BF862 Preamp

[xrk971]

Update to the above Aksa Lender pre FFT, when implemented in real PCB as designed by JPS64, distortion went down to below 0.001% THD.

 

[sarathssca]

1. Do we need the single BF862 in gain stage to be matching with the one in the other channel of the stereo preamp?
2. For getting 4 matched BF862 for the buffer stage, approx how many BF862s we need to procure and do test matching?
3. Why are we using asymmetric power supply here ie positive and negative rails of different voltages, unlike more commonly used +/-10v or +/-12v (where both +ive and -ive rails have same voltage with opposite polarity)?

 

[AndrewT]

I bought 600 lsk170.
I found many matched twins, in various grades of matching accuracy from 0.25% to 5%.
I did not find any quads.

BTW, that is matching over a range of Id, not selecting by measuring Idss.

 

[juma]

1. Do we need the single BF862 in gain stage to be matching with the one in the other channel of the stereo preamp?
2. For getting 4 matched BF862 for the buffer stage, approx how many BF862s we need to procure and do test matching?
3. Why are we using asymmetric power supply here ie positive and negative rails of different voltages, unlike more commonly used +/-10v or +/-12v (where both +ive and -ive rails have same voltage with opposite polarity)?

1. Yes
2. In 500 pcs. of BF862 I have bought at Mouser almost all have the Idss in range of 13-14mA so matches were abundant in that quantity (all JFETs delivered to me were on the same production stripe). Others have different experiences.
3. Already explained - please read the thread from the beginning. Actually, you can do it without negative rail - see post #346. The price to be payed is introduction of input coupling cap.

 

[xrk971]

I made a small quick and dirty mockup of the actual circuit the BF862 will be used in and then measured the current through a 1k drain resistor (part of actual circuit) at the voltage it will be used at (16.4v) and can get pretty amount of pairs (40%) with better than 1%. Trick is to push down on JFET with enough force to make contact yet not make it fly off with a pop. The eraser of a pencil is good for this.

 

[veloplex]

Hi XRK971,
could you drop the shematic of your "mockup" please? You messure the Idss current through/over the 1k drain resistor?
Christoph

 

[sarathssca]

sismik,
there is an easier way to make this pre (see attached schematic).
That way we don't need the negative PS anymore and the gain is easily adjusted by changing only the value of R3 (in reasonable range: from 100R for 17dB gain to 470R for 6dB gain).
Of course, this demands the use of an input cap (C1) but since it's of small value (47-100 nF) it's easy to fit the good one (WIMA MKP10 or whatever one likes).

Can we increase the gain to around 28-30db to use with unity gain current buffers like pass F4 or Mofu? Do we need to increase the supply voltage to higher level and cascode the gain BF862 FET to keep safe from higher supply voltage?

 

[juma]

Can we increase the gain to around 28-30db to use with unity gain current buffers like pass F4 or Mofu? Do we need to increase the supply voltage to higher level and cascode the gain BF862 FET to keep safe from higher supply voltage?

For goals you set a different design is needed - have a look around, there's plenty of them on this forum.

 

[xrk971]

The AKSA Lender and Melbourne/Yarra preamp were specifically designed to drive the F4 or MoFo with sufficient gain and voltage swing to be able to hit clipping with low distortion from the preamp.

AKSA's Lender Preamp with 40Vpp Ouput GB

Melbourne Daughterboard for M2X

The YARRA Preamplifier/HPA for Melbourne DB Group Buy

 

[graham355]

Hello,
I an fairly new here.
I am trying to build Juma's preamp from post 23.
I am unsure what type of capacitors to order for the 5 off 0.47uF caps. They don't show polarity so I know they aren't electrolytic.
For the electrolytics I was looking at Nichicon UFG 50V and 80V versions.
For the 1.5Uf, Digi dont stock UFG. Will a UMF 35v be ok there ?
Thanks for your help.
Graham.

 

[graham355]

DIY Preamp help please

Never-mind, it seems most of the important components are unavailable now.
I am looking for a suitable 'up to date' DIY preamp to drive DIY Pass ACA's.
They are currently connected to TV through a cheap Prozor DAC. The volume is a bit low on some programs. If connected to my Ipad I have plenty of volume.
I am assuming the DAC output is a bit low.
I assume I need a preamp as If I understand correctly, a buffer does not amplify ?
I like to make my own PCB's where possible.
Thanks again,
Graham

 

[DougL]

I assume I need a preamp as If I understand correctly, a buffer does not amplify ?

You are spot on about the buffer.

Doug

 

[veloplex]

Hi,
when BF862 was discontinued we had a big group bye. What to do with >150 BF862 in stock leads me to Juma's 862 Pre

Some years ago I made these PCBs

To lazy to match all the bf862 delayed the project until last week

Fed by a Jung Didden SperReg. First listening are very promising.

Thanks for sharing this design Juma. If you are still located in Berlin, feel free to come by.

Christoph

 

[thimios]

Yes you should
Here you are:

Hi Juma,why i can't open this with SP6?
Do you have gerbers available?
Thanks.
Regards from Greece!

 

[juma]

....
Thanks for sharing this design Juma. If you are still located in Berlin, feel free to come by.
Christoph

Thanks Christoph, glad you like it

-------------------------------------------------------------------------------------------

Hi Thimios,
the .zip archive from the post #54 contains Sprint Layout 5.0 file.
GBR file attached (inside the zip archive).

All the best !

 

[thimios]

Many-many thaks juma!!

 

[thimios]

Hi Juma ,sorry if i will stress you again.
Do you have a pcb for this preampliifer?
I have some 2sk170BL from a donation and i look around to built something.
What would your recommendation?
Thanks!
thimios.

 

[juma]

Sorry Thimios, that is an old circuit, I couldn't find the PCB - it was probably built point to point.
I'd recommend this:
https://www.diyaudio.com/community/threads/lsk-pre-baf-2013.244106/post-4254886
https://www.diyaudio.com/community/threads/lsk-pre-baf-2013.244106/post-4543386

 

[thimios]

Thanks juma.
I have a pcb by prasi. 🙏
The problem is that i have 2SK170 but not 2SJ74.
I have two pairs only, for a future project Nelson Pass F4.
I have read somewere your's recommendation about using a pair of k170&j74 in the preamplifier circuit and skip this from the F4 input?
I have 2SK246Y & 2SJ103GR only...
Thanks for all!
Thank you prasi!
thimios.