Set Gain for Audio Amp

Put the radio in the motor room and the speaker in the office.

You can adjust the gain with a potentiometer, say 50 kOhm to devide your 1 volt into something lower. Your can also make a divider with 2 resistors, say 47 kOhm and 1 kOhm - this will attenuate the voltage about 47 times.

Some experimenting may be necessary since audio levels are logarithmic. The potentiometer give you more flexibility.

Hello to Lyn and DVMDSC, Re the location of the radio... this is not possible. The CFA (Victoria's fire authority), will NOT allow anyone to fool with their equipment. If this was allowed, I would take a feed from the "hot" end of the pot and then drop its signal with an attenuator (or pot) and Bob's your uncle.

The other suggestion regarding the pot, that will not automatically adjust itself for varying signal levels, set by the operators in the radio room (office).

I know there is a way to do this but I am unable to remember what it is called. If I could remember, I would Google it. It's a bugger getting old.

Rod. J

What you want is a audio compressor, in Europe also named compander. It limits the dynamics and is meant to reduce distortion in audio recording studios.

The disadvantage is when you used it your voice becomes dull and flat. Is there that much difference in audio output of your operators? Perhaps they need some microphone technique training.

Loud voices can speak from a bigger distance into the microphone and that helps, unless you are in a small toilet room.

A audio compressor is not cheap and the more you compress the worse the quality gets. A divider is definitely better (and also a part of the input of the compressor to chose a optimal work set point.

Seems like your only option is to bury the speaker in a trapbox, or use a high ohms impedance speaker, spray it with foam or screw it behind a panel with a small adjustable hole.

Hi, seems people do not read the question properly. What I ask here is help for a radio 'RECEIVER'. Nothing to do with a microphone.... If anyone has a website that I might visit to get an answer, then I would be very grateful. If you can, thank you.

Read the question again yourself. Then try to re-phrase it: without making any assumptions about what you know about the problem.

I thought dvmdsc's suggestion of an AGC (Automatic Gain Control) chip sounded like a good idea irrespective of the source of the signal.

Thank you for explaining a little more. I am not that bright.

You have no radio operator permanency? Are you using VHF? Try to get good stuff to communicate. This seems to be a question for the manufacturer for your communications supplier. Here on the island, we monitor the sea traffic and it is in the best interest of the ships to adjust their equipment, gain and squelch to get the messages through. Perhaps you need proper equipment and training. At least one person in your outfit should be able to communicate and forward the content.

You can try a horn speaker to restrict the audio bandwith or an equalizer - to work the acoustics for speech or voice on possible background noises in your garage and otherwise I pray with you.

Thank you anonymous hero, you have the problem down exactly. The radio in the office is a fixed item. It has a volume control, which is standard for all radio's. I have built several D.I. boxes over the years for muso's, this is OK for a set input level. My problem here is slightly different however. We are a volunteer fire fighting force in Victoria, there are many people who use the radio and listen to what fires are burning in the general area. Some of these people, myself included, are a little hard of hearing. We turn the radio up a few notches, others who fill out reports, have no interest in listening and therefore turn the radio down a few notches. Therein lies the problem. What I need is a simple AGC device that will compensate for a "small-ish" variation in signal level. This would keep the volume in the motor room to an even level. Not too loud for those with reasonable hearing, not too soft for us oldies. My background has been in the audio industry, but that was too many years ago and my files and circuits that were used are long gone.

Thanks to you for an excellent answer, unfortunately not quite what I need though.

Regards, Rod. J.

He usually mostly is always right.

I wonder if you could relocate the radio or rearrange the room to better suit everyone's needs?

Inserting an AGC or more likely a compressor to have a min/max range for the radio volume would likely require you to invasively modify the radio.

The other option is an external speaker. Personally, I find a better high quality auxiliary speaker really improves the intelligibility of the audio.

I use an old Heathkit HS-24 on my scanner and it vastly improves the sound, making it less harsh. You can find these at Hamfests cheap. This is a low cost way to go as a first step.

Does your radio have a jack for an external speaker?

Also, just relocating the speaker in the office may help.

I just do not see what that DI box is doing different than an attenuator in his case.

There is just a impedance match and a symmetrical (mostly a small impedance transformer) device arrangement but nothing AVC - automatic volume control.

2 resistors or a potentiometer in his case do exactly the same. It needs just some experimenting.

The DI box has also fixed stepped outputs.

Impedance is usually matched with a small transformer, but, more importantly it also adds isolation. The isolation can be a real necessity sometimes.

That is correct, but I guess the receiver is also a battery powered one or equivalent and if not, it probably has a transformer in it for the power supply.

When you mean DC protection, that can be done with a series capacitor.

The risk indeed is a lot higher with e.g guitar amplifiers especially with tubes. It is also done to not have the hum induced by earth faults or differences in polarity.

He can always use a DI box, but he also needs to compress to a satisfactory level.

Even a circuit with one transistor can do that. Thank you. D

AH -

It may also solve the OP's problem by purposely mis-matching the impedance on the speaker that he wants to attenuate? As you have mentioned, normally speakers have an impedance matching transformer that are multi-tapped. Since the matching of impedance will affect the power delivered to the speaker as its load, then it is very possible the OP can just try to adjust the volume by finding an acceptable impedance mis-match in this particular case?

I fear we are entering into confusion between two different kinds of audio amplifier and two different line levels here. A normal domestic or commercial audio unit, e.g. CD player, tuner, or microphone preamp, produces an output at line level, nominally 1V RMS, for the line input of a power amplifier. The output of the amplifier is matched to a speaker impedance of generally 8 ohms (may be 2 or 4 ohms, especially in car audio setups). These speakers do not have impedance matching transformers; they are of fixed impedance. This line level is not to be confused with the 70/100V line level output from amplifiers designed for public address systems. The speakers in these systems do have tapped transformers, to allow them to draw variable amounts of power from the line. This arrangement allows one amplifier to supply lots of power to speakers in open areas in a building and less power to speakers in the corridors. However, these step-down transformers are autotransformers, and do not provide any isolation. The 100V line is strictly do not touch, and even the 70V line is to be treated with great caution.

The OP states; " I am not too sure what level out the radio gives, possibly up to a volt or maybe more."

You are correct. I was referring to your statement "normally speakers have an impedance matching transformer that are multi-tapped" which is not the case except for PA speakers.

phph001-

With the OP not providing all pertinent details, the statement I posted was in conjunction with one of AH's comments. In my comment, it was suggested that it is also possible to attenuate the audio output of an amp by purposely doing an impedance mismatching scheme.. If it can easily be implemented, it will be a very low cost way of achieving what the OP wanted to accomplish!

Why not use a pair of those powered speakers that plug into a laptop PC? You might need to get an adapter jack or splice the wires.

Or buy another radio and tune it to the same station.

Thank you Usbport. As I have already built an amplifier to raise the signal, I don't need a set of powered speakers. I have installed a horn speaker in the motor room at a height that will cover the entire area. (about 20 foot high) and angled down toward the centre of the room. I will try a D.I. box as they are not hard to build. Also try to incorporate some AGC. This is about as close as I am able to get from this thread.

Thanks to all who gave constructive answers to my query.

Regards, Rod. J.

DVMDSC, Brilliant. This looks exactly what I am after. Since I have most of the bits in the "junque" box it will be a simple matter to knock up a breadboard to fine tune it, so to speak. Will use a TL074 for the opamps and I'm sure I still have the old but good old MPF102 J-fet

Thanks for the effort and the circuit, very much appreciated. Hope someday I might return the favour, as long as it's not too far in the future, I may not last that long..

:-)

Rod. J.

Hi Jacko.

I am also in Vic - in fact work for a radio mob. Your problem is fairly common.

It would be easier if you could tap into a fixed level audio output in the radio - then feed a volume contorl and your external Amp.

That way you would have two independent volume controls and speakers. Sorry if you already looked into this.

Hello Johny 451, quite correct. However the CFA has very set issues with people playing with their radios. The thing however, has an external speaker terminal, so no problem hooking a lead to that. I agree with your comment about taking a feed from the "hot" end of the volume control, this would be a great fixed level to work with, but the blokes in power would have a "hissy fit" if anyone started pulling the covers off their radios. BOTHER....

I am in the Bendigo area in case you were wondering..

Kind Regards, Rod. J.

I hate to mention it after all these suggestions, but would it not be simplest just to install another radio?

Hi, php001. The quick answer is no.! Reason: The price of these radios is quite steep, somewhere in the order of $1200+ then there is the problem of being ALLOWED to have this radio, not an easy thing to achieve.

Believe me, it had passed through a few minds, however, the powers that be will NOT part company for such a trivial (in their minds) use.

Thanks for the idea all the same.

Regards, Rod.J.

When you put an external speaker into the jack it will disable the internal speaker.

Here is a possible simple, cheap fix for the problem…

Install two external speakers at the desired locations and put an L-Pad on each external speaker. L-Pads are nothing more than a volume control for a speaker.

Crank up the master volume on the radio to a level louder than you normally would want to listen to it and then adjust each speaker's L-Pad to the desired listening level.

Hi anonymous hero, the radio has a terminal strip for the extension speaker. This in no way affects the internal speaker as it is not a 'jack' with break contacts inside. I am now sure that I have reached the proper conclusion for this problem. My sincere thanks to all who have sent in relevant information

Regards, Rod. J.

One of the simplest methods that you can use to control the volume level of the local radio speaker is with a low resistance potentiometer.

Remove one of the wires to the speaker and connect this wire to one end of a 50 ohm pot. Connect the speaker to the movable arm of the Pot. Connect the other end of the pot to the other side of the speaker.

Now you can adjust the radio volume to satisfy the the extension volume and adjust the local speaker volume to any level you desire.

Hello Marv. Yes I totally agree with your suggestion and have already wired in just such a pot. My question related to the low level of audio, so I decided that an outboard amp. would be needed. The crux of the matter was cutting the output from the radio from ~1Volt, to around 50mV to suit the input to the amp. I believe I now have the circuit required to do just that. Thank you for your input.

Rod. J.

Have you had any luck? That looks like the right kind of circuit - nice and simple. Designing audio compressors can be tricky, but fortunately this isn't a very demanding application. You'll need quite a slow response time, which should help keep it stable. It looks like the "normal" gain of that circuit should be quite high though? You might need to fiddle some values to avoid having to attenuate excessively at the input. I have even seen "passive" compressor circuits, recitfying the audio signal and using that do drive a JFET shunt. Something that simple might even suit you in this situation, especially since you have a relatively high starting voltage. Can't remember where I found the schematic though, it was years ago...

Hello Tomizett, so far I have only breadboarded it up. According to the CRO it looks promising, the input needs padding down a little though. Next time I'm in town I will grab a few low ohm resistors to make up a " T " pad. About 8 Ohms on the top 2 resistors and a 1 Ohm on the shunt resistor. I know this sounds harsh but even a volt from the SigGen looks massive compared to .05V needed. I will certainly post the final outcome here when it is ready for a trip to the fire station and in service.

Regards,

Rod. J.

A quick search turned up the rather cunning passive compressor I remembered seeing: http://www.electroschematics.com/138/dynamic-compressor-passive-components-no-power-supply/ although I wouldn't expect too much of its fidelity, it's an interesting circuit. Of course, the simplest answer is the old classic of wiring an incandescent lightbulb in series with the extension speaker, but regulation may not be tight enough and you're limited by the choice of bulbs that are available to you. Personally I think I'd be tempted to drop R9 in the above schematic to 10k, as I can't see why you'd want the extra gain, but I'm sure a little experimentation should reveal an appropriate gain structure.

Hello again Tomizett. 2 things come to mind here, R9 ? there are only 6 resistoes in the circuit, not counting P1. The actual circuit needs a little thought to understand, so I'll stick with what I have and will store the circuit away for a rainy day when I might breadboard it to see what it does, also how it treats a sine wave on the CRO.

Thanks for the input, it is appreciated.

Rod. J.

He meant R9 in the circuit in post #15.

Sorry, yes - that's exactly what I meant. For some reason all the line breaks have been stripped from my posts, so they're not always that easy to read!

AHA!! The penny drops, thanks to both for the clarification. Some days I'm a little slow, the other days I'm totally asleep.

Regards,

Rod. J.