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12AU7 Hybrid Headphone Amp

post #1 of 14
4/30/09 at 7:01pm
Thread Starter 
So I had been looking to build this headphone amplifier for some time, designed by Roggom over at DIY Audio Projects. I got all the circuit work done last year with some help from a couple guys at AudioKarma who know their tubes.

One suggestion I had was that the heater coil is 20ohm and could be used to replace the 20ohm 5W resistor in the schematic. I figured I'd stick to the schematic and play around with the resister-heater idea another day, as I plan to do some rolling with this amp.

I'm stoked-- this is my first tube amp build ever-- it's a modest start, I think, and being partly SS I'm a more comfortable with it. I had a couple stutter-starts that stalled on me, but I've got a good feeling about this one.


Philip.


P.S. I picked up a nice piece of Red Oak in the 75% bin at Rona today that I plan on making the case out of.
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post #2 of 14
4/30/09 at 7:04pm
Thread Starter 
I stayed up an entire night and wired up the circuit exactly as shown in the schematics. I was getting some 60Hz hum in one channel (sounded the same as the lighting) and nothing out the other channel. It turns out that it was my power supply and that was fixed when I came up with a Canon switch-mode supply from an old printer!

Going to take a closer look at it, but I think one MOSFET is dead, as it didn't heat up as well as the working channel did... then I realized I had it installed backwards

It sure as hell looks pretty cool, I think!


Philip.


Photos;


Green is ground, red is +12V.


Black wires go to the MOSFETs and the red / black / yellow go to the plate / grid / cathode and the green wires are for the heaters. I left the wires long for testing.
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post #3 of 14
4/30/09 at 7:33pm
Thread Starter 
Here's some photos of the circuit under test hooked up to some test speakers-- keep in mind that this is a headphone amp, however it works fine with some efficient speakers, too! I'm also getting some interference-- during quiet passages I can hear CBC Radio 2 in the background. I'm going to chalk that up to the long wires and test leads.

On the desk;

Overview of the circuit board, power switch and indicator, and the source.


Philip.
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post #4 of 14
4/30/09 at 7:36pm
Thread Starter 
THEN, while I was cutting up the oak and planning the construction of the box I found these sweet heatsinks (that I've seen on ebay at much more than what I paid) at the surplus shop;


Wooo!!
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post #5 of 14
4/30/09 at 7:38pm
Thread Starter 
I've got the box finished up and I found some acrylic that I had laying around that I'll use for a top rather than a piece of metal (hard to find in my part of town). I plan on putting a 45° chamfer on the acrylic and painting it with a few coats of hammertone. It should be good

Here's where I am so far with the box. It's made of Red Oak;


And I'm starting to lay out the acrylic;

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post #6 of 14
4/30/09 at 7:38pm
Thread Starter 
And here's where I am right now.

I can mount both MOSFETs to the same heatsink, right?

I was up until 3.30a last night because I wanted to get it done and laid out. The two black circles represent the volume knob and the headphones/speaker switch.

Ta da!



What do you guys think so far?


Philip.
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post #7 of 14
4/30/09 at 8:09pm
I could not even do 3% of what you have, so to me it looks GREAT!
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post #8 of 14
5/1/09 at 12:54pm
Wow, it looks fantastic. Can't wait to see the final finished product.
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post #9 of 14
5/20/09 at 12:35am
Thread Starter 
I've been laggy, but I've been working a lot at my job lately!


Excuses...
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post #10 of 14
5/20/09 at 2:28am
Quote:
Originally Posted by thefragger View Post
And here's where I am right now.

I can mount both MOSFETs to the same heatsink, right?

I was up until 3.30a last night because I wanted to get it done and laid out. The two black circles represent the volume knob and the headphones/speaker switch.

Ta da!

What do you guys think so far?


Philip.
Nice job Phillip it's an interesting design you're working on there, can't wait to see it finished. How does it sound?

I like those heatsinks -- nice find. They look almost identical to my MSS heatsink but without the 2 holes. I did mount both MOSFETs to the same heatsink and it handles the temp easily (bit toasty, but that's expected).
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post #11 of 14
5/20/09 at 3:39am
That is way cool. Very nice, I cant wait to read impressions and see it when its finished.
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post #12 of 14
3/18/10 at 6:26pm
Tempted to build this as I have a couple of extra power adapters and an extra 12AU7 tube. Any reason not to use IRF610 instead of the IRF612 here?

Uploaded schematic in case page goes down.
LL
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post #13 of 14
3/24/10 at 3:21pm
Any interest at all in this cute little hybrid?

Even if there isn't much interest, can somebody enlighten me what I would want to achieve with P1 (the 100K trimpot)?

Thanks
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post #14 of 14
3/24/10 at 3:51pm
P1 is used to set the idle current for the output transistor. Set it to 100Kohm before installing, and slowly adjust until desired current is achieved.
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