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Old 9th September 2009, 12:11 AM   #1
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Default Resistance coupled cascaded amplifier

Just wondering how the circuit shown in the pdf would work for a headphone amp.

http://www.mif.pg.gda.pl/homepages/f...37/1/12AL8.pdf

Cheers

Lar
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Old 9th September 2009, 12:24 AM   #2
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Default hmmmmn - is it worth it?

40mW @ 10% THD?
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Old 9th September 2009, 01:13 AM   #3
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Quote:
Originally Posted by aardvarkash10 View Post
40mW @ 10% THD?
That would be at full output so for 20mW the THD should be far lower.

Not sure how many mW it would take to run some head phones.

Also I'm sure the circuit could be tweaked.

Cheers

Lar
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Old 9th September 2009, 01:34 AM   #4
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Default ok, well if you are inclined to try...

I would CCS load the pentode with a LM device (you are limited to 30v DC by the tube anyway, so a 317 would probably do it) with your headphones para-coupled off the anode.

You'll need a 28-32V DC supply, but the current loading is not very taxing, so a simple voltage trebler off a 12V AC wallwart would probably get you there.

You will also have to seriously reduce the input voltage from any modern device with a 2V-pp line output voltage - a voltage divider at the input would acheive that.

Accept that you will need to be driving a pair of high impedance (600 ohm) phones - 32 ohm ain't gonna cut it here!

If you already have the tubes, your project cost to a breadboard stage is maybe 10 bux - give it a go!
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Old 9th September 2009, 01:48 AM   #5
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Came across a nice quad of Sylvania 12AL8s.

Nice looking tube anywho with long black box plates.

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Lar
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Old 9th September 2009, 02:09 AM   #6
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Default alternatively...

mu-follower could be interesting... http://www7.taosnet.com/f10/mustage.html
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Old 9th September 2009, 02:14 AM   #7
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Quote:
Originally Posted by aardvarkash10 View Post
mu-follower could be interesting... http://www7.taosnet.com/f10/mustage.html
Seeing that the 12AL8 is a triode/tetrode would this work?

Cheers

Lar
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Old 9th September 2009, 02:21 AM   #8
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Default yep

in fact the article goes into that in some detail - input to the triode, pentode (tetrode) stacked on top, output from the cathode of the pentode. Also check out valvewizard's mu-follower page at http://www.valvewizard.co.uk/

In essence a mu-follower is a CCS loaded triode, but this way its all glass, not glass and sand. Hey, if you have four tubes, you could build both and do a side by side comparison! Cost you twenty dollars now...
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Last edited by aardvarkash10; 9th September 2009 at 02:26 AM.
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Old 9th September 2009, 02:25 AM   #9
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Quote:
Originally Posted by aardvarkash10 View Post
in fact the article goes into that in some detail - input to the triode, pentode (tetrode) stacked on top, output from the cathode of the pentode. Also check out valvewizard's mu-follower page at http://www.valvewizard.co.uk/
Oooo, cool. Will have to print that one off a give it a good read.

Thanks

Lar
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Old 9th September 2009, 02:33 AM   #10
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Originally Posted by diamondsouled View Post
Oooo, cool. Will have to print that one off a give it a good read.

Thanks

Lar

Yeah - watch out for the seriously funny typo's throughout the article... A little interpretive skill is required, but not much.
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