# audio amplifier 50 watts minimum

Discussion in 'Homework Help' started by ajdizonm, Mar 1, 2010.

1. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
Hi. I'm Arman and I'm a student. We are required to submit a final project;

Audio Amplifier 50 watts minimum using only discrete transistors.

I planned it using class AB configuration and it still wont reach the minimum.

I use a DC Power supply +15 and -15 output...

Can somebody help me??
start again from scratch...anyone?
thanks.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
What is the nature of your problem? As a hint, study difference amplifiers and operational amplifiers.

3. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
no matter how i design the circuit it wont reach 50watts power output. I use discrete transistors.BJTs

4. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Try to use higer supply voltage. Can you post your design also?

5. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0

Here. I assume an AC Vin signal, 5v max voltage, 40khz.

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
40 KHz is well above the audio spectrum. have you looked at other designs for amplifiers? That should give some guidance - especially if you analyze how they operate.

7. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The first rule of thumb in amplifier design is not to try and get some large gain in one step(stage). What do I mean? Well think about going from say 400 mV(RMS) at 50mA(RMS) for a power level of (0.4*0.05 = 20 mWatts) to 50 Watts. That is a gain of 10*log(50/.02) = 34 dB. That is quite a bit of gain for one stage. It will be better to get that 34 dB of gain in two or three stages.

A better approach would be two stages of say 20 dB and 14 dB or even three stages of say +14 dB, +10 dB, and +10 dB. That's my \$0.02 your mileage may vary.

ajdizonm likes this.
8. ### daviddeakin Active Member

Aug 6, 2009
207
27
For a simple design like that the power output will always be limited by the supply voltage to:

P = V^2 / 2R

With +-15V supply and a 4 ohm load load you can't get more than:

15^2/(2*4)= 28W.

You would need greater than +-20V supply to achieve 50W into 4 ohms, or more than +-28V for 50W into 8 ohms.

9. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
What would be the configuration in the output stage that would likely compliment if I were to use class AB configuration on the driver stage?

I only know concepts about the configurations, and I lack knowledge how to use/ put them all together... to make a sensible amplifier...

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
Your amplifier does not amplify because it is just two emitter-followers that have no voltage gain.

You need a voltage amplification stage, not a driver stage because your transistors are darlington transistors that already have a driver transistor driving the output transistor.

Because your transistors are both darlington transistors they operate in class-B, not class-AB because two darlington transistors must be biased with 4 diodes but you have only 2 diodes.

ajdizonm likes this.
11. ### daviddeakin Active Member

Aug 6, 2009
207
27
The bible for this type of amplifier is this one by Douglas Self:
Maybe your local library has it?

See page 151 which shows typical output stages with voltage amplification, and also page 75 onwards.

ajdizonm likes this.
12. ### hobbyist Distinguished Member

Aug 10, 2008
774
65
Are there any constraints whatsoever, to work under??

Such as VCC, load impedance, voltage or power gain,...ect...

Or is the only constraint 50 watts min
using discrete components only??

13. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
yup. 50watts minimum using discrete components only...

14. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
I have another question...

we've found a ckt that can produce a minimum of 50 watts... now, it is a bit of an issue for us, locally,that we can't find a proper speaker that can take an average of 80 watts out with 8 or 4 ohms impedance.

The question is, is it possible to series a resistor before the speaker so that the power output can be minimize? so as it can't damage the speaker?

thanks!!!!

Another thing,
What possible equivalent transistors are there for:

NSDU57 - PNP 2W 100v 70MHz 1A TO-202

MPSA70 - PNP TO-92

MD8003 - NPN
TO-78

Last edited: Mar 7, 2010
15. ### Audioguru New Member

Dec 20, 2007
9,411
896
A 50W amplifier has an output of 20V RMS into 8 ohms.
A speaker has a resonant frequency that makes it sound "boomy" if its resonance is not damped by the extremely low output impedance of a modern amplifier. So if you connect a resistor in series with a cheap speaker to reduce its power then it will sound "boomy".

PA systems use a transformer at each speaker to reduce the power fed to each speaker.
But their voltage is much higher than 20V so if you use one the power to a cheap speaker will be very low.

Home entertainment stores sell a switched transformer for homes that is mounted as a wall switch.

You listed extremely old transistors. So the circuit you found must also be extremely old.
Why not use a modern circuit? Aren't you supposed to design the amplifier yourself instead of simply copying one?

16. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
presentation is on friday, besides, its not the only subject that i'm enrolled. Instructors are getting a bit fierce. they demand this and that, we dont have much time.

were doomed. T_T

17. ### ajdizonm Thread Starter New Member

Mar 1, 2010
8
0
Is there any equivalent for and old MD8003/MD8002 NPN transistor?

We have managed to find equivalent for the other two..
thanks

18. ### Audioguru New Member

Dec 20, 2007
9,411
896
Thousands of new transistors can replace very old and obsolete transistors.
Simply calculate the max voltage, max current, minimum current gain, max frequency response and max power dissipation of the transistors needing replacement and select new ones having the calculated spec's.

EDIT:
www.datasheetarchive.com shows that they are old low power dual transistors. An audio amp circuit does not need and does not use dual transistors.