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Good idea to connect 2 x 30 watt speakers to a 2 x 50 watt amp board??  

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kanaaudio
(@kanaaudio)
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15/07/2020 7:10 am  

I need your help : would it be a problem to connect 2 x 30 watt speaker units to a 2 x 50 watt bluetooth amplifier board? I am thinking in terms of damage if the volume gets too high, or something?

The speaker units are Dayton Audio ND91-8.
They have Power Handling (RMS) : 30 Watts and Power Handling (max) : 60 Watts.

The amp board is Dayton Audio KAB-250v3

Thanks in advance!


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TVOR-Ceasar
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15/07/2020 10:43 pm  

Devil's in the details.

Specs on the speakers is 30W RMS, 60W max., impedance 8 ohm

Specs on the amp are 50W (x2) @ 4 ohm

Since the speakers you want to use are 8 ohm, that means that the power you will be getting from the amp will actually be around 25W. That means you should be good to go with that combination.

If you want any further explanation on how power works, just speak up and I'll do my best to concisely elaborate.

-Charlie


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kanaaudio
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16/07/2020 4:57 am  

Wow thats really helpful, thank you for that, I really appreciate it!


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123Toid
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19/07/2020 1:10 am  

@kanaaudio

In general it probably wouldn't have been an issue either.  We would need more information.  But unless you turn up the volume all the way and leave it there, you would most likely be fine.  PS Audio did as good video explaining the volume control here:


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kanaaudio
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20/07/2020 7:38 am  

@123toid Thank you for that. I'll admit I was one of those people as explained in the video, who believed the volume control worked the other way around. In any case, I was afraid of having an amp able to send too much power to speakers not able to handle the stress. Glad my combo is compatible. But I guess that means I can't use the version of the ND91's with 4 ohm?

Is the reason for the different ohm's, 4 and 8, that is allows a way to pair speakers with amps? Or whats the reason behind it?

 


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TVOR-Ceasar
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26/07/2020 2:58 am  

Hey guys. It's been a hectic week and I just didn't have the oompf left after work to put in. Now that I've had a bit of a "mental health" day, I'll give it a go.

That video from Paul makes a good point. Almost every amp out there is based on a fixed gain design. Sure, there are some that are variable, but they are rather rare, and they are usually only variable to a point, meaning they are fixed at some minimum value and varying up to the design maximum.

Anyway, to get to your question, I'll need to explain about how an amp drives a speaker. The 2 main major factors in use are voltage (V) and current ([A]mps). Voltage determines how far the cone should move (how loud) and current determines if the amp can continue to play that loud under the speaker load. 

In another post, Nick (123TOID) gives a brief explanation of what watts are - Volts x Amps = Watts. For example, 20V x 5A = 100W. Simple math.

To measure the wattage of an analog amplifier (Class A or AB), you need to hook up the speaker or dummy load and use an oscilloscope to find the point of maximum clean output, just before distortion. Then you read the RMS voltage, square that and divide the result by the ohm rating of the speaker. An easy scenario is: 8 volts RMS clean into an 8 ohm speaker. 8^2=64, 64/8=8 watts. If you get 8 volts RMS into a 4 ohm speaker, that would then be 64/4=16 watts 

Now, if the amp cannot deliver enough current into the load, it will pull the voltage down, resulting in less clean power and earlier distortion. This has to do with the relationship between current, voltage, and resistance, in the case of AC (audio), impedance, which is expressed in Ohms. There's a lot to it, but the short of it is if there isn't enough current it will make the voltage sag. 

Okay, your question - the case for different ohms. Originally, drivers were of a much higher impedance. As a matter of fact, the crystal earpiece is in the 1000's of ohms. In the early days a 16 ohm speaker was considered extremely low. When very powerful tube amps became available, and then better solid state started to mature, 8 ohms was the defacto.

4 ohms were mostly used for automotive applications, simply because the voltage was often limited to close to battery levels. The lower impedance allowed for more current to flow, depending on how rugged the voice coils were made. This allowed for more power to be delivered at a lower voltage. Usually the quality was not as good as home systems that relied on 8 ohm outputs. Gradually, home systems could deliver more  current, so the lower impedance drivers could be used. 

At this point, there really isn't much difference in quality between them. You just have to make sure your amp can deliver the power to the speaker up to full ratings. If in doubt, the higher impedance driver will give you a margin of safety for your amp, as can be seen way up above in the calculations.

Class D amps work differently than Class A, A/B. They use frequency modulated Pulse Width Modulation to create the power to the speaker. It may sound confusing, the FM/PWM output, but it can be followed if you think of it as using an FM transmitter to drive a digital speed controller to a motor. There is a carrier frequency well above the audio range that gets pushed higher and lower to the input signal (music). At the end of the amp is a filter that strips away the carrier signal to recreate the audio. 

To make it change in volume, you need to change the duty cycle of the carrier wave. 0% duty cycle means no volume and 100% duty cycle means full volume. I know I'm throwing terms out there that may seem like strange ideas, but they are easily researched and well documented. Needless to say, this makes it difficult to measure actual power like detailed above, so generally you take the manufacturer's numbers for granted. And you absolutely must make sure you pay attention to ALL the specs used to define the output power. 

I hope this helps clear up some questions. If you would like, I'd suggest watching JohnAudioTech on YouTube. He demonstrates how to do what I said above, and makes it pretty clear. Just look for almost any of his "amp" videos.

-Charlie


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123Toid
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27/07/2020 12:15 am  

@tvor-ceasar

Fantastic write-up!  Thanks for doing that.


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TVOR-Ceasar
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27/07/2020 1:15 am  

@123toid

Absolutely my pleasure. Way back when I joined, I stated I am willing to share anything I know, and am willing to learn what others know. The community can only become stronger if we all grow together. That's my hope for this "Audio Family" you've started. Thank you for that.

Best to you and the fam.

-Charlie


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kanaaudio
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28/07/2020 5:21 am  

@tvor-ceasar THANK YOU so much, I really appreciate you taking the time!


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TVOR-Ceasar
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28/07/2020 2:17 pm  

@kanaaudio

You are absolutely welcome. Always a pleasure to help out if I can.

-Charlie


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