# Audio amplifier chip for 4w, 45 ohm speaker

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#### fluc

Hi everyone,

Looking for some advise for an intercom system that I am working on.

I want to use 4W, 45 ohm speakers at the stations. I will have a +12V and -12V voltage availlable.

The different amplifier chips that I looked at untill now are made for 2, 4, and 8 ohm. Looking at the specs sheets, it seems that if I increase the impedance output od the power of the amplifier the power decrease.

Is there any way that I can amplify my audio signal in my station without having high voltage supply?

Kind regards,
Luc

#### valleyman

A +/-12V supply can offer at most an output peak voltage of +/-12V
12V peak = 8.5V rms

and P=Vrms^2/R

So P= 8.5^2/45 = 1.6W

which will be the theoretical max output power you could get. The chips wont quite achieve that but shouldnt be too far off

I'm assuming the speakers are very sensitive so 1.6W might be plenty and I'd think the audible volume difference between 1.6W and 4W is not very big

#### fluc

Thanks for the respond valleyman and the theory.

Would you recommend any specific chip?

Maybe it I might better using a 8 ohm speaker? The chip would give me a higher power output.

#### theAnonymous1

Is there any reason you can't use a step-up transformer?

#### valleyman

You should look at the sensitivity of the speakers. Using a lower impedance speaker will allow you to get more power from a chip but if the 45ohm speakers have a high sensitivity (say around 105-115dB) then 1.6W is going to be very loud and trading for 8ohm speakers of lower sensitivity will allow the amp to deliver more power without actually making it any louder

#### BWRX

valleyman said:
A +/-12V supply can offer at most an output peak voltage of +/-12V
...
So P= 8.5^2/45 = 1.6W

A bridged amp will allow for twice the output voltage swing (and 4x the output power - so you'd have 6.4W available), and will work very well with higher impedance speakers.

I would suggest bridging two LM1875s for this application.

#### fluc

valleyman said:
You should look at the sensitivity of the speakers. Using a lower impedance speaker will allow you to get more power from a chip but if the 45ohm speakers have a high sensitivity (say around 105-115dB) then 1.6W is going to be very loud and trading for 8ohm speakers of lower sensitivity will allow the amp to deliver more power without actually making it any louder

I checked the specs and they say: ''the 45 ohm impedance provides the higher sensitivity required for proper talkback operation in intercom.''

The SPL 1W/1m is 88dB. Does this mean that with 1 watt output at the speaker and 1 meter away I would get 88 dB?

#### BWRX

fluc said:
The SPL 1W/1m is 88dB. Does this mean that with 1 watt output at the speaker and 1 meter away I would get 88 dB?

Yes.

#### fluc

BWRX said:

I would suggest bridging two LM1875s for this application.

Looking at the specs of the LM1875. They recommend a supply range of 16-60V. I have +12V and -12V. Would this still work?

#### valleyman

fluc said:

I checked the specs and they say: ''the 45 ohm impedance provides the higher sensitivity required for proper talkback operation in intercom.''

The SPL 1W/1m is 88dB. Does this mean that with 1 watt output at the speaker and 1 meter away I would get 88 dB?

This sounds like marketing jargon to me. 45ohm impedance is not required to reach 88dB sensitivity and many low impedance drivers have greater sensitivity than this.

It may be a better solution to look into getting lower impedance, higher sensitivity drivers and avoid having to bridge 2 chips.

The supply voltages for this chips mean |V+| + |V-| so a +/-12V supply is actually 24V and LM1875 would be fine

#### Dr_EM

My interpretation is that the "higher sensitivity for talkback operation" refers to higher microphonic sensitivity of the speaker when its presumably used as the pickup as well as the output device.

#### fluc

theAnonymous1 said:
Is there any reason you can't use a step-up transformer?

I did not look into this option yet. Price will probably be the determining factor. I plan to have several intercom on the system. How much do a step up transformer cost?

valleyman said:

This sounds like marketing jargon to me. 45ohm impedance is not required to reach 88dB sensitivity and many low impedance drivers have greater sensitivity than this.

It may be a better solution to look into getting lower impedance, higher sensitivity drivers and avoid having to bridge 2 chips.

The supply voltages for this chips mean |V+| + |V-| so a +/-12V supply is actually 24V and LM1875 would be fine

Thanks for the explanation of the supply voltage range. I did not catch this when I looked at the specs sheet.

I guess I will have to do some testing to see which speaker is more appropriate, an 8 ohm or a 45 ohm. I am trying to have a simple circuit.

The intercoms will be wall mounted. The speaker have to be sensitive enough to pick up the voice(like a microphone) from someone about 20 feet away during talk back operation.

Dr.EM said:
My interpretation is that the "higher sensitivity for talkback operation" refers to higher microphonic sensitivity of the speaker when its presumably used as the pickup as well as the output device.

This is the way I understood it.

#### Sangram

Paid Member
If you have 8 ohm speakers and +/-12 volt supplies, another good alternative is the TDA 2030. This is usually very cheap and can get the job done for intercom use. I used 25 of them as an paging/announcement system in an office. The whole thing cost less than a hundred dollars to build, and as it was only for paging, required minimal heatsinking. I remember clamping it to a 3mm aluminum sheet. The chips spent most time idling, so it never even got warm.

For 45ohm speakers the task is a little more difficult, but probably you're looking at a 100volt system with transformers attached to each speaker. I used amps like those in my school auditorium (man, was that a long time ago!) and they worked fine. Those were 32 ohm speakers though, and I don't know that you can get transformers for 45ohm.

Of course, I've never used a chipamp to drive a transformer so I'm not really sure if it will work. Those were purpose-built push-pull solid state amps (with the transformer built into the amp circuitry).

#### EWorkshop1708

BTL is great for getting the most voltage out of your powersupply, and into your speaker.

A simple IC with BTL outputs will make any speaker loud on 12V even a high impedance speaker (with less watts of course)

I just set up a small amp using a HA13158A 12V car radio IC with outputs for 4 speakers in BTL, all from only one chip, so it's easy to hook up, and doesn't need any fancy wiring or circuits to bridge IC's together. Just hook up 3 power wires, 5 grounds, inputs and outputs, and mute and standby to 12V and you have a complete 4 channel amp.

Datasheet here: http://www.datasheetarchive.com/HA13158-datasheet.html

It's a powerful IC amplifier.
I almost burned up a 5W 8 ohm speaker with one of the channels when the amp drove into clipping for a few seconds, and I started smelling hot coil

#### fluc

I will surely checkout the possibilities of using a TDA2030 or a Bridge Tied Load chip.

#### east electronics

there is a much better easier

effective way to solve your problem ....and also do that without creating a all new circuit for it .....

you can use any of the amplifiers you choose that fullfill your power requirements the only thing you have to do is connect a transformer to the output of your amplifier .....

transformer will effect of course the quality of your sound but since you construct inetrcom that is not really importand is it ????

a variety of these transformers also tamper included meaning that also you will have ajustable Z you can find in the audio bussines that construct 100v amplifiers and related periferals ( i actually have a gozillion of these trafos and if you want i can send you as many as you like the only thing you will have to pay is the postage )

100v amplifier is not a typing mistake its very correct if you are not familiar with this technology google about the theory and you will find that this the best solution

best regards sakis

#### EWorkshop1708

Have you tried a 120V or 240V mains transformer, but used "backwards"? Drive the signal into the secondary coil, and use the higher voltage output from the primary?

#### east electronics

you are in the right way

you can also check a company called australian monitor .....

in our country since we have a lot of hotels that use this kind of technology ( its a way to match impentance in order to drive from one amplifier more than 20-50 speakers )

so we have both imported but also greek made a lot odf these trafos .....

i also as i said have a lot of them in a very good quality and proper ratings ..... i presume even if i wnat to give this free to you still there is a long way to travel in kanada ha ha ha

EWorkshop1708 Have you tried a 120V or 240V mains transformer, but used "backwards"? Drive the signal into the secondary coil,

the ratio to be used is something 1:5 so mains trafo not really a good idea ( what is mains in canada ?? 220 or 110 ) dont know .....

also trafos made for 100v amplifiers and speakers do not really use the same material since is made for audio ( thats why i think that my trafos are special made )

but any way .....you are in the right track

#### east electronics

i have a boy

that works in my company .... he has a very good computer background ....

he is good when it comes to networks or any other problems that might be related to software ......

so ...he also things as "informatic" as they call him where he comes from ..... so his solution for most of problems is format ....

may be to him but not to me ...there is always a few things you can do before you format a pc .....

my point :

so many nice audio people looked at your post managed to send you to the moon by providing a bunch of circuits .....

but missed the obvious ..... ....

some times i enjoy things like that

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