dúvida - trabalhando com dois amps

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dúvida - trabalhando com dois amps

Mensagem por renatogaragerock em Ter Jun 22, 2010 9:56 am

Pessoal,
Tive a chance de testar amplificadores da marca Kustom e achei que eles chegam bem próximos do que quero, considerando ainda que estou de mudança e será mais viável vender meu equipamento (em Campinas) para comprar novos (em Manaus).
Os amplificadores desta marca trabalham melhor em volumes quase que no limite (ele é pré-valvulado e a potência tem um sistema que emula power amps valvulados), e o modelo que mais gostei é de 200W, potência que com outras marcas vi que é suficiente para ensaio, mas que às vezes deixa a desejar para shows. Como o preço deste equipamento está legal e existem opções 1X15 e 2X10 pensei na possibilidade de pegar um de cada e trabalhar com o baixo em dois amplificadores diferentes.
O que queria saber é se há algum cálculo ou alguma aproximação de qual seria a potência equivalente final. Tipo "usar dois amplificadores de 200W ao mesmo tempo equivale à massa sonora de um de 350", ou algo assim, para que eu tomasse uma decisão sobre esta futura e próxima compra.
Obrigado e abraços,
Renato
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Re: dúvida - trabalhando com dois amps

Mensagem por renatogaragerock em Ter Jun 22, 2010 9:59 am

Apenas queria complementar que este cálculo seria para o uso dos amps sem estarem ligados um ao outro, com um cabo contendo bifurcação.
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Re: dúvida - trabalhando com dois amps

Mensagem por renatogaragerock em Ter Jun 22, 2010 10:07 am

Apenas para um ponto de partida achei essas informações. Não estão claras se é para algo em série, paralelo, etc...

1.4 - Actual vs Effective Power
If we assume that our 100 Watt amplifiers will be handling exactly the same peak amplitudes with typical program input, then we have a total of 200 Watts for the combined program material. So, where does the magic come into this? This amp combination will sound (and measure) as if it were 400 Watts - twice as much "effective" power as there is real power. For this to make sense, we need to back track a little.

Imagine a sine wave signal of 100Hz at an amplitude of 28V RMS. For an 8 ohm load, this equates to about 100W (98 actually). The same amplitude at 1000Hz will be exactly the same power. Now add the two signals together, in the same way that signals add together in music. We are interested only in the peak amplitude, the RMS value indicates that the power is only 3dB higher, but it is only when an oscilloscope is used that the true picture emerges.

We will now see a low-frequency waveform, with a higher frequency waveform superimposed - the high frequency signal will be riding up and down the path of the low frequency signal. If we were to perform a calculation (or simply measure the combined signal with an oscilloscope), we will see that the peak amplitude has doubled. The effective RMS value (most multimeters will get this wrong unless they are true RMS types) is 40 Volts, and this would imply 200W. Although this is the real RMS voltage, it totally underestimates the amplifier power needed to reproduce it cleanly. An oscilloscope shows 80V peak for the same waveform, so the amplifier must be capable of passing an 80V peak signal - a 400W amplifier.


Figure 2 - Addition of Waveforms

To illustrate this point, Figure 2 shows two signals, each of 1 Unit peak amplitude. As can be seen, when the two are combined, the amplitude is much greater. The maximum peak amplitude is now 2 Units - double the peak voltage and four times the peak power of each signal individually. Power increases as the square of voltage, so twice the (peak) voltage is four times the power. Real ('RMS') power increases by 3dB or double the power, but this is a misleading figure and cannot be used. An oscilloscope is essential.

Note: Peak-to-peak amplitude is actually double the values quoted above, but since amplifiers are generally symmetrical (capable of equal positive and negative voltage swings) it is more convenient to simply refer to the peak amplitude only.

This is not to say that the actual music will be symmetrical. It isn't, but it is completely unpredictable in nature. As a result, it is possible (for example) to set up an amplifier asymmetrically and adjust the phase to suit with a switching circuit, since it will change. AM radio actually does this (or they used to) - a circuit is used to switch the phase so that slight over modulation causes more transmitter power, but never reduces it below the acceptable minimum. I shall not be going into details, since I believe few audiophiles would find this acceptable - I know I wouldn't.

All signal sources have the same characteristics as shown above in Figure 2, even a solo voice or musical instrument. In these cases, the fundamental frequency forms the low frequency component, while the harmonics 'ride the wave' as it were. Not surprisingly, the 'equal power' frequency will change (often dramatically) from the 250 to 350Hz range quoted above, but the basic principle does not alter.

Completely beside the point (but interesting anyway) is that in many musical instruments, the harmonics are actually at a greater amplitude than the fundamental. (File this away under 'Useless Information'.)

Note: It must be explained here that the 3dB effective power increase is the absolute maximum that can be obtained. In most cases it will be less - I have examined sections of music where the power gain was less than 1dB, and it can be reasonably safely assumed that the real gain will lie somewhere between 1-2dB in most cases. The real figure depends a lot on the type of music, the actual crossover frequency, and the peak to average ratio of the two separated signals. Just this topic alone is sufficient for a complete article in its own right.
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