Audio amplifier chip for 4w, 45 ohm speaker - diyAudio
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Old 28th March 2008, 07:05 PM   #1
fluc is offline fluc  Canada
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Default Audio amplifier chip for 4w, 45 ohm speaker

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
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Old 28th March 2008, 09:14 PM   #2
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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
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Old 29th March 2008, 02:05 PM   #3
fluc is offline fluc  Canada
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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.
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Old 29th March 2008, 03:56 PM   #4
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Is there any reason you can't use a step-up transformer?
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Old 29th March 2008, 05:01 PM   #5
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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
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Old 29th March 2008, 05:55 PM   #6
BWRX is offline BWRX  United States
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Quote:
Originally posted by valleyman
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.
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Old 29th March 2008, 08:30 PM   #7
fluc is offline fluc  Canada
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Quote:
Originally posted by 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
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?
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Old 29th March 2008, 08:31 PM   #8
BWRX is offline BWRX  United States
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Quote:
Originally posted by fluc
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.
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Old 29th March 2008, 09:21 PM   #9
fluc is offline fluc  Canada
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Quote:
Originally posted by BWRX


I would suggest bridging two LM1875s for this application.
Thanks for the reply BWRX.

Looking at the specs of the LM1875. They recommend a supply range of 16-60V. I have +12V and -12V. Would this still work?
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Old 29th March 2008, 09:33 PM   #10
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Quote:
Originally posted by fluc


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
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