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Another Completed OTL Headphone Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4160
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Author:  Nodric [ 24 Nov 2012, 12:36 ]
Post subject:  Another Completed OTL Headphone Amp

After a great deal of work and learning I gave birth to a beautiful baby amp :D Below is the schematic and parts list, and a bunch of photos.

It sounds incredible with a pair of Sennheiser HD650s!

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The Schematic I Used

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This is one variation of the Headwize designs, but I believe is the same as used by the top $$ Woo designs :?:

I am quite surprised when I added it all up. $156.00 plus tubes. I already had the tubes, but dealer prices for the set are about $90 for the 3. You also have to factor shipping and taxes on top of my list.

The transformer is also excluded as I had a NOS one, but a suitable Hammond is about $80. My transformer was 225-0-225, and had 5v 2a and 6.3v 5a windings. The heater supply needs 2.8a at 6.3vdc which draws about 4a from the AC winding, so size accordingly. The B+ sits around 52ma, and hardly flickers even at full gain.

I also had the tag strips, but they are cents each. I also tracked down some black button head/allen M3 machines screws to mount all the sockets and tag strips. Damned hard to find black screw heads!

So assuming buying all parts, total cost is around $326.00 plus a few bucks for misc, but most of you will have some tubes, and even find an old tranny. Just watch those amperes on the tubes! I chose tubes for form as much as function.

The heater supply is just a bridge rectifier using the Schottky diodes, with the 33000uf cap as input, and then a single 5W 0.15R resistor in each leg of the DC, and then the 22000uf cap. Again the resistor values depend on the transformer used, so adjust for correct DC volts when the heater load is connected.

The B+ filter has a 6.8uf cap as input, and then 2x330R 2W and 1x1.25k 5W resistors to divide the filter up between the 2x100uf caps, and the final 470uf cap. The latter is probably too big, but I had it on hand so I used it :) The resistors will need to be adjusted for your transformer to get around 150-160vdc when load is present.

I used Mouser and Digikey, switching between either due to price, or just because I was placing an order anyway. I also used CD DIst which is a trade only supplier, but you can source the small parts list from elsewhere.

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It started life like this

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And developed into this

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Top left is the B+ section. The 1.25K WW resistor was needed as I changed the tranny for a NOS one I had and had to drop the voltage.
Tranny to the top right of course.
In the middle you have the heater rectification.
Lower left is the output section. Note the TO220 Resistors. 25w in such a tiny package, and non inductive 1% tolerance!
Bottom right is the preamp stage.

I have no idea if my layout skills are any good, or dreadful, but the amp is as quiet as the grave at idle. No hum at all! <1mv on the heaters, and around 1.5mv on the B+.

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Yes those heaters are DC, with less than 1mv of ripple thanks to 33000uf and 22000uf of capacitance!

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I still need some little brass screws, missing in the corners those stunning RCA sockets!

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Tube lineup is currently - 5V4 - 6AS7 - 6922 I also need a brass nut for the jack socket.

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Author:  HT Performance [ 24 Nov 2012, 12:41 ]
Post subject:  Re: Another Completed OTL Headphone Amp

Looks very good mate!

Strange to me, there is no interstage capacitor between the tubes section.

Does it work OK?


Cheers,

Miguel

Author:  Nodric [ 24 Nov 2012, 13:00 ]
Post subject:  Re: Another Completed OTL Headphone Amp

I didn't design the amp schematic, only the B+ and heater sections, but it works wonderfully. Incredible detail in all music, if fact so much so that if you feed it a poor quality CD it will be unforgiving! No Beats by Dr Dre yuk here :)

Author:  mwhouston [ 24 Nov 2012, 14:41 ]
Post subject:  Re: Another Completed OTL Headphone Amp

@Modric: Excellent build. Looks an absolute stunner. I built a 6AS7 power amp with just two tubes and loved it. A friend loved it more and bought it from me. I have six or so more tubes so this project interests me.

Author:  Nodric [ 24 Nov 2012, 19:38 ]
Post subject:  Re: Another Completed OTL Headphone Amp

mwhouston wrote:
@Modric: Excellent build. Looks an absolute stunner. I built a 6AS7 power amp with just two tubes and loved it. A friend loved it more and bought it from me. I have six or so more tubes so this project interests me.


Thanks for the compliments. If you need any data I have feel free to ask. I'd like to see your 6AS7 power amp schematic.

N

Author:  mwhouston [ 24 Nov 2012, 20:58 ]
Post subject:  Re: Another Completed OTL Headphone Amp

Nodric wrote:
mwhouston wrote:
@Modric: Excellent build. Looks an absolute stunner. I built a 6AS7 power amp with just two tubes and loved it. A friend loved it more and bought it from me. I have six or so more tubes so this project interests me.


Thanks for the compliments. If you need any data I have feel free to ask. I'd like to see your 6AS7 power amp schematic.

N

Follow through this thread.

Author:  Suncalc [ 24 Nov 2012, 21:58 ]
Post subject:  Re: Another Completed OTL Headphone Amp

mwhouston wrote:
Follow through this thread.

I'm pretty sure that Mark meant this one (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2243).

Author:  mwhouston [ 24 Nov 2012, 23:27 ]
Post subject:  Re: Another Completed OTL Headphone Amp

Suncalc wrote:
mwhouston wrote:
Follow through this thread.

I'm pretty sure that Mark meant this one (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2243).

Yes that one.

Author:  EXILE [ 25 Nov 2012, 07:32 ]
Post subject:  Re: Another Completed OTL Headphone Amp

This is my current head amp. It is basically the same but with the 6as7 paralleled

Author:  Geek [ 25 Nov 2012, 08:14 ]
Post subject:  Re: Another Completed OTL Headphone Amp

Awesome work, Nodric! :o :D

But I have just one picky.... I see those 3.3K resistors just standing on the terminal strip. They have a free-air rating of about 3W or so and are only 25W rated when heat sinked to the chassis.

Cheers!

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