Ok
I will order TL072.
I have another question. Can I put potentiometer between Cf and R1 so I can lower the gain if I need to? The problem is that this preamp will be part of PA system and I can't rely on volume control on the laptop because ppl are always walking to the laptop pumping up the volume
And... 300mA @9VAC (12,6VDC) transformer wound be enough to power the preamp right?
I will order TL072.
I have another question. Can I put potentiometer between Cf and R1 so I can lower the gain if I need to? The problem is that this preamp will be part of PA system and I can't rely on volume control on the laptop because ppl are always walking to the laptop pumping up the volume
And... 300mA @9VAC (12,6VDC) transformer wound be enough to power the preamp right?
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You could use a pot like that. You would need to take care in the wiring to it (keep it very short) to avoid it picking up stray noise.
Always keep R1 in series with the pot, even if you fitted something like a 100k control. R1 prevents the gain going to high and the opamp crashing out when you get to crazy gain settings.
Always keep R1 in series with the pot, even if you fitted something like a 100k control. R1 prevents the gain going to high and the opamp crashing out when you get to crazy gain settings.
Ok, thank you very much. I just got parts for the filter(s). I will add filters and a pot.
What part should be improved and what are the benefits? I like the way it plays, sound is clear and natural.
EDIT:
I added the filters, resistors and a pot (stereo one for both gains).
Gain controll works and there is no noise when no source connected (some noise was gone when I changed switching psu for traditional regulated one).
My amplifier has some sort of signal detection. It detect`s if there is signal and if there is no signal it "cuts" the inputs so that even if you have nothing connected to it and if you touch the jack with the hand there is no hum. There is even no hiss if music is not playing and volume controll is turned all the way up.
Well this function is not working when preamp is connected to it.
Is there a way to make a simple circuit that would detect if there is input signal on preamps input and if there is no it would "cut" the outputs on the preamp?
I think that this could be done by using relays on the output and transistors to drive the relay coils?
OK, its a valid design...... and everyone has their own ideas 
My comments would be that the OPA1622 isn't diy friendly in VSOM (very small outline form). Its only 3 x 3 mm in size.
You've gone for a voltage gain of 27 (28.6db) in the three final stages. That's pretty high and I think more than the design brief called for.
Not sure on the twin gang pot arrangement. There are valid arguments not to pass the opamps bias current from the input pin directly through a wiper (noise and unreliability as the contact moves) although the arrangement would in practice work. You would be unlucky for both wipers to generate an OC condition at the same time. If that happened the opamp would crash out with a swing to one of the rails.
The circuit would need considerable redesign for single rail operation.
No DC blocking is incorporated which for a general application is always a good idea imo.
My comments would be that the OPA1622 isn't diy friendly in VSOM (very small outline form). Its only 3 x 3 mm in size.
You've gone for a voltage gain of 27 (28.6db) in the three final stages. That's pretty high and I think more than the design brief called for.
Not sure on the twin gang pot arrangement. There are valid arguments not to pass the opamps bias current from the input pin directly through a wiper (noise and unreliability as the contact moves) although the arrangement would in practice work. You would be unlucky for both wipers to generate an OC condition at the same time. If that happened the opamp would crash out with a swing to one of the rails.
The circuit would need considerable redesign for single rail operation.
No DC blocking is incorporated which for a general application is always a good idea imo.
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9vac 0.3A transformer >diode bridge >1000uF cap >12v voltage regulator >1uF cap
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