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Post by JohnH on Mar 27, 2022 14:37:13 GMT -5
I've done some pot scraping. I found It's nice and controlled if a meter is kept clipped across the ends, reading resistance. Then, you can watch the ohms increase scrape by scrape. If you want a log taper, I'd work on the low half first, adding however much needed there, then the upper half to arrive at the final total.
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swirling
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Post by swirling on Mar 27, 2022 21:01:31 GMT -5
Remember, ALL guitar pickups are active. If you want to add a filter to a guitar, you have to buffer the generator coil first. Uh ... not really. "Active" is a term reserved for devices that require an external source of power to control or modify signals. (Transistors,tubes, integrated circuits, etc). It's a bridge too far to view a guitar pickup (transducer) as an active device. Passive pickups are reactive, meaning the internal impedance is more complex than simply resistance. It also has inductive and capacitive components. Also, it's not strictly necessary to buffer the signal from a passive pickup to add a filter. However, since passive filters are lossy, we need to consider the insertion loss and/or how the passive filter will work in concert with internal impedance of the pickup. I'm sorry. I'm not familiar with the local vernacular. When I was in school, voltage and amperage sources were considered active, resistors and capacitors were passive. |
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Post by reTrEaD on Mar 28, 2022 0:49:41 GMT -5
I'm sorry. I'm not familiar with the local vernacular. When I was in school, voltage and amperage sources were considered active, resistors and capacitors were passive. It isn't a matter of local vernacular, although admittedly there is some disagreement among academics as to whether voltage and current sources should be characterized as active or passive. But even if we consider a voltage source (and at the core, an electromagnetic pickup can be modeled, starting with an AC voltage source), a passive electromagnetic pickup looks nothing like an ideal voltage source. An ideal voltage source supplies any current necessary to maintain the specified voltage across its terminal. Such a source would require no buffering if a filter was connected to it. The problem with passive pickups is not the voltage source, it is the high and complex impedance presented by the internal resistance, inductance and capacitance. Active (this isn't local vernacular, the term is recognized world-wide) guitar pickups use active devices (BJTs, FETs, or ICs) and a DC voltage source (battery) to amplify or buffer the signal. Their output impedance is orders of magnitude lower than typical passive pickups and that impedance is simple. Primarily resistive. |
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swirling
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Post by swirling on Mar 28, 2022 23:00:02 GMT -5
Again, I'm sorry. I misspoke - a guitar pickup is an alternator, not a generator.
Why is this important? Because if you think a pickup is passive, you'll expect to see a mere -6dB rolloff from the insertion of a capacitor.
I just think it helps to realise that a pickup does produce electricity and the amount is dependent on the load at any given frequency.
In other words, yes, the "complex impedance presented by the internal resistance, inductance and capacitance" creates a load-dependent resonance that creates the tone of the pickup. Our volume and tone pots create this load, and thus give us control over the resonance and we model the amp input as 1 Meg (or 47 something if we like Fenders).
If you don't plug a guitar in, can you still read a voltage across the volume pot?
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Post by unreg on Mar 29, 2022 0:33:56 GMT -5
If you don't plug a guitar in, can you still read a voltage across the volume pot? Guitar pickups put out tens to hundreds of milliVolts AC. 1,000 milliVolts equals 1 Volt. See tag line below!  I read elsewhere that passive guitar circuits output a (1) volt max. 🙂 EDIT: Yes, but you have to make sure that you connect EVERYTHING wrong. ;D ;D  No, a passive electric guitar develops 1 volt AC maximum. The guitar in and of itself won't shock anything (well, maybe a little if you stick the other end of the cord in your mouth, play real hard, and do the "9 volt battery tongue test"). Oh, come on, every kid tested 9 volt batteries this way. |
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Post by reTrEaD on Mar 29, 2022 3:28:15 GMT -5
Again, I'm sorry. I misspoke - a guitar pickup is an alternator, not a generator. I have no idea what you're trying to say here. Feel free to explain the difference between an alternator and a generator and how that applies to this discussion.Why is this important? Because if you think a pickup is passive, you'll expect to see a mere -6dB rolloff from the insertion of a capacitor. Maybe that's what YOU would expect, but I would NOT. As I clearly stated in a previous post, the internal impedance of a pickup is more complex than a simple resistor. An electromagnetic pickup already has internal series resistance and inductance, and has capacitance in parallel with the output. Adding a very small capacitor in parallel with the pickup will decrease the amplitude of the pickups resonant peak, broaden it, and shift the resonant frequency downward in frequency. The rolloff above the resonant peak is already present. I just think it helps to realise that a pickup does produce electricity We've already established this. What part of "an electromagnetic pickup can be modeled, starting with an AC voltage source" did you not understand? and the amount is dependent on the load at any given frequency. The amplitude (voltage) of the signal produced by the pickup is dependent on the amplitude of the movement of the string in the magnetic field, the intensity of the magnetic field, the size and number of turns in the coil, etc. The attenuation of this voltage and the effect on frequency response is not only dependent on the load. The internal impedance of the pickup is important. I can't even begin to stress how important this is. We can indeed add filters but if the impedance of the filters are too low, they will adversely affect the amplitude of the signal and the frequency response because of the pickup's internal impedance. If the impedance of the filter is too high, the cumulative load presented by the tone pot, volume pot, and amplifier input will adversely affect the amplitude of the signal. Unfortunately there isn't really a goldilocks zone where complicated filters don't present significant losses. That's why the tone circuits Yogi and I are working on in this thread, take advantage of losses already created by rotating the volume control counter-clockwise. If you don't plug a guitar in, can you still read a voltage across the volume pot? Of course you can. Why would you even ask? Is this some snarky setup to insist the we must call passive pickups "active"? You can tilt at windmills if you so choose, but I won't be participating. |
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Post by Deleted on Mar 29, 2022 3:46:11 GMT -5
My days of doing electronics at university inductor is passive.
How I see a pickup.
It is strange feeling having it as a energy source (but only when a string is hit)
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swirling
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Post by swirling on Mar 29, 2022 11:43:04 GMT -5
My days of doing electronics at university inductor is passive. How I see a pickup. It is strange feeling having it as a energy source (but only when a string is hit) Yeh, see, I was building dulcimers and putting pickups in them long before that. What I had to compare them to was loudspeakers. I was also building those. This was before Thiele et al, so we didn't have elegant equations. A resonance was like a slope where you can pickup speed on your bicycle. It was like free energy. It never, ever, occurred to me that a guitar pickup should be flat.  Then again, in those days most transducers weren't flat at all. Getting a few octaves out of a loudspeaker was a major accomplishment. Now, I look at an FFT sweep, and it's obvious they were trying to make pickups flat with the volume and tone controls set to '5'. They came pretty close by ear, I'd say. Anyway, you say it's a strange feeling to think of a pickup as more than an inductor, but it is, just like a microphone. It just uses inductance rather than leverage. I could write a white paper all technically correct and notated, but I prefer to speak in slang just to arouse those 'strange feelings'. Sort of in line with Asimov's "that's funny....' In this case, I was hoping to give frets a springboard to understanding "load" as it related to pickups. |
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Post by Deleted on Mar 29, 2022 12:17:58 GMT -5
I know I'm getting I trouble for this I'm not in to a beating chest completion ok kiddo I know I ignore people who I feel are too simple so don't get upset |
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swirling
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Post by swirling on Mar 29, 2022 13:39:40 GMT -5
I know I'm getting I trouble for this I'm not in to a beating chest completion ok kiddo I know I ignore people who I feel are too simple so don't get upset Cute trick, putting words in my mouth. JohnH, I hope you have fun with your new friends. |
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