LED biased EL84 headphone amp - Page 6

baggystevo82 · 9th March 2005, 02:44 PM

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

richwalters · 9th March 2005, 03:11 PM

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

baggystevo82 · 9th March 2005, 03:19 PM

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

baggystevo82 · 26th March 2005, 04:20 PM

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?
Cheers,
Steve

SY · 26th March 2005, 06:23 PM

The max voltage is plate-to-cathode. So it's probably a good idea not to go over 300 even with cathode biasing.

mark_titano · 16th April 2005, 11:50 AM

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

Tekko · 16th April 2005, 12:55 PM

Do the LEDs dig to the music ?

26th March 2005, 06:23 PM	#55
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	The max voltage is plate-to-cathode. So it's probably a good idea not to go over 300 even with cathode biasing. __________________ If there's a sucker born every minute, where do the rest of them come from?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
<WTA>Tube biased by LED	arwan_zoel	Tubes / Valves	1	24th October 2008 02:33 PM
12AU7/EL84 Headphone Amp	sorenj07	Headphone Systems	8	5th February 2008 04:44 PM
The Importance of Being Biased	Nelson Pass	Pass Labs	74	26th July 2007 01:58 PM
OLT Headphone amp useing two EL84's & a 12ax7?	ak_47_boy	Headphone Systems	3	24th December 2006 07:58 AM
#19 biased class A	Joel	Tubes / Valves	33	20th December 2002 05:38 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

9th March 2005, 03:11 PM	#52
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	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

9th March 2005, 03:19 PM	#53
baggystevo82 diyAudio Member Join Date: Dec 2003 Location: Herefordshire	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

26th March 2005, 04:20 PM	#54
baggystevo82 diyAudio Member Join Date: Dec 2003 Location: Herefordshire	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? Cheers, Steve

16th April 2005, 11:50 AM	#56
mark_titano diyAudio Member Join Date: Oct 2004	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

16th April 2005, 12:55 PM	#57
Tekko diyAudio Member Join Date: Jan 2005	Do the LEDs dig to the music ?