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I am measuring 18 milivolts DC at the input jacks to my phonograph preamp with nothing connected to it. My thinking is that there should be zero voltage present at the inputs. Otherwise there could be overload or distortion when the MM cartridge is hooked to this input. After all, my cartridge (Ortofon OM10) only puts out 4 milivolts itself. My VOM is not the finest but I can see the DC voltage fall to zero and back up again when I disconnect and reconnect the AC power
Follow Ups:
A voltage at the input of any amplification often points to a defective part. In a tube unit, most often this is a bad tube (the scenario Victor points out only occurs if there is also an engineering error).In a solid state unit, it is possible for FET input devices to develop leakage. In any case there should be no voltage at the input whatsoever (although it is acceptable for a voltage to 'seem' to be there, but then fall to zero as you continue to observe it).
If the condition is persistent, I recommend service. Very small voltages like this almost always mean the unit is not performing as intended!
Thank you all for your input. I have sent the preamp back to Parasound. They were willing to check it out for me and we will see what they have to say about it. I first noticed problems with what I thought were distortion/overload/frequency imbalance. Changing cartridges made little difference. The resistance at the inputs were correct at 47kohm, but I did notice the small DC voltage present.
I use a digital multimeter only when absolutely needeed. Most of the time they tell the truth, but some are sensitive to RF and most will feed you numbers under almost any conditions.Analog meters are less accurate and can also be fooled, but a cross check for sanity can't hurt.
Oh, dear...Presumably when there is nothing connected to the input, you are measuring the input bias current flowing through the 47k resistor.
When you connect your cartridge, with its low resistance, that 18mV number will drop proportionally.
If a tube amp, then maybe you just read some grid current. Like 0.4uA.
Its a solid state preamp, a Zphono by Parasound. I think a tube amp would have a capacitor to block any DC voltage from apearing on the inputs and wouldn't a solid state amp be the same?
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