Looking for a OTL tube headphone amp design - diyAudio
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Old 18th November 2012, 03:34 AM   #1
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Default Looking for a OTL tube headphone amp design

I am a total newbie to tubes and would like to make a headphone amp. I would prefer OTL (output transformer less) and OTL (overall transistor less). Output transoformers are a headache and I might have to import these, significantly increasing costs.

I have only two headphones at present, realistic pro 30 (32 ohms, 98db/mw, 100mw) and koss porta pro (60 ohms, 101 db/mw). I might get some more headphones, maybe yamaha orthos or sextetts. Impedance could be all over the place, 32 ohms to 600 ohms. I would like the amp to work with all of these.

I have the following tubes with me. If there is a very good design without these, i could try to get some other different tubes, but would prefer if I could manage with some of these.
ECC81, ECC82, ECC83, ECC88, 12AU7A, 5751, ECL86, PCL86, ECL82, PCL805. I have some more, will list those later.

For rectifier, I could get my hands on some of these - gz34, 5au4, ez81, PY88, 5U4, 5y3

For power transformers, it will be difficult to get a ready made trafo, I could get one custom wound. I could easily get my hands on an isolation trafo or a step down to US voltage trafo (230-110v). For heater, i can easily get a separate trafo of integer voltages 5,6,7,8... volts. Geting 6.3v etc would be an issue though, if thats what is needed, I would have to get it custom wound. While that also is not a big issue, its just time consuming. pre built ones will be EI, but for custom wounds, i can go with any of EI, torroidal, R-cores. Any special care needed to use multiple power transformers?

If it can also do preamp duties, that will be great, but eventually I would make a no gain preamp anyway.

The headphone amp should have a volume control. I would also prefer a simple design. I have a problem doing layout of complex p2p circuits, these very quickly become a mess and get out of hand.

Please suggest some design that can fit these needs. I would also need a power supply design to go with these.
Thanx a lot guys.

Edit: I dont have any high efficiency speakers, so I wouldnt use this to drive speakers.

Last edited by doors666; 18th November 2012 at 03:41 AM.
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Old 18th November 2012, 08:10 AM   #2
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With 32 ohm headphones you need bigger tubes then the ones on your list.

Or you can use a bit of solidstate assistance.

There are several good hybrid designs around.

Other route to go is output transformer but that was not what you asked for.
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Old 18th November 2012, 08:46 AM   #3
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Quote:
Originally Posted by flatheadmurre View Post
With 32 ohm headphones you need bigger tubes then the ones on your list.

Or you can use a bit of solidstate assistance.

There are several good hybrid designs around.

Other route to go is output transformer but that was not what you asked for.
Can you suggest some suitable output tubes for this, I can try to find them locally for decent rates.

I have nothing against transformers, they are just PITA to acquire. If I can use some general purpose ones (normal stepdown types) that are easy to get, then no problems. If nothing else works out, I will try to import some. I would try to avoid transistors though.
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Old 18th November 2012, 09:08 AM   #4
Stixx is offline Stixx  Germany
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The original Chumoy-Morgan Jones headphone amp which is an improved version of the "legendary" Earmax was designed to work down to 32 Ohm impedance headphones. I built one around 2005 (with help from the Master himself) and it worked flawlessly (but on higher impedance phones ) I later sold it to a guy with Grado's... and he reported good results!

The old Headwize forum is long gone, but you can find the original article here --> The Morgan Jones Mini Tube Headphone Amplifier | HeadWize

The schematic how I built it can be found here -->

http://s11b.directupload.net/images/...7/w2y7bh54.jpg

Most everything else will take much bigger tubes or would have to be hybrid.. like the previous poster said.

Last edited by Stixx; 18th November 2012 at 09:11 AM.
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Old 18th November 2012, 01:15 PM   #5
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The 6as7/6080 comes to mind.

Mains toroids work as OPTīs but itīs a trial and error cause some work great and some donīt.

Another option is 100v speaker line transformers but again itīs a gamble bandwith and perfomance is not often specified.

If you have a local dealer willing to let you try them out you can cliplead them into the cicuit and measure/listen to find out if they work.
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Old 18th November 2012, 10:18 PM   #6
tikiroo is offline tikiroo  Australia
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I have designed and constructed an OTL headphone amplifier based on the 6GF7 (novar equivalent to 6EM7) which is a dual dissimilar triode originally used for TV vertical oscillator and output duty. It uses a CCS loaded LTP followed by a futterman output stage with LED bias and is heavily influenced by the Cavalli Audio Bijou design. The standing current is 50 mA which allows around 100mW into 32 ohms, plenty for most applications. Also plenty of voltage swing for higher impedance headphones (variable feedback to set the gain/output impedance)

I've been meaning to post the design but haven't got around to it yet. The design is not exactly simple, it uses two different B+ voltages as well as a negative supply for the CCS.
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Old 19th November 2012, 02:50 AM   #7
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Thanx guys, looks like there are not many options for OTL for 32 ohm headphones. I will try to see if I can find any of these tubes locally. I will send email to some of the transformer manufacturers to find out if they can make the trafos and at what price, some of them advertise that they make audio transformers. What kind of specs do I need to provide to them for this.

If I do manage to find trafos, are there are any good designs that I can use with the tubes i have. I have ecc88 and ecl86 the most.

This looks like its going to take some time, I will probably start the preamp thread also as that might be simpler and I might be able to finish that one first.
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Old 19th November 2012, 08:39 AM   #8
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The ECL86 in some kind of parafeed with a mains toroid for OPT is one way to go.

Hereīs a link i found:

Parafeed Headphone Amp

As a start for tinkering.
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Old 19th November 2012, 05:51 PM   #9
Stixx is offline Stixx  Germany
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There was also an ECL82 PP-headphone amp using simple toroids as output transformers by Diyaudio member 'Shoog'.... somewhere on the forum.
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Old 20th November 2012, 12:56 PM   #10
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I own a pair of Akg 550s which also present a 32 ohm load. I just finished the OTL design shown here

Aikido Push-Pull

I don't know what the mains voltage is in India, but the 6080/6as7 loves 120ish volts, so All I needed was a cheap isolation transformer.

It has a very nice sound, and gets extremely loud
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