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Old 26th April 2013, 09:28 PM   #1
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Join Date: Mar 2010
Default Micro amp - all-tube, direct heated, portable and IT FITS

After almost going blind soldering everything and stuffing it into an impossibly inappropriate mint box here is a battery powered headphone amp sounding surprisingly good for the size of OPTs.
Everything runs off one battery and it plays decent 9 hours on one charge when using 3400 mAh cell (B+ set at 27V).
Component values were set by listening exclusively since I have no scope (aiming for slightly more gain then smartphone directly to headphones) so proper measurements would probably lead to some other choices but the way it sounds now more than surpassed the expectations.
Other than the tubes and resistors everything was recycled so monstrously looking coupling caps (which look tiny in grown-up amps) are there just because they happened to be present at construction time. I also regretted this decision as used components don't like being soldered again, to which burn marks on the insulation can testify.
Attached Images
File Type: jpg Finished1.jpg (479.1 KB, 949 views)
File Type: jpg Finished2.jpg (617.2 KB, 856 views)
File Type: jpg Finished3.jpg (537.8 KB, 783 views)
File Type: jpg FinishedTop.jpg (808.0 KB, 791 views)
File Type: jpg BuildStart.JPG (930.3 KB, 775 views)
File Type: jpg BuildEarly.jpg (332.7 KB, 324 views)
File Type: jpg BuildBottom.jpg (527.6 KB, 305 views)
File Type: jpg BuildLate.jpg (566.5 KB, 358 views)
File Type: jpg Fitting.jpg (703.6 KB, 374 views)
File Type: jpg RunningOpen.jpg (595.4 KB, 344 views)

Last edited by dovla; 29th April 2013 at 01:17 PM. Reason: typo correction
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Old 26th April 2013, 09:34 PM   #2
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Join Date: Mar 2010
Default + schematic

And schematic attached as well. There was uneven gain between the two 1zh29b tubes and that was sorted by different screen resistors (one channel 20K, the other 12K). The picture shows how the values were chosen
Attached Images
File Type: png Amp_head_micro.png (56.8 KB, 646 views)
File Type: jpg TestValues.jpg (426.2 KB, 593 views)
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Old 26th April 2013, 11:00 PM   #3
lexx21 is offline lexx21  United States
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Join Date: Mar 2011
Location: North Carolina
Very cool build. I can easily see how you would go cross eyed building that. Nice job.
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Old 27th April 2013, 07:23 AM   #4
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Location: London
I can see arguments for and against you as a brain surgeon....
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Old 27th April 2013, 10:16 AM   #5
JLH is offline JLH  United States
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Location: Indianapolis, IN
That's great stuff. I love seeing people build things like this. It's nice seeing different things.
Ah, how beautifully the orchestra sounds before a rain! In a dry sunny day there is no way for the instruments to sound this way!
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Old 27th April 2013, 10:57 AM   #6
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Join Date: Mar 2010
Default Airport safe

Even though it clearly says on top cover what the device does ("for throat AND VOICE") I still have to give myself some extra time when traveling.
So far the record for is held by Heathrow airport - 3 security screeners, 3 armed policemen and 2 special branch agents were deliberating for 20 minutes before one remembered to ask me what it was (another dismal statistic - none of them could recognized Manu Chao playing - never heard of him, to be precise)
On the bright side, one of the security personnel in Munich asked where to buy an amp like this.
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Old 27th April 2013, 01:33 PM   #7
TheGimp is offline TheGimp  United States
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Location: Johnson City, TN
That is Great!

Reminds me of all of the transistor projects I did in Sucrets Throat Lozenges Tins.
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