Easiest way = digital multimeter with transistor function and hfe readout. They are cheap...and easy to get. Like this:
https://www.amazon.com/Neiko-40508-...t=&hvlocphy=9033284&hvtargid=pla-436226440025
https://www.amazon.com/Neiko-40508-...t=&hvlocphy=9033284&hvtargid=pla-436226440025
I got a cheap multimeter with hfe measure at the nearest tool store. 12 dollars each! NPN shows the numbers 610~687 and PNP 436~470. I have no idea about the differences in range.
What I need is 6 of them each, so I grouped with nearest values and decided to use.
The problem is you want the p to match the n...ideally within 10% or so. You have good matches p to p and n to n but not between the two. The preamp will still work with the matches you have but matched p to n would probably be better and have lower distortion.
I had much closer matching with bc550/560. All were 475-525 hfe. Do you have different grade parts of p and n channel? I believe the suffix after the part (like a,b,c) is a indicator of hfe range.
Do you know the values of your jfets?
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Hi Hikari1, they are both C grade.
I do not know if can trust this cheap multimeter. I will find out if my there are any other multimeter that I can use.
Maybe I should go with BC560 550 circuit. Is this the circuit?
Thank you for your advice.
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Juma's handrawn schematic is the one to use with V grade jfets. The resistor values of r2/r4 are appropriate for V grade jfets.
You may need to play with the values of r3/r5 to get 10-15ma through 2nd stage bjt. You could in fact use the bjt's you have now if you are willing to trim r3 and r5 to achieve equal current through each side (10-15ma). but matched parts is easier.
Also, in the handrawn schematic you can see there is no P1 pot to control symmetry between jfets and adjust DC offset. That's fine if you are using an output cap. The other schematic (not handrawn version) shows the P1 pot to control offset if your goal is no output caps. Even with the pot, DC offset may fluctuate and you may need caps if your power amp is DC coupled.
My Amp is DC coupled and I am not brave enough to omit the caps. At least not yet. That is a goal for 2019....with either servo or better thermal contact between output devices.
You may need to play with the values of r3/r5 to get 10-15ma through 2nd stage bjt. You could in fact use the bjt's you have now if you are willing to trim r3 and r5 to achieve equal current through each side (10-15ma). but matched parts is easier.
Also, in the handrawn schematic you can see there is no P1 pot to control symmetry between jfets and adjust DC offset. That's fine if you are using an output cap. The other schematic (not handrawn version) shows the P1 pot to control offset if your goal is no output caps. Even with the pot, DC offset may fluctuate and you may need caps if your power amp is DC coupled.
My Amp is DC coupled and I am not brave enough to omit the caps. At least not yet. That is a goal for 2019....with either servo or better thermal contact between output devices.
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I assume you are talking about putting the pot in series with resistor; it should work but you need to measure across both resistor and pot to calculate current. You are wanting 10-15ma in second half of preamp...first half (jfet) depends on jfet idss.
I would put pot at r4 and/or r2 and balance jfets that way.
But bc550/560 are so easy to get and cheaper than good pots.
I would put pot at r4 and/or r2 and balance jfets that way.
But bc550/560 are so easy to get and cheaper than good pots.
Yes, I supposed current can be checked across just 10r resistor after the pot.
You should still use resistors or pot on jfet sources. Use 10r resistors and you should still be able to mull dc offset with pots at r4 and/or r2.
Also, did you know bc550 is the low noise version of bc547? Maybe it’s better?
You should still use resistors or pot on jfet sources. Use 10r resistors and you should still be able to mull dc offset with pots at r4 and/or r2.
Also, did you know bc550 is the low noise version of bc547? Maybe it’s better?
It will make me feel guilty if I do not use the "low noise version"......
Isn't that what this hobby is all about?