# Audio amplifier design

Discussion in 'Homework Help' started by Rubs, Oct 19, 2011.

1. ### Rubs Thread Starter New Member

Jun 20, 2011
16
0
hey all...i have to design an audio amplifier...the input voltage would be 5mV(p-p) and the load is a 10W, 4ohm speaker...i'd be using a class AB output stage biasing it with Vbe multiplier...and im not sure what type of amplifier to use for this very high gain (3576V/V) at the intermediate stage... i was thinking of using two cascaded common emitters or just one cascode amplifier...which one would be better? or if u hav any other suggestion plz tell me...

2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Is this homework? There is a dedicated section on this site about homework. If it is so, it always helps if we know more about your curriculum background.

3. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
10W RMS into 4 ohms = 6.33V RMS
2 x 6.33V = 12.66V

I would use a +/-15V supply @ ~3amps (6.33V / 4 = 1.6amps), which is fine for use with op amps and provides enough headroom for decent distortion numbers.

5mVpp = 1.77mV RMS

6.33V / 0.00177 = 3576 of gain

if the power amp has a gain of 20, then 3576 / 20 = 179

I would have an input op amp with a gain of 10, and a second stage with a gain of 20 (200 total) and a volume pot to ground before the power amp.

10*20*20 = 4000 which will allow for pot adjustment and variations.

So, use a dual op amp for the first two stages that drive a power amp IC (LM1876) with a gain of 20; all using a +/- 15V supply.

4. ### Rubs Thread Starter New Member

Jun 20, 2011
16
0
my teacher has told not to use an op amp unfortunately any other suggestion?

5. ### Rubs Thread Starter New Member

Jun 20, 2011
16
0
no its not a homework its a project..we are supposed to design an audio amplifier

6. ### Audioguru New Member

Dec 20, 2007
9,411
896
Of course it is homework for school.
The very simple amplifier can be made with only 7 transistors but with fairly high distortion. With more transistors then it will have less distortion. Design it and if it doesn't work then we will help you.

Oct 22, 2011
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8. ### Audioguru New Member

Dec 20, 2007
9,411
896
You found an old tutorial. Push-pull outputs of amplifiers have not used an input transformer for at least 50 years.

Audio amplifiers do not operate in class-B because it has awful crossover distortion. They operate in class-AB.

The push-pull complementary output transistors are not an amplifier because they produce no voltage gain. They are the output stage of an amplifier.

9. ### chrisw1990 Active Member

Oct 22, 2011
543
41
umm, further down the page, theres a part on A-B?

10. ### Audioguru New Member

Dec 20, 2007
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896
It shows two input coupling capacitors exactly like the little Philips amplifier made in 1955. Audio amplifiers have been DC-coupled without coupling capacitors for many years.

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Oct 22, 2011
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12. ### Audioguru New Member

Dec 20, 2007
9,411
896
The very old circuit shows coupling capacitors like the 1955 amplifier used.

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13. ### chrisw1990 Active Member

Oct 22, 2011
543
41
oh ok then, my apologies! used an amplifier just like that, minus the caps last year, doesnt really matter anyway, just removes some of the bass =]

14. ### Audioguru New Member

Dec 20, 2007
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When the value of a coupling capacitor is too smalll then it forms a simple highpass filter that removes some of the bass. When the value is correct then there is plenty of bass. The value is easily calculated.

Here is the push-pull output stage with a driver transistor replacing one resistor:

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