Hi! Please help me with certain issues in LM1875 chipamp design! - Page 2 - diyAudio
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Old 4th September 2012, 01:09 AM   #11
sreten is offline sreten  United Kingdom
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Hi,

The audio bandwidth you set an amplifier to has very little to do with
the amplifiers gain bandwidth product, which is all about the stability
of the amplifier at various gains, and as as you reduce closed loop
gain you also need to reduce open (not closed) loop bandwidth.

I suspect you don't really understand what the numbers mean.

Otherwise you'd be talking about phase and gain margins.

rgds, sreten.
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Last edited by sreten; 4th September 2012 at 01:11 AM.
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Old 4th September 2012, 11:51 AM   #12
AndrewT is offline AndrewT  Scotland
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Look at the datasheet.
Build a fully componented version. Include all the optional parts. They really are necessary for good performance when the odds are stacked against the amplifier.

Then learn what you did and how they affected performance. When you understand the bits around the outside of the chipamp, you may even want to experiment with some of the values of those surrounding components.

I'd suggest that only after that, you come back and start asking about what goes on inside the chipamp.
There are some here who work in the IC industry and some who designed the National chips. They are a wealth of information.
But first understand what is needed to get a chipamp working properly.
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Old 5th September 2012, 11:13 AM   #13
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I dont even understand your question just reading it hurts my head :/ cant wait to start building amps
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Old 6th September 2012, 12:50 PM   #14
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Quote:
I suspect you don't really understand what the numbers mean.
Yes! Yes! Mr.Sreten your suspicion is 200% correct. Please, Can you really make me understand what the numbers mean?

Quote:
Otherwise you'd be talking about phase and gain margins.
I really wanna talk about phase and gain margins. But, i cannot do this without your help and support.

Thanks.

Last edited by noddy55; 6th September 2012 at 12:56 PM.
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Old 6th September 2012, 12:55 PM   #15
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Mr. stenwo123,

Please, don't wait for my questions and their possible answers.
They are endless in a quest to build something very near, if not exactly perfect. Plz, Carry on with your project. Follow k50 diagram and consult the datasheet. And, come back to this forum after, if you feel any problem. Share your experience with all of us along with some great pics. We will be happy to see your project working live. And, also let me know if i can help you in any way. There are lots of people who will really help you here.

Thanks.

Last edited by noddy55; 6th September 2012 at 01:02 PM.
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Old 9th September 2012, 10:28 PM   #16
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Well,

I don't feel that Point 1, 3, 4th last part, 5, 6 and 7 are anyway related to chipamp internal design. They are all pertaining to the external k50 kit circuit.They are still not answered. I wonder if anyone at diyaudio could really help me understand them?.

Thanks.
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Old 10th September 2012, 01:45 AM   #17
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Since my earlier answer you've posted the schematic so I can see how to answer a couple more of your questions.

Firstly about there being two high pass filters - they are rather different. The first one (the input HP) is a true HP and its to protect the chipamp against DC on its input terminal. The feedback HP is not a true HP in that below a certain frequency the response no longer keeps falling. In the technical parlance we say there's a lower frequency pole which cancels out the zero and leads to a flat response. Its because of this flat response at lower frequencies that we also need the input HP.

The answer to question 5 is no.

The remaining questions don't seem interesting enough for me to answer them, perhaps others will chip in. No.6 isn't a question at all. No.7 contains a false premise - there's no need to do impedance matching at audio frequencies.
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Old 10th September 2012, 03:46 AM   #18
benb is offline benb  United States
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Quote:
Originally Posted by noddy55 View Post
7.R1(1K) with R2(1M) forms the asymmetric L-Pad 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?
I'll take a shot at this. As stated, there's no need for impedance matching, and this isn't meant to be an "L pad" anyway. The dB loss is low enough to be insignificant.

We can calculate the loss. Firstly, at audio frequencies C1 is effectively a short circuit (look up the formula for capacitive reactance, and calculate it for this cap at 20Hz and 20kHz), so the bottom side of this "L pad" is actually R2 in parallel with R3. Since R3 is so much lower than R2 (1/10th its value), the effective value is only slightly lower than the lower value, or the value of R3. With R1 this forms a voltage divider of R3/(R1+R3), or 22/23, or just slightly less than 1. Using the voltage-ratio-to-dB formula (I'm too lazy to go through it, but it's not too complicated) it would probably come out in the ballpark of 0.1dB, or (for the purpose of figuring out the gain of this circuit) practically no loss.

But of course that's not what these resistors are there for. R2 is to bleed off any charge on C1 and on any output capacitor of the unit that's driving this amp. R3 provides a DC bias/ground path for the chipamp's input so the output is at (or very near) 0V with no signal. Not sure what the designer was thinking with putting R1 in, though it may offer some protection for an excessively high input signal voltage. Perhaps the chipamp data sheet says something about this.

I think this is a case where "a little knowledge is dangerous." You seem to know a little bit about electronics, but there are big gaps in your knowledge. Get some old textbooks on DC circuits, and then on "AC circuits analysis," plow through them and then you'll know how to do the calculations I described. Doing approximations and knowing when to ignore things (such as R2 for the input loss calculation) is more a matter of feel and experience, of seeing the resistor ratios and doing a gross in-head calculation that tells you "these resistors give insignificant signal loss, so chances are they're there for other reasons," or even "I've seen this type of circuit before with opamps, so I see C1 isolates DC, R3 biases the + input to 0V,..." Then for some of the other questions there are textbooks on the basics of opamps. Don't get the latest current editions used in college electrical courses unless you're actually taking the course - older editions of used textbooks will do fine and are available for much cheaper, virtually for free. There are even websites that teach this stuff.
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Old 10th September 2012, 03:58 AM   #19
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1) Build and measure it or use a SPICE modeling program.

3) R4/C3 sets the low frequency rolloff and reduces DC offset at the output because it reduces the gain to 1 (from pin 1 of the IC) at DC. C1 prevents DC on the input from getting to the to the speaker. In general the network with the highest corner frequency sets the low frequency rolloff.

4) A small capacitor in parallel with R2 can set the upper frequency rolloff. Otherwise the gain and GBW product with determine the HF rolloff.

5) No

6) R3 and R5 set the closed loop gain by the formula Av=1+(R3/R5). These should be chosen so as the capacitance at pin 2 of the chip and any parasitic capacitance of the PCB do not adversely affect the frequency response.

7) The general formula to convert a voltage ratio to dB is Av(dB)=20*LOG10(V/V). R2 is intended to prevent thumps through the speaker if the source is connected while the amp is powered on. It can be used as a voltage divider if desired. Remember R2 is basically in parallel with R3 for AC frequencies. R1 provides some protection against bad inputs, static discharge and Radio interference.
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Old 10th September 2012, 03:59 AM   #20
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Default Recommended literature

Here's my favourite book on understanding opamps - its out of print by the looks of it but there are a few used copies going - bag one while they're still available -

Amazon.com: Used and New: Intuitive IC OP amps (National's semiconductor technology series)
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