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Chip Amps Amplifiers based on integrated circuits

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Old 14th December 2011, 04:22 PM   #11
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Since you adapted all values from the datasheet, you can assume that it will work OK like that.

And yes, you should use film caps in the signal path, where possible and affordable. For Cout that is clearly out of the question, although a small film cap in parallel to it will be an improvement. That leaves Cin and C4. Cin's 10 F make it already quite expensive, and C4 is usually an electrolytic due to its size anyhow. In both cases you could achieve the values by paralleling cheap big electrolytics with small and affordable film caps.
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Old 14th December 2011, 08:45 PM   #12
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Why such a large cap on the output. Does running a single supply add that much DC into the signal? I should have v1 of the pcb finished later tonight.
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Old 15th December 2011, 12:28 AM   #13
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I updated the schematic and cleaned up the names and some other small stuff. I also completed v1 of the pcb. I added a .1uf film cap in parallel to Cin Cout and Ci. Higher values are much larger in size. If this is not sufficient changes can be made. For the electrolytics, mainly Cin and Cout are these meant to be polarized as seen in the data sheet or bi-polar?

I am sure the component placement could be stuffed in a little tighter but as it is this is a relatively compact design and I really like the symmetry of the board.

Once the design is ready I will edit the silkscreen on the pcb and make it pretty.

Click the image to open in full size.

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File Type: jpg pcb1.JPG (127.5 KB, 171 views)

Last edited by crazifunguy; 15th December 2011 at 12:33 AM.
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Old 15th December 2011, 10:22 AM   #14
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Quote:
Originally Posted by crazifunguy View Post
Why such a large cap on the output. Does running a single supply add that much DC into the signal?
In a single supply amp the output is at half the rail voltage, e.g. for 42 V rails at 21 V. The size of the cap is due to its filter function. It forms a high-pass filter with the load, according to f = 1 / (2*PI*C*R), where R is the speaker impedance. A smaller capacitor could audibly cut the bass, depending on the speaker.

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Originally Posted by crazifunguy View Post
I also completed v1 of the pcb.
It is advisable to route power supply and output traces as far away as possible from the input to avoid crosstalk and (worst case) oscillation. Where you cannot avoid that they cross, they should do so at right angles. Take a look at the reference design in the datasheet to see how it is done.

Watch that the big electrolytics are placed in a way that still allows you to reach the LM4780 with a screwdriver. Otherwise it will be difficult to fix it to the heatsink.

It is OK to connect the two mute pins in parallel and only use one capacitor and resistor for both channels. By the way the mute resistor seems to be missing in your design.

You should also add a 100nF ceramic cap next to the power supply pin and another 10 F electrolytic no too far away from that. See chapter Supply Bypassing for that.

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Originally Posted by crazifunguy View Post
I added a .1uf film cap in parallel to Cin Cout and Ci. Higher values are much larger in size.
The Pareto principle works here. You can achieve 80 % of the result with 20 % of the investment. The next 20 % of investment bring you to 96 % of the result, the next 20 % investment to 99 %, and so on. The more money and PCB space you invest in those caps the smaller is the gain, but gain there is. I would qualify the electrolytics as the first 20 % of investement, parallel film caps as the next 20 %, have film caps of 1 % or more of the electrolytics value are another 20 %, replacing the electrolytics entirely with film caps another 20 %.

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Originally Posted by crazifunguy View Post
For the electrolytics, mainly Cin and Cout are these meant to be polarized as seen in the data sheet or bi-polar?
Polarized.
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Last edited by pacificblue; 15th December 2011 at 10:27 AM.
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Old 15th December 2011, 05:54 PM   #15
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I updated the schematic and pcb adding in the capacitors for te Supply Bypassing. I dont plant to use any mute switch so the resistor and switch was left out of the design. I am going to build this board as a prototype and testing platform while I rework the PCB. Are there any final changes that I should make?
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Old 15th December 2011, 06:06 PM   #16
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You need the resistor, because current must flow to unmute the amp. Without the resistor the amp remains silent.
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Old 15th December 2011, 08:19 PM   #17
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Do I use Vcc or Vcc/2 for Vee?

RM ≤ (|VEE| − 2.6V)/l where l ≥ 0.5mA

^Good ol' Ohms Law
42V Rm=78.8K ohms
21V Rm=36.8K ohms
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Old 15th December 2011, 08:37 PM   #18
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In a single supply amp Rm connects to GND and then you calculate with 21 V, because the amp's GND pin is connected to Vcc/2. It is OK to use 1 k / 1 V. That makes the math easier, and it leaves a good safety margin for voltage drop and component tolerances.
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Old 15th December 2011, 10:35 PM   #19
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Unfortunately the majority of the output from a spike system chip severely under-run on voltage, is exacerbated hard clipping, that is not especially practical.

Instead of so much noise, why not use a simple chip that you can bridge? Perhaps 2 of these: http://www.st.com/internet/com/TECHN...CD00236549.pdf
The little brother of that chip has the single rail bridge application in its datasheet. See figure 18. http://www.st.com/internet/com/TECHN...CD00000145.pdf
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Old 16th December 2011, 12:31 AM   #20
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Quote:
Originally Posted by danielwritesbac View Post
Unfortunately the majority of the output from a spike system chip severely under-run on voltage, is exacerbated hard clipping, that is not especially practical.

Instead of so much noise, why not use a simple chip that you can bridge? Perhaps 2 of these: http://www.st.com/internet/com/TECHN...CD00236549.pdf
The little brother of that chip has the single rail bridge application in its datasheet. See figure 18. http://www.st.com/internet/com/TECHN...CD00000145.pdf
I see the point that you are making in that running at 1/2 the supply voltage will be running the chip close to the lowest supply voltage. I do not have the knowledge currently to design a circuit that will run on a single supply and also BTL. I would however like to gain more information on this. The LM4780 was a good starting point and I will still build the amp for a proof of concept/test platform. I would like to explore other chips and circuits that can be used with a single supply that will give good results as well. This project has pleanty of room to evolve and will be a good foundation for a learning platform on chip amps for me. The bridge application for the "little cousin" is for a split supply.

Could it be as simple as setting up the single supply in the same configuration as the split supply bridged?

Last edited by crazifunguy; 16th December 2011 at 12:54 AM.
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