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Old 25th May 2016, 02:17 PM   #1
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Default How to modify a class D amp to work below 20hz?

I am using an audio amp and speakers for a very unconventional method. I want to compress a fluid with a speaker to mimic a heart beat. That means a sub 1Hz response is required.

However, most amps aren't rated below 20Hz. As I understand it, I can remove the output capacitors on a class D amp to remove it's low pass filter. In this case it's output range goes from DC to it's rated maximum. I would just need to ensure that my input power is clean to reduce noise.

Would this be a viable option?
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Old 25th May 2016, 02:33 PM   #2
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I'm not aware that Class D amps have 'output capacitors'. That term normally relates to AC coupled conventional designs.

Most Class D amps use a bridge configuration with the speaker directly coupled between two outputs stages that are driven in antiphase with respect to each other, however there is normally AC coupling at the input which would roll the response off. Any caps on the output of a Class D amp are there for filtering all the noise and hash normally produced by such output stages. The speaker current does not flow in these caps.

Do you really need Class D for this ? I'm thinking a simple 'chip amp' would suffice here.
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Old 25th May 2016, 03:03 PM   #3
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Originally Posted by Mooly View Post
I'm not aware that Class D amps have 'output capacitors'. That term normally relates to AC coupled conventional designs.

Most Class D amps use a bridge configuration with the speaker directly coupled between two outputs stages that are driven in antiphase with respect to each other, however there is normally AC coupling at the input which would roll the response off. Any caps on the output of a Class D amp are there for filtering all the noise and hash normally produced by such output stages. The speaker current does not flow in these caps.

Do you really need Class D for this ? I'm thinking a simple 'chip amp' would suffice here.
I may have confused what someone told me about class D and Class A amps.

Well I'm driving about 160 W. I'm not sure I could get something with that wattage on a chip.
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Old 25th May 2016, 03:11 PM   #4
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OK, well Class A amps are usually smaller low powered designs designed for out and out fidelity. They are spectacularly inefficient and run very hot. Class D is the other extreme, very efficient but not the ultimate in fidelity.

160 watts is a lot of power (assuming you mean the proper text book definition of it being rms or root mean square) and would normally be done by a conventional class B type design... and they cover most audio type amplifiers. That kind of power still means pretty big power supplies and heatsinks.

'Chip amps' are commonly available up to 50+ watts rms, some go higher.

Perhaps if you could give an idea of the type of speaker you are trying to drive then we might have a better idea what would be best.

(to give you an idea of 'power', most audio listening on a good hifi system is done at only a couple of watts or so)
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Old 25th May 2016, 03:19 PM   #5
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I am using 8 Dayton Audio exciters. They are 20w rms and 4 ohm impedance. I'll configure them in a parallel/series such that the overall impedance is correct for the amp I select. 1 channel is sufficient.

The choice behind the driver(speaker) is convoluted and not really open to change. So the amp must follow it.
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Old 25th May 2016, 03:33 PM   #6
Mooly is offline Mooly  United Kingdom
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Interesting (speaker design isn't my specialty and I haven't seen anything like that before).

I think the bottom line comes down to how hard you actually need to drive these to get the results you need. I couldn't begin to guess at that... and you might find its a lot less than you think. So much will depend on how you couple these to the fluid... I'm guessing you might have some flexible membrane that the speaker works into.

My advice, if you are starting from zero on this, would be to try a small DC coupled chip amp design and see if that gets you in the right ballpark on this. You would at least get a feel for what is possible or not.

I don't know what your construction abilities are... would you be making the amp yourself or would you be looking to modify an existing offering such as a kit.

I'm thinking something like a TDA2050 based design could work here. Its simple and has a basic maximum current drive of around 5 amps before it limits. At such low frequencies you are really looking at the basic properties of ohms and amps rather than complex impedance curves over a wide frequency range and so on.

Interesting, I'll look in again later.
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Old 25th May 2016, 03:56 PM   #7
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I'd rather not build an amp if I don't have to, since I have no experience with it. I will do that if it comes down to a necessity.

I'll keep looking into this. Thanks for the clarification on amp design.
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Old 25th May 2016, 04:14 PM   #8
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You could use an array of 8 el cheapo class-d-modules, each one firing a single speaker.
To extend to sub-audio-frequencies, you have to change input coupling caps, and the bootstrap caps as well. As a rule of thumb, you might increase all capacitances by a factor of ten.
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Old 25th May 2016, 04:22 PM   #9
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Originally Posted by voltwide View Post
You could use an array of 8 el cheapo class-d-modules, each one firing a single speaker.
To extend to sub-audio-frequencies, you have to change input coupling caps, and the bootstrap caps as well. As a rule of thumb, you might increase all capacitances by a factor of ten.
That is an interesting solution.

So you're saying i should short the input and and bootstrap caps, and then increase all other caps on the board by a factor of ten?
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Old 25th May 2016, 05:15 PM   #10
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Quote:
Originally Posted by nate san View Post
I am using an audio amp and speakers for a very unconventional method. I want to compress a fluid with a speaker to mimic a heart beat. That means a sub 1Hz response is required.
However, most amps aren't rated below 20Hz.
You don’t have to use a sub Hertz sinusoidal signal for that.

You can get away from the low frequency limitation of the amplifiers by driving the amplifier -any audio amplifier- with a pulse repeating every x seconds, where x is the time period of the heart pulse you want to imitate (e.g 1.5s is the time period of 40 beats per minute, 0.7s is the time period of 80 beats per minute)

So, you don't need to modify an amplifier for doing your experiments. (what you suggested as a mod in your first post, won't work anyway)
George
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