QuestionQUESTION: I just bought some PA speakers and am not sure which amp i should get to drive them. The speaker specs are:
Impedence: 4-8 Ohms
Input Power (RMS) 20 - 200W
Max Power (Peak) 600W
Frequency Response 45 - 20KHZ
Sensitivity 93+/- 2db
Cabinet Size approx 500 x 300 x 300
8" Driver Speaker
I'm looking at this amp:
Proaudio DJ Series Pro Power Amplifier - 2 Channel - 2000W
http://www.oo.com.au/Proaudio_DJ_Series_Pro_Power_A_P19786C1525.cfm
The power specs are:
Power Each Channel: 8 OHM - 2 x 250W, 4 OHM - 2 x 500W 8 OHM Bridge - 2000W
My questions:
1. if the speakers are set to 8ohms, will this amp output only 250W to each one? And, set to 4ohms, 500W to each? My maths is basic and even I can see there's a few watts missing here ;) So I would need something more powerful? Or have I bought speakers that are more powerful than I need (house parties, sometimes outside, up to around 250 people).
2. The low end frequency of my new speakers is 45Hz. If I bought a sub woofer in the future, I would want an amp that could drive this as well. Does this amp have the power to drive this too?
3. Does 2 channels mean there is only 2 outputs?
4. Does it make a difference to power/channel if speakers are wired in series or in parallel?
5. What is an ohm bridge? (8 OHM Bridge - 2000W)
6. Is it better for speakers to run them at 8ohms?
7. The amp also says it's freqency response is 50/60 Hz, but others I've looked at express it as a range more like eg. 20Hz-20KHz. Is this something else?
So many questions! I hope you can answer some of them for me.
Thanks in advance for your help,
Tabitha
ANSWER: My questions:
1. if the speakers are set to 8ohms, will this amp output only 250W to each one? And, set to 4ohms, 500W to each? My maths is basic and even I can see there's a few watts missing here ;) So I would need something more powerful? Or have I bought speakers that are more powerful than I need (house parties, sometimes outside, up to around 250 people).
A: Speaker is not "set"; it has only one impedance. In your case the specification says it will range from 4 to 8 ohms over the frequency range of its normal operation. So, assume each speaker is a 4 ohm unit. You did not say how many speakers you have or are planning on using. If you have two of them, you can connect each one a channel of the amplifier. If you are planning on using more than two you must connect them in either series or parallel. See these diagrams to give you an idea of what different connections are possible:
http://www.bcae1.com/spkrmlti.htm
These are not powerful speakers. From the specs it appears they are rather anemic, but if you will give me the make and model and how many your are using I can give you a more clear idea of their ability to do the job AND how to hook them up to your amplification system.
2. The low end frequency of my new speakers is 45Hz. If I bought a sub woofer in the future, I would want an amp that could drive this as well. Does this amp have the power to drive this too?
A. Pretty much you can ignore the power ratings and frequency response figures given for speaker systems. There are no reliable standards on which such things are measured and the figures given are only very broad indicators made up by the sales department of the manufacturing company. Rather, you must determine their qualities by getting them installed in your system and run listening tests to see how they will perform. These speakers, without knowing more about them, I cannot tell much, but they don't sound like they are suitable for outdoor use.
Based on what I see from the speaker specs you will need a subwoofer system in order to make the system good for music. The sub system would need its own amplification and subwoofer/crossover in order to fill out the system design.
3. Does 2 channels mean there is only 2 outputs?
A. Two channel means there are two amplifiers within the set.
4. Does it make a difference to power/channel if speakers are wired in series or in parallel?
A. Yes, it makes all the difference in the world. You must connect them properly to the amplifier to match the impedance for optimum performance. If the impedance load on the amplifier is higher than the rating on the amplifier you are safe, but not getting maximum transfer of power. If the impedance load on the amp is lower than the rating of the amp you can cause damage to the amplifier.
5. What is an ohm bridge? (8 OHM Bridge - 2000W)
A. Bridging allows the two amp sections within the amplifier to be bridged into one channel; then you have only one amplifier output - but with much higher output power capability, as you see on the specs for the amp you are looking at.
6. Is it better for speakers to run them at 8ohms?
A. Best speaker impedance is that which optimizes the power transfer from the amplifier to its rated load impedances.
7. The amp also says it's freqency response is 50/60 Hz, but others I've looked at express it as a range more like eg. 20Hz-20KHz. Is this something else?
A. The 50/60 Hz rating is the operating line frequency that the amplifier will support. Some countries power systems are 60Hz and some countries are 50Hz; your amplifier will work in either country.
The amplifier you have chosen seems entirely adequate for the kind of audiences you refer to. The question you must consider is whether the speaker systems you have selected are going to be adequate and are they the right ones for the kind of acoustic performance required for the variety of jobs you are thinking of.
Now, I await the make and model of the speaker systems and how many your are planning to use so we can determine if they are adequate for the acoustic space you are powering up (not only power and frequency response is important but radiation pattern, especially when used in large spaces and out of doors) and how to connect them wisely to your amplifier.
C
---------- FOLLOW-UP ----------
QUESTION: Hi C, ok I think I’m following you.
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SPEAKERS
I assumed speakers ran at either 4 or 8 ohms and there was a switch. So you make your base assumption 4 ohms, and the speaker will adjust the impedance if it receives more power. This protects the speaker.
These is the speakers I bought:
http://cgi.ebay.com.au/Pair-HEAVY-DUTY-2-Way-8-PRO-600W-PARTY-DJ-PA-SPEAKERs_W0Q
There is no brand or model. I’m hanging my head in shame. I know, I know, assembled with cheap parts in some factory in China or Korea. I just got excited about buying some speakers and went ahead after a small amount to research. I wish I’d found this web site first!!
These came in a pair, so I have two of these speakers
I will want to add a bass speaker soon
Why do you say the speakers are not powerful or suitable for outdoors? My assumption (based on very little knowledge and research) was that a lot of PA speakers I reviewed were only 250 or 400, so 600 sounded a lot. I guess it’s not just about the wattage. Would you be able to explain the other factors involved?
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AMP
I’m glad I’m onto something good here at least. There’s also a lesser model of the same amp with a total output of 1000W, but for an extra $40 it makes sense to get the better one.
When you say: “The sub system would need its own amplification and subwoofer/crossover in order to fill out the system design”, is the amp not powerful enough to run a subwoofer as well? Could you run 2 speakers in series from one channel and the subwoofer from the other?
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By the way, I do have some other speakers I just picked up for free recently. I tested them, and at first there was no sound. I thought they were broken until I turned the volume right up, and they kind of kicked in. They sounded great. That was when I realised that they needed a more powerful amp to drive them than the domestic one I was using for testing.
I cant see a brand on these there’s a model number and specs:
K-7150-3
8ohm nom.
max input 150W
95db/w.m.
woofer 30cm
mid 12cm
horn 3 x 65mm
Do you have any knowledge of these ones?
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Thanks again and all the best,
Tabitha
AnswerThe speakers you purchased are not professional enough for true music reproduction. They may be good enough for voice reinforcement but with and 8 inch woofer element they cannot put out much strong bass that you will need for bass reproduction.
You must use a separate amplifier to drive a subwoofer because of it limited frequency range and its high consumption of power compared with the higher frequency range of the other music spectrum. It is not a good idea to drive a sub with the same amplifier that is driving the main full range channels.
You have a misunderstanding about impedance. A speaker impedance is a function of frequency. When the rating is 4 ohms it is a nominal and usually a minimum value. At high frequencies the impedance curve rises and may go as high as 20 or 30 ohms. It is not switchable. It is what it is. When the manufacturer specifies it as 4 to 8 ohms it means they are not too sure and they may use different 8 inch bass drivers depending on their supplier.
Best wishes,
C
