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Re: OT impedance

6/30/2004 2:57 AM
SpeedRacer Re: OT impedance
Sorry if I'm confusing things!  
I hope that graph on the site helps.. it helped me a ton. (I'm a visual learner..)  
re: testing/experimenting - FWIW I am completely from this school. Couple reasons:  
1 - There is, IMHO, no good math to tell you what will 'sound good'. Math will make it work and keep it from humming or bursting into flames, but what cap values and voltages will take you to sonic nirvana? Who knows.  
2 - you're building a musical instrument, not a lab reference amp (which has a far more narrow design scope) A MI amp has to "feel" right, has to have the right dynamics and tone and response.. how can you possible define these things in an equation or refine them down to a cap value or bias setting..  
3 - you know what you like. (don't discount this!)  
4 - The best circuits out there were hit on by experimentation and (I think) accident. There is a lot left to discover IMHO.
7/1/2004 9:36 PM
Steve Rathmann
I spent a bit of time looking over the graph and your article last night.  
I would indeed like to be able to do enough of the math to be able to make it work flame free! What I feel the math provides is a start point narrowing things down to sensible parameters. It also shows the relationship of variables in the system. This can help in understanding how changes to one aspect will affect everything else.  
The graph definitely makes the flat listings of "typical operating" values seem a lot less pedantic. The visual aspect helps a lot especially in illustrating the affect of load. However it did raise a couple of other questions if you don't mind...  
Would it then be possible to use a load line to determine the operating point? My thinking is that I would be able to take any voltage chart and draw a line from a specific point (for max current)on the Eg=0 curve to the Eg=max point at my plate voltage and then pick the midpoint to determine my bias point. I could then also take the reciprocal of the slope for the line i drew to determine the load I would want.  
What I'm probably missing from my scenario is the notion of max dissipation. How would I determine this if I had a voltage chart that didn't display the dissipation curve?  
So then if I am going to run push-pull I can move the bias point further down the load line to allow greater voltage swing of the grid since approaching cutoff of one tube will result in increased current in the other.  
Looking at this also makes me realize from a strictly engineering standpoint that it seems almost silly to run things class A if running push-pull.  
Finally, when setting up a cathode biased output stage, does the voltage at the cathode directly represent ths bias voltage as seen in this chart? Or would I basically set the operating point by determining the idle *current* I want then figure the current through the resistor via ohm's law. These things seem to be the same to me, but I'm sure there's probably something I'm missing.  
Thanks again for your comments and help.  
6/30/2004 12:07 AM
Steve Rathmann
What are tonal differences between even and odd order harmonics?  
If I apply the notion of harmonics based on music theory the "even" ones are all octaves of the fundamental and can add brightness without producing harshness and the "odd" ones can add complexity, but as you increase higher order odd harmonics things can become harsh (approaching square wave clipping). Is this more or less correct?  
Which ones tend to get cancelled in a push pull arrangement?  
6/30/2004 2:48 AM
Diff betw even and odd harmonics is much like you state: evens are considered 'musically related pitches" whereas odds can do some nasty things to your output signal. In THEORY, evens will cancel in a balanced push-pull output section. There is lots of cool math to prove that out.. in reality no output section is so well balanced - certainly not when being overdriven like crazy.. Loads are not resistive, signals are not constant single pitches at constant levels and tubes are not matched for dynamic transconductance (they are matched for idle current in hopes that the rest of the operational characteristic will be 'close enough'). The real world is too hard to model, so we use lots of fudge factors so we can at least write something in the text book! ;)  
In the general case, one of the things we're all supposed to like about tube amps is that the vast majority of their harmonic distortions are lower order (2nd, 3rd, 4th) and so closer to the musical pitch and less objectionable to the ear.  
All that said, it appears from various bits of research that some combinations of even and odd order harmonic distortion is desireable - all evens is not particularly nice, nor is all odd (you can build circuits to generate all even HD etc) In a nutshell, 2nd Harmonic distortion is flattening of the trough of your output waveform.. you could run a single diode to ground, or bias a single tube really cold (Soldano?) or.. you get the idea.  
Last bit of confusion - none of this THD talk mentions something I consider (big whoop) to be critically important - IM or Intermodulation Distortion. EG you take a 1kHz signal and a 400Hz signal, and not only does your 2nd HD add 2kHz and 800kHz, but now you also get 1.4kHz and 600Hz (sum and difference) signals perhaps as well.. your 2 notes are now a chord (of sorts) IM is like the 'ringing' you get when you tune a guitar. If the IM is productive musically, you can get some amazing effects. If it is not, chords go muddy and double-stops can sound awful. One way I test amps is playing the break from "La Grange" - that little 2 note walking bassline bit will uncover bad IM in a heartbeat.
6/30/2004 5:22 AM
Bruce /Mission Amps

"EG you take a 1kHz signal and a 400Hz signal, and not only does your 2nd HD add 2kHz and 800kHz, but now you also get 1.4kHz and 600Hz (sum and difference) signals perhaps as well.. "
OK, do you also get a new subharmonic of the original 400Hz mixed with the new 600Hz, resulting in a new tone at 200Hz... (sum and the differences.)  
6/30/2004 5:06 PM
, do you also get a new subharmonic of the original 400Hz mixed with the new 600Hz, resulting in a new tone at 200Hz... (sum and the differences.)  
Yes.. at lower and lower amplitudes, but they should be there. And the new tones created get acted on by regular HD as well as IM AFAIK. The mirror looking into a mirror effect. I suspect that the additional products have amplitudes that fall off exponentially or something liek that. Don't actually know though.  
Here's a clearer explanation I just googled from  
"Intermodulation distortion begins with additional frequencies being produced by the sum and difference (addition and subtraction) of two original frequencies. For instance, two frequencies of 2 kHz and 8 kHz may have been produced by an audio system. Intermodulation distortion would create two additional signals at 10 kHz (the sum or addition of 2 and 8 kHz) and 6 kHz (the difference between the numbers or subtraction of 2 from 8 kHz).  
Each of these new signals, the first parts of intermodulation distortion, would then develop harmonics of their own (creating harmonics from 6 kHz of 12 kHz, 18 kHz and so on while producing harmonics of 10 kHz at 20 kHz, 30 kHz and son on). To further confuse the issue and add even more distortion, these harmonics (the harmonics created from the sum and difference frequencies of the original two frequencies) would then create their own sum and difference distortions (for example, the first harmonics located at 12 kHz and 20 kHz would create a sum frequency of 32 kHz and a difference frequency of 8 kHz).  
Obviously, many additional frequencies are added to the audio output creating a complex pattern of distortion. From the two example frequencies at 2 kHz and 8 kHz would come distortions (additional frequency signals) at 4, 6, 8, 10, 12, 16, and 20 kHz within the range of human hearing and many more beyond our ability to hear. Each of these tones at new frequencies are less powerful or have a smaller amplitude than the original two signals. "
6/30/2004 7:48 PM
Steve Rathmann
I know it's not exactly what you meant a couple posts ago, but you described the "ring" effect of an acoustic guitar. It's an apropos word here though since IM distortion is what happens in a ring modulator.  
I wonder if this is at the heart of what we like about tubes and transformers and what's missing in SS amps.  
The IM distortion description makes me think of chaos theory which describes dynamic systems not as random but extraordinarily complex where a minute change can result in a huge non-linear net effect. The fact that the input is changing constantly (i.e. actual guitar playing!) means the *quality* of the distortion changes dynamically based on the harmonic content, rather than simply the *amount* of distortion based on amplitude.  
I have definitely heard the "ghost third" when playing just the root and fifth of a chord through a cranked amp. As well, sometimes the "phantom" fundamental when playing fourths can send shivers down my spine.
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