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output impedence of CF? and a 6L6 question
output impedence of CF? and a 6L6 question
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Old 24th June 2003, 01:06 AM   #11
Positron is offline Positron  United States
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Join Date: May 2003
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Default Hi Ben..

Still working away on the headphone amp I see. Keep up the good work. I think you'll learn alot here, and in your experimenting.

Like mentioned above, the 6L6 will give plenty of power for your headphones. The good side is the tube will last forever if you run it easy.

Take care and all the best.

ps. Ask all the questions you want. There are no bad questions here.
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Old 24th June 2003, 08:34 PM   #12
PRR is offline PRR  United States
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Location: Maine USA
Hi, TG---

> in a cathode follower config, the Zo is reduced by a factor of about 5.1... correct?


The Zo of a cathode follower is 1/Gm (in parallel with any cathode resistor if significant).

6L6 at usual bias has Gm=5,000µMho, Z0 is about 200 ohms. 12AX7 at common operating point is Gm=1,000µMho, Ro is about 1KΩ.

Rp and Rk (1/Gm) are related by Mu (Amplification Factor). Your assumption of "5:1" would be true on a tube with a Mu of 5. Tube Mus range from 2 to 100. Most pentodes, when triode-wired, run a Mu of 10 or 20.

> in order to drive lower impedences well enough, we need a Zo quite a bit lower than the Z of the loudspeaker.

No, on several points.

If you are actually unfortunate enough to need every scrap of power you can make, then you want to Match the large-signal impedance. Sometimes this is not an exact match: best power transfer from a Triode is about 2Rp, not Rp as you might think. And although the cathode has a much lower impedance than the plate, the maximum power transfer is still at Rp to 2Rp, _not_ Rk. So a 6L6 wired as a cathode follower should be loaded in 2K to 5K ohms, not the 200-400 ohms you might think from the cathode impedance.

Many older speakers were designed to be fed from a Pentode plate, hi-Z source, and they worked just fine. The current (last 40 years) fad is to use very-low-Z sources, mostly to control box size.

I think here you are talking headphones. It is not clear if headphone designers generally assume low-Z drive or not. My impression is that several 100-600Ω headphone models are flatter with low-Z drive, but I don't know if this covers all cases. Generally we do not need the super-damping that loudspeaker amps have; damping of 3 or 10 flattens the headphone resonance quite adequately. I do know I have not yet seen a 32Ω can that cared one whit about its source impedance.

Hi-Fi headphones impedances are generally lower than tube impedances. We can start an analysis by assuming the load is a short on the tube, so we don't have to squint at tube curves to get some initial approximations.

Bias a 6L6GC to an extravagant 200V 50mA on the tube, another 200V across the cathode resistor. When you smack the 6L6 into clipping, its current will vary from 0mA to 100mA. The cathode resistor draws a fairly constant 50mA. The difference, +50mA to -50mA, goes to the load.

With 32 ohm phones, 50mA in 32Ω is 1.6V peak, 3.2V peak to peak. (Note that this is hardly louder than a 2-AA battery WalkMan.) Assuming sine-wave tests, the RMS power is about 40 milliWatts.

With 300Ω cans, 50mA makes 15 volts peak. This is comfortably less than our 200V biases on tube and cathode resistor, so the error in assuming that the load is "a short" is small. We will really get 14V peak if we do the math. That's 10V RMS, or 333 milliWatts. 300mW is a heck of a loud noise in most 300 ohm cans.

Overall, a cathode follower usually makes a crummy power amplifier. Drive voltage is MUCH higher than the same tube working with a plate load. Power into a low impedance load is no better with cathode loading, and usually worse (Langford-Smith has an analysis in the big red Radiotron). Grid drive voltage is so extreme that it is impossible to make a driver running on the same B+ smack a cathode follower to full output, and even with a reasonable extra B+ on the driver the driver distortion usually exceeds the distortion of a CF or a plate-loaded amp (which does not need such extreme drive).

If you do not need every scrap of power, don't mind some waste heat, a direct coupled voltage-amp and power cathode follower does make a cheap and cheerful headphone amp. HeadWize forum has some plans. Some of these use the 6080/6AS7 lo-Mu triode, but slight changes would adapt it to use 6L6. A single 6L6 makes about half the current of a 6080, but that would still be plenty for the higher-Z headphones.
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