Microphone Amplifier - Electronic Circuits Projects Diagrams Free

nicho12 · 29th April 2008, 06:15 PM

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 · 29th April 2008, 09:33 PM

Use preamplifier http://sound.westhost.com/project13.htm to drive power amplifier http://web.telia.com/~u85920178/use/afamp-4w.htm. 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 · 29th April 2008, 09:52 PM

yeah I know how to reference my sources

looks good, will look at it in a bit and check it out thanks mneary

nicho12 · 3rd May 2008, 05:24 PM

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** · 3rd May 2008, 06:45 PM

Quote:

Originally Posted by 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?

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 · 6th May 2008, 09:58 AM

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 · 6th May 2008, 02:06 PM

Quote:

Originally Posted by nicho12

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 · 6th May 2008, 02:17 PM

what is the purpose of the 1 ohm resistors?

audioguru · 6th May 2008, 02:36 PM

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 · 6th May 2008, 08:27 PM

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

nicho12 · 6th May 2008, 09:58 PM

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 · 6th May 2008, 10:13 PM

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 · 6th May 2008, 10:42 PM

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 · 6th May 2008, 11:23 PM

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 · 6th May 2008, 11:30 PM

Quote:

Originally Posted by Gaston

what is the purpose of the 1 ohm resistors?

With this resistors change class of amplifier.

29th April 2008, 06:15 PM	(permalink)
nicho12 New Member	Microphone Amplifier 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

6th May 2008, 02:36 PM	(permalink)
audioguru Experienced Member	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. __________________ Uncle $crooge

6th May 2008, 10:42 PM	(permalink)
audioguru Experienced Member	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. __________________ Uncle $crooge

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29th April 2008, 09:33 PM	(permalink)
mneary Experienced Member	Use preamplifier http://sound.westhost.com/project13.htm to drive power amplifier http://web.telia.com/~u85920178/use/afamp-4w.htm. 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.

29th April 2008, 09:52 PM	(permalink)
nicho12 New Member	yeah I know how to reference my sources looks good, will look at it in a bit and check it out thanks mneary

3rd May 2008, 05:24 PM	(permalink)
nicho12 New Member	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

6th May 2008, 09:58 AM	(permalink)
nicho12 New Member	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

6th May 2008, 02:17 PM	(permalink)
Gaston Experienced Member	what is the purpose of the 1 ohm resistors?

6th May 2008, 08:27 PM	(permalink)
mneary Experienced Member	Goal apparently is to drive 3.2 ohm speakers. Would this change anything?

6th May 2008, 09:58 PM	(permalink)
nicho12 New Member	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

6th May 2008, 11:23 PM	(permalink)
mneary Experienced Member	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.