12AU7/12BH7 Headphone Amp

the thermionic watercooler

Postby TomMcNally » Thu Apr 26, 2007 12:24 pm

Hi Soren -

The schematic still shows the old values - is that the right link ?

Looks pretty cool - I have a power transformer I've been wanting
to do something with, might be a fun project. Since there are no
OPT's, in can fit in a smaller box.

... tom
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Postby sorenj07 » Thu Apr 26, 2007 1:42 pm

Yeah I was going to re-draw the thing but the coupling capacitor is the only thing I'm changing so I figured I wouldn't bother. Parts are arriving soon, I can't wait to get it going. 10x6x2" chassis with top metal cage :)
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Postby TomMcNally » Thu Apr 26, 2007 8:35 pm

So what do you think Soren ? 7" x 4.5" x 2" choke underneath

Image
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Postby sorenj07 » Thu Apr 26, 2007 8:57 pm

wait, did you just build that thing in the space of 8 or so hours? If so, wow. Either way, it looks pretty sweet. It's one of those Hammond cast-aluminum stomp boxes right? I like the old-school Triad-looking PT also. How does it sound btw?

(I bet it was a lot easier with headphones with an impedance 9.5 times as big as mine Yellow_Light_Colorz_PDT_21 22uF would be more than enough, and you could scrape by with ~12 if you wanted)
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Postby TomMcNally » Thu Apr 26, 2007 10:27 pm

Well so far I've only spent about 1/2 hour on it ... it's not wired up.
I just thought it looked cool and I'd show you what I was going
to do in the next few days. It might be a little packed underneath,
so smaller caps will be good. Yep, it's a die cast pre-painted
Hammond box I got on eBay a year or so ago. It has some
DB-25 connector holes in the back plate, and I'll mount an
IEC connector there for power. It should be fun !
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Postby sorenj07 » Fri Apr 27, 2007 12:19 am

yeah, I hope it sounds great! I'll post a little pictorial of me putting the new one together, probably. I have SO MUCH on my plate right now, though. Amp for my parents, amp for a friend, my monoblocks, amp for that friend's parents, this headphone amp, a Tek 2213 that needs fixing, the crossovers in my C-V D7's that need replacing... the list goes on!
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Postby erichayes » Fri Apr 27, 2007 1:41 am

What about school, Soren? Or sleep? Sounds like you're stretched tighter than a Trojan blown up at a bachelor party.

It's Spring . . . go out and appreciate the sight and smell of the season for a couple of minutes each day. It's amazing what it can do for your thought process--not to mention your lifeline.
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Postby sorenj07 » Fri Apr 27, 2007 10:23 am

thanks for the concern :) in any case, my last day of school was today. i had some last work to take care of this morning, but now things will be a bit more relaxed Yellow_Light_Colorz_PDT_11
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Postby TomMcNally » Sun Apr 29, 2007 1:21 pm

Soren -

That power transformer isn't big enough, I'm getting about 200 volts DC,
so time to for me to rethink. I thought it was 240-0-240 but it's 120-0-120 ... grrr

... tom
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Postby sorenj07 » Sun Apr 29, 2007 2:52 pm

hmm, why not run the PT in a 240-0 bridge? You can easily cut the voltage down to the 315V B+ since the total current in the circuit is something like 32mA.. I also noticed no balance pot? I guess that if you can find some matched 12AU7's, you should be good to go. I don't really trust these sketchy eBay 7AU7's.
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Postby TomMcNally » Sun Apr 29, 2007 3:14 pm

Yeah a bridge would be simple enough. I didn't have much time to mess with it yesterday, I worked on it while reloading XP on my friend's laptop, and I didn't even get any solder on it !

I think the gain will be balanced enough without a balance pot, and I could always add a trimpot to tweak if necessary. I hope my junkbox has enough resistors close enough to work. Maybe I'll work on it some more tonight after work. So far the AC wiring and power supply (pending mods) is done, I used a small 5 henry choke I had and two 100 mfd caps, and will add more capactitance as I go. The input wiring and pot is done up the the first tube, as well as the ground buss. The sockets I used have a center pin, so I jumped from tube to tube to the PS. I am going to use the last tube as output for both channels, I think that will be better, ground current flow-wise, rather than splitting it across two tubes. Do you see any drawbacks ?
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Postby sorenj07 » Sun Apr 29, 2007 6:06 pm

Honestly, whichever works more simply for you should be OK. I haven't even built the thing, but the way I plan to do it, I'll split one 7AU7 for the gainstage, then use one tube per channel in the CF. No particular reasoning behind it, and it might change depending on how I lay out the PSU.
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Postby WA4SWJ » Wed May 02, 2007 5:36 am

Hey guys,

Just wondering how this amp sounds. Finished it yet? If you remember I was running the circuit through my SPICE simulations and was getting screwy results. It would be interesting to know that the analysis was incorrect.

I have an Aikido board from John Broskie that I have not built yet. I might analyze that circuit too and see what happens with the results. If that comes out screwy too, I'll stick with passive circuit analysis until I can load in some better tube models.

Let us know how it works out.
Ed Long
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Postby EWBrown » Wed May 02, 2007 7:42 am

I just finished up "stuffing" one of the Octal Aikido boards a couple days ago, JRB had the partial kits on sale for $99 last month, and I also got one of the 20K step attenuator kits as well, kind of an unusual approach, but at $29 for the kit, it seems a bargain...

For the output coupling caps (the board accomodates two sets, with independent outputs, I used some of the Green Monster Russian 2 uF PIOs, and for the other set, the included 0.68 uF 400V WIMA caps. THose were intended for PSU bypassing, but I have more similar ones to choose from...

The biggest hamfest / swapfest in the NE ares is this weekend Friday and Saturday , at the Deerfield NH fairgrounds, in Deerfield NH, look up "NEARfest" on the net. I'll dig up the site and add it here. Always lots of good cheap parts bargains and "freebies". One man's trash is another man's treasure...

From the ARRL Letter:

NEAR-Fest to continue "Hosstraders" tradition in New England: The New
England Amateur Radio Festival ("NEAR-Fest") will take place Friday and
Saturday, May 4-5, at the Deerfield, New Hampshire, Fairgrounds, Route 43
approximately 15 miles northeast of Manchester. NEAR-Fest will continue the
tradition begun years ago by the now-discontinued "Hosstraders Tailgate
Swapfest" -- a New England tradition for more than 30 years. NEAR-Fest will
take place spring and fall at the original Hosstraders location. Gates open
Friday at 9 AM. Tickets are $10 per person ($5 after 3 PM Friday) and $10
per vehicle into the flea market. Commercial vendors will also be on site.
There are no sellers' fees or space limitations, and there's ample free
parking outside the flea market. There's no charge for camping, but RV
hookups (power and water) are $30. Check the NEAR-Fest Web site
http://www.near-fest.com/ for additional information and updates.




/ed B in NH
Real Radios Glow in the Dark
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Postby TerrySmith » Wed May 02, 2007 4:20 pm

I also have the Aikido board, a 9-pin, it's about 3rd in the project line.

I'll probably use plinkers like 8CG7, 7AU7's etc. I wonder if a PAS power trannie will work with it?

One of the local guys has an Aikido, and it sounded very impressive!
T. Smith
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