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Old 13th December 2010, 10:00 PM   #1
miallen is offline miallen  United States
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Default Purpose of series input resistor in guitar preamp?

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
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Old 13th December 2010, 10:23 PM   #2
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they're 'grid stoppers' :Grid Resistors
although I suppose they've got a new name when applied to ICs??
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Old 13th December 2010, 11:58 PM   #3
miallen is offline miallen  United States
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Very interesting. That's exactly what I wanted to know.

Thanks,
Mike
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Old 14th December 2010, 12:01 AM   #4
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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.
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Old 14th December 2010, 04:42 PM   #5
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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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Last edited by indianajo; 14th December 2010 at 04:44 PM.
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Old 14th December 2010, 05:24 PM   #6
DF96 is offline DF96  England
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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.
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Old 15th December 2010, 12:51 AM   #7
Enzo is offline Enzo  United States
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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.
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Old 15th December 2010, 12:59 AM   #8
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Quote:
Originally Posted by Enzo View Post
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.
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Old 15th December 2010, 10:04 AM   #9
DF96 is offline DF96  England
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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.
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Old 16th December 2010, 11:23 PM   #10
miallen is offline miallen  United States
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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."
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