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Post subject: Amp Theory 101
Posted: Sun Feb 10, 2013 1:31 pm
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Hey guys, I'm hoping you can teach me something or at least get me started on the path toward understanding.

Situation:

50 Watt amp, one 8 Ohm 112 cabinet makes "X" number of decibels sound pressure.
or
50 watt amp, two 8 Ohm 112 cabinets (exact same cab as above, parallel = 4 Ohms) makes more decibels than one cabinet.

This much I already know but ....

Is the extra sound pressure due to more speakers? Or is it due to the amp putting out more power due to the lower impedance? Or is it a combination of both.

If it's a combination of both, then what happens if you run two identical 50 watt amps (one is a slave) each amp pushing one of the above 8 Ohm 112 cabinets. Same number of speakers and twice the power but also twice the impedance. Does the power and impedance cancel each other out? What happens to the sound pressure level compared with one amp, 2 speakers, lower impedance?

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 3:59 pm
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Unlike changing speaker loads on a solid state amp, connecting an 8 ohm load to the 8 ohm tap of a tube amp OT will result in the same power output and sound pressure level as connecting a 4 ohm load (assuming the speaker sensitivities are the same) to the 4 ohm tap of a tube OT

If I remember correctly, two speakers playing at the same loudness level will result in an overall loudness level 3 dB great than one speaker playing by itself. It is irrelevent if the two speakers are connected to one amp or two amps.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 5:34 pm
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OK, I just used that situation as an example. I didn't know it would be different for tube vs S/S.

The real situation is solid state P/A amplifer and cabinets. One amp driving two cabs per side. I got wondering about this because I keep seeing everywhere that P/A amps will always put out different wattages depending on impedance. For example my little P/A amp is rated at:

350 watts per side into 8 Ohms @ 20 Hz - 20 KHz, 0.1% THD
500 watts per side into 4 Ohms @ 20 Hz - 20 KHz, 0.1% THD
700 watts Bridged Mono into 16 Ohms @ 20Hz - 20kHz, 0.1% THD
1000 watts Bridged Mono into 8 Ohms @ 20Hz - 20kHz, 0.1% THD
1500 watts Bridged Mono into 4 Ohms @ 20Hz - 20kHz, 0.1% THD

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 5:34 pm
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BMW-KTM wrote:
...Is the extra sound pressure due to more speakers? Or is it due to the amp putting out more power due to the lower impedance?...


If you are talking about the same amp (8 ohm amp) and comparing a 4 ohm 2x12 load to an 8 ohm 1x12 load, the amp will actually put out less power when connected to the 4 ohm cab due to the impedance mismatch. The extra sound pressure is due to more speakers.

If you use two 50 watt amps, each with it's own matched impedance 1x12 cab, obviously you will get more power (100 watts) and thus more volume than using two speakers on one 50 watt amp as described above.

Impedance and power of speakers are two totally independent parameters having nothing to do with each other. Speaker sensitivity (ie, 98 db) has much to do with perceived volume also. If using two amps, each with their own speaker, the impedance of the speaker on one amp has nothing to do with the sound of the other amp, even when one amp is a slave to the other.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 5:39 pm
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BMW-KTM wrote:
...350 watts per side into 8 Ohms @ 20 Hz - 20 KHz, 0.1% THD
500 watts per side into 4 Ohms @ 20 Hz - 20 KHz, 0.1% THD
700 watts Bridged Mono into 16 Ohms @ 20Hz - 20kHz, 0.1% THD
1000 watts Bridged Mono into 8 Ohms @ 20Hz - 20kHz, 0.1% THD
1500 watts Bridged Mono into 4 Ohms @ 20Hz - 20kHz, 0.1% THD


My answer above was based on your first scenario. Your second scenario is completely different, and has to do with how the amp is connected internally. I shouldn't have presumed that you were talking about a tube amp either. A SS amp will put out more power as you lower the impedance, and has to be rated enough to handle the lower (lowest) impedance that will be used.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 5:48 pm
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I think shimmilou and I are saying the same thing. I also assumed you were talking tube amps as that is mostly what we deal with here.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 6:01 pm
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OK but what happens when you use more amps with the same number of cabs? Does the added impedance and associated lower power output cancel out any SPL increase that might be gained by adding the extra amplifier?

My amp numbers appear to be unusual, BTW. Most amp specs I look at seem to have a more directly proportional correlation between wattage and impedance. Example: I saw one that was rated at 500W per side at 8 Ohms and 1000W per side at 4 Ohms.

If we were talking about one of these amps with a more directly proportional correlation would it be of any benefit to split up the cabs and add an amp?

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 6:07 pm
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The impedance of two separate amps have no affect on each other. It doesn't matter if you have one amp with a 2 ohm speaker and another amp with a 16 ohm speaker, each amp is independent of the other and their sound outputs will be additive as long as both are in phase with each other. Whenever you connect multiple speakers to one amp, the power output from the amp is divided proportional between the speakers according to each speaker's impedance.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 6:51 pm
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And if you balance each amp/speaker set so that they all have the same output SPL, the overall SPL will increase by 3 dB for each set added, regardless of power output.

For example, each set is capable of 100 dB SPL. You have three sets of amp/speakers. Two sets will be capable of 103 dB SPL. Three sets will be capable of 106 dB SPL.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 9:09 pm
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I don't think I'm being clear enough.

I am currently running two 8 Ohm cabinets in parallel which should give me a 4 Ohm load. Now, because the load has been reduced from 8 to 4 Ohms with the addition of the second cabinet the amp is now putting out more wattage than it did under an 8 Ohm, one cabinet load and it is definitely noticibly louder than when I was running one 8 Ohm cabinet with that same amp. This all makes sense to me because I am running more speakers and more wattage which should give me more sound pressure. I still want even more volume yet. I want to consider getting a second amp but not getting any more cabinets. My worry is that if I get a second similar amp and run two amps instead of one and they both see an 8 Ohm load because I give each amp one of the cabinets then the lower wattage they push due to the increased impedance might not result in much of an increase in volume. Does this make sense? Is this in fact the case?

Both cabinets will be able to handle the power the amps put out. There is no issue there. The issue is that "what the amp puts out" will be lower thus negating the effect of adding a second amp if I don't also add more cabinets ... if I am understanding it correctly.

My first instinct says more amps equals more power equals more volume but the change in impedance throws the more power thing out the window.
Or does it?

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 10:54 pm
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I think that I see now......maybe. For purposes of this discussion, forget about impedance for a minute, it is irrelevant, and focus on wattage per speaker/cab and total wattage. The wattage per speaker/cab, and number of speakers/cabs will determine loudness in this case. Since you are talking about using the same speakers/cabs for both scenarios, consider the following.

1) One amp, 500 watts per side into 4 Ohms (two speakers/cabs per side), you have 250 watts per speaker/cab, 4 speakers/cabs and 1000 watts total.

2) Two amps, one amp 350 watts per side into 8 Ohms (one speaker/cab per side), and another amp 350 watts per side into 8 Ohms (one speaker/cab per side), you now have 350 watts per speaker/cab, 4 speakers/cabs and 1400 watts total.

Scenario 2 will be louder. Is that basically what you are asking?

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 10:59 pm
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I think shimmilou and I have answered your question.

You listed the following power outputs for your amp:

350 watts per side into 8 Ohms @ 20 Hz - 20 KHz, 0.1% THD
500 watts per side into 4 Ohms @ 20 Hz - 20 KHz, 0.1% THD
700 watts Bridged Mono into 16 Ohms @ 20Hz - 20kHz, 0.1% THD
1000 watts Bridged Mono into 8 Ohms @ 20Hz - 20kHz, 0.1% THD
1500 watts Bridged Mono into 4 Ohms @ 20Hz - 20kHz, 0.1% THD

For simplicity's sake, let's only look at the unbridged 8 and 4 ohm ratings. First, notice that the increase in power from 8 ohms to four ohms is only about 1.5 dB. A 3 dB power increase would be double the power or 700 watts at 4 ohms. Safety features and current output capacity usually prevent a solid state amp from achieving the theoretical double power increase when the impedance load is halved. A 1.5 dB increase in power is a noticable increase in power but it is not significant. Sound pressure wise, a 1.5 dB increase in SPL is again noticable, but not significant. Let's look at what happens as you increase power to the speaker.

Let's assume that one 8 ohm speaker has a sensitivity of 100 dB SPL with a 1 watt input, measured at 1 meter. The power to SPL relationship is as follows:

1 watt = 100 dB SPL
2 watts = 103 dB SPL
4 watts = 106 dB SPL
8 watts = 109 dB SPL
16 watts = 112 dB SPL
32 watts = 115 dB SPL
64 watts = 118 dB SPL :shock:
128 watts = 121 dB SPL :shock: :shock:
256 watts = 124 dB SPL :shock: :shock: :shock:
300 watts = approx. 125.5 dB SPL :shock: :shock: :shock: :shock:
500 watts = approx. 127 dB SPL :shock: :shock: :shock: :shock: :shock:

So we see that at maximum output with an 8 ohm load, the SPL is about 125.5 dB SPL.

Halving the load impedance results in only about a 1.5 dB SPL increase to about 127 dB SPL.

I hope you never reach levels above 115 dB SPL. 8)

What does that tell us?

Well, it tells us that a single amp operating at 4 ohms is only about 1.5 dB SPL louder than a single amp operating at 8 ohms (the number of speakers is irrelevant).

Now if you have two amps putting out 300 watts into one speaker each, each speaker is capable of outputting about 125.5 dB SPL. If you sum those outputs acoustically, the total SPL is about 128.5 dB SPL (3 dB SPL increase) or only about 1.5 dB SPL louder than a single amp operating with a 4 ohm load.

So, the question to ask yourself is, is the cost of a second amp worth only about a 1.5 dB increase in SPL? I know what my answer would be. :wink:

Depending on the sensitivity and power handling capability of your speakers, you have more than enough power available to deafen the entire audiance.

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 11:02 pm
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shimmilou wrote:
.... 1000 watts total.

.....1400 watts total.

Scenario 2 will be louder. Is that basically what you are asking?


By only about 1.5 dB SPL as per my calculations. :wink:

Time for bed. Good night. :D

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Post subject: Re: Amp Theory 101
Posted: Sun Feb 10, 2013 11:22 pm
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Great minds think alike? :)

Yes, you explained better, more detail, and I agree that it's not much louder. It actually sounds like a wimpy increase when you explain it, but my explanation sounds much louder....400 more watts! :lol:

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Post subject: Re: Amp Theory 101
Posted: Mon Feb 11, 2013 12:19 am
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So my assumptions were correct. I won't see much of an increase in volume by adding another amp.

My P/A was always powerful enough for the rooms I played in the past but now that I'm in this new outfit and they want to play an outdoor gig come summer on a farm for few hundred people I'm thinking I (we) need more. This will take some more thinking.

Thanks for letting me pick your brains, guys.

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