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BF862 based SE Class A Headamp without the HEAT
BF862 based SE Class A Headamp without the HEAT
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Old 18th December 2016, 11:04 PM   #11
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
I have let the amp burn in overnight. Still running and sounding great. Case temp is barely warm to the touch after being on for hours. I am waiting for the 5v USB charger battery to arrive. Looking forward to seeing if it can indeed be portable battery powered. I am debating whether or not to go in and tweak DC voltage up to get more head room. I think at max volume I detect clipping with my headphones. So currently at 17v now. Maybe take it to 20v.

Last edited by xrk971; 18th December 2016 at 11:06 PM.
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Old 19th December 2016, 11:28 AM   #12
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
I just noticed that MMBFJ310 (SMT version of J310) has 60mA current capability. Could a similar amp with just two pairs be biased at 100mA then? It would be more compact as fewer parts. Is performance of J310 not as good as BF362 musically?

http://www.arrad38.fr/datasheet/J309_J310.pdf

Also, just using a single pair through hole J310 could give similar current as four pairs of BF862. Although heat dissipation could be an issue on single TO92 at 50mA.

Last edited by xrk971; 19th December 2016 at 11:31 AM.
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Old 19th December 2016, 03:14 PM   #13
needtubes is offline needtubes  United States
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Quote:
Originally Posted by xrk971 View Post
I just noticed that MMBFJ310 (SMT version of J310) has 60mA current capability. Could a similar amp with just two pairs be biased at 100mA then? It would be more compact as fewer parts. Is performance of J310 not as good as BF362 musically?
MMBFJ310 has 24-60mA IDSS, which is the "idle current" when Vgs=0, so I wouldn't exactly call that "current capability" for the typical MMBFJ310, which will have less than 60mA IDSS. You may have trouble finding matched pairs if IDSS does actually vary that much.

Note that forward transconductance of MMBFJ310 is only 18 maximum compared to 45 typical of BF862. I would expect you to need almost three times as many of the MMBFJ310 to maintain a similar output impedance as the BF862's, despite their higher IDSS.

Also, how do you plan to cool a tiny SMT MMBFJ310 passing 60mA at ~10V?

Last edited by needtubes; 19th December 2016 at 03:19 PM.
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Old 19th December 2016, 05:16 PM   #14
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Good points, guess I stick with BF962's then. It't a slick little FET that is not so expensive.
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Old 19th December 2016, 05:29 PM   #15
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Good job, guy.

May you say me how did you do the transfer from the paper to the copper side of the PCB, of the toner ink? I know it can be used an iron, but I doubt about time, and quality of the transfer, and which process did you do to ensure that the transferred toner doesn't peel off.

Thanks in advance.
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Old 19th December 2016, 06:17 PM   #16
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Quote:
Originally Posted by Osvaldo de Banfield View Post
Good job, guy.

May you say me how did you do the transfer from the paper to the copper side of the PCB, of the toner ink? I know it can be used an iron, but I doubt about time, and quality of the transfer, and which process did you do to ensure that the transferred toner doesn't peel off.

Thanks in advance.
I use a laser printer to generate the black images onto adhesive backed vinyl film (applied to paper) used to cover shelves and other surfaces. Then preheat copper with iron for 2 minutes, then lay image face down on copper and cover with another piece of paper and apply iron for 2 minutes. Remove and run under cold water, then peel vinyl off at sharp (near 180 deg angle) to leave transfer behind. Touch up holes with black Sharpie marker.

This is what I use.

https://www.dollartree.com/Con-Tact-...6003/index.pro

Very crisp features can be obtained if done right. I got it after 2nd try. Start with small projects like a preamp. Take for example my BF862 preamp by Juma. Lettering and traces are very clear. This one was better than my head amp.

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Old 19th December 2016, 06:31 PM   #17
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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OK, thanks for your entry. Did you try the same method for double sided PCB's?
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Old 19th December 2016, 08:32 PM   #18
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Have not attempted double sided yet. I imagine it can work. Just apply both transfers at once and flip sides half way through iron press. Registration of holes on both sides is tricky. Maybe drill a few holes to line up transfers before ironing.
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Old 19th December 2016, 08:51 PM   #19
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
I took the little amp to work and all my coworkers who listened were blown away with how good it and the DT880's sound together. Great demo track is Nils Lofgren's "Keith Don't Go."
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Old 20th December 2016, 05:37 AM   #20
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Default Added Juma Easy Peasy Cap Mx

I added Juma's Easy Peasy Cap Multiplier (using IRF540 MOSFET and 10mF 25v output cap). I also adjusted the DC step up to produce 20.0v output after the cap Mx. The dynamic headroom really went up another notch and cannot detect clipping at highest volume setting now. Also, bass authority seems to have improved dramatically with the cap Mx. Running 20v across about 50mA is about 1w dissipation in the JFETs per channel. Cap Mx is dissipating about 200mW for 4v drop and 50mA current, per channel. So 2.4w total heat generation so should be manageable.

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Last edited by xrk971; 20th December 2016 at 05:44 AM.
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