Hi,
Regarding the schematic in Figure 1 on this page:
100W Guitar Amplifier (Mk II)
What is the purpose of the 2.2k series resistor at the non-inverted input of the initial op amp?
Mike
Regarding the schematic in Figure 1 on this page:
100W Guitar Amplifier (Mk II)
What is the purpose of the 2.2k series resistor at the non-inverted input of the initial op amp?
Mike
they're 'grid stoppers' :Grid Resistors
although I suppose they've got a new name when applied to ICs??
although I suppose they've got a new name when applied to ICs??
It probably serves little or no purpose on the input to a modern op-amp.
I have designed loads of op amp gear and never used them.
I have designed loads of op amp gear and never used them.
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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Given that there are already higher value series resistors at the inputs I'm not sure what the 2k2 does. All I can think of is that if the 47nF cap is physically large (unlikely) then the 2k2 might reduce pickup of RF.
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.
But the owner of that site generally knows what he is doing. Why doesn't the OP email him and ask? That would save a lot of speculation here. Once we know the answer, we can debate whether we agree with him.
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.
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."
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.
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.
If my math is right (big if), and if the OPAMP has a 5pf input (OPA2134) the filter the 2,2k res makes with 5 pf has a corner at 15 Mhz. So it seems to do as advertized.
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
Duke
I did some tests with mobile phones close to IC-analog circuitry and found that 1~2 kOhm resistors before op-amp input pins greatly improve noise immunity by breaking the input loop antenna. Since then I add these in all my circuit designs.
The initial post referred to an op amp. If RFI suppression is used with an op amp input, it uses a resistor in series AND capacitor in shunt. The resistor is needed in case some idiot connects a speaker output from something else to the preamp input.
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.