Purpose of series input resistor in guitar preamp?

If somebody plugs the output of another guitar amp in the input, it toasts the resistor and perhaps protects other things. If 1/8 watt, much cheaper than a fuse. I got a big discount on my CS800S PA amp because the input resistors were toasted (& nothing else). That scenario is my best guess at what happened. There are lots of 1/4 phone plugs on stage. Some are guitar outputs, some are amp outputs headed for the speakers, which before special Speak-on connectors invariably had 1/4 phone jacks. Garage bands still buy the cheaper 1/4 phone version of amps & speakers.
 
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How about this: the designer took a basic tube preamp circuit and "converted" it to op amps, part by part. SO it has the same basic configuration as a couple of triode stages. So in that sense, I bet it really is a grid stopper.

I would agree as the amp already has an input resistor before the coupling capacitor.
 

miallen

Member
2009-05-29 7:44 pm
Ok, I asked the author of the schematic and he replied:

"The resistor is an RF "stopper", and helps prevent RF pickup and demodulation. The general scheme is described in the amp design article.

Despite what's claimed in the forum, it isn't simply transposed from a valve circuit, but is quite deliberate. The resistor has to be physically very close to the opamp input for it to work properly."
 
"The resistor is an RF "stopper", and helps prevent RF pickup and demodulation. The general scheme is described in the amp design article.
."

But the op amp input capacitance is neglible and certainly isnt a very good filter with that value resistor.

The usual way to do it is an RC input filter that filters out RF and other out of band signals.
 
But effective or not, efficient or not, we now do know WHY it was there.

You really need to fix the problem or the next time some with a radio transmitter goes by you will pick up their conversation.

The first Maplin amplifier in I built in 1980 picked up local police broadcasts a treat.
That was a bit embarassing when i was running my disco at the time.
 
It will get rid of mobile phone interference, which these days can be the main culprit. The preceding coupling capacitor is plenty big enough to act as an antenna at those sort of frequencies. A metal case will not prevent microwaves from leaking in as they sneak through the joints (unless they use finger stock to make them RF proof, which nobody does for audio amps).
 
OK it's an old thread but answer is not given. The purpose of the serial resistor in the input op is to protect against an excessive cuurent which is destructive (see datasheet, absolute maximum rating) -> 10mA is the standard value for modern op. With a 1k resistor, max voltage at input can be 10V & with 10k, max is 100V (the + or - op inputs are virtuel grounds so ohm's law is applicable easily).
 
In the TUBE world, the input resistor 47k to the GRID and 1Meg to ground was a low pass ckt to get rid of RADIO interference. This also is used for FET Opamp’s circuits.
Duke

without any C or L in the circuit (where impedance depends on frequency), I don't think you will cut any RF signals ;)

The input resistor is a protection against overvoltage & overcurrent (in a opamp, max input current I=10mA so a 1k R protects against 10V at the input & 10k against 100V. It could be used with clamp diodes to have a reliable protection of opamp input). same thing for jfet (but max input current before destruction could be higher than for an opamp). I don't know for tubes but I think it is same thing. The input resistor could be used too for different input level configurations because with the resitor to ground, it is an attenuator.