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preamp get a hum when volume is at full or 80% is thier away to stop it??

prorms

Member
2006-11-15 2:28 am
preamp get hum when volume is at full or 80 % .even when thier
is nothing connected to the amp ill still get hum i know its the tube but iam not so sure on how stop it also if i keep the volume at 50% and hook up anther opamp again to make it louder the hum still can be heard is thier away get the hum to stop????
 

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kevinkr

Administrator
Paid Member
I hope the op-amp circuit is not built as shown.. Feedback is always to the inverting input.. Plus as designed the input impedance of that op-amp as an inverting circuit is only 33K - doesn't work so well with a 1M pot. Use the non-inverting configuration with the gain you need instead that way you really have a 1M pot that is not loaded down. (The non inverting input should have an input impedance many times higher than the thevenin equivalent of the pot wiper at worst case of 250K.)

Given the miniscule currents in the 12AU7A at the low supply voltage used reducing the 1M pot to anything less than 500K would probably give bad results. You need additional shielding and make sure the pot body is grounded.
 

kevinkr

Administrator
Paid Member
prorms said:
i put on inverting input feedback work great for when nothing is connected to amp makes a whisle noise resistor are 820 to the ground 4.7k to the output and - side of the opamp

And the pot is connected to the non-inverting (+) side of the op-amp right?

Whistling as described sounds like another term for oscillation. The connections at the (-) input of the op-amp must be very short for good stability. A small cap - say 30pF - 100pF in parallel with the 4.7K feedback capacitor will compensate for higher than expected input capacitance if your layout is not so good. Finally make sure that you have a power supply bypass cap to ground right on the supply pins of the op-amp.
 
In your above pre, the supply is 12V.....

You also have very high plate resistors, therefore very little current flow and need high value pot or the gain will suffer badly.

I have experimented with tubes at 12V and devised this gain stage followed by a follower that can supply a 100K pot fairly easily....
 

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