I think your choices are unrealistic - to get that much gain you'll have to use very high I/V resistor (2k2 - R9 in sch. post #66) which determines output resistance of the preamp. 2k2 won't work great with your amp's Zin of 47k. If I were you I'd go for gain of 12dB max (R9=1k).
Think about it for a while and get back when you find out answers for questions 1. (without that I can not calculate current values for the rest of the circuit) and 4. (9V/V is a gain value not the output swing - you have to know how much voltage you need to drive your amps to full power and than add 100% headroom for quieter recordings/sources)
Think about it for a while and get back when you find out answers for questions 1. (without that I can not calculate current values for the rest of the circuit) and 4. (9V/V is a gain value not the output swing - you have to know how much voltage you need to drive your amps to full power and than add 100% headroom for quieter recordings/sources)
Juma, it will help a lot if you can please post values of components to be used for BL grade JFETs as those are the ones, most people possess or have access to. I also have BL devices and am interested in building the line stage after completing the power amp you posted which I am working on at the moment. Thanks.
OK,
input sensitivity of DH-220 is 1.55Vrms so I say our target is 3Vrms of output swing (meaning that voltage dividers at BJTs' bases should provide the Vds of about 10V) and the gain of 9dB (R9=680R).
Now we need just a JFETs' Idss data. If it's between 7-10mA the schematic in attachment is for you. If you want CCSs just put them instead of R1,R11 and set them to about 16mA.
It would help even more if BL range weren't so wide (6-12mA).
The idea is to run JFETs at about 80% of Idss and provide 8-10mA more for BJTs so our CCS (or R4,R5) should be calculated for that current. For example:
we have JFETs with Idss=10mA so we expect them to have Id of about 8mA with Rs=10R (approximative degeneration with P1=20R). Add about 9mA for BJTs and that's it - CCS (or R4,R5) should supply about 17mA (both CCS in one channel should be matched to same current). Matching and thermal coupling will help to obtain more stable DC offset.
Attachments
Juma preamp combo (JFET/BJT)
I'm playing with an idea to merge JFET (post #66) and BJT (post (#101) layout on the same PCB. That possibly solve a problem for people who cannot easily obtain JFETs. I also added two optional CCS from some other schematics.
Does it make any sense to organize a new GB using such design?
Of course, Juma if you have anything new to suggest please let me know I can try to add it in new layout.
I'm playing with an idea to merge JFET (post #66) and BJT (post (#101) layout on the same PCB. That possibly solve a problem for people who cannot easily obtain JFETs. I also added two optional CCS from some other schematics.
Does it make any sense to organize a new GB using such design?
Of course, Juma if you have anything new to suggest please let me know I can try to add it in new layout.
Attachments
Yes, MT is Multi Turn variable resistor.
And yes, fixed resistor should be 82 ohms, not 100 ohms. Then, with MT in middle position, equivalent resistance will be about 70 ohms, very similar to resistance of 68 ohms (emitter resistor of Q3).
So, if you have, for example, 6 mA on Q1 transistor, 5 mA will flow through fixed resistor (82 ohms) and only 1 mA through MT (500 ohms, middle position), which is much lower than current limit of MT.
And yes, fixed resistor should be 82 ohms, not 100 ohms. Then, with MT in middle position, equivalent resistance will be about 70 ohms, very similar to resistance of 68 ohms (emitter resistor of Q3).
So, if you have, for example, 6 mA on Q1 transistor, 5 mA will flow through fixed resistor (82 ohms) and only 1 mA through MT (500 ohms, middle position), which is much lower than current limit of MT.
Of course, it's much better to use exact values as in Juma's schm, but I'm Poor Serbian Man, so I'm used to compromises, because my whole life is big compromise.
I must say, I drew this pcb for myself, not for public use. It's not finished yet and not verified, but it may be useful for Eric.
I must say, I drew this pcb for myself, not for public use. It's not finished yet and not verified, but it may be useful for Eric.
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