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Old 2nd November 2011, 08:46 AM   #1
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Default bi-amping doubts

hi all
i want to purchase power amps for bi-amping application. my doubt is can i use different amplifiers for woofers and tweeters? i mean is it ok to use 100w/ch stereo amplifier for woofers and 50w/ch stereo amplifier for tweeters? will there be any issue due to tonal differences in the amplifiers.
also i have come across following affordable power amp kits on ebay. pls let me know if anybody have used any of them. what are your opinions about quality.

TDA7293 stereo amplifier board Assembled AMP 85W+85W | eBay

QUAD405 Stereo Audio Power Amplifier Board Assembled | eBay

Assembled H-140 Power Amplifier with Power supply Board | eBay

LM4702+2SA1943+2SC5200 Audio power Amplifier AMP Board | eBay

Audio Power Amplifier X2+ Pre-Amplifier + Power Supply | eBay


thanks
pranam
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Old 2nd November 2011, 01:29 PM   #2
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Yes, it seems right to use smaller amps for higher frequencies. I do.
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Old 2nd November 2011, 01:36 PM   #3
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Originally Posted by Cal Weldon View Post
Yes, it seems right to use smaller amps for higher frequencies. I do.
is there a formula/procedure to determine how much power would be required for high frequencies for given power for low frequencies?
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Old 2nd November 2011, 03:28 PM   #4
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Frequencies under 4kHz or 5kHz need roughly the same power. From 5kHz you can reduce the power by 10dB. So if you have crossover around 6kHz, you'll be able to manage with just a10th of power over 6kHz.

However if cross-over is around 2kHz, you'll need same power above and below 2kHz.

For a cross-over around 4kHz, you should be able to manage with half power for tweeters.

Check Peak frequency response for common audio for more information.

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Originally Posted by soundnovice View Post
is there a formula/procedure to determine how much power would be required for high frequencies for given power for low frequencies?
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Old 2nd November 2011, 03:48 PM   #5
wrankin is offline wrankin  United States
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So the best answer is "it depends". As always, Wikipedia is the keeper of all knowledge...

http://en.wikipedia.org/wiki/A-weighting

Notice in the graph how much more energy is needed to reproduce the bottom octaves over something in the "middle" (say above 200-300 Hz). Also what is the content of the music you like listening to? Is it bass-heavy?

Are you going to be running the stock crossovers (sounds like it)? Then you don't have to worry about difference in driver sensitivity.

A lot of people bi-amp (IMHO) specifically because they do want the "sound" of he amps to be different. Have a big-bruiser of a transistor amp running the lower frequencies and something with lower power but having a better/preferred high frequency response (eg. tubes, FETs, et. al.) for the upper octaves.

Depending on where the crossover point is (usually 2-3kHz or thereabouts) you could probably have about a 10:1 difference in the power ratings of the two amps, especially if you are listening to material with significant low frequency (<50Hz) content.

-bill
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Old 2nd November 2011, 04:21 PM   #6
AndrewT is offline AndrewT  Scotland
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Originally Posted by sudhir81 View Post
From 5kHz you can reduce the power by 10dB. So if you have crossover around 6kHz, you'll be able to manage with just a10th of power over 6kHz.
Rubbish.

You are suggesting that irrespective of driver impedance and sensitivity that one can and should use 1/10th the power for a driver operating at >6kHz, one will obtain similar performance over a range of music types and over a range of preferred volume (SPL).

Take an example:
10W main amplifier, 1W treble amplifier.
90dB/W @ 1m 8 ohm bass mid driver, 88dB/W @ 1m treble driver.
Maximum SPL @ 2.5m listening distance from the two drivers will be:
bass/mid 92dB, treble 80dB.

Provided the volume of the treble content never goes above 80dB the drivers stay in unison. The gain of the two amplifiers will be adjusted such that the frequency response of the combined driver set is near flat. That automatically limits a flat frequency sample to the limit set by the treble of 80dB.

If one asks the "system" to play louder than 80dB with a flat frequency sample then the treble channel will overload and the reproduction quality will be ruined.
I contend that the peak SPL from all the drivers should be roughly the same, to allow all types of music to be reproduced in the domestic environment.
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Old 2nd November 2011, 04:29 PM   #7
wrankin is offline wrankin  United States
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Quote:
Originally Posted by sudhir81 View Post
Check Peak frequency response for common audio for more information.
Very interesting article - but I am more than a little confused (and skeptical) about their methods. They keep referring to their measurements as "SPL" which implies that they measured the sounds acoustically. But in the article it does not seem like this is the case:

Quote:
If we feed audio coming out of a CD player to a frequency spectrum analyzer, what we'll see is an instantaneous distribution of frequencies.
and
Quote:
Frequency points were collected from the peak SPL graph obtained by running a frequency spectrum analyzer in peak hold mode. These presented in a tabular form below. Relative SPLs are shown corresponding to different frequencies.
So it looks like they collected their data from some unknown "frequency spectrum analyser" connected to the analog signal output of a line source. This is most definitely NOT SPL. In addition their frequency data intervals are all over the place - one data point for the top octave, five data samples for the next octave down and back to a single data sample for the entire 1k-2k octave. I can't held but question both their knowledge of signals and acoustics as well as what type of "spectrum analyzer" would return these results (is it measuring peak signal or actual power within those intervals, which would be very different things).

It would be easier (and more accurate) to run multiple FFT's against the raw music data and combine the results. I would definitely be interested in seeing that sort of data. If anyone has a reference to those kind of results (AES probably) I'm would love to see them.

Thanks, and take care.

-bill
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Old 2nd November 2011, 08:53 PM   #8
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Power is a difficult question in bi-amping. I run all speakers in fully active mode with no passive crossovers. I agree with AndrewT that you should be primarily concerned with achieving a roughly equivalent SPL from each driver.

Using the sensitivity of the driver, you can establish what the required gain difference should for the amplifiers. For instance, if we use a 90 dB/W @ 1m midrange and a 96 dB/W @ 1m tweeter, we will need an additional 6 dB of gain in the amplifier for the midrange (twice as much voltage applied across the driver). Note that higher frequency drivers typically have higher sensitivities, and therefore require less gain.

Now that we have figured out our gain requirements, we can examine our power requirements. The impedance of the driver becomes important at this point due to Ohm's law. The actual power provided to the driver by a voltage source (read ideal amplifier) is determined by (voltage^2)/z, where z is the complex impedance of the driver. Often, drivers are chosen to operate in a range where 'z' is fairly constant, and is assumed to be constant at the nominal value (typically 4-8 ohms). For our rough back of the envelope calculations, this nominal value will be sufficient, and will be referred to as 'z'.

I would make an estimate of the power level that you want, multiply it by 'z' of the driver in question, and square root that number to determine what voltage supply will be necessary for you to achieve your power goals.

Most amplifiers spec power in watts, which will not provide sufficient information to determine if they are suitable for use in your system. More important is the maximum voltage they can supply, and the max current. Power= volts*current. Keep in mind that as the impedance of the driver goes down, the current must go up to maintain the proper voltage drop. This is where most amps fall short. The output stage will have some current limit that it cannot surpass without overheating, this will limit the range of voltages and impedances that are compatible with it.

As a word of caution, beware of ebay sales. Be aware that you may end up with an amp more suited to an ipod than a hifi sound system. There are several sources of amp kits that have proven reliable, for starting, chipamps produce decent sound for an incredibly cheap price.

Louis
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Old 3rd November 2011, 04:39 AM   #9
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Originally Posted by riverwarrior View Post

As a word of caution, beware of ebay sales. Be aware that you may end up with an amp more suited to an ipod than a hifi sound system. There are several sources of amp kits that have proven reliable, for starting, chipamps produce decent sound for an incredibly cheap price.

Louis
can you suggest me some reliable amp (chipamp) kits?
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Old 3rd November 2011, 04:45 AM   #10
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It is true that a 2 KHz crossover HF amplifier has similar voltage swing requirements to the LF, but remember that dome tweeters tend to actually be 8R (often more due to padding resistors), while woofers are usually 4R. This means that the LF amplifier is actually working much harder
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