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previous: Mike D. Ken,If you put all 3 resist... -- 7/16/1999 7:55 PM

Re: O'connor books/mistakes & corrections?

7/17/1999 3:05 AM
moocow	Re: O'connor books/mistakes & corrections? My annoying, arrogant, and rambling comments: 1) 36.86k and .22 uF gives 19.6 Hz. That should be .022 uF. 2) I was at a boring meeting today and derived the equations for bypassed and unbypassed Rk: The equation for the equivalent "R" is indeed: Rg + (RL \|\| rp) as Mike D stated. When Rk is unbypassed: Rg + (RL \|\| (rp + (A+1)Rk)) (A+1)Rk should be added to rp, not simply Rk. The UT guy is wrong. 4) These equations are derived by first drawing the equivalent small-signal circuit model. An ideal AC voltage source is applied to the model and the resultant AC current is calculated. The ratio between AC voltage/AC current is the equivalent resistance. The UT guy states that the equivalent resistance "is the value that an ohmmeter would read...". This is a DC value, not AC. This is why he is wrong. 5) There isn't much of a difference whether the cathode resistor is bypassed (6.97 Hz) or unbypassed response (6.77 Hz). Still, if an equation is posted on this BBS, it should be the correct equation. 6) None of this is accurate if the cathode bypass capacitor has its critical frequency near that of the coupling capacitor. The equations for Cc only work when the critical frequencies are widely seperated. As luck would have it, they two freqencies are very close together in a Fender amp. 7) Because of (6), the best way to analyze these circuits is with PSpice or other circuit simulator. If you really want to know what's happening inside of these amps, you must use a circuit simulator. Those tone control simulators ignore the EQ contribution of the rest of the amp, which is what we're discussing here. 8) The innacurate equation for R = Rg \|\| Rl is popular because it is a synthesis equation, not an analysis equation. That is, the equation is used to choose a coupling capacitor, not determine its cutoff frequency. You use they equation by choosing a cutoff frequency and calculate a capacitance. Then use the next largest value. You never use the value you calculated, so the slight error doesn't matter much. 9) Audio people don't really care where the cutoff frequency is, just as long as it is very low. If they designed for a 2 Hz cutoff and got a 1 Hz cutoff instead, it would be no big deal. Even thoug it is a 1-octave error, it will be inaudible. They can get away with these innacurate rules of thumb, so these rules can still be found in Morgan Jones' book, Audio Cyclopedia, and other otherwise reliable reference texts. Amp gurus then read these books and repeat the equations in their own, without ever having derived them on their own. 9) To find the actual cutoff frequency, we need to use either exact equations or simulations to analyze our favorite guitar amp circuits. These circuits were tuned by ear, so their cutoff frequencies are often in the audible range. In this case, if we designed for a 200 Hz cutoff frequency but wind up with 100 Hz, the difference will be noticable. 10) If someone wants to ignore these equations altogether and tweak their amp by ear, that's fine. In the final analysis, your ear is what really counts.

7/17/1999 3:05 AM

moocow

Re: O'connor books/mistakes & corrections?

My annoying, arrogant, and rambling comments:

1) 36.86k and .22 uF gives 19.6 Hz. That should be .022 uF.

2) I was at a boring meeting today and derived the equations for bypassed and unbypassed Rk:

The equation for the equivalent "R" is indeed:

Rg + (RL || rp)

as Mike D stated.

When Rk is unbypassed:

Rg + (RL || (rp + (A+1)Rk))

(A+1)Rk should be added to rp, not simply Rk. The UT guy is wrong.

4) These equations are derived by first drawing the equivalent small-signal circuit model. An ideal AC voltage source is applied to the model and the resultant AC current is calculated. The ratio between AC voltage/AC current is the equivalent resistance.

The UT guy states that the equivalent resistance "is the value that an ohmmeter would read...". This is a DC value, not AC. This is why he is wrong.

5) There isn't much of a difference whether the cathode resistor is bypassed (6.97 Hz) or unbypassed response (6.77 Hz). Still, if an equation is posted on this BBS, it should be the correct equation.

6) None of this is accurate if the cathode bypass capacitor has its critical frequency near that of the coupling capacitor. The equations for Cc only work when the critical frequencies are widely seperated. As luck would have it, they two freqencies are very close together in a Fender amp.

7) Because of (6), the best way to analyze these circuits is with PSpice or other circuit simulator. If you really want to know what's happening inside of these amps, you must use a circuit simulator. Those tone control simulators ignore the EQ contribution of the rest of the amp, which is what we're discussing here.

8) The innacurate equation for R = Rg || Rl is popular because it is a synthesis equation, not an analysis equation. That is, the equation is used to choose a coupling capacitor, not determine its cutoff frequency. You use they equation by choosing a cutoff frequency and calculate a capacitance. Then use the next largest value. You never use the value you calculated, so the slight error doesn't matter much.

9) Audio people don't really care where the cutoff frequency is, just as long as it is very low. If they designed for a 2 Hz cutoff and got a 1 Hz cutoff instead, it would be no big deal. Even thoug it is a 1-octave error, it will be inaudible. They can get away with these innacurate rules of thumb, so these rules can still be found in Morgan Jones' book, Audio Cyclopedia, and other otherwise reliable reference texts. Amp gurus then read these books and repeat the equations in their own, without ever having derived them on their own.

9) To find the actual cutoff frequency, we need to use either exact equations or simulations to analyze our favorite guitar amp circuits. These circuits were tuned by ear, so their cutoff frequencies are often in the audible range. In this case, if we designed for a 200 Hz cutoff frequency but wind up with 100 Hz, the difference will be noticable.

10) If someone wants to ignore these equations altogether and tweak their amp by ear, that's fine. In the final analysis, your ear is what really counts.