# Megaphone modifications

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##### New Member
Gday all. I bought a cheap Chinese megaphone a year or so ago, had some fun with it out 4wding, but being cheap it broke being made of junk plastic. Its the usual one with four buttons, the first is power, second is music, third and fourth is record and playback of a few seconds of speech.

Anyway, im making a new housing for the horn speaker from aluminium, and want to start from scratch with the driver electronics.

I reverse engineered as best as possible the original electronics. It became hard due to two COB cards which are non-identifiable. One contains the musical tune and the other is the voice recorder i assume.
The amp that was used is the A2025 16-pin package. In the datasheet for this IC, it mentions a max output of 5watt. Is this the usual max a hornspeaker can take, I would think it could take at least 20watt? Assuming it is, i was thinking of using the TDA1905 which i can get from my local Jaycar which is also a 5W amp IC seemingly similar to the A2025.
Hooking the 1905 is simple enough using an electret mic, but what i also want to do is also play music thru the megaphone from an iPod or similar source. This is where i get confused, how can I feed sources from both a mic, and a mp3 jack into the amp? Am i correct in assuming that the mp3 player is too big a signal for the amp?

#### audioguru

##### Well-Known Member
I don't know what is an "A2025" because a letter or two are missing.
The TDA1905 has an output of 4.0W at clipping when it has an 18V supply and an 8 ohm speaker. If the supply is less than 18V then the output power is much less.

A microphone needs to have a preamp. An MP3 player does not need a preamp.
Music sounds awful when played by a horn speaker.

##### New Member
It has A2025 written on it, but when i look it up its listed as TEA2025. It has no preamp so does that mean it differs greatly from the TDA1905?
On one side of the mic in the megaphone, it goes to ground, on the other lead it goes through a 0.1microfarad ceramic, then a 82k resistor, then a slide pot of unknown resistance, and finally into pin 10 (input pin).

#### audioguru

##### Well-Known Member
A TEA2025 is completely different to a TDA1905.

A TEA2025 is a dual amplifier that is probably bridged in the megaphone where its output at clipping is 3.76W into 8 ohms with a 9V supply.
The TEA2025 has more voltage gain than a TDA1905 so maybe a preamp is not needed for its microphone.

You must isolate the microphone from the speaker somehow to avoid acoustical feedback howling.

##### New Member
Thanks for youre help so far audioguru.

Ok i will use the TEA2025 as thats what was intended to be used in the megaphone. You can get em here also from the looks, just order it online.
So how do i now run an audio input from mp3 player into the TEA2025? I know the music will sound crap, but i have a few specific uses for it.

#### audioguru

##### Well-Known Member
Two resistors or a logarithmic volume control will attenuate the MP3 player's output so its level is low enough.

##### New Member
Ok sweet. So for connecting the mic, i earth one side of mic and the other goes thru predescribed cap and res, then thru pot and then to (+)input pin.
Then for the mp3, I connect shaft of 3.5mm mono jack to earth as well as pin 3 of pot to earth, then pin 2 of pot to (+)input of IC, then pin 1 of pot to tip of 3.5mm plug?
From what i have read through on other parts of this site, would 10k pot be enough? I wouldnt really need log would I? Linear should be ok for one off adjustment would it not?

#### audioguru

##### Well-Known Member
The mic or MP3 volume control must not connect directly to the input pin of the TEA2025 and short its bias voltage to ground. Instead they must connect to the input capacitor. Look at the datasheet of the TEA2025.

The mic might be dynamic or electret. If it is electret then it must be powered.

You need a switch to select mic or MP3 volume control.

If you use a linear volume control then it will be extremely difficult to adjust the MP3 level to about 1/33rd of its output. Use a 10k log volume control.

##### New Member
Here is a basic diagram of the amp section of the original megaphone circuit. The mic section just doesnt seem right to me, I would think the mic should be connected to Vs not grounded, and yet it worked?

I know these are simple questions but im always learning.
How do you know that you can just connect a microphone to the input of the TEA2025, or say the LA4270 (another suggested)? How do you just connect the mic, connect Vs to one side of electret, and other side to input?

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#### audioguru

##### Well-Known Member
Your mic is not powered so it is not an electret type. It has an 82k attenuating resistor which would reduce its level much too low.

The input current of the TEA2025 at the volume control will cause "scratching noises" when the volume control is turned. An input coupling capacitor is needed.

#### Attachments

• TEA2025 amp.PNG
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##### New Member
Alright so i have added the power circuit to the electret mic as per a diagram i found online, not quite sure what value to use for resistor for 12volt, 2.2k seemed to be the one most used.
I also have the input capacitor added. Is this ok?

#### audioguru

##### Well-Known Member
The little TEA2025 will melt if its supply is as high as 12V when it is bridged. The datasheet shows 9V max.

You still have a coupling capacitor missing at the input of the amplifier. Look at the datasheet!
Your electret mic should be powered from 10k resistor and the 10k resistor should be powered from a filter made with a 1k resistor in series and a 47uF capacitor to ground.

##### New Member
I have been searching for a decent explanation about electret mics but have yet to find one i quite understand. This is the best i have found yet though from THIS page.
Is this what you are talking about when powering the mic?

Code:
        +---------------------------- battery +ve (3 to 12 Volts)
|
2k2 R1
|
o---------- 10uF ------o----- output
|+                     |
CAPSULE                  10k R2
|-                    |
+----------------------o----- GND, and battery -ve

EDIT: This forum really doesnt like ascii

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#### audioguru

##### Well-Known Member
You should not power a mic directly from a battery that powers the amplifier because the amplifier's current causes the battery voltage to jump up and down. Instead the resistor feeding the mic should be powered from a filter (a resistor and capacitor).

Most electret mics use 0.5mA. If you use a 9V supply and have 4.5V across the mic then the resistor feeding the mic is 10k and the resistor of the filter is 1k.
Use 2.2k and 220 ohms if the supply is only 3V.

#### Attachments

• electret mic power.PNG
7.1 KB · Views: 856

##### New Member
Have i got it about right this time? You reckon the 10k pot for adjusting mp3 vol is enough? Thanks again for youre help with this

#### audioguru

##### Well-Known Member
Now your circuit should work fine.

#### bernards6

##### New Member
hi, I am new in this website. wish that I can find some good help, I just need to use an mp3 and use a 5w speaker , supply power will be 9v or 12v if I use the tda1905 will work good ? And if yes where can I find a detaled diagram thanks

#### audioguru

##### Well-Known Member
The datasheet for the TDA1905 has the schematic of its amplifier circuit and a pcb layout.
With a 9V supply its power at clipping into an 8 ohm speaker is only 1.2W.
With a 12V supply its power at clipping into an 8 ohm speaker is only 2W and it gets extremely hot.
Flea power.

#### bernards6

##### New Member
so what is your suggestion to use for a 5w

#### audioguru

##### Well-Known Member
For 5W at clipping into 8 ohms the amplifier needs a supply of about 22V.
A bridged pair of amplifiers will have an output at clipping of 5W into 8 ohms if their supply is about 12V.

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