Need More Audio Output from Internal Speaker

[Tinkertek]

Hi,

I have a Radio Shack Pro-34 handheld scanner. I would like to modify it so that the audio output is louder. I did some research and found out that it currently has a .3 watt speaker. So, I found a 1 Watt speaker of similar dimensions at Mouser: AS04508MR-3-R PUI Audio Speakers & Transducers

Now I need to figure out how to modify the audio amplifier section to produce louder audio. According to the schematic, looks like it is currently using two 2SC2712 transistors. I wonder if there is another transistor or 2 that can basically be dropped in to increase the audio output. I’d like to have the least amount of modification as is possible. The pertinent section of the schematic is attached. Can you think of any other transistor which would kick up the output from .3 watts to maybe 1 watt?

Thanks so much !
Paul

 

[RickS31]

As an electronics engineer, just throwing a higher power transistor will not increase audio output. In fact, it will probably be less since higher wattage transistors usually have lower gains in favor of providing more output drive current. Output power is based on the biasing of the output stage, feed voltage and hence current flowing through the output transistors and then the speaker. Basically, not to intimidate but rather inform, it is a whole design process itself. You would have to have the design experience to redo the whole driver/output stage of the receiver not to mention the power supply. Not a light undertaking. Besides the pertinent part of the schematic you attached is not the audio output stage. Follow the center tap on the volume control (one with the arrow on it) to that stage.

The easiest solution is to pick up a cheap 5 watt monaural amplifier and plug it's input into the headphone jack with a suitable 3.5mm headphone plug on one end and an RCA phono plug on the other. A suitable external speaker hooked to the outboard amp's output should give you tons of volume. I know that's not a portable solution but even with the redesign, trying to modify the internal circuit boards would be almost impossible.

 

[KC5EIB]

If you are wanting to do this to be able to hear the scanner in the car, get an amplified speaker like
Midland® CB/Amateur/Marine Radio Extension Speaker RS Catalog #: 55016223 for $19.99
Cobra® HighGear HG-S100 Extension Speaker Model: HG-S100 RS Catalog #: 55013039 for $19.99
or If you can find them in the stores
Amplified Mobile Extension Speaker RS Catalog # 2100541 on clearance in many stores at$ 19.07

If planning to use in the house, get a low priced set of amplified MP3 or computer speakers.

 

[RickS31]

Or Doug's solution too. A good set of suggestions.

 

[Tinkertek]

Ok I've found the correct portion of the schematic which controls the audio. It's a LM386 IC. Below is the pic. Based on the above feedback, it sounds like it really won't be possible to increase the audio output using internal parts.

If anyone has any ideas, please reply. Guess I'll need to look for an external solution as mentioned above

Thanks !
Paul

 

[kruser]

Ok I've found the correct portion of the schematic which controls the audio. It's a LM386 IC. Below is the pic. Based on the above feedback, it sounds like it really won't be possible to increase the audio output using internal parts.

If anyone has any ideas, please reply. Guess I'll need to look for an external solution as mentioned above

Thanks !
Paul

One other thing to consider is battery life if you are planning on using the 34 as a portable as it was designed for. If you did find a way to boost the audio to 1 watt or more, I'd expect you would eat through batteries fairly fast. If you intend on using it in a car or at home than the suggestions by Rick and Doug are perfect and will give you what you are after.

I've also used the cheap amplified computer speakers that you can buy for $5 or so online and they actually work great for boosting scanner audio. Just need to be careful and let the external speakers do the work and not crank the scanner output way up. Most of the cheap computer speakers are looking for a line level signal. Your correct schematic shows the headphone output to be at full speaker level unlike modern scanners today were they add a resistor in series with the headphone output to keep the output at a much lower level. I think that is why the modern scanners work so well with the cheap amplified computer speakers. A true amplified speaker made for radio as mentioned in the posts above is the best way to go if you have a way to power it.

I've never worked on the Pro-34 before but if you are sure the speaker at Mouser will fit without any effort then it may be worth a shot to see if the Mouser speaker is louder. I doubt it could ever produce the volume I suspect you are looking for however but it may be worth a shot if it is an easy and cheap swap. It may just give you enough of a boost that you are happy with it. Then again, it may make things worse especially if the impedance is not the same between the old and new speakers.
I see the PUI is only $2.66 so what the heck! I’d try it and see how it sounds. If it will drop in without modifying the housing then you won’t be out much.

 

[Tinkertek]

Thanks to everyone for the suggestions. I was surprised to get such detailed responses. Very nice indeed. I'm going to order the 1 watt speaker and see how it sounds. Thanks ! Paul

 

[N8IAA]

Paul, don't know how you program the radio, but, Win500 has an option in the conventional and Tsys objects portion of the program. It is on the far right hand side under the AGC and Supertrack check boxes. Audio boost works for conventional frequencies, as well as, analog and digital systems. Increases the audio quite a bit. Don't know if the other software providers offer the same in their programs. Have used WinXXX ever since I've owned GRE scanners from RS.
HTH,
Larry

 

[SkyPager]

I have noticed that many consumer electronics have limited the audio output; because some people don't know what a volume knob is used for. Try usng a piece of wire and shorting the audio ground (larger part of the headphone plug) to the antenna jack. I did this with my PRO-96 and it gave a noticable increase.
The audio boost selection does also help.

 

[RickS31]

In looking at the schematic and the design specs for LM386 audio amp IC, you are pretty much getting the max audio out of that circuit. That's about 1/4 of a watt. A more efficient speaker MAY help. Before you tear anything apart I'd have to be pretty convinced that the replacement speaker was going to help. It is an 8 ohm speaker. I'd try hooking the replacement to the scanner via the headphone jack first just to see and run an A/B test between the two.

Anyway, the best solution is the outboard amp'ed speakers suggested by Doug. There is literally nothing you can do circuit wise to beef up the existing audio. And, there is no audio boost capability like what is in the newer scanners (96 & 106). You can see all the PRO 34 scanner specs at:

Realistic PRO-34 Programmable Scanner (200-0135)

Good luck,
Rick

 

[Tinkertek]

Thanks for the feedback folks. The Pro-34 is a really old scanner, I think late 80's vintage It doesn't have any computer serial interface. Though it still works mint and has pretty much everything I need (except trunking). Just ordered some Eneloop rechargeable batteries for it (those things are mint). I also ordered the 1 watt speaker today.

If the speaker doesn't help ( I suspect it won't), I may try to build one of these 1 watt amplifiers:
http://www.electronickits.com/kit/complete/ampl/ck701.pdf

Then my only issue will be trying to squeeze it into the case somehow (probably impossible). I could look at finding surface mount components (except for maybe the large electrolytic and the IC)

Take Care,
Paul

 

[budevans]

Paul,

I have a Pro 34 and I have an external 5 watt CB speaker with 1/8" plug that works great. The Pro 34 has no problems driving that speaker. I still use the Pro 34 and external speaker when I go to the Cleveland Air Show. It's loud enough even outside that I usually end up with a crowd of people around me.

FYI: Radio Shack has the Uniden® 5-Watt External CB Speaker on sale for $9.99.

 

[W2NJS]

You might want to know that just because a radio has speaker rated at 1 watt does not mean that the radio puts out 1 watt of audio; actually it's usually far less. A very good and quite expensive line of commercial two-way portable radios claims an output of 700 milliwatts which means 7/10ths of a watt and at that level they are quite loud. Portables, be they scanners or two-way HTs, are carefully engineered for specific levels of audio performance vs. battery life and there is no free lunch when it comes to changing these factors. You either live with the radio the way it was designed or you make changes which will usually change the performance of the unit.

I vote for using the external amplifier fix; it will get you what you want with no risk of harm to the radio itself.

 

[RickS31]

If the speaker doesn't help ( I suspect it won't), I may try to build one of these 1 watt amplifiers:
http://www.electronickits.com/kit/complete/ampl/ck701.pdf

Then my only issue will be trying to squeeze it into the case somehow (probably impossible). I could look at finding surface mount components (except for maybe the large electrolytic and the IC)

In looking at the specs of the above kit amp circuit, the output of it is about 1/4 watt continuous, the same as the circuit you already have in there. The one watt output is probably just before the IC destroys itself. Also, the input impedance of the chip is 10K ohms. Driving from an 8 ohm source like the output of the existing circuit will certainly over drive the kit IC and it will frankly sound like crap with no benefit. In other words, no real help there.
Sorry,

 

[nodak1crdet3]

Here is another possible fix to your problem. I bought a SCOSCHE FMT4/FMT4R Universal FM Transmitter from Wal-Mart for about 13 bucks. You can plug this into your scanner (3.5MM jack) and play the audio thru your cars radio, on certain FM freqs. It works really well to.

 

[Tinkertek]

Rick,

Do you know of any IC which would provide close to the 1 watt output? I see lots of them out there and I just assumed they would provide the full rated output. However guess that very much depends on the level of the input. I was thinking of tapping into the input of the LM386 chip and using that as the input to the new amplifier. I would then essentially disconnect the internal LM386 ampl and associated components and use my own amp. At this point, now it seems that I'm on a mission to get this to work !!!

Now looking at the LM4860 IC.

Thanks,
Paul

 

[RickS31]

The LM4860 is a good IC choice for 1 watt output with an absolute minimum of external components. Kept in it's tolerance ranges, it also does not need any heat sinking. A couple of drawbacks to it are that it has only a max power supply voltage of 6 volts DC. The 34 has 9 volts via it's 6 AAs. You will have to get the voltage down to a maximum of 5 volts to the chip (for roughly 1 watt output and not to exceed the chip maximums).

This chip also has a quiescent current/power draw of around 1/2 of a watt. This means it's going to drain your batteries about 3-4 times faster than the regular scanner alone would.

Keeping all the above in mind and armed with 4086's data sheet, it certainly would be worth tooling around with if you have the equipment and ability to solder and work with SMDs. Please note also that the solder temps and times are low so you definately need to solder fast and CAREFULLY.

As to hooking it up, you shouldn't need to disable any of the 386's circuitry. A fast and easy way to do this is to tap the scanner audio for input to your new circuit, directly off the center wiper tap of it's volume control and with a good ground route that to the new circuit. Disconnect the internal speaker from the LM386 and put it on the output of your new circuit. This will effectively disable the 386 internal chip. The only other caveat with this method is the squelch may or may not work. It is the transistor on the schematic labeled Q19. It effectively grounds the audio (mutes it) when signal level goes below your squelch settings. You may have to get the new circuit audio input directly from the input pin of the LM386 for squelch to continue to work normally.

Anyway, the above method of hookup would allow you to easily restore the scanner to original.

Hope that helps.

Rick

 

[pro92b]

The datasheet indicates quiescent power is 35 milliwatts. If it was 1/2 watt the chip would be quite hot while doing nothing. The datasheet also says the maximum dissipation with an 8 Ohm load and 5V supply is 0.625 Watt.

There are two major problems with using this chip in a PRO-34. One is that the output is bridged, not ground referenced. The speaker and headphone jack will be floating at 2.5 volts and that limits what can allowably be connected to the jack.

The other problem is the supply voltage mismatch with the battery voltage. To convert 9V to 5V for the audio IC will waste a lot of battery power.

 

[RickS31]

Oops, a couple of "my bad". You're right on the quiescent current/power. Draws somewhere around 7 ma in quiet state. That's with no load so it'll be somewhat higher with a speaker. Second, I forgot about the fact that the headphone jack and speaker are ref'ed to ground. That could be remedied by either floating the headphone jack & speaker (for higher gain) or designing it as single-ended amp with the output cap and have lower gain.

I mentioned the max chip voltages in my post and, yes, it is somewhat of a waste. May be better to find a higher voltage chip.On a positive note, the amp would run to a lower voltage if a proper regulator is provided. Again, though not the best chip for this but cheap and few external components as well as heat sinking.

 

[pro92b]

Running the chip single ended cuts the output power by a factor of four compared to bridge mode. That puts us back at the same output power we get from the LM386.