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Old 9th March 2005, 02:44 PM   #51
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Quote:
Originally posted by richwalters

Also anode load in first stage is nice and low, again ideal.
Hehe, just been working on a slightly modified first stage with only one led biasing, and a 15k load. This reduces distortion of the first stage at 5v p-p output to around 0.03% THD, down from over 0.06% that it was in this part of the circuit earlier (with 2led's biasing and a 10k load). Is this right to do?
Cheers,
Steve
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Old 9th March 2005, 03:11 PM   #52
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You could be getting close to the cut off curve.....if you swapped for another 6N1P perhaps the thd results won't be so favorable. Give it a whirl....but consider tolerance and spread in the curves......I will have to look up this tube.......

richj
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Old 9th March 2005, 03:19 PM   #53
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Hi,
ok I'm down to overall amp THD of 0.017% at 10mA ignoring the 2nd harmonic now. Still around 0.7% with though I guess this is down to the Cathode follower. Hopefully the 2nd harmonic won't be so important...
Steve
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Old 26th March 2005, 04:20 PM   #54
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Back again!!! More rethinking going on....
I think I'll still stick with the LED biasing, but moving accross to transformer coupled output now I think...
Just a short question to start with though, as a triode, EL84 maximum plate voltage = 300v. Does this mean that I can use a higher b+ (like 450-500v) so long as the grid doesn't go negative enough to exceed this rating?
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Steve
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Old 26th March 2005, 06:23 PM   #55
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The max voltage is plate-to-cathode. So it's probably a good idea not to go over 300 even with cathode biasing.
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Old 16th April 2005, 11:50 AM   #56
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I've just find this page:

http://www.turneraudio.com.au/htmlwe...evisedst70.htm

looking at the output stage cathode bias I've seen the zener diode string across each cathode RC network. Its purpose is to limit the rise of voltage in the cathode bypass caps when the amp works hard in class AB. Can this be worthwhile for standard cathode bias?

Mark
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Old 16th April 2005, 12:55 PM   #57
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Do the LEDs dig to the music ?
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