Max9744 dc amplification

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Ivanpianobcn
 
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Max9744 dc amplification

Post by Ivanpianobcn »

Hi there.

I want to amplify ac typical audio signals AND DC signals from some amplification board. I'm thinking the Max9744 https://www.adafruit.com/products/1752 could be the right one, as it seems it doesn't have dc blockers. Am I right?

The thing, is that I'm trying to draw images on a CRT (old fashion) TV. With the AC typical audio signals I can draw symmetrical things like https://www.youtube.com/watch?v=qr7rB5JRBuk those, but I want to actually draw more complex things.

I just bought a soundcard that can be used as a cv/gate controller, so it's able to play DC signals http://www.motu.com/products/motuaudio/ultralite-mk3. I just need to amplify those DC signals.

This amplifier is able to do so?

Thanks a lot

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adafruit_support_bill
 
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Re: Max9744 dc amplification

Post by adafruit_support_bill »

Unfortunately, this board does have DC blocking caps. I don't think t will work for your intended use.

Ivanpianobcn
 
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Re: Max9744 dc amplification

Post by Ivanpianobcn »

Thanks for your fast answer. Anybody know where can I get a board without those DC blocking caps?
Or maybe a board that have DC blocking caps, but it's really easy to remove them, and without them will work well?

Thanks again

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adafruit_support_mike
 
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Re: Max9744 dc amplification

Post by adafruit_support_mike »

You'll probably want to go with an analog video amplifier.

A class-D amp would do horrible things to video signals. It samples the input at 300kHz then converts those samples to PWM output. Any information above 300kHz would be lost, and any information between 150kHz and 300kHz would get aliased to a lower frequency.

Your best bet is to get an amp designed for the job: http://www.mouser.com/Search/Refine.asp ... ricing%7c0

Ivanpianobcn
 
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Re: Max9744 dc amplification

Post by Ivanpianobcn »

adafruit_support_mike wrote:You'll probably want to go with an analog video amplifier.

A class-D amp would do horrible things to video signals. It samples the input at 300kHz then converts those samples to PWM output. Any information above 300kHz would be lost, and any information between 150kHz and 300kHz would get aliased to a lower frequency.

Your best bet is to get an amp designed for the job: http://www.mouser.com/Search/Refine.asp ... ricing%7c0

Thanks Mike, but I'm not trying to amplify video signals.

I'm really working with audio, but I need this audio to have DC offsets all the time. I don't know anything about video amplifiers, but I just think (I'm wrong here?), that with and audio amplifier without DC blocking caps I would be perfectly able to amplify those signals.

Thanks again

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adafruit_support_mike
 
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Re: Max9744 dc amplification

Post by adafruit_support_mike »

Removing the decoupling caps would allow the MAX9744 to see DC offsets, but that wouldn't solve all your problems.

The main problem is that the output is a differential signal which doesn't translate to DC easily.

Each of the MAX9744's output channels is made from an H-bridge. One side of the bridge is always HIGH and one is always LOW. When the wave is HIGH, the left side of the H-bridge will be Vcc volts higher than the right side. When the wave is LOW, the left side will be Vcc volts lower than the right side. That makes the effective voltage swing of the square wave 2Vcc.

If the voltage between the input pins is 0v, the output will be a square wave with a duty cycle of 50%. That square wave is the modulated form of your DC GND rail.

To convert the output back to DC you'd have to subtract the 300kHz carrier from the signals on each side, combine what was left, then integrate the result. That's a lot of work, and there are lots of places where errors can creep in.

Overall, it would be much easier to use a linear amplifier like an op amp. What voltage and current ranges do you need from the output?

Ivanpianobcn
 
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Re: Max9744 dc amplification

Post by Ivanpianobcn »

I see that would be a pain to make it work...

I really don't know what voltage and current ranges I need on the outputs. But I know, that with this 20W MAX9744 chip it was enough, and more than enough amplification for my purpose.

So, better solution would be an op amp. Any place I can buy one? You adafruit have some op-amp product?

Lots of thanks for your help.

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adafruit_support_mike
 
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Re: Max9744 dc amplification

Post by adafruit_support_mike »

We carry the TL2642 dual op amp: https://www.adafruit.com/product/808

It works for supply voltages up to 6v, can accept rail-to-rail input, can produce rail-to-rail output, and each op amp can handle up to 80mA.

Are you comfortable setting up the feedback loop to get a certain amount of gain from an op amp?

Ivanpianobcn
 
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Re: Max9744 dc amplification

Post by Ivanpianobcn »

Okay, that looks good. I could start trying out this one, and if I need more amplification output, get a more powerful one I guess.

I guess I'm comfortable setting the feedback loop, but, the thing is:

I would actually love to find a "finished" product. One that I don't need to solder parts, pieces, do the circuitry, etc. There nothing like this?

If not, I'll start trying with the TL2642 dual op amp, but it will take me quite a lot of time to make it work...

Thanks Mike

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adafruit_support_mike
 
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Re: Max9744 dc amplification

Post by adafruit_support_mike »

I doubt you'll find a preassembled analog amplifier that can handle DC without paying bench equipment prices.

It's easy to package the most complicated part of an amplifier circuit (that's what op amps are), but the parameters are what get you.. gain, input range, output range, DC offset from input to output, polarity, etc. Everyone wants something different, so a one-size-fits-all package would be about 5% op amp and 95% knobs to adjust all the values that are settable.

Ivanpianobcn
 
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Re: Max9744 dc amplification

Post by Ivanpianobcn »

I see, okay okay, So, I will start with the TL2642 and find problems and solutions step by step.

Thanks for your help. I guess I'm gonna be asking more whenever I receive the op-amp and I have time to start building my "amp".

Thanks

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