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Chip Amps Amplifiers based on integrated circuits

Question about designing Chip amps
Question about designing Chip amps
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Old 24th April 2018, 05:13 PM   #11
crisroxmysoc is offline crisroxmysoc  Canada
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Quote:
Originally Posted by Fast Eddie D View Post
I think he doesn't understand.

You can certainly improve upon the datasheet circuits. Some data sheet circuits have rather high low frequency poles. You can improve this. You can put bigger bootstrap caps in for chips that use them. You can use premium parts. You can parallel or bridge them. You can configure them as a Howland current pump. You can use them in a two stage circuit (composite) with a premium op amp in the front end. You can compensate for their input capacitance.

Etc.

Half the people here have done at least one of these improvements.
I really appreciate that thank you!

It looks like all you guys are converging to the same conclusion, let me give you more background on me.

I have already built a LM1875 Amplifier. The problem I was having was horrible distortion, which I believe it was ruled out being on a breadboard, and not using non to shorter leads.

I am an electrical engineer with a heavy background in control theory, so I know about pole frequencies and what not. I love this DIY community, as I think the concept of making an amplifier is beyond word I can express.

If I were able to make my own amplifier I would cry ahhah, I just want to be knowledgable as this guy here. But he leaves so many questions unanswered, for example designing components (no LPF, or HPF), why he needs a low pass filter for open loop configurations, and in order to add zeros/poles within the amplifier you need to add it in the feedback loop? I haven't worked control with amplifiers so this is a new concept I usually do it for electrical systems or other systems. When he is looking at the output power of the amplifier he just looks at the datasheet?


In the end, I just want a good place to get started, in how to design components in an amplifier, how to know which components I need in an amplifier, how to do control theory on an amplifier, how to get my wattage out of an amplifier.

Thank you all for your kind words, hopefully this fills in more blanks
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Old 24th April 2018, 06:25 PM   #12
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by crisroxmysoc View Post

But he leaves so many questions unanswered, for example designing components (no LPF, or HPF), why he needs a low pass filter for open loop configurations, and in order to add zeros/poles within the amplifier you need to add it in the feedback loop?
That is a composite amplifier. The 3886 is controlled by the op amp. The 3886 is dog slow compared to the op amp. This will degrade the phase margin to the point of oscillation. The low pass filter "slows down" the op amp to reduce gain to below unity at the frequencies where the phase margin is degraded. This is no different than employing an interstage capacitor (often referred to as "Cdom") in a discrete design.

Where the closed loop gain crosses the open loop gain, you want the slope to be 20 dB/decade for stability. With an op amp driving a 3886 the slope is 40 dB/decade where you need it to cross. This will be unstable. But by limiting the bandwidth of the signal from the op amp, you move the open loop response curve to a point where the intersection is at 20 dB/decade.

When you superimpose the curves to examine phase/frequency response it's called a Bode plot. If ever a picture was worth a thousand words then this is it. You can research this and learn a lot. Walt Jung (my mentor) examined this topic exhaustively and presented it in a way that is very easy to understand. "IC Op Amp Cookbook" is highly recommended, if you can find it.

Last edited by Fast Eddie D; 24th April 2018 at 06:29 PM.
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Old 24th April 2018, 06:57 PM   #13
GOR3 is offline GOR3  United States
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Quote:
Originally Posted by crisroxmysoc View Post
I just want to be knowledgable as this guy here.
Look at that guy's CV. That's how he got to be knowledgeable.
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Old 24th April 2018, 07:11 PM   #14
crisroxmysoc is offline crisroxmysoc  Canada
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Originally Posted by GOR3 View Post
Look at that guy's CV. That's how he got to be knowledgeable.
Just read it, rip me ahahahh
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Old 24th April 2018, 07:16 PM   #15
FauxFrench is offline FauxFrench  France
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Thank you for telling us about your background and your LM1875 experience. Control theory is very relevant for amplifiers and your previous LM1875 experience should be replaced by an amplifier that works.

Your first posting left the impression that you were asking how to design an amplifier in all its details and, as I have replied, all details encompass many considerations. Then you looked at one of the most elaborate LM3886 constructions available, that of Tom from Neurochrome. You regret you cannot follow Tom's considerations in details and that may be intended since that knowledge has given the possibility for Tom to finance his life for years during studies through income from Neurochrome. Most of us do not understand all the details either. Tom's LM3886DR is no starter design if you look in the details. When you understand all he writes in details, you will help us. Fast Eddie gave you some detailed information you should appreciate for a start.

I suggest you start with an LM1875 design that works. We can help you making such an amplifier work and you can apply your control knowledge when we go along. But accept for a start that all control aspects will not be available to you in the sense that those designing the LM1875 chip issued a datasheet in which you have all the information you need to make it work, but not all information about loop details they have implemented inside the IC. Amplifier IC's are sold on "need to know" basis in order not to help competitors too much.

For a start, you need a power supply. Do you have a symmetrical power supply with 15-25V (+ and -) output?

On the chip: if this project is mostly meant to teach you about making an amplifier work, you do not need to make two for stereo. The second amplifier won't teach you much new. Do you have an LM1875 chip ready?

Do you have a reasonable selection of resistors and capacitors available such that yo do not need to buy every time a component is changed or added?

What type of breadboarding are you used to work with?
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Old 24th April 2018, 07:38 PM   #16
crisroxmysoc is offline crisroxmysoc  Canada
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Quote:
Originally Posted by FauxFrench View Post
Thank you for telling us about your background and your LM1875 experience. Control theory is very relevant for amplifiers and your previous LM1875 experience should be replaced by an amplifier that works.

Your first posting left the impression that you were asking how to design an amplifier in all its details and, as I have replied, all details encompass many considerations. Then you looked at one of the most elaborate LM3886 constructions available, that of Tom from Neurochrome. You regret you cannot follow Tom's considerations in details and that may be intended since that knowledge has given the possibility for Tom to finance his life for years during studies through income from Neurochrome. Most of us do not understand all the details either. Tom's LM3886DR is no starter design if you look in the details. When you understand all he writes in details, you will help us. Fast Eddie gave you some detailed information you should appreciate for a start.

I suggest you start with an LM1875 design that works. We can help you making such an amplifier work and you can apply your control knowledge when we go along. But accept for a start that all control aspects will not be available to you in the sense that those designing the LM1875 chip issued a datasheet in which you have all the information you need to make it work, but not all information about loop details they have implemented inside the IC. Amplifier IC's are sold on "need to know" basis in order not to help competitors too much.

For a start, you need a power supply. Do you have a symmetrical power supply with 15-25V (+ and -) output?

On the chip: if this project is mostly meant to teach you about making an amplifier work, you do not need to make two for stereo. The second amplifier won't teach you much new. Do you have an LM1875 chip ready?

Do you have a reasonable selection of resistors and capacitors available such that yo do not need to buy every time a component is changed or added?

What type of breadboarding are you used to work with?

I do appreciate it, I appreciate every comment you guys leave trust me!

I agree with your philosophy on how I should get started.

I pretty much have a lab here, and I do have a LM1875 Chip on end. I have a basic breadboard kit.

Dont have a split power supply only problem, I have a DC power supply bench withe one terminal

Last edited by crisroxmysoc; 24th April 2018 at 07:49 PM.
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Old 24th April 2018, 08:27 PM   #17
GoatGuy is offline GoatGuy  United States
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Interesting para-discussions. Advice, strategies, references, linkies.

But interestingly, not much about “how — in principle — to do it right”. Well perhaps I might bear upon a few receptive readers to offer principles.

(1) BREADBOARD your first attempts.
You can make all kinds of learning mistakes without really doing much damage to anything except your Ego. You'll likely learn some about the following recommendations.
(2) TIGHT PACK your layout.
Can't over-stress the sensitivity of the feedback-which-creates-gain-and-stability loop, and how any long lines inside that feedback loop becomes pickups for RF noise, for self-oscillation signal, yada, yada, yada. The more tightly you pack your chip amp and its critical feedback/control loop components, the happier your circuit will be.
(3) DON'T STRETCH the gain of the stage.
There might be a few “rules of thumb”, but in essence, do not attempt really high gain one-chip amplification. This is what a preamp stage is good for. Just let the chip-amp stage be something that'll raise “nominal standard input” (0.775 volts RMS) to “nominal rated output” (which might be 20 V RMS, as an example). Do the math: 20 ÷ 0.775 = 26× gain. This isn't much.
(4) GET USED TO THE MATH that's involved.
Use your calculator. Make spreadsheets. Don't keep them! Just toss 'em. Make more. The idea of “repetition makes the mind grow keener” is actually really true. Make a bunch, make some more, and you'll be able to do the formulæ from memory!
(5) OBTAIN CHEAP-BUT-GOOD test equipment.
Minimally, I advise having at least 3 multimeters. 2 of them, digital. 1 if you like, an older “needle meter”. Its surprising how useful they are at averaging varying input signals. You don't need terribly high accuracy you know, whilst experimenting. Ballpark is OK. Ballpark on a orders-of-magnitude kind of scale. ±3% is fine for all but the most demanding “matching required” designs.
Also get ahold of a USB based oscilloscope module + its software. Sure: there are gazillions of absolutely great full-sized near-antique analog and digital scopes on EBay. But seriously? A $200 USB module with 2 or 4 channels is going to get everything done you want done. And its compact. No downside.
Third, you need a standard signal source. You can use the output jack of the same computer that's running the digital scope software interface! Kind of convenient. There are hundreds of signal generator software apps for … very little money. Worth the investment? Pretty much!!! Or again, you can “EBay” yourself into a fine, fine old analog signal generator or two. They look heck-of-impressive on your test bench.
Fourth, drop the coin on some nicer soldering and “3rd hand” tools. A Weller-style low wattage precision soldering iron is a must. A all-direction clamp is good. Even wooden clothespins are great at times.
Fifth, drop a “relatively tiny” amount of money into plussing-up your box-of-standard-components. NOTHING is more dementing than to be at 2:30 AM, bleary eyed, and need a 2.7 kΩ resistor. And not have one. Nothing. The old “5% series” of resistors, the 20% series of capacitors, a bunch of standard diodes (1N4007 & successors) is your meal ticket. Get them. 5 to 25 each. More, especially, of the middle-range values.
And perhaps zeroth … get a BREADBOARD. It doesn't need to be über-fancy. Just a nice 100 mm by 250 mm socket-board. It'll need a bunch of variously colored pieces of pre-cut hookup wire (necessarily mostly solid core), so get some rolls of that. I found a place that had a 10-pack of 100 foot rolls, solid core, 24 gauge. Great for breadboards.
Lastly, your hand tools. Tiny needle-nosed pliers. Tiny wire cutters. A real, honest-to-good wire-stripper that isn't a piece of ACE hardware junk. A magnifying glass. A bunch of electrical tape.
_______

I'm sure subsequent posters will offer more advice along this line. But it is good. You need to MAKE (sig gen) precise signals to SEE (oscilloscope) the evolution of the signal from input to output, from output back to negative feedback input. To SEE the ripple on your power supply, and the lack of ripple on the output due to good chip-amp CMRR (common-mode ripple rejection).

And don't feel back just “tearing it up” (wholesale removing everything from the breadboard) and starting afresh. Stuff happens. Don't try to build a castle using a deck of poker cards.
_______

Welcoming you to your inevitable success:
GoatGuy
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John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink
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Old 24th April 2018, 09:06 PM   #18
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by crisroxmysoc View Post
I do appreciate it, I appreciate every comment you guys leave trust me!

I agree with your philosophy on how I should get started.

I pretty much have a lab here, and I do have a LM1875 Chip on end. I have a basic breadboard kit.
There exists a type of very popular wire boards where you enter each component in holes in a plastic board (with contacts underneath the holes) and connect the components with wire jumpers. These boards are less suited for amplifiers because the critical components are often not located close enough to the chip to avoid self-oscillation.

Do you have a symmetrical power supply?

I suggest you start making the amplifier (in mono version) described on page 2 of the LM1875 datasheet. It is named "Typical Applications".
Try to leave the leads of the components a bit long such that they can be re-used, EXCEPT for C3, C4 and R4. They need to be placed very close the the LM1875 with short leads, else you risk self-oscillation!
You must find a piece of aluminum profile or plate to fasten on the LM1875 as heatsink. Make sure it does not touch anything else because the LM1875 cooling pad is not insulated from one of the 5 pins.


I will shortly describe the function of each of the components:


R1/C1/R2 form the input circuit. C1 is called a coupling capacitor and serves to bring the AC signal (sound) to the LM1875 input, even if there is a DC difference between the input from the source and pin 1 of the LM1875. R1 makes sure C1 is charged to ground level on the input side, even without a source connected. R1 is not critical and may be 100K or so if that’s available. R2 is a bias resistor recommended by the IC manufacturer and serves to bring the IC input potential to a suitable level (here probably ground level). Together with C1, R2 determines the low frequency cut-off point for the amplifier, thus, how low frequencies the the amplifier will pass through.


C2/R3/R4 form the feedback circuit. C2 has got a high impedance compared to R3 at very low frequencies such that the amplifier will not try to amplify very low frequencies. In the audio range, the values of R3 and R4 determine the amplification. With 20K and 1K the amplification is 20K/1K=20 times. Somewhere in the datasheet it is described that the gain must be minimum 10 (20dB) for stability. Thus, you could leave R4 as 10K and the gain would be 10K/1K=10 times. If you use 4K7 as R4 the gain would be 4.7 times but then the manufacturer warn you the amplifier may oscillate. These components are always connected to what will act as the inverting input (here pin 2).


C3/C4/C7/C8 are power line decoupling capacitors. The small 100nF capacitors are very important for HF decoupling and should be placed as close to the LM1875 pins as possible to avoid self oscillation. The 100nF capacitors are often chosen as ceramic capacitors because they have good HF properties. C7 and C8 are not critical in value as long as they are minimum 100uF. 1000UF, 2200UF or even 4700uF are often used. They serve to keep the charge ready for the amplifier when the amplifier needs to reproduce a high volume signal at the output. They are buffer capacitors and of the electrolytic type.


R5 and C5 form what Neurochrome calls a Zobel network. It serves to compensate the amplifier for certain changes in the speaker impedance. On the Internet you can read more about Zobel networks. Why these component values? The manufacturer has studied the LM1875 needs carefully and concluded these values to be the best. We believe him.


That was it and you now have a first overview of the design.


One very useful rule being valid for an amplifier designed as a power Operational Amplifier (like the LM1875): The amplifier will put its output such that there is no voltage difference between the two input pins (pins 1 and 2). With your knowledge of regulation systems, you can try to make simple calculations of how the output will vary when the signal in the input (pin 1) is varied.


If you connect the components as described in this schematic, it is 99% certain you have made your first working DIY amplifier.


Let us know when you have made the circuit and we will advise you how to test it in a rather safe way.

Last edited by FauxFrench; 24th April 2018 at 09:22 PM.
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Old 24th April 2018, 09:55 PM   #19
AudioLapDance is offline AudioLapDance  Canada
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A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics

Goes through a lot of the calculations for components.

Cheers,
Jeff
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Old 24th April 2018, 10:05 PM   #20
FauxFrench is offline FauxFrench  France
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Just a correction of my calculations of gain above. It is a non-inverting coupling so the gain will be 1+R4/R3, not just R4/R3.
With 20K, I come to 21 and with 10K, it should be 11. With 4K7, the gain should be 5.7 if not oscillating.
Sorry for the inconvenience.

Last edited by FauxFrench; 24th April 2018 at 10:09 PM.
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