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Old 6th June 2003, 10:07 PM   #21
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OK... so I am to try it... Well, with the limited budget I have, that can get hairy.

The only two tubes I can find any info about are the 6DN7 and 6EM7. The 6EM7 has a 750-ohm Ra, so it's output impedence would be quite a bit lower than my headphones, though it has a much higher gain. (60 on first half)

Am I correct in assuming the high gain goes first, followed by the lower gain as output? And, the output half gets higher voltage... what, 200V? 250? First half gets about what as well? I have some difficulty decifering the tube terminology. What mA rating tranny would I need? If I read the data sheets correctly, Ip was VERY high, maybe reaching 80 on one of these tubes (forget which). On a dual triode? Gee... That would require a lot more current that $30 or so can buy.

What is the main reason that you want the output impedence lower than the impedence it is driving on OTL's? Does it increase THD?

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Old 6th June 2003, 10:25 PM   #22
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Electromagnetic loudspeakers rely on being driven from a low source resistance to achieve correct bass response. Headphones were traditionally driven via large resistors from the output of power amplifiers, so they didn't have this benefit. However, modern equipment with headphone sockets drives them from dedicated amplifiers, which inevitably have low output resistance. The question is, have the headphone manufacturers taken any notice?
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Old 6th June 2003, 11:53 PM   #23
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6EW7 data: http://hereford.ampr.org/cgi-bin/tube?tube=6EW7
6FD7: http://hereford.ampr.org/cgi-bin/tube?tube=6FD7

Bypassing the cathode resistor on the first stage is optional - if you need the gain, do it. Easier to deal with too much gain then not enough...

About 50-60 mA will do it for the transformer - 20 mA x 250 V is 5W, output stage is good for 10. One way is to use the scheme on Fred's page: http://www.dogstar.dantimax.dk/tubestuf/miniblok.htm
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Old 7th June 2003, 12:13 AM   #24
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Quote:
Originally posted by trespasser_guy
OK... so I am to try it... Well, with the limited budget I have, that can get hairy.
Nahh, it's no problem!

Quote:
The only two tubes I can find any info about are the 6DN7 and 6EM7. The 6EM7 has a 750-ohm Ra, so it's output impedence would be quite a bit lower than my headphones, though it has a much higher gain. (60 on first half)
You might need that gain, well then again maybe not.. I don't know. If nothing else you can burn it off with global NFB or voltage dividers.

Quote:
Am I correct in assuming the high gain goes first, followed by the lower gain as output?
Yes.

Quote:
And, the output half gets higher voltage... what, 200V? 250? First half gets about what as well? I have some difficulty decifering the tube terminology.
The CF I designed for +250V. You could use +200V for the first stage.. it doesn't really matter.

Quote:
What mA rating tranny would I need? If I read the data sheets correctly, Ip was VERY high, maybe reaching 80 on one of these tubes (forget which). On a dual triode? Gee... That would require a lot more current that $30 or so can buy.
Nope. Look at my graph again - it's for its own operating point, has nothing to do with published values for typical operation. (After all this isn't typical operation for a vertical amplifier!)
It shows Iq (quescent (fancy word for the DC or standing value) current) = 20mA. So for stereo you need 40mA, plus whatever the driver stage takes. 50mA total should suffice.

Quote:

What is the main reason that you want the output impedence lower than the impedence it is driving on OTL's? Does it increase THD?
No, it doesn't have much to do with THD; it's more of a frequency response issue. Speakers are never ideal resistors, instead complex combos of electrical and mechanical reactance (in a nutshell, reactance is resistance that varies with frequency, such as a capacitor or inductor's response). An 8-ohm speaker might be 8 ohms at 400Hz, but as low as 4 or 6 ohms at 200Hz, and 12 or even 16 at maybe 100 or 50Hz...
The reason for having a high damping factor (DF = Rl / Zo) is to level these bumps, giving a more even response.
On the flipside, you might well prefer a constant-current output (one with a high Zo) instead.

Tim
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Old 23rd June 2003, 04:40 AM   #25
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I might want to add that the 10EM7 is actually quite similar to the 6EM7...other than the filament voltage...and I used the 6EM7 to build a 1W amp once...sounder quite good...abit low on bass though...
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