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Old 28th March 2007, 12:41 AM   #1
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Default 1 watt per channel tube headphone amp

Good evening,
I was looking at a August 1961 Electronics World magazine and found a construction article that might be of interest to some of you. A 1 watt per channel tube headphone amp using 2 6AU6 and 1 12AU7 tube. I scanned the article and uploaded to my website. There are 2 files, 1 687K and the other 910k big. You can download them at:

http://www.tuberadios.com/temp/headphone1.jpg and
http://www.tuberadios.com/temp/headphone2.jpg

Regards,
Sal Brisindi
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Old 28th March 2007, 01:28 AM   #2
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Very nice, but don't most more modern headphones have a much higher impedance? Seems like if nothing is plugged in here it still may cause damage to the finals.
I've been meaning to ask this exact question too, at some point, how do you get a normal old fashioned tube amp powered up, and add a headphone jack that bypasses/disconnects the speaker(s) being connected, and still have it not mess up. Is a special OPT needed with a high-impedance tap, as well as also possibly have the headphone jack kick in some kind of resistive load in parallel to the finals OPT tap to fake it out & be safe?
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Old 28th March 2007, 03:18 AM   #3
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Quote:
Originally posted by frank754
don't most more modern headphones have a much higher impedance?
my Sony MDR-V6's are 32 ohms.
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Old 28th March 2007, 07:36 AM   #4
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Hi frank754,

Quote:
I've been meaning to ask this exact question too, at some point, how do you get a normal old fashioned tube amp powered up, and add a headphone jack that bypasses/disconnects the speaker(s) being connected, and still have it not mess up. Is a special OPT needed with a high-impedance tap, as well as also possibly have the headphone jack kick in some kind of resistive load in parallel to the finals OPT tap to fake it out & be safe?
I built myself a small triode push-pull amp which can deliver about 1 Watt of 100% class A into nominal 8 ohms load. I use it for dual purpose, either to drive small bookshelf monitors of headphones of any load. A 1/4" stereo headphone connector with internal switches is located at the front, which disconnects speakers and adds a dummy load resistor of 10 ohms at the secondaries. The headphones are operated in parallel to the dummy load.

Using headphones of any impedance from 50 ohms to 600 ohms will lead to load impedances the output stage sees from 8,5 ohms to about 10ohms, which is near perfect (much better than any speaker impedance response!).

Here is the story and here is the schematic.

The sound is very good (full triode PP, 100% class A, minimal gNFB).

For me, when using low impedance headphones like my 50 ohms Sennheiser HD595, IMHO this method beats "big CF OTL" headphone amps using EL84 or even 6AS7 hands down, which run out of steam and distort with my low Z headphones.

Besides that, nice dual purpose, since it is a real amp (although flea power) capable of decent listening levels in my study room

Regards,

Tom

P.S.: Thank you, Sal, for sharing this interesting article!
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Old 29th March 2007, 07:50 AM   #5
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Would there be anything wrong with using the Lundahl LL1680 with the 6AU6 + ECC82 amp?

http://www.lundahl.se/pdfs/datash/1680.pdf
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Old 29th March 2007, 09:12 AM   #6
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I would suggest a simple mosfet output stage, like the SOHA hybrid, due to the similar clipping behaviour of the mosfet, it does not adulterate the sound.

I also did an opamp output stage, but it comes second to to fet in sonic class.
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Old 6th April 2007, 12:05 AM   #7
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Quote:
piffpaff asked


What do you people think about the negative feedback in this circuit (using the 6AU6)?

http://www.tuberadios.com/temp/headphone1.jpg

IMO, that 1961 circuit has potential. Approx. 1/3 of the O/P voltage is returned. Naturally , some tweaking is in order.

I've stated repeatedly that NFB is a tool, not a crutch. Best results are obtained when the circuit is reasonably linear open loop. Let's look at improving the linearity. Replace the 'AU7 with a 'FQ7 ('SN7 equivalent), which is inherently more linear. Pentodes are most linear when g2 B+ is regulated at a fraction of anode B+. So, set up a 0A2 gas discharge regulator as the g2 B+ supply.

Notice that the 'AU6 cathode resistors are not bypassed. For proper pentode operation, a cap. should connect g2 to the cathode, which is not at AC ground.

The interstage coupling caps. are too small. 100 nF. is about right.

Mr. Voss took steps to control hum, but he didn't have access to present day parts. Use a regulated (7812) VDC supply for the heaters. No hum from that quarter!

Let's use a tube rectifier and CLC filter for the B+. SS diode switching noise becomes a non-issue. Either a 6X4 or a 7Y4 will do NICELY.

Allied Electronics catalog number 227-0001 looks like a suitable power trafo. Use the 6.3 VAC winding to power the rectifier's heater.

A matter of concern to me is the 'AU6 cathodes at 0.9 V. RC coupled circuitry and A2 operation are a bad mix.
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