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Old 11th February 2010, 11:48 PM   #1
GregH2 is offline GregH2  Australia
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Default A few chipamp configuration questions

Hi all,


I've been looking at some of the commercial kits available to try and get a handle on typical resistor values for gainclones. The one that seems to vary the most is the input resistor which varies from 220R to 1k, while the feedback resistors are universally 22k and 680R.

I am interested to know the relative merits (if any) of these different values for the input resistor.

Additionally, some place the 22k resistor from the input to ground before the input resistor and some place it after the input resistor. Is there any advantage to either method?

Your advice is appreciated.

Greg.
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Old 12th February 2010, 11:09 AM   #2
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Quote:
Originally Posted by swordfishy View Post
I am interested to know the relative merits (if any) of these different values for the input resistor.
Judging from the values you are refering to the blocking resistor. The datasheet describes its function like this.
Quote:
Prevents currents from entering the amplifierís non-inverting input which may be passed through to the load upon power-down of the system due to the low input impedance of the circuitry when the under-voltage circuitry is off. This phenomenon occurs when the supply voltages are below 1.5V.
The bigger the resistor, the better the protection and the bigger its potential bad influence on the sound.


Quote:
Originally Posted by swordfishy View Post
Additionally, some place the 22k resistor from the input to ground before the input resistor and some place it after the input resistor. Is there any advantage to either method?
If the blocking resistor (1k) is placed before the input resistor (22k to ground) they form an attenuator. That reduces the input voltage by 1/23. If the blocking resistor's value was higher, you could use that as a limiter with powerful sources. With the blocking resistor after the input resistor you don't loose any input voltage, which may be an advantage with weak sources. No big difference either way.
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Old 13th February 2010, 11:24 AM   #3
GregH2 is offline GregH2  Australia
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Great thanks. Just what I needed to know.
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Old 15th February 2010, 01:56 PM   #4
AndrewT is offline AndrewT  Scotland
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and you can combine that input protection resistor/blocking resistor with a small capacitor to create an RF filter to help the chipamp perform as intended.
I've seen 100r to 2k2 for that resistor.
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