BF862 Preamp

[ElFishi]

yeah, I've seen those, but I wanted to drop the caps and I'm not looking for a headamp. Just think you can use the same concept for more current drive like in your HA.

 

[xrk971]

I don't mind caps. They give me peace of mind from the dreaded full DC rail hitting the voice coil of my headphones. I have enough DC coupled amps to know I am playing with Fire if I don't have a protection circuit. I can't risk frying my Beyerdynamics now.

 

[VuGiaA9]

Could I use 2sk170 in this preamp? I have no experience in smd parts

 

[kpsthakur]

I believe yes...reason we want to use BF862’s are that 2SK’s are becoming rare and BF’s are in production and way cheap


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[propitious]

Could I use 2sk170 in this preamp? I have no experience in smd parts

Do not be afraid of soldering a BF862. You can ruin ten of them and still be cheaper than a 2sk170. First of all, once you can see it, you can solder it, so you need a magnifying system. I use an inexpensive version of one of those headband magnifying jewelry visors like a watch repairman uses (wait...most people do not know what a watch repairman is!). Anyway, if it is to solder a BF862 onto a TO92 pinout, you can buy some premade SOT23 adapter pcb's on ebay or whatever. It is safest to break them apart before you solder the parts on them. I hold the adaptor pcb's in a pcb vise. Next, google and watch several You Tube videos on how to solder SMD, until you start feeling like "maybe I can do this." One way I do it is to apply paste soldering FLUX (not soldering PASTE which has a short shelf life) to the 3 pads and stick the BF862 in place, apply a small amount of solder to the small chisel tip, and then touch one lead with the chisel tip just for a moment, so that the BF862 is tacked on. Now the rest is easy. There are many other threads on this. Once you do this you will feel so empowered that there might be no limit to what you will tackle next.

 

[kpsthakur]

[emoji106]


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[JPS64]

where do I place volume potentiometer and what for value?

Doing Layout published soon.

JP

 

[juma]

At input
10k

 

[JPS64]

Thanks Juma.

First attempt, potentiometer (attenuator) missing.

Micromatch (red connector) for power supplies.

JP

 

[ElFishi]

Shweet!

You can turn the fets around so the gate faces away from the board, they are symmetric regarding drain and source anyway. If you then make the little boards and the gate contact pad bigger it can dissipate some heat.

 

[JPS64]

BF862 custom adapter board (panel) not finished yet.

Waiting for ELMA switch correct 3D model (promised by support).

With Lightspeed Attenuator.

Some place on the panel, perhaps CUBIE (1, 2 or 3) including supplies?

JP

 

[JPS64]

still working on BF862 preamplifier (Zen Mod Iron Pre on ice).

Talema Eagle symbols wrong, new defined, working on 3D models.

Left cm2 for:
- xrk971 BF862 SE headphone amplifier?

Waiting for suggestions.

JP

 

[georgehifi]

BF862 custom adapter board (panel) not finished yet.

Waiting for ELMA switch correct 3D model (promised by support).

With Lightspeed Attenuator.

Some place on the panel, perhaps CUBIE (1, 2 or 3) including supplies?

JP

Good to see that you kept any heat sources (transformers, regulators ect) well away from the led/ldr's, make sure you have vents above the transformers to release any heat build up.

Cheers George

 

[VuGiaA9]

Do not be afraid of soldering a BF862. You can ruin ten of them and still be cheaper than a 2sk170. First of all, once you can see it, you can solder it, so you need a magnifying system. I use an inexpensive version of one of those headband magnifying jewelry visors like a watch repairman uses (wait...most people do not know what a watch repairman is!). Anyway, if it is to solder a BF862 onto a TO92 pinout, you can buy some premade SOT23 adapter pcb's on ebay or whatever. It is safest to break them apart before you solder the parts on them. I hold the adaptor pcb's in a pcb vise. Next, google and watch several You Tube videos on how to solder SMD, until you start feeling like "maybe I can do this." One way I do it is to apply paste soldering FLUX (not soldering PASTE which has a short shelf life) to the 3 pads and stick the BF862 in place, apply a small amount of solder to the small chisel tip, and then touch one lead with the chisel tip just for a moment, so that the BF862 is tacked on. Now the rest is easy. There are many other threads on this. Once you do this you will feel so empowered that there might be no limit to what you will tackle next.

Thank you. I will try once

 

[sergiu2009]

Intrigued by good reports (Scott Wurcer and EUVL) on BF862 JFET I also got some. The idea was to build a preamp based on JBOZ and B1 concepts with gain of 2 (enough to drive F5) and ability to drive 5K next stage impedance. Also, 6-7 V of undistorted output signal is needed.
This is what I ended with and it's all I hoped for, it sounds great, better than 2sk170 in same topology - I suppose it's due to much higher transconductance and smaller parasitic capacity. The only "problem" is that BF862 is an SMD part but that's easily cured with small adapter PCB.
This is the preamp and PSU schematic:

Hello Juma and thank you very much for this wonderfull preamp. I got almost all the parts needed.
Please tell me, is there any posibility to get 10-15v of output signal from this design without altering it too much?
Thanks in advance
Sergiu

 

[juma]

... is there any posibility to get 10-15v of output signal from this design without altering it too much? .....

No. Cascoding would be inevitable. More gain would be needed too and that would compromise linearity - in other words, a whole new citcuit...

 

[sergiu2009]

Thank you very much. Actually is perfect for my Hiraga amp, but not for a power follower amp that i want to test..
Can you please sugest a preamp designed by you (simple as this one) that can swing 10-15v?
Thanks

 

[juma]

I am sure I published a couple of those too but I don't remember in which threads. Search function should help...

 

[xrk971]

Thank you very much. Actually is perfect for my Hiraga amp, but not for a power follower amp that i want to test..
Can you please sugest a preamp designed by you (simple as this one) that can swing 10-15v?
Thanks

AKSA's Lender Preamp with 40Vpp Output

In the above thread Juma suggested this circuit which looks very good too. I have not had a chance to build Juma’s yet but expect it to work very well like all his circuits do.

But I can confirm that the Aksa Lender Preamp sounds superb and easily puts out 20vpp at 0.002%THD. Here is 20vpp into 7kohm load and running 22dB gain.

 

[sergiu2009]

Thank you very much xrk. This is exactly what i was looking for.