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Old 26th April 2004, 02:28 PM   #1
mgreene is offline mgreene  United States
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Default Reduce Input Gain Question?

Got my battery powered NIGC running over the weekend. Quite impressed with the sound, (1mv and 11mv offset.)

However, I built it to use with an active pre and it is TOO LOUD. I have a TVC output on my pre (does not fully attenuate the signal) and 93dB speakers. I used the same values as on Peter's monoblock schema. Just a 220R input series resistor to + and a 22K to ground.

I read here that the feedback resistors set the gain. How much attenuation can I expect by increasing the 680R from (-) to 1K or more?

I did try different values on the input series resistor - I happened to have a 3.6K Caddock and I thought it may have sounded better overall (?) , but didn't reduce the gain much. Any caveats I should consider, e.g., ratios of the input resistors?

Thanks

Mike
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Old 26th April 2004, 03:48 PM   #2
sss is offline sss  Israel
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if u are using not inverted gc then:
take the value of the resistor connected between the output of the chip to the negative input and divide it by the value of the resistor connected between the negative input and ground
if u will show a schematic it will be much easier to explain
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Old 26th April 2004, 03:58 PM   #3
mgreene is offline mgreene  United States
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Hi, Thanks for the reply.

I am using the schema here:

http://www.diyaudio.com/forums/showt...546#post265546
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Old 26th April 2004, 04:08 PM   #4
sss is offline sss  Israel
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thats better
so the gain of your amp is 22000/680 = ~ 32
u can increase the value of 680r or reduce the value of 22k

dont play with the other input resistors hough
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Old 26th April 2004, 05:43 PM   #5
mgreene is offline mgreene  United States
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Thanks,

I get it now.

Mike
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Old 26th April 2004, 06:07 PM   #6
sreten is offline sreten  United Kingdom
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Note that if you reduce the gain via feedback you
are also reducing the stability margin of the amplifier.

sreten.
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Old 26th April 2004, 07:19 PM   #7
mgreene is offline mgreene  United States
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I am open to other suggestions - althought I hve found several references from those who have apparently used 1K and 22K in the feedback loop with no problems.

I was thinking of rigging up an 1 step "attenuator" like a 10K series resistor and a shunt resistor to halve the gain. Any suggestions on values to accomplish this?

Mike
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Old 27th April 2004, 02:06 AM   #8
sss is offline sss  Israel
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its better to lower the gain of an amp then to use input attenuator , lower gain = better performance , anyways imho changing the gain from 30 to 20 or 10 wount do nothing to stability
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Old 27th April 2004, 10:48 AM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by sss
its better to lower the gain of an amp then to use input attenuator , lower gain = better performance , anyways imho changing the gain from 30 to 20 or 10 wount do nothing to stability
In your opinion stability is not affected ?

Opinion doesn't come into it. Gain margin and phase margin do.

sreten.
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Old 27th April 2004, 11:32 AM   #10
sss is offline sss  Israel
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Quote:
Originally posted by sreten

In your opinion stability is not affected ?
well i think if u'll change the gain to 20 or 10 there wount be any stability problems
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