Author Topic: FM Radio  (Read 495 times)

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Offline Sparky661

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FM Radio
« on: November 08, 2019, 06:00:49 pm »
Hello everyone. Complete rookie here. I am attempting my first project and am struggling to find some answers. I want to take the guts of an alarm clock and put them into a cool looking tin box to make a customized radio. This means upsizing the speakers.
My question is, what in the already completed PCB must also be upsized?
I am putting in a bigger pot and adding a 7W PA. Anything else?
Oh, do I match the pot size with speaker size in ohms?
 

Offline w2aew

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Re: FM Radio
« Reply #1 on: November 08, 2019, 08:17:12 pm »
Assuming that you are referring to the volume control pot, you probably don't need to change it. Volume controls are typically designed before the final amplification stages, so they are not subject to the high power the speakers see.  If you are adding a separate power amplifier, you can likely still use the existing volume control to adjust the radio's output in order to control the volume coming from the new PA.
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Offline james_s

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Re: FM Radio
« Reply #2 on: November 08, 2019, 08:46:45 pm »
I would probably try to pick off the audio signal from the existing volume control. You might even find that the original amplifier is adequately powerful with reasonably efficient speakers anyway though. Larger speakers can be more sensitive than smaller ones and actually require less power to make more sound. A lot of speakers designed to handle a lot of power are not very sensitive at all though. If you can find the specs it will normally be in dB/W.
 

Offline Audioguru again

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Re: FM Radio
« Reply #3 on: November 09, 2019, 01:31:27 am »
My clock radio had a tinny-sounding 3" speaker. It produced no bass sounds and its screech was loud but most music and voices were not loud.
I increased the value of its output coupling capacitor and connected it to a speaker I bought that had a 6.5" woofer in a sealed enclosure and a poor quality tweeter.

I calculated then added a port on the enclosure for the woofer and I added a hifi tweeter. I added a good crossover network.
Now it sounds louder and clearer with good bass and treble.
 

Offline Sparky661

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Re: FM Radio
« Reply #4 on: November 09, 2019, 07:53:30 pm »
Thank you for the reply.
 

Offline Sparky661

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Re: FM Radio
« Reply #5 on: November 09, 2019, 08:03:07 pm »
Thanks for replying. I know nothing about nothing, so everything is a learning experience for me haha.
I was planning on upsizing the existing speaker to a four inch and then adding a second one. My concern is that will be too much of a load for the system. So I have been scouring the internet to try and understand the different components to figure out what I may have to upsize and am not having any luck. There is lots of information on how to build a tiny radio with a tiny speaker, but I am struggling to figure out what to do in order to acommodate two or three times the speaker output. Maybe the 7w PA is all I need???
 

Offline Sparky661

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Re: FM Radio
« Reply #6 on: November 09, 2019, 08:07:24 pm »
Thank you for your reply. That sounds awesome, but is all Japanese to me. Is there a course  book or website you could recommend for me to learn things like that? I viewed the thread on starter books and while I am grateful for all of the information people have shared, I am getting lost trying to find what I am looking for.
 

Offline Zero999

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Re: FM Radio
« Reply #7 on: November 10, 2019, 08:28:55 am »
Thanks for replying. I know nothing about nothing, so everything is a learning experience for me haha.
I was planning on upsizing the existing speaker to a four inch and then adding a second one. My concern is that will be too much of a load for the system. So I have been scouring the internet to try and understand the different components to figure out what I may have to upsize and am not having any luck. There is lots of information on how to build a tiny radio with a tiny speaker, but I am struggling to figure out what to do in order to acommodate two or three times the speaker output. Maybe the 7w PA is all I need???
As long as the speaker's impedance (this is specified in Ω and is often printed on the back of the speaker magnet) is the same or greater than the original one, it won't be a problem. The power rating of the speaker is not how much power it will draw from the amplifier, but the maximum amount of power it can take before being damaged. The amplifier will be designed to drive a minimum load impedance and going lower could result in damage or the protection circuitry activating which would distort the sound.

This is due to Ohm's law:
P = V2/R

So if your amplifier outputs a voltage of 1V and your speaker impedance is 8Ω:
P = 12/8 = 1/8 = 0.125W

Reducing the speakers impedance to 4Ω, will double the power dissipation, hence why the risk of damaging the amplifier.

If you replace the speaker with a physically larger one, with the same impedance as the original, the power delivered by the amplifier will not change, but it's likely it will be louder and sound better too as the larger speaker will be more efficient than the original one.

Adding another speaker to the output will result in more power dissipation. The total impedance seen by the amplifier again can be calculated using Ohm's law, more specifically the formula for resistors in parallel:
R = (R1*R2)/(R1+R2)

Hint: if R1 = R2, then the resistance is simply halved, so if you have two 8Ω speakers, the impedance will be 4Ω.
 

Offline Sparky661

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Re: FM Radio
« Reply #8 on: November 11, 2019, 02:15:34 am »
Thank you for taking the time to explain this. I am researching speakers and am not learning anything like what you are sharing. If you had to teach this stuff to someone who knows nothing like me, where or what would you suggest I start reading?
 

Offline Audioguru again

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Re: FM Radio
« Reply #9 on: November 11, 2019, 04:35:02 am »
You are talking about sounds from an FM radio so first learn about your hearing:
1) Frequency response. Up to about 25 years old we can hear frequencies from 20Hz to 20kHz. high frequency hearing loss normally occurs with higher age.

Then learn about speakers:
2) A little speaker has trouble producing frequencies below maybe 300Hz (look at its datasheet). Then the sound is "tinny" and is missing bass sounds.
3) A large speaker (called a woofer) is louder than a little speaker and produces good bass sounds but usually has trouble producing high audio frequencies.
4) A high frequency speaker is called a "tweeter". it is damaged if it is fed low frequencies so it needs a "crossover network" to pass high frequencies to it but block low frequencies.
4) A speaker must be in an enclosure to prevent sounds from its rear from cancelling sounds from its front. The enclosure should be designed using spec's or a recommendation from its manufacturer.

How did you select 7W of power? Because you will be far away from the speaker?
 

Offline Zero999

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Re: FM Radio
« Reply #10 on: November 11, 2019, 08:59:37 pm »
Thank you for taking the time to explain this. I am researching speakers and am not learning anything like what you are sharing. If you had to teach this stuff to someone who knows nothing like me, where or what would you suggest I start reading?
For the electrical side, start by learning Ohm's law.
https://www.google.com/search?client=firefox-b-d&q=Ohm%27s+law

As far as acoustics are concerned: it's a bit more tricky. The size of a speaker generally governs how efficient it is at different frequencies, with physically larger units being better than smaller ones. In the case of small speakers designed for a wide range of frequencies, compromises will have been made, so they won't be as efficient as speakers designed for a narrow bandwidth. Generally a larger, wide bandwidth speaker will be more efficient, than a smaller one, especially at lower frequencies and is why you can get an improvement, just by changing the speaker, without increasing the electrical power output from the amplifier.
 


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