Will i got any advantages if I change Q24, Q26 (BD139, BD140) to something like KSC2690 or 2SC3902?
What's their role in the circuit?
What's their role in the circuit?
I don't expect advantage but you can try. I wanted easy available transistors for this preamp. This is bootstrapped cascode, small transistors defined the quality and bigger one takes power dissipation.
Hello Dadod
I am now placing the gainwire into a new case and upgrading some components.
I got a 4 pole "short before break" rotary switch that I was planning to use as a stepped attenuator but now I decided to use it instead as a input selector switch..... hope the "short before break" action does not cause any issues.
I also need new volume pots and am wondering what should be the best impedance....
10k
20k
50k ?
Would you please help me decide ?
Best
Ricardo
I am now placing the gainwire into a new case and upgrading some components.
I got a 4 pole "short before break" rotary switch that I was planning to use as a stepped attenuator but now I decided to use it instead as a input selector switch..... hope the "short before break" action does not cause any issues.
I also need new volume pots and am wondering what should be the best impedance....
10k
20k
50k ?
Would you please help me decide ?
Best
Ricardo
Hi Ricardo,
Nice to hear that you move ahead. You can use 10k or 20 k, I think that 50 k is to high. I use 10 k but cheap one, for the moment no enough money to buy stepped one of good quality.
BR Damir
Yes, specially as with some DC current through it in DC connection as normal. If you don't want that DC current insert 1uF polypropylene cap between the pot and the preamp input.
Because of that small DC current the quality of the pot is very important.
I'm about to order parts for this preamplifier.
Looking at the bom list i see many trans.non stocked and other (irf640)very expensive.
Are alternatives for parts as 2n5457,2sk170 irf640 irf9640?
Looking at the bom list i see many trans.non stocked and other (irf640)very expensive.
Are alternatives for parts as 2n5457,2sk170 irf640 irf9640?
The 2N5457 ought to be pretty easy from Central Semiconductor (Mouser distributes). Linear Integrated Systems makes an LSK170, not sure who distributes them.
A lot of leaded parts are being phased out, unfortunately.
2n5457 is just a CCS set to about 2 mA, BF245A could be used with Idss from 2.5 to 3 mA, or any similar n-jfet. Instead IRF640/9640 could be used IRF540/9540. Only for 2SK170 I don't know replacement, as it should be n-jfet with very low gate-source cut-off voltage.
Instead of 2SK170 I am using 2SK117GR. It works even better because it catches up the final voltage in much less time.
Now working on input offset....
If I do correctly understand, input offset voltage is originated by beta mismatch between the two input devices.
One way to correct this would be to place a resistor between emitter and base of the bjt that has higher beta.
I placed a trimmer (100k) between the two emitters and bases according to the sim.
As the simulator does not help I modified one emitter resistor (R41) just to produce input offset and playing with the trimmer (R32 - R35) I am able to eliminate input offset.
Please see the sim and let me know if it should work in reality....
If I do correctly understand, input offset voltage is originated by beta mismatch between the two input devices.
One way to correct this would be to place a resistor between emitter and base of the bjt that has higher beta.
I placed a trimmer (100k) between the two emitters and bases according to the sim.
As the simulator does not help I modified one emitter resistor (R41) just to produce input offset and playing with the trimmer (R32 - R35) I am able to eliminate input offset.
Please see the sim and let me know if it should work in reality....
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