on Apr 30, 2016 3:02:38 GMT -5
JohnH, If your circuit works as you explained I see no reason for having a varicap circuit built into what is basically a stereo cable. I'd rather have it built into a stompbox pedal and use my regular guitar cable instead. It appears to me that C2 is in the bootstrap loop, but not C3. The flow of current in C2 can be counteracted but I don't see how to compensate for the existence of C3. More opamp gain will only cause more active signal to be mixed in with the signal from the pickup (across R4||C3). I'm not familiar with spice, would it make a difference for the results in spice if the opamp was replaced with a second voltage source? A diy project that's related to the subject |
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Post by JohnH on Apr 30, 2016 6:11:50 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.
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Post by perfboardpatcher on Apr 30, 2016 13:05:29 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. |
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O.M.F.G. ..... I am so speechless. I mean, I don't know where to begin, so I think it'd be best if I don't say anything for awhile yet.... 
