hello - I'm working on some toa rx-7 input strips, they have a transformer and a discret opamp front end. the opamps (4558dx) before and after the fader have a 47pf cap between the +/-inputs. can some one shead some light on what these do? Arthur
47pf cap across the +/- input of an opamp
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In general a capacitor between + and - minus input attenuates differential input signal at HF. I suspect it is a strategy to increase HF stability.
4558 don't have much HF performance to speak of, so rolling it off up there is no great loss, but I would lose them if you upgrade to modern higher performance opamps.
JR
In general a capacitor between + and - minus input attenuates differential input signal at HF. I suspect it is a strategy to increase HF stability.
4558 don't have much HF performance to speak of, so rolling it off up there is no great loss, but I would lose them if you upgrade to modern higher performance opamps.
JR
bcarso
Well-known member
Often in audio opamp circuits something of order of that value, as a leaded part, is series-resonant somewhere useful for the most obnoxious high-freq interference. I've seen 39pF most commonly.
As discussed by some authors (I think Walt Jung in particular IIRC), JFET input amps are of order 100x less susceptible to RF rectification effects than bipolars, so in many cases they won't need the input caps.
As discussed by some authors (I think Walt Jung in particular IIRC), JFET input amps are of order 100x less susceptible to RF rectification effects than bipolars, so in many cases they won't need the input caps.
I don't know how common this is, you need hundreds of mV of RF to get rectification in a bipolar LTP and if they use darlington inputs or emitter degeneration even more voltage than that is needed (bi-fets can handle volts of RF, at least in theory). This is made worse by very slow opamps and high noise-gain.
That schematic's liberal use of several times larger feedback capacitors than the input shunt is arguably counter productive against that input cap keeping the RF CM to both opamp inputs.
I only encountered this once in production inside a small mixer that I inherited. We got some service complaints about RF in one of the buses. I didn't want to go inside and revisit some other engineer's design with a major tear up and redo, so I dropped a BIFET into the bus amp socket with a quick engineering change notice, and never thought about it again.
In general if you bandpass your inputs and decouple outputs, you shouldn't find hundreds of mV of RF in your opamp inputs.
JR
That schematic's liberal use of several times larger feedback capacitors than the input shunt is arguably counter productive against that input cap keeping the RF CM to both opamp inputs.
I only encountered this once in production inside a small mixer that I inherited. We got some service complaints about RF in one of the buses. I didn't want to go inside and revisit some other engineer's design with a major tear up and redo, so I dropped a BIFET into the bus amp socket with a quick engineering change notice, and never thought about it again.
In general if you bandpass your inputs and decouple outputs, you shouldn't find hundreds of mV of RF in your opamp inputs.
JR
