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# Electrical Engineering/Audio Amplifier frequency rosponse

Question
QUESTION: I will appreciate your input on my objective which is:
To convert the factory frequency response of a class D audio amplifier rated 5000 OR 10000 watt which is somewhere from 2hrz to 150Hrz  to a higher like 2Khz to 20Klz, No music or speakers are involved just sine wave, the input will be 1-3 volts from a BK PRECISION signal generator model 3011B 50 ohms and the out put of the amplifier will be inserted into 2 ground probes in to the earth, the ground resistance is between 100-200 ohms

ANSWER: IF: You really meant the bandwidth was 2 hz to 150 hz then you may not be able to get it up to 20 khz. It will depend upon the sampling frequency of the class D amp.

IF: You meant the original bandwidth of the amp to be 2 hz to 150 khz then you can achieve the 2khz to 20 khz range.

It would be easiest to perform the filtering on the input to the amp but it could also be done at the output.

HOWEVER:  I am confused a little when you say the amplifier is rated at 5kw or 10kw.  To get this kind of power into a 100 -  200 ohm load would require very high voltages.  If you wanted to drive 5kw into a 100 ohm load you would need a driving ac voltage output from the amplifier of 700 volts!  And 10kw to the load would require 1000v!!  Not a very practical design.  If you are working with an amplifier that produces those wattage into an 4 ohm or 2 ohm speaker type loading then that makes more sense but the amount of power capable of being driven into the 100 to 200 ohm load would be much less.

So, I need some clarification as to the amplifier sampling frequency and whether you are looking to drive the ground plane with power or voltage!

Let me know more.

---------- FOLLOW-UP ----------

QUESTION: Thank you very much for your respond, I do appreciate your input very much,
All Class D amplifiers are made to have a frequency response 250Hrz and bellow,
Yes my sine wave working signal is 2kHz to 20klz into the amplifier from a signal generator and particulaly a BK PRESISION model 3011B
I have used common audio amplifiers but the most I can get out of them is around 80V AC measured with a volt meter from the ampl output,(not barely enough for my objective)
I heard that Class D amplifiers have some very unique charachteristics as they use minimal DC voltage to drive 5000 watts, The problem is their frequency respond it is very low in relation to the one I am using, That is why I try to find a way if it is posible for a class D ampifier to be changed to my working range, I don't know, if there is only a part in the amplifier or a module or something that filters out high frequency if that particular part can be changed or replaced then it may deliver what I need, Maybe??
Regards
Yanis

Not sure I understand your quest.  If you want to put that 5kw to the load you must get the voltage up there as outlined in the previous reply.

Most class D audio amplifiers are designed to work in the 12v environment (automobiles) and they utilize internally a converter to bring the dc voltage to a higher level in order to produce the higher power output.  The rails of the amplifier output must be able to swing high enough to get out the power you want.

If your amplifier has a sampling frequency of 250khz then it would be no problem getting the frequency response to 20khz out - unless the output devices are heat restricted by design choice.

All automobile class D amps that I know of are rated into a 2,  4 or 8 ohm load.  Getting the voltage up to a higher load could be accomplished by using an step up transformer at the output I suppose but I haven't seen a suitable transformer for what you want; you may have to have is custom wound.

But back to the beginning: If you want to get the full power delivered into the 100 to 200 ohm load then you must get the output voltage up there.

I am at a loss to understand the statement "I try to find a way if it is posible for a class D ampifier to be changed to my working range, I don't know, if there is only a part in the amplifier or a module or something that filters out high frequency if that particular part can be changed ".  If you are feeding it a sine wave and you want to get a sine wave out then I see no need to restrict its frequency response.  An amp with 250khz sampling will, according to the nyquist rule be able, theoretically, to give you response of a sine wave input out to 125 khz (1/2 of the sampling frequency) and all is well.

If you want to limit the bandwidth of the amp - and I can't see why that is necessary if you are feeding into it a sine wave and getting out a sine wave - then a low pass filter at the output of the amp can be added. Or, the internal design of the amplifier circuitry can be adjusted to limit the bandwidth as well.

I suppose your quest is to get sufficient ac voltage output from  the amplifier to drive the ground probes. I suspect you are really using the amplifier as a voltage feed to the ground probes.  In a class D amplifier that can supply 5kw into a 2 ohm load (which is typical of a class D automobile amp) the output voltage would need to be 100 volts ac [140v ac peak to peak]. [Ohms law calculation where P = V*2/2] The dc feed line to the output switching devices in the class D amp would need to have rail voltages of +70 and -70 volts dc.[1.4 times the rms sine wave].  The 12v feed would need to be converted into a +,- 70 volt supply line.  It's ohm's law;  we can't violate OHMS Law.

Am I making sense of this to you?  Let me know if you need power drive or voltage drive to the ground probes....  That is at the core element of your quest, I think.

Help me out here what you really want to achieve - as I think you are confused about power, voltage and frequency response elements of your project design.

Let me hear back so I can get on the same plane with your needs.

Electrical Engineering

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#### cleggsan

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