• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Micro amp - all-tube, direct heated, portable and IT FITS

dovla

Member
2010-03-15 7:32 am
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.
 

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dovla

Member
2010-03-15 7:32 am
+ 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 :)
 

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dovla

Member
2010-03-15 7:32 am
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.