Calculating RC time constants...and why you should.

Info for maintaining and tweaking your amp to perfection.

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Xtrchessreal
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Post by Xtrchessreal » Tue May 30, 2006 4:57 am

Rather good information. A good example of the so called sub harmonics is the end of "Working Man" By Rush. Alex Lifeson does that weird sounding bend-up sequence that gains speed in its repetition but has a strange sub harmonic that actually sounds like it is bending down ward.

Physically the bending note is ascending away from the other but also the overlapping harmonics are both cancelling and amplifying each other at what you could call nodes. The effect to the ear is a percussive type sound that resembles glass bottles being pressed together only in reverse if that makes sense. (Such that the bottles are at rest initially and then vibrate away from each other.)

Some of that was enhanced in the studio mix down but in general you would not hear that if you filtered everything below your low E.

I know its an old post but I just read it.

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Post by MattPete » Mon Apr 23, 2007 7:39 pm

flemingmras wrote:And that's due to the harmonics below the string's fundamental frequency. But NO...those aren't important remember? :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:
There are no harmonics below the fundamental. No matter how hard I bash or whisper sweet nothings to the low E string, it will never produce a 40Hz frequency when in standard tuning.

Xtrchessreal
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Post by Xtrchessreal » Thu Apr 26, 2007 11:18 pm

MattPete:

Hey now, did you read any of this stuff or are you just trying to cause trouble?

There is the tone Hz of the note, the tone of the string type and material, wound, smooth,...tone of the fret as you do a tremolo or vibrato, the tone of the wood in the neck, the glue, the screws,...etc. Then there is the tonal conflicts these each make as they impede the Hz note you plucked. You can have a fundamental 80 Hz all you want but the note you hear is all of the above mixed together and does not look anything like a pure tone.

A fundamental can be halved or doubled, thirded or tripled, etc. it can have many irrational divisions as well. Your statement about whispering to your E is not right and in general - false.

X

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Post by MattPete » Sun Apr 29, 2007 6:37 pm

Xtrchessreal wrote:MattPete:

Hey now, did you read any of this stuff or are you just trying to cause trouble?
Not trying to cause trouble at all. But subharmonics would seem to go against FFT analysis and all that I had learned about music and vibrations. I tried wrapping my head around the idea of subharmonics, and I just can't imagine a string vibrating at a period longer than it's length. Sure, there are harmonics, and it's the harmonics that make each instrument sound unique, but I couldn't figure out how interference and cancellation by these higher-frequency harmonics could lead to a harmonic below the fundamental frequency. But maybe my knowledge is lacking.

Thinking I might have missed something, I did a google and wiki search. The only things I came up with were (1) a violinist that had managed to make subharmonics using a unique bowing and fingering technique, and (2) subharmonic distortion. As far as subharmonic distortion goes, the only references I could find discussed subharmonic distortions occuring at the loud speaker. I later found a reference to (3) "synthetic bass" occurring in single-ended power amps (Radiotron handbook, p 616, 676), which is due to a strong 2nd harmonic, but the impression I got from this discussion is that it is a perceptual error similar to the missing-fundamental.

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Post by parkhead » Thu Nov 22, 2007 11:15 pm

MattPete wrote:
flemingmras wrote:And that's due to the harmonics below the string's fundamental frequency. But NO...those aren't important remember? :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:
There are no harmonics below the fundamental. No matter how hard I bash or whisper sweet nothings to the low E string, it will never produce a 40Hz frequency when in standard tuning.
sorry to bring up an old thread:

take your guitar plug it into an amp turn it up loud. Don't play simply mute all of the strings ... then handle the guitar. All of those noises you are hearing are subharmonics and part of the character of the guitar and amp that you hear along with the note you chose to play.

Hendrix at monterey ... at the beginning of wild thing he shows the world just how much music he can find in the subharmonics of a strat
the subharmonics are not part of the tuned note of the string

they are the note existing withing the guitars wood under string tension

like imperfect reflections on water the wood & string reflect images of the played note back and forth creating the thing we call tone ... without the wood a guitar string would vibrate like a sine wave

in addition to all of this going on, the actual act of picking a note can have a frequency as low as 1 cycle per second, but most people practice at higher metronome settings! Displacing the string with the pick produces a sound well below the lowest available "tuned" sound of the string this is still a huge part of the "note" and the part a guitar synth has the most trouble "tracking" because the information is so low frequency and complex

p

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Post by MattPete » Fri Nov 23, 2007 11:08 am

parkhead wrote:... the subharmonics of a strat
the subharmonics are not part of the tuned note of the string p
by definition, harmonics are relative to a note. What you are refering to are not subharmonics of the played strings, and if they exist, would usually be discordant with the played notes.

http://www.till.com/articles/PickupResponse/index.html

I suppose that you could have a guitar body that resonates at, for example, 50Hz, so that if you tapped it, it would naturally vibrate at 50hz. But, that wouldn't be a subharmonic (it would be the 1st harmonic of the body). I guess it would act kinda like a drone note, but since the pickups pickup the vibrations of the strings and not the body, I doubt it would be very audible. I supposed that if the body is vibrating, the pickup will be vibrating along with it and moving relative to the strings, but I would imagine it would be a very subtle effect.

You can also have missing fundamentals, but by definition, the fundamental is physically missing. Any fundamental that is perceived is a psychophysical illusion. So, you can play the open low-E and A strings together, and they may imply an A an octave lower, but that's an illusion.

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Post by Billy Batz » Fri Nov 23, 2007 4:36 pm

There must be something going on. When you boost a graphuic EQ slider at 100Hz you can hear the difference in the 'low end' of the highest D note on a guitar and thats well below that notes fundamental. CHanging a bypass cap from 330u to 50u is audible in the highest notes and thats well below their fundamental. So theres something your missing in the explanations.

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Post by MattPete » Fri Nov 23, 2007 4:43 pm

Billy Batz wrote:There must be something going on. When you boost a graphuic EQ slider at 100Hz you can hear the difference in the 'low end' of the highest D note on a guitar and thats well below that notes fundamental. CHanging a bypass cap from 330u to 50u is audible in the highest notes and thats well below their fundamental. So theres something your missing in the explanations.
You're assuming that the 100hz slider doesn't affect frequencies above 100Hz. Depending on it's design, there might be quite a bit of slop and overlap with other frequencies.

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Post by Billy Batz » Fri Nov 23, 2007 4:49 pm

True but that must be a huge slope for a graphic- that D is around 2.349kHz- and that still doesnt address the second part.

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Post by toner » Fri Nov 23, 2007 7:36 pm

more geek speak:

I think that comparing harmonics to frequency response has caused some of the debate here. They're not the same thing.

The fundamental note alone does not determine the lowest perceivable frequency. It obviously affects the amplitude of various frequencies but thinking of a note's fundamental (or even "sub-harmonics") as a high-pass point is not accurate. Frequencies well below a musical instrument's lowest fundamental exist and can be manipulated.

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Post by AJW » Wed Apr 16, 2008 3:40 pm

I just found this online R-C filter calculator that seems to work accurately: http://www.muzique.com/schem/filter.htm

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Re: Calculating RC time constants...and why you should.

Post by roberto » Fri May 16, 2008 8:07 am

VelvetGeorge wrote: For your typical .68/2.7k combination the formula is this:

F= 1
------------------------------- = 87Hz
6.28 * .00000068 * 2,700

Meaning that every frequency above 88Hz is at full gain for that stage.
This is not completely correct. As you can see here: http://aikenamps.com/RC-FHz.htm
the frequency rolloff is higher than calculated as above. This is due to the fact that the real resistance to ground is dependant on the anode resistor.
Keep a typical 12ax7 for exemple: Ra=62500 and mu=100, and Rp= anode resistor = 100k
The real resistance to ground will be the parallel of the physical resistor (ie 2k7) with the internal resistance, calculated as:

Rk'= (Rp+Ra) / (mu+1)= 1k61

So 2k7//1k6 is 1k, and then 2k7//680n with 100k Rp has a cutoff frequency of 232Hz.
As you can see different tubes have different mu and Ra (that depends also on voltage), and this can change the cutoff frequency of the amp (ie more mu means more gain and more midrangey)
MattPete wrote:There are no harmonics below the fundamental. No matter how hard I bash or whisper sweet nothings to the low E string, it will never produce a 40Hz frequency when in standard tuning.
So how can you explain that 330u on plexis sounds different than 100u?
4u7 should be enought to obtain full guitar bypass, but the amp sounds WAY different. Fourier was wrong? NO, but an amp is an interaction of purely electronic with some physical elements as speakers, strings, and so on, and this produce some harmonics below the fundamental.

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Post by dewey342 » Tue Jun 03, 2008 7:40 am

Love this sight-Thanks George!!! :D

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Re: Calculating RC time constants...and why you should.

Post by jerrydyer » Fri Dec 19, 2008 12:44 pm

what about 330k 330pf what does that start rolling ogg at? Sorry only took one year of math.
tanx
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Re: Calculating RC time constants...and why you should.

Post by Roe » Sat Dec 20, 2008 10:20 am

jerrydyer wrote:what about 330k 330pf what does that start rolling ogg at? Sorry only took one year of math.
tanx
1462.2hz possibly, depends on the circuit obviously
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