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Author Topic: AF amplifier for exteral speaker?  (Read 4358 times)
NEVBEN
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« on: March 16, 2012, 02:03:40 PM »

I'm looking at the KENWOOD KES-5 external speaker.  It's specified to handle 40W peak and works at 4 ohms impedance like the output of my mobile unit.  But my mobile unit's output is 3W at 4 ohms.  That's fine for the internal speaker, but obviously I need more for the external speaker.  Where does it come from?  What does the AF amplifier in the mobile unit really put out?  Mine's a FT2900R, but I looked at 8900R's and Kenwoods and they all specify 2W at 8 ohms.  Where do I get the ~25-50W I want to drive my external speaker without a ton of THD?  I read the reviews of external speakers like the KE-5 and SP-50 and nobody mentioned an amp.  I looked at car audio amplifiers but most of the single channel amps are for sub woofers, over-powered and not always full range.  I found a nice 60W two channel amp but it's a bit pricey.
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NA4IT
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« Reply #1 on: March 16, 2012, 03:31:47 PM »

Used siren unit with the "radio repeat" feature... may be as high as 100W...
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W0GSQ
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« Reply #2 on: March 16, 2012, 04:33:19 PM »

I use an old commercial radio surplus GE MASTR amplified speaker. It is so loud it will run you out of the vehicle if you turn up the volume.
Still see them on E-Bay for cheap.
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K6LCS
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« Reply #3 on: March 16, 2012, 08:05:57 PM »

>> ... 3W at 4 ohms.  That's fine for the internal speaker, but obviously I need more for the external speaker. 

Absolutely not. Mount that speaker well, and you'll never desire full volume from that 3W. We have no idea what kind of car you are using, but in my Ford Ranger, I have one speaker mounted behind my head and on the column between the doors on the driver's side of the car. Never desired more amplification from it at all.

Now, then, if you mount the speaker further away, not only will it possibly irritate passengers, but you'll need more volume. (grin)

Clint K6LCS
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Clint Bradford, K6LCS
http://www.k6lcs.com
M6GOM
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« Reply #4 on: March 17, 2012, 08:08:09 AM »

Wattage doesn't tell you how loud it is. Two different speakers can have completely different dB levels for the same wattage. The speaker you use will be what determines the ultimate volume you hear - the size of the magnet, the design of the voice coil, even the design of the box the speaker is in all have an effect on the volume.

The radio specs merely tells you what power it'll output and what impedance the amp wants to see on the output.

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NEVBEN
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« Reply #5 on: March 17, 2012, 08:39:41 AM »

The speaker must go under the driver's seat.  There are no other options (other than the car's audio speakers).  This is an offroad vehicle that can get very loud but isn't always loud.  It's packed with gear and just about every square/volumetric inch is accounted for.

If I wear a helmet, I will use headphones, but I like the no headphone option.

I understand speaker sensitivity (SPL @ 1m per watt).  We could talk about SPL, but it's common practice to estimate the SPL you'll get based on how many watts (peak or continuous RMS) you have driving.  It's given that for voice frequencies, we don't need 1000W like for 20Hz, but there's no speaker that has such high sensitivity that 3W is going to give a reasonably high SPL.  Besides that, if I crank the built-in amp up to 3W (or whatever), it nears 10% THD.  So even if 3W were enough, I'd want a 30W amp so I didn't get so much THD by cranking it to max all the time.  Of course some amps are better for THD at different portions of their overall rated capacity but it's true, the Yaesu 3W amp is at 10% THD when cranked.  That's bad.  It might be good for 1W with reasonably THD.
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W0GSQ
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« Reply #6 on: March 17, 2012, 10:31:02 AM »

The GE MASTR speaker is a bit big to fit under the seat but the Motorola amplified speaker is thin enough to do so. They are both VERY loud. Several articles on the 'net on how to make the very simple mod to the Motorola speaker so it can be used with non-Moto radios.
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AA4PB
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« Reply #7 on: March 17, 2012, 01:42:54 PM »

If you really need more power to get the SPL out of the speaker that you require then you are going to have to add an external audio amplifier. There is no simple way that you are going to get the transceiver to put out more audio power.

I would do some experimental tests with your speaker first. Usually just cranking up the SPL in a noisy environment lets you hear but you may not be able to understand what is being said. Usually it is better to get the speaker closer to your ear in order to be able to understand in a noisy environment. Under the seat is probably not a very good location for a speaker, even if you crank up the SPL so that you can hear it.

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KG4NEL
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« Reply #8 on: March 17, 2012, 02:11:51 PM »

Older 12V car audio amps, from the mid '90s and back, may be what you're looking for. If they're Class A/B, they're fine for running full-range - it's the class D amps with internal LP filters you have to watch out for.

In most cases, you can bridge the 2 channels and end up with a single channel.

This one might do the trick: http://www.ebay.com/itm/Blaupunkt-BSA-40-MS-Car-Audio-Amp-2-20W-/260979720371?pt=Car_Amplifiers&hash=item3cc39a80b3#ht_1315wt_992
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NEVBEN
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« Reply #9 on: March 17, 2012, 04:02:25 PM »

That Blaupunkt looks perfect.  I'm going to look for something like that, maybe bid on that one.

I know under the seat isn't ideal, but that's the space I have.  Since I don't even have a remote faceplate, you can believe I tried to work it out better but I don't have an ideal place for the head unit, much less a speaker.  If I want to see what channel I'm on, I have to get out and look -- can't even see between my legs, which is a good thing because there'd be no point to driving around that way.  So I drive around with the scanner on but I don't transmit that way.  I have it more or less set up as a vehicle-mounted portable.  That's another reason why driver-oriented speaker placement isn't necessarily ideal.  Because quite often I use it standing outside.  Ideally I'd be able to remote it to an HT like I mentioned in another thread, but technically and legally and with the expense involved, I don't think it will happen.  So good, loud audio is nice to have -- but I don't need a PA system or anything, that would be obnoxious.  Thanks all for the tips.
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KC2RGW
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« Reply #10 on: March 24, 2012, 09:22:14 AM »

You do NOT need an amp for that speaker....plug it in and try it first, you'll see.

It's a great speaker, perfect for wanting to hear your radio through an open car window 50 yards away too.

I run an older Icom speaker that is similar in a Jeep Wranger.  With the windows down at 70mph and the music absolutely blasting, it is plenty loud to hear repeaters if I want to with the stock audio output of a mobile radio.
« Last Edit: March 24, 2012, 09:23:47 AM by KC2RGW » Logged
N6GND
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Posts: 338




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« Reply #11 on: March 26, 2012, 09:00:34 AM »

I'm wondering just how loud the environmental noise (wind plus vehicle) plus the radio sound is. My guess is that the sound levels are close to, if not in excess, of levels that are likely to cause permanent hearing loss.

There is lots of permanent hearing loss these days just from wind plus vehicle noise. For example every motorcyclist (with helmet) who travels at more than very modest speed (40 mph) is likely to be losing hearing. Most of us always wear earplugs for this reason.
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KC2RGW
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« Reply #12 on: March 26, 2012, 12:12:32 PM »

I'm wondering just how loud the environmental noise (wind plus vehicle) plus the radio sound is. My guess is that the sound levels are close to, if not in excess, of levels that are likely to cause permanent hearing loss.

There is lots of permanent hearing loss these days just from wind plus vehicle noise. For example every motorcyclist (with helmet) who travels at more than very modest speed (40 mph) is likely to be losing hearing. Most of us always wear earplugs for this reason.

I would estimate in excess of 110db SPL, I don't do it often, but just illustrating that the speaker should be _plenty_ loud to overcome that.
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MDNITERDER
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« Reply #13 on: March 26, 2012, 12:23:59 PM »

I have a class D mono amp at 700 watts rms at 4 ohms u can try haha jk, 3 watts sometimes just is not enough, /I have looked for small 35-50 watt mono amp for PA speaker for my boat. I have even thought of building one.

 if you find one pass the info.

John
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WB6BYU
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Posts: 13029




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« Reply #14 on: March 26, 2012, 02:35:19 PM »

I've built small audio amps using the LM383 or TDA2002 audio amplifier chips.
If you need more power than that, use a pair in push/pull, or one of the higher
power devices.  Circuits are relatively simple and provides plenty of audio.
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