Microphone Amplifier

[nicho12]

Hello all,
I have a little issue here, I've been tasked with creating a Microphone Amplifier using discrete components (i.e. No opamps).

I have required specifications for the device, but i'm not really sure how to go about doing this..
The spec is:
Must be suitable for use with range of common microphone types
Capable of driving 3.2ohm 0.4w speakers

Does anybody have any experience with this? Clues would be very much appreciated or if anyone knows a full on solution, then brilliant!
Thanks for your help
N

 

[mneary]

Use preamplifier . You may have to adapt the preamp for electret mics, and the 4W amplifier needs to be modified for 0.4W.

Be sure to give proper credit to the authors when you turn in your work. Your teacher probably knows how to use a search engine.

 

[nicho12]

yeah I know how to reference my sources
looks good, will look at it in a bit and check it out thanks mneary

 

[nicho12]

Ok I've pretty much made the whole circuit, except I cannot find for the life of my any 1ohm resistors that are specified in the amplifier circuit diagram.
do you think they are really necessary to have the two sets of 1 ohm in parallel? would I be okay just shorting that part of the circuit or is there a better solution?

Also couldn't get hold of a 2700 capacitor so Im using a 2200 and a 470 in parallel, should work right?

Do you know what the best way would be to alter it to amplify suitably for a 0.4w loudspeaker?

cheers,
Nick

 

[Nigel Goodwin]

Ok I've pretty much made the whole circuit, except I cannot find for the life of my any 1ohm resistors that are specified in the amplifier circuit diagram.
do you think they are really necessary to have the two sets of 1 ohm in parallel? would I be okay just shorting that part of the circuit or is there a better solution?

The resistors are crucial! - DO NOT LEAVE THEM OUT!

1 ohm resistors are pretty commonplace, they are used in parallel to create 0.5 ohms, if you could find 0.5 (or 0.47) you could use those instead. All those three values are easy to find.

 

[nicho12]

o lol one of my classmates insisted they were useless and shorting that part would work fine!! lol
ok Ill try and get hold of some 1 ohms or 0.5 ohms
what about editing it slightly to make it 0.4w output? any clues on that one?
thanks for your help!
Nick

 

[audioguru]

what about editing it slightly to make it 0.4w output? any clues on that one?

The circuit is designed for 4W output so it uses a high supply voltage and many transistors for high current. A 0.4W amplifier would use a lower supply voltage and only 3 transistors.

0.4W RMS is a peak-to-peak voltage of 5.1V and a peak current of 318mA into 8 ohms.
If the amplifier has a power supply voltage of about 7.5 V and the output transistors are 2N4401 and 2N4403 then the max RMS output power into an 8 ohm speaker will be around 0.4W.

I simulated it. It has only 3 transistors so its distortion at 0.4W is about 10% and is lower at lower levels. Its voltage gain is about 18.

 

[Gaston]

what is the purpose of the 1 ohm resistors?

 

[audioguru]

The 1 ohm resistors add voltage to the base-emitter voltage of each output transistor so that there is less difference in current between transistors that have a 0.65V base-emitter voltage and transistors that have a 0.68V base-emitter voltage. You want about 15mA in the output transistors to reduce crossover distortion and 10mA in 1 ohm is a voltage of 0.015V.

Without the 1 ohm resistors, some output transistors that have a low base-emitter voltage will have an extremely high current all the time.

 

[mneary]

Goal apparently is to drive 3.2 ohm speakers. Would this change anything?

 

[nicho12]

this looks brilliant thanks will test it all out tomorrow, and thanks for the info on the resistors!
unfortunately we cant get hold of any 3.2ohm speakers, the only suitable ones are around 7ohm to 8ohm
would the circuit still work with something like these?
thanks for your help!
n

 

[audioguru]

I forgot about the 3.2 ohm speaker.
I reduced the values of some resistors so that more current can flow, increased the value of the output capacitor and decreased the battery voltage.
Now the amplifier provides 420mW into 3,2 ohms.

 

[audioguru]

I also haven't seen 3.2 ohm speakers for about 30 years. You must have an old text book.

The amplifier needs to be designed for the load. My 400mW amplifier that I simulated has an 8 ohm speaker.

 

[mneary]

Your original post said 3.2 ohms, which is highly unusual these days. audioguru designed a circuit that works with 8 ohms. Now that you are changing to 8 ohms, the original circuit is perfect.

The leading cause of redesigns is inaccurate specs.

 

[sniper007]

what is the purpose of the 1 ohm resistors?

With this resistors change class of amplifier.

 

[audioguru]

The amplifiers I simulated are class-AB because the output transistors have about 15ma of idle current.

The 1 ohm resistors even out the huge spead between the base-emitter voltage of different transistors.

Without the 1 ohm resistors then the output transistors will either operate in class-B with high crossover distortion or in class-A with a high idle current and lots of heat.

 

[nicho12]

awesome! you guys are geniuses!
this helps loads..but do these updated circuits still need a preamp? will the previous pre amp posted work?
cheers
N

 

[nicho12]

oh and btw that stuff about the 3.2ohm speakers coming from an old text book...it actually came directly from my lecturer, who pretty much is an old text book! haha

 

[audioguru]

The amplifier circuit I simulated with the 8 ohm speaker has a voltage gain of 18. Its output voltage at 400mW is 1.79V so its max input voltage is 1.79V/18= 99mV.
A microphone has an output level of from 2mV to 10mV so it needs a peamp with a voltage gain of at least 49.5.

 

[Gaston]

i thought a class A amplifier used a transistor that conducted for 360 degrees and an output capactor to get the positive and negative swing. as opposed to the push pull arangement of the class B ?