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Chip Amps Amplifiers based on integrated circuits 

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28th August 2012, 12:39 AM  #1 
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Join Date: Feb 2012

Hi! Please help me with certain issues in LM1875 chipamp design!
Dear Sir,
I am really confused about certain issues with chip amp design. I am sure you could help me. 1.How we can determine the exact bandwidth of a particular design like LM1875 K50 Kit circuit diagram? 2.If we want to design a bandwidth of 20Hz to 20Khz, then lower 3db cut off should be one decade below the required frequency i.e 2Hz and better if we can go up to 0.6Hz. What about the higher frequencies 3db cut of? Do we need to go one decade or more with them also? ie. 20Khz*10 = 200Khz? 3.We have two high pass filters (C1/R3 and R4/C3) in k50 circuit diagram. Why two? What if we set both of them to different 3db cut of? Which one of them will be effective? How do they both affects each other? 4.There is no Low pass filter in K50 diagram.How we limit the higher frequency to say 20Khz(required) or 200Khz (need to be set to get the actual required)? If we do not set the higher frequency limit, by any means what will be the effective higher frequency, in such a scenario and how we will determine this effective frequency? 5.We determine the value of c3(22uf) in conjunction with R4(10k) with a formula: fl = 1/2 * pie * F *C Is the determination and final selection of this capacitor value is also affected by R5(180K) or R3(22K) in any way? 6.Please, recall the relation of R3(22K) with R5(180K). 7.R1(1K) with R2(1M) forms the asymmetric LPad atteunator as well as impedance matcher.How we will determine the actual atteunation performed by these two resistors in decibel(db)? and, with what direction (higher side/or lower impedance side) the impedance is matched with these two resistors in k50 diagram? I want to know the way if and how i could change these values for a different configuration but with the same effect? 8. GBP is a gain and bandwidth product which always remains constant, no matter we change the gain or the bandwidth. If we increase bandwidth, gain will decrease and viceversa. We know that: GBP = Gain*Bandwidth, or Bandwidth = GBP / Gain. Also, Bandwidth / 1000 = Bandwidth in Khz. GBP for LM1875 is 5.5Mhz. So, i found certain values (assuming GBP=5Mhz) based on the above formula and k50 diagram: R5(Feedback resistor)/R4(Feedback Shunt Resistor) 150k/10k (gain 16.00V/V) (bandwidth 313Khz) 47k/2.7k (gain 18.41V/V) (bandwidth 272Khz) 180k/10k (gain 19.00V/V) (bandwidth 263Khz) 47k/2.2k (gain 22.36V/V) (bandwidth 224Khz) 68k/2.7k (gain 26.19V/V) (bandwidth 191Khz) 56k/2.2k (gain 26.45V/V) (bandwidth 189Khz) 100k/3.3k (gain 31.30V/V) (bandwidth 160Khz) 47k/1.5k (gain 32.33V/V) (bandwidth 155Khz) 100k/2.7k (gain 38.04V/V) (bandwidth 131Khz) You can see the product of gain and bandwidth is always near about 5Mhz or constant. Now my questions or confusions are: i). Do this 5Mhz GBP is always automatically itself attainable all the time? Can't we have a lower GBP at a particular time? ii). If we decrease the gain by Rf/Ri ratio, will it automatically(due to GBP contant rule) increase the bandwidth, without touching any high pass or low pass filters in the circuit? or, iii). If we insert a low pass filter at the input of the amplifier, thus only intending to change/limit the bandwidth, will it also automatically(due to GBP constant rule) change the gain set by Rf/Ri? Thanks. Last edited by noddy55; 28th August 2012 at 12:47 AM. 
28th August 2012, 01:23 AM  #2 
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Join Date: Nov 2003
Location: Brighton UK

Hi,
Learning is about asking the right questions, not just asking questions. Loaded questions seeking confirmation aren't really questions at all. You haven't provided a link to the circuit in question. rgds, sreten.
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28th August 2012, 01:59 AM  #3  
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Just cherry picking the more interesting questions (ones not requiring the schematic details) here...
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28th August 2012, 10:00 PM  #4 
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Sorry! Mr sretan, my apology,
Here is the circuit diagram for K50 LM1875 Audio Kit. Thanks. 
28th August 2012, 10:34 PM  #5  
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Thanks! Mr. abraxalito for some of the answers.
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Thanks. Last edited by noddy55; 28th August 2012 at 10:41 PM. 

29th August 2012, 04:42 AM  #6  
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__________________
“A new scientific truth does not triumph by convincing its opponents ... but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”  Max Planck 

29th August 2012, 11:42 PM  #7  
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Quote:
In short, we can set the gain and bandwidth product of overall circuit to any value, so far as it lies in the range of 5.5Mhz. Quote:
Sorry! but i remain always confused about this decibel(db) thing. I still did not understood .1db at 17Khz. Can you please, elaborate more about this decibel(db) thing for me? Thanks. 

29th August 2012, 11:59 PM  #8  
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“A new scientific truth does not triumph by convincing its opponents ... but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”  Max Planck 

30th August 2012, 01:31 AM  #9 
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Location: Hamilton, was Ottawa (Canada)

Check the chipamp forum for recent posts about the 3886.
There is a good discussion on the proper sizing of the input cap and its relationship with the capacitor in the feedback network. As well as comments regarding a high freq cap you can put accross the inputs. I have the K50 but our very own Chipamp.com has a much tighter PCB. You get 2 compact 1875 boards and a power supply board. Good luck and keep us posted! Cheers, Jeff PS the chipamp site is a bit rusty but they seem to be stirring again. Last edited by AudioLapDance; 30th August 2012 at 01:34 AM. 
3rd September 2012, 10:14 PM  #10 
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Thanks, Mr. abraxalito,
Point 3, 4th last part, 5, 6 and 7 above are still not answered. If anyone can? Then Please. Can't go for more questions until. Thanks. 
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