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Combining stereo amplifier channel outputs to mono.

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fordgtlover

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A little background first:

I am building a guitar talkbox. I found an old amplified PC speaker that uses the TEA2025 amp chip. This chip is rated at about 2.5 Watts per channel stereo output. Unfortunately, this is just a little underpowered for my needs.

Question:
Can I simply take the outputs from both Left and Right channels and connect them to one speaker for a combined output of about 5 Watts?

If so, is there anything else I need to do to ensure the amp or speaker don't blow up?

Alternatively, the chip will work in bridged mode. Should I just modify the circuit board to run it in bridged mode.
 
Just paralleling the outputs won't make any difference - it will still only produce the same power into the same impedance - the only difference is that it would produce twice the power into half the impedance.

Bridging it produces no more power either, just an impedance change - so if the amplifier produce 2.5W per channel (total 5W) into two 4 ohm speakers, bridging it will give the identical 5W but into one 8 ohm load. Using a 4 ohm load would try to produce 10W, but would probably kill the chip as each half is seeing 2 ohms and trying to produce double it's rated output.
 
The TEA2025 does not have a heatsink (it is in a DIL case) and produces a max of 2.5 Whats per channel or 1.1 Watts per channel.
The 2.5 Whats is when its volume is turned up way too high so the output is square-waves. The 1.1 Watts is at clipping with low distortion.

The difference in loudness between 2.5W and 5W is very small. Your hearing's response to loudness is logarithmic so it takes 10 times as much power to sound twice as loud.
So you probably need an amplifier that produces 10 Watts or 25 Whats.
 
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So it seems that there is no real opportunity to up the output from the amp.

The other option I have been considering is to increase the input signal from the guitar using a preamp. I found a simple one here based on the J201 JFET.

I hope this will boost the signal enough to allow the little amp to get a bit louder.

The amp achieves acceptable volume when I'm running my iPod into it, but is just not loud enough with the guitar plugged directly. Hopefully the 3dB increase from the preamp will be enough.
 
^

Thanks for the suggestion Nigel. By 'adding a capacitor across R2' I take it to mean bypass (sorry, I'm still learning the lingo)? Is that correct?

Is there a simple formula or rule of thumb to apply for the value? The origial circuit notes 'Bypass R2 with a large value electrolytic capacitor for more gain', but I am unsure what is meant by a large value in this context.

I may be able to offest some of the amp's low power by connecting it to a higher sensitivity driver than I had intended.

Thanks for all the help so far.
 
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fordgtlover said:
^

Thanks for the suggestion Nigel. By 'adding a capacitor across R2' I take it to mean bypass (sorry, I'm still learning the lingo)? Is that correct?

Yes, it would 'bypass' the resistor at AC.

Is there a simple formula or rule of thumb to apply for the value? The origial circuit notes 'Bypass R2 with a large value electrolytic capacitor for more gain', but I am unsure what is meant by a large value in this context.

The value depends on the bass response you want, try a 22uF for a start and see how that sounds. If the gain is too much, then put a resistor in series with the new capacitor.
 
Now we learn that yout source is an electric guitar that needs an amplifier with a very high input impedance . Your amplifier has an "ordinary" input impedance that loads down the guitar's pickup and reduces its level.
Try the FET preamp, it will increase the gain of the system but the max output will still be fairly low power.
 
^
Sorry audioguru, I though that my original opening statement 'I am building a guitar talkbox' would indicate that the source would be a guitar (assumptions, eh?).

I apologise for any confusion or assuming that everyone would know what a guitar talkbox is.

It doesn't really need to be loud. Apparently, it is common to run these things off 5W or so. I have ordered some J201s off ebay, so I'll build the preamp when they arrive using the 22uF cap as suggested.

Thanks to all for the advice.
 
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I guess "a talkbox" is a tube with guitar audio in it that is fed into your mouth and you "talk" the music. Then a microphone picks up the sound from your mouth.

Your amp is 1.1W. It is much less than the 5W amps that other people use.
 
^
That's the one.

Given that this project is for a friend of my teenage son, and the friend doesn't have much money, I was hoping to get away with using the parts I have available for lowest possible cost. If it is just not loud enough I'll have to find a slightly bigger amp.

Will the J201 run off AC? The spec sheet doesn't seem to mention either AC or DC. Does this mean it will operate on either? If it can run off AC I'll just take power from the existing 10V transformer in the PC speaker to power the pre-amp.

cheers
 
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fordgtlover said:
^
Will the J201 run off AC?
Of course not! The schematic for the preamp shows +9V and ground to power it. It uses the same +9V and ground as the TEA2025 power amp.
 
A Talk Box This is a guitar and amp driving a midrange horn driver with a tube attached to the diaphram instead of a horn shaped sound output fixture. The open end of the tube is placed in your mouth and the guitar sounds are shaped by your mouth as if you were talking. Some commercial examples of this kind of talk box are: the Heil Talk Box, Electro Harmonix Golden Throat, the Banshee & a blast from the past, Kustom Electronics "The Bag" (I saw Jeff Beck use one of these, live, back in 1977.
) Is this what you are talking about? If it is than you will need alot more power to drive a compreshoin horn driver. Andy
 
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