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Author Topic: Power Amps/ Monitor Setup  (Read 482 times)

Ever Cruz

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Power Amps/ Monitor Setup
« on: July 10, 2019, 12:13:00 pm »

I am a beginner when it comes to PA systems and am just trying to help out my church.

I have a QSC GX7 power amplifier. I saw on the manual that it gives 8ohms of power per output BUT when I checked diagrams within the manual i see two 8ohm monitors connected to output. If i'm not wrong connecting two 8ohms speakers in a series would raise the impedance to 16ohms right? Isn't that bad for the power amp?

Anything helps

Thanks,
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Taylor Hall

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Re: Power Amps/ Monitor Setup
« Reply #1 on: July 10, 2019, 01:06:15 pm »

First, ohms are not a rating of power, they are a rating of resistance. The power rating of your amp will be in watts which in the case of your amp is 725/1000W at 8 and 4ohm respectively. I would make sure that the monitors you plan on using are capable of handling that kind of power.

Typically, speaker cabinets are wired to work in parallel when two or more are chained together. So you would actually be lowering the impedance to 4ohm with two 8ohm cabinets. It's easy enough to test with a multimeter, just wire everything up EXCEPT the connection to the amplifier and see what kind of resistance you get. It won't be exactly 4/8/16/etc ohms, that value changes depending on what frequency the speaker is playing.

Working at higher impedances isn't tacitly 'bad' for an amp, but it is incredibly inefficient (unless specifically designed to work at higher impedance values) and will make the amp work harder to produce the same kind of output of lower impedance loads. If your amp isn't rated for the impedance load you're creating, then it's best not to run it like that to keep everything happy and the magic blue smoke inside your gear where it belongs.
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Chris Grimshaw

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Re: Power Amps/ Monitor Setup
« Reply #2 on: July 10, 2019, 07:02:15 pm »


Working at higher impedances isn't tacitly 'bad' for an amp, but it is incredibly inefficient (unless specifically designed to work at higher impedance values) and will make the amp work harder to produce the same kind of output of lower impedance loads. If your amp isn't rated for the impedance load you're creating, then it's best not to run it like that to keep everything happy and the magic blue smoke inside your gear where it belongs.

Since most amplifiers are voltage sources, running higher impedance just means less current. That'll usually mean less heat in the amplifier, so it'll actually be happier.
The amp will have to swing more voltage to get equal power into a higher impedance load, but that's not the end of the world.


In the PA world, though, we run things in parallel. If you have N identical speakers of Z ohms, the impedance seen by the amp will be Z/N.
For example, 4x speakers, 16ohm each. 16/4=4ohm.

Chris
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Ken Webster

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Re: Power Amps/ Monitor Setup
« Reply #3 on: August 09, 2019, 05:39:09 am »

Since most amplifiers are voltage sources, running higher impedance just means less current. That'll usually mean less heat in the amplifier, so it'll actually be happier.
The amp will have to swing more voltage to get equal power into a higher impedance load, but that's not the end of the world.


In the PA world, though, we run things in parallel. If you have N identical speakers of Z ohms, the impedance seen by the amp will be Z/N.
For example, 4x speakers, 16ohm each. 16/4=4ohm.

Chris

That only works by accident.  ie the specific case where each load is the same.

The correct rule for determining parallel impedance is:
1/Z = (1/Z1 + 1/Z2 + 1/Z3....... + 1/Zn)

I would say that in the PA world, we connect things according to the schema that is appropriate to the circumstances.  That doesn't necessarily mean parallel in every case.

To the OP:

Here is the user manual for that Amp.
https://www.qsc.com/resource-files/productresources/amp/gx/q_amp_gx_usermanual.pdf

Congratulations on the QCS amp BTW, I appreciate manufacturers who are honest and up front about their products.

More precisely, speaker load on an amplifier is impedance (Z), not resistance (R).  Resistance is a DC load while Impedance is an AC load.  As music amps produce an audio signal, that is a voltage waveform akin to AC except that the waveform is a complex sum of many frequencies at varying amplitudes.  Essentially though, it is an alternating current (AC) so impedance is the correct term.  While you may see speaker terminals labelled Ė black and + red, this is just a convention where the Ė (black) terminal is a common earth (stable reference voltage) and the +(red) terminal is amplified signal that can be a dangerous voltage.  (Amplifier output impedance is extremely low and offers no inherent over current limitation at all.  So be warned.)  As impedance varies with frequency, for convenience it is generally stated as a nominal impedance at a standard frequency.  To make things easy, whatever DC resistance you measure across the speaker terminals with an ohm meter is as close to the nominal impedance as makes little difference.  Itís just that sometimes itís important to have an understanding of what is actually going on.

In theory (assuming a perfect amplifier), if the nominal impedance is halved, then the power doubles.  That is 750 Watts into 8 ohms load becomes 1500 W into 4 ohms and 3000 W into 2 ohms etc.  However, amps are not that perfectly simple and you may note the power ratings for your GX7 on page 3 do not follow this rule.  Reasons for this include the amps capacity to deliver power, distortion and clipping as power increases but you will sometimes see the above stated in literature.  The spec on that page states Watts at 0.1% clipping and under this limitation you have 750W into 8 ohm, 1000 W into 4 ohm and 600W into 2 ohm.  This tells me that a 2 ohm load is of no advantage and therefore best avoided while a 4 ohm load will yield the most power and therefore volume while an 8 ohm load will be nearly as loud at lower current and heat.

It should be noted that 2 ohms is very little load and most amps wonít handle that.  It is best to stick within the load ratings the manufacturer states for the amp.

The next side to this is what is the speakers continuous power handling?  Generally speakers will have a recommended power range.  As you already have an amp and speakers and assuming both are more than adequate for the room, the safe setup is to have speakers connected so that the amps power is within the total power handling of the speakers.  If you connect two speakers in parallel per channel, the amp will be capable of delivering 1000W per channel.  I don't know what your speakers can handle but for the sake of argument, if the speakers can handle 600W between them, this would mean the amps is capable of burning them out in this configuration.  If you just had one speaker per channel at 8 ohm, the amp could deliver 750 W into a single 300 W speaker, also blowing the speaker.  If the speakers are in series, then you have a 16 ohm load which isnít specified in the manual but interpolating from what we do know, Iíd estimate the amp capable of delivering around 560 W into 16 ohms with a total speaker capacity of 600W which is safe.

There are alternate opinions on power matching but it depends on specific circumstances.  For example, if the speakers are only adequate for the room, you will typically be driving them close to their power capacity.  In this case some people like to use an amp that can exceed the speakerís continuous power capacity by say 50%.  This ensures that the amp is not clipping the signal at high levels which helps to keep voice coils from overheating.   While this provides a degree of protection in one aspect, you better be sure you never exceed the speakers power handling capacity.  Itís a commonly touted approach though.  Iíd rather choose speakers and amps that exceed the needs of the room so you never go near these limits.

I like QSC because they generally qualify their specs to be meaningful.  Things like power at some industry standard such as %THD.  Many manufacturers donít which leaves a large degree of uncertainty.


Regards, Ken
« Last Edit: August 09, 2019, 08:13:57 pm by Ken Webster »
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Steve M Smith

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Re: Power Amps/ Monitor Setup
« Reply #4 on: August 09, 2019, 07:23:26 am »

Working at higher impedances isn't tacitly 'bad' for an amp, but it is incredibly inefficient (unless specifically designed to work at higher impedance values) and will make the amp work harder to produce the same kind of output of lower impedance loads


I disagree.  It won't make the amp work harder, it will just use up its headroom quicker.  With a higher impedance load, it will work less hard as it will be pushing out less current for the same voltage output.


Steve.
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Chris Grimshaw

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Re: Power Amps/ Monitor Setup
« Reply #5 on: August 09, 2019, 07:38:58 am »

That only works by accident.  ie the specific case where each load is the same.


If you look at my post again, I did include that qualifier.

Chris
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Ken Webster

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Re: Power Amps/ Monitor Setup
« Reply #6 on: August 14, 2019, 04:12:07 am »

If you look at my post again, I did include that qualifier.

Chris

That's true Chris, I guess I didn't I didn't pick up on it stating a special case.
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Luke Geis

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Re: Power Amps/ Monitor Setup
« Reply #7 on: August 14, 2019, 09:33:16 am »

I wouldn't say that running at higher impedances is bad or inefficient. In the end, it is just a conversion of energy. Technically speaking and amplifier is happier running at higher impedances. This is because the exchange of energy is in favor of current instead of voltage. The amp receives a fixed voltage and rate of current from the wall, so it is easier for it to manage the speaker's needs with higher impedance loads. When you start running lower impedances @ 4 ohm's or lower, the amp is converting current for voltage. The amp is literally starting to pull current from the power source in order to create the voltage needed to power the speakers. This means the power rails in the amp, which are a fixed voltage, are now needing to be maintained with power more rapidly. This higher exchange in energy increases heat, increased heat reduces efficiency. Or put another way, when running at higher impedances, the amplifier doesn't need to convert energy into voltage and the amp can more easily replenish it power rails because its power source can easily keep up. When running at lower impedances, the amp is converting current directly into a voltage to power the speakers. This conversion demands more work from the amps power rails to maintain output power. More work means more heat.

4 ohms is probably the best compromise between the two extremes. In terms of power density, 4 oms produces the most power and does it reliably. I.E. if you need high wattage, a 4-ohm load will provide you with the highest wattage that you can exploit for the longest period of time.

A 16-ohm load will provide you with a modest output wattage reliably for the longest period of time, while a 2-ohm load will give you a very high level of wattage for shorter periods of potential time. Remember, heat kills, lower impedance loads requires less energy conversion and therefore creates less heat.
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