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xrk971 Pocket Class A Headamp GB
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Old 12th August 2019, 08:38 PM   #1911
xrk971 is offline xrk971  United States
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Decrease R4 a little bit to lower the bias current. You are at 6.9v / 117.5ohms is 58mA bias current. Not too hot actually. I have run as high as 70mA. But if you want to run at 5.8v which is 50mA that’s fine too. In order to reduce the resistance of R4 in small increments, parallel a large value like 1k or 820R etc with the R4 nominal value of 47R. It will take trial and error.

Alternatively, you can use matched FETs and the bias current will be the same.
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Old 13th August 2019, 06:00 PM   #1912
potatopie17 is offline potatopie17  United States
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Hi xrk971 I lowered r4 to 35 ohms got it down to 6 volts and now both run at about 54-56 degrees celsius after about 1 hour. One channel sounds like really open and lively. the other channel sounds dull with little bass and little treble and is quite a bit lower volume? missmatched transistors? Bad cap? I went ahead and ordered a matched set from you today. What idss range of transistors is your headphone amp designed for? Ive built a Nelson Pass F5 amp kit from diy audio store 2 years ago

and its input fets were designed for 6-12 ma idss. Just wondering.
Thanks

Benjamin and btw whats your name?
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Old 13th August 2019, 06:40 PM   #1913
xrk971 is offline xrk971  United States
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Hi Potatopie,
Thanks for ordering the matched FETs. I match them in-situ (with similar operating DC setpoints - to get the true actual current matched). Typically they will the provide the MOSFETs with 50mA (low), 60mA (med) to 70mA (high) bias current range. You can select low, medium, or high. High will let you drive lower impedance headphones. I would guess actual Idss to be on 10mA to 15mA range.

As to why one channel now sounds weak and distorted after the resistors mod? Might be that you have a cold solder joint or the JFET or MOSFET may be going bad, or your batteries are dead. It requires at least 15v from the two 9v to sound decent and undistorted. Try new batteries. For this reason using rechargeable Li-ion batteries makes sense. You can’t get more than 2.5hra out of alkaline cells but 4-5hrs from 600mAhr 9v Li-ion cells.

Cheers,
X
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Last edited by xrk971; 13th August 2019 at 06:44 PM.
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Old 13th August 2019, 07:02 PM   #1914
potatopie17 is offline potatopie17  United States
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xrk971 I'm using 21700 5 amp lithium batteries and the amp is running at 16.45v after 1 hour of use. and it was sounding like that before the resistor mod. Ive also upgraded the 2200uf 16 volt caps to 10,000 hr 2200uf 25v low esr 12.5 by 25mm caps. not using the power plug on amp. I'm using the amp for my dad he is hard of hearing and using 32 ohms in the ear speakers balanced armature. also I added silmic bypass caps on the output caps too 10uf. I ordered a bunch from mouser. hopefully its just the transistors. what is the safe operating temp of output transistors?

Thanks

Benjamin
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Old 13th August 2019, 08:03 PM   #1915
xrk971 is offline xrk971  United States
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You can run 80C - just be careful not to burn yourself. They are made of glass and epoxy remember - no harm will come from operation even at 100C but that’s going to heat up your nearby caps etc above their 85C rating. But temps of 65C are no problem.
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Last edited by xrk971; 13th August 2019 at 08:17 PM.
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Old 15th August 2019, 04:08 PM   #1916
ItsAllInMyHead is offline ItsAllInMyHead  United States
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I finally built my PCA this morning. It took me a while to get BF862s, and then I went on vacation. X - I can't thank you enough for answering all my questions and posting the updated BOM to the forum.

I need to freshen my breath a bit to get rid of the mints, wire up the batteries (Running off two lab supplies for the moment), and punch the case.

I think it sounds great with a set of Meze Classics. I'm sure the lab supplies aren't ideal, but if it sounds this good in this situation, then I know I'll be thrilled with batteries.

As a relatively new builder, I found the most challenging aspect of the build to be the physical positioning and lead bending for the larger caps. Odd, I know. For the next build, I may try a hot plate, heat gun, or some other method for the SMD soldering, but for one-offs, I don't mind hand soldering. Fired up with great tunes in both channels on the first shot. Excellent thread to learn the aspects of the build! I read it end to end. Thanks to X and all those that contributed. It makes things much easier for noobs. A few glamor shots attached. I'll post again after I get the batts hooked up tomorrow and get the case punched etc.

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File Type: jpg IMG_1406.jpg (704.7 KB, 104 views)

Last edited by ItsAllInMyHead; 15th August 2019 at 04:25 PM.
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Old 15th August 2019, 04:18 PM   #1917
xrk971 is offline xrk971  United States
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IAIMH,
Nice work!
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Old 15th August 2019, 05:51 PM   #1918
zman01 is online now zman01  Bangladesh
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Nicely done ItsAllInMyHead, enjoy!
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Old 16th August 2019, 12:23 AM   #1919
xrk971 is offline xrk971  United States
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I just ordered another set of PCA boards. This time in blue with ENIG finish. Should make the SMT soldering a bit easier as the pad surfaces are very flat. I think there are probably 200 green PCA’s out in the wild. Time for a different look.
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Old 16th August 2019, 12:26 PM   #1920
ItsAllInMyHead is offline ItsAllInMyHead  United States
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X - Love that idea. The boards are fantastic as-is, but any tweaks will always help. There are a few part spacings that are a tad tight for hand soldering (couldn't fit my tweezers between the parts), but seriously fantastic boards.

Thanks to X and the Zman for the kind words.

Listened a bit with the Meze Classics. Then, since this is for on the go, I tried a few sets of IEMs (16 ohm). They sound phenomenal. No issues at all with the lower impedance phones which is what I needed this for. Built using the same components in the BoM most recently posted.

Finished pics. Rechargeable batts coming later today, but I couldn't resist.

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