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Amplifier circuit problem

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Mcanderson

Thu Sept 14 2017, 10:51AM

Registered Member #61727 Joined: Thu Aug 10 2017, 05:12PM
Location: Berlin, Germany
Posts: 4

Hi,

In my first post- need help or advice for designing speaker amplifier circuit.

I have to design a speaker amplifier circuit for my car (E30-SEDAN1994). We are told the impedance of the speakers and the minimum voltage they require and also the voltage source. After researching I have found that the LM386N-4 is a popular choice. According to a datasheet of LM386N-4. I've seen many circuit diagrams but what I don't understand is how to pick the voltage gain needed for the amplifier.

I originally thought that you would need to know the input audio signal voltage and when you multiplied that by the gain that would need to give the minimum voltage required by the speaker, but it appears that what matters is the current which I don't get.

And are you able to design such a circuit without knowing about the audio input signal? If you do need to know about it, what is it you need to know about it and why?

I would greatly appreciate any help as I am very lost.
Thanks

Conundrum

Thu Sept 14 2017, 11:24AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Yes, just make it variable.

hen918

Thu Sept 14 2017, 09:41PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

I think you are taking a different meaning to "current amplifier". To amplify current you can increase voltage, but to do so without sufficient current available will be pointless; what limits the output voltage is the current and clipping occurs. So what they probably mean is amplify voltage only slightly (or not at all), but ensure that there is sufficient current available to drive the speakers. (The tiny ICs in your phone will drive a pair of headphones, but if you tried driving a load with a much lower impedance, the tiny IC inside would be unable to supply the current, and very little would happen)

This results in a current amplifier without having to think about current.

The input to most (all?) audio amplifiers is a varying voltage signal, so yes, you could build a voltage to current amplifier when you know the impedance of the load (usually 4 or 8 ohms per speaker), but in practice it would be the same as a voltage amplifier: if the impedance is constant when the voltage increases so does the current and vice versa. (Think Norton and Thevenin transformations).

Sulaiman

Thu Sept 14 2017, 11:24PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Mcanderson;
I suggest that you follow Figure10 in the datasheet that your link leads to,
if maximum gain ('volume' control setting) is insufficient then follow Figure 12
(you could have a HIGH/LOW gain switch to toggle the capacitor in or out of circuit)

I like the LM380 compared to the LM386 ... higher power output using similar supply voltage and loudspeaker.

BUT if you search eBay for "12v stereo amplifier" your brain will explode;
very cheap, high power, low distortion digital audio power amplifiers.

hen918;
In the specific case of a loudspeaker there is a HUGE difference between voltage and current drive,
due to the frequency dependance of impedance, especially at resonance.
e.g. I can (or at least, used to be able to) clearly hear the difference that damping factor makes.

Hazmatt_(The Underdog)

Fri Sept 15 2017, 04:02AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

The LM386 is a 1W amplifier that distorts a lot depending on what speaker load you use, 8 ohm will distort more than 6, but again its only 1 watt.

Anything over 6W for a car per speaker would be more desirable.

I used the LM386 as a headphone driver and it worked great, but you had to be very careful with it or you would cause significant hearing damage, and it was always set on low volumes.
Other than a small driver it has limited practical use.

For cleaner signals set the gain to x20, not x200.

Mcanderson

Sun Sept 17 2017, 09:23AM

Registered Member #61727 Joined: Thu Aug 10 2017, 05:12PM
Location: Berlin, Germany
Posts: 4

Thanks a lot, all of them for your kind reply and guiding me.

Should I use LM380 instead of LM386 and the internal voltage gain of 20?

Regards
Christian

Sulaiman

Sun Sept 17 2017, 10:48AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

hazmat made a good point regarding the power level;
in a quiet environment 1W audio into a good loudspeaker is loud enough, typ. 85 to 90dB SPL
but in an suv, with typical automotive loudspeakers, 10 to 100 W would be more suitable.

An LM386 based amplifier is always handy to have around,
check out the cost via eBay e.g.

(sort of takes the fun out of it though)

For general purpose 12V audio I bought one of these

(the speaker terminals are fragile)

This looks too good to be true

even more

There are so many to choose from.

I can't even buy the hardware for those prices via 'professional' outlets.

In my experience, if it is not in an enclosure it will not last long,
so consider how you will make the box, do the printing/labeling, fit connectors and controls etc.......

It may be worth purchasing a ready made unit if only for the box, knobs, connectors and printing
then put your own amplifier inside.

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