[sdiy] Op-amp versus noise

[David Ingebretsen]

Thanks Neil.

David


>> -----Original Message-----
>> From: Neil Johnson [mailto:neil.johnson71 at gmail.com]
>> Sent: Sunday, March 23, 2014 2:47 PM
>> To: David Ingebretsen
>> Cc: synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] Op-amp versus noise
>> 
>> Hi Dave,
>> 
>> > I just used a basic non-inverting configuration with 22k in series
>> > with a 100k trimmer in the feedback and a 30k from "-" to ground. 22p
>> > cap in parallel with the feedback "resistor". The opamp is an NE5532.
>> 
>> First case:
>> 
>> The NE5532 is a bipolar op-amp so the input bias currents are appreciable.
>> Generally the effect of these bias currents is to produce unwanted offset
>> voltages.  First check you can do is to see what the DC level of the op-amp
>> output is with no signal.  If you see millivolts or more then that will cause
>> scratchy noises on the pot.
>> 
>> Also make sure you have a resistor from the +ve input to ground, and
>> ideally this circuit should go inbetween the 10uF capacitor and the pot.  I
>> might also add a second capacitor after the op-amp to block offset voltage
>> from finding its way to the pot.
>> 
>> I would start by reducing the feedback resistors by a factor of ten, that will
>> reduce offsets due to the bias current flowing in such large resistors, and
>> thermal noise.
>> 
>> Second case:
>> 
>> Could it be a dodgy connection?  Was the output amplitude as expected?
>> Again, you would want to block any bias currents from flowing in the pot as
>> that will cause scratching sounds.
>> 
>> Hope this helps.
>> 
>> Neil
>> --
>> http://www.njohnson.co.uk