LSK pre - BAF 2013

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prasimix

prasimix

Member
Joined 2009
juma said:
Yes, P2 sets the current through CCS (Q6) in rage from 7mA to 14mA to make it equal with the current through BF862 (Q5). When the equilibrium is achieved we have 0V DC at the output.
Before we do anything else, tell me what is the voltage drop across R16 (6R8) so we can determine how much current runs through the BF862 ?
Don't forget to short-circuit the input when setting/measuring.
Click to expand...

86mV drop or 12.65mA. Other channel where I can go down to zero current is 9.13mA.
 
prasimix

prasimix

Member
Joined 2009
juma said:
Obviously, there is a Idss difference between BF862s. Also, the Vbe of your Q8 is a bit lower than expected, but never you mind - just lower the value of R21 to 22R (or solder from beneath another 47R in parallel with the existing one) and all should be fine.
Click to expand...

Thanks, 27R was enough. Now I have zero DC offset, and 13.8mA on that channel. Maybe instead of P2=50R and R21=47R we can suggest in the future P2=100R and R21=22R?
 
K

koja

Member
Joined 2009
ever think of FW B2-like circuit?

Juma,

Can I challenge you (and others) to come up with a proposal or share your qualified opinion on the feasibility of something :rolleyes::

I do not believe Papa ever shared a DIY version of a First Watt B2? ( i.e. a buffered splitter with adjustable relative gain). I may need this to drive two ACAs on top of each other if one proves underpowered.

Do you think the LSK-pre/phase splitter could be adapted to this application, and/or does the gain control with this topography lend itself to an easy adjustment (like for example in Aleph P)?

Thanks a lot! Hvala puno.

S.
 
K

koja

Member
Joined 2009
LSK for B2-like application

Thanks Lynn,

this is really where I was going with the question.

Zen, I know that two full blown preamps with high enough input impedance to parallel as load on the source could do it, but I was looking for the simplest integrated solution where one side of the splitter is committed to a fixed gain, and the other one is still tunable within reason. i imagine Papa built his B2 around B1 concept by adding gain to one side, but i thought LSK circuit could provide for a good platform to build one as well? not sure though if gain adjustment in LSK could be done by simple controls (like Aleph P for eaxmple calls for trimming r in just one place?).

thanks guys.
 
L

lhquam

Member
Joined 2010
Trimming R6 or R9 in post #271 would be all that is required. R9 would probably be the best choice since it would interact less with the H2 adjustment set by P1.

koja said:
Thanks Lynn,

this is really where I was going with the question.

Zen, I know that two full blown preamps with high enough input impedance to parallel as load on the source could do it, but I was looking for the simplest integrated solution where one side of the splitter is committed to a fixed gain, and the other one is still tunable within reason. i imagine Papa built his B2 around B1 concept by adding gain to one side, but i thought LSK circuit could provide for a good platform to build one as well? not sure though if gain adjustment in LSK could be done by simple controls (like Aleph P for eaxmple calls for trimming r in just one place?).

thanks guys.
Click to expand...
 
Ryssen

Ryssen

Member
Joined 2003
Yeahh,got me a situation here...
Started the dual LSK build (for first time) and messaured the voltages,the +-24v are ok but the +- 33v are about +-18v,if I disconnect the preamp from the PSU i get +-33v.....

I use the 2sk246/2sj103BL combo unmatched,tried with a sinuswave and theres a sinsuwave on my scope at the output,and theres allso a music output.
Thought I start with adjusting the pots so P2 is for zero offset but how do I ajust P1?

And where do I start messaure on the preamp to see what pulling the current?
 
juma

juma

Member
Joined 2005
Ryssen, if your preamp's current draw drops the +/-33V to +/-18V something is wrong with the PSU - please check the value of R2,R11 (re. PSU sch. from the post #66 of this thread) and the voltage drop across each of them with and without preamp connected.
PSU voltage shouldn't have such a big sag under load - first try to correct that and then start adjusting DC offsets of the preamp (P1) and buffer (P2)
BTW, what's the Idss of your JFETs?
 
juma

juma

Member
Joined 2005
It looks OK - the preamp consumes 27-30mA.

Please check the following PSU voltages (with and without preamp connected):
1. DC voltage between the plus poles of C5 and C9
2. DC voltage between the minus poles of C6 and C10
3. DC voltage between the ground and the point where R1 and R2 meet.
4. DC voltage between the ground and the point where R10 and R11 meet.
5. AC voltage between the plus poles of C1 and C2
6. AC voltage between the minus poles of C3 and C4
 
Ryssen

Ryssen

Member
Joined 2003
Ok,I´ve managed to locate the fault,the gateresistors (R5,R7)where missing.....:rolleyes:
So after that the voltages out from Q1 and Q2 where + - 36v but after chaning the trafo to a 2x15v one (before 2x18v) they where + -33v.
However the offset in one channel is 10mv and the otherone 200mv and can´t be adjusted any lower.
Is there anything that can be done about it?

Btw,it drives my HD650 headphones really good..
 
juma

juma

Member
Joined 2005
Ryssen said:
...However the offset in one channel is 10mv and the otherone 200mv and can´t be adjusted any lower.
Is there anything that can be done about it?...
Click to expand...

Of course, but please tell is that offset voltage measured on preamp's or buffer's output ?
If it's the preamp you are talking about, you better use P1=200R as per schematic in post #67 because 2sk246/2sj103 have much lower gm than 2sk170/2sj74.
 
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