Need help with my Ruby Amp

[dudemcfly]

Hello everyone, I am planning to build a ruby amp and I have a few questions. First of all, is that amp capable of powering two 4 inch speakers? It might be useful to know I will be using a 12 volt instead of the 9 volt like runoffgroove mentions. Also, I previously made a little gem amp and had trouble adding an led light, the 9 volt was to much and they burnt out. I thought of adding a resistor but, wouldnt that reduce the already small amount of power going to the speakers? Where di I even add the light in the circuit? And what kind of enclosure is best? Does it even need to be in an enclosure like a speaker, or can it be partially open in the back like some amps? Sorry, I'm new at this, but I really appreciate your help. Thanks in advance.

 

[4pyros]

The LM386 used will run off 15 volts DC Max so 12 volts should be fine. It is a 1 watt amp so I would use two, one for each speaker. LEDs need current limiting resistors. The case is up to you but I would put them in the speaker cabs like computer speakers. Andy

 

[dudemcfly]

Ok thanks 4pyros, so where in the circuit should I add the second amp, how would I wire it? These speakers are rated 15 watts, should I consider using another amp besides the LM386? If so which? What kind of resistor specifically do you recommend?

 

[audioguru]

The LM386 used will run off 15 volts DC Max so 12 volts should be fine. It is a 1 watt amp

No.
The datasheet shows that with a 9V supply and an 8 ohm speaker it clips when its output is 0.56W and its heating is 0.52W.

It shows that with a 12V supply and an 8 ohm speaker it clips when its output is 0.66W (barely louder than 0.56W) but its heating is 0.82W and it will be very close to its max allowed temperature.

The datasheet lists it with a 32 ohm speaker when its supply is 16V so it cannot use an 8 ohm speaker with a 15V supply.
If it drives two 8 ohm speakers in parallel and the supply is 9V then each speaker gets only 0.15W and the IC heating is 0.8W. It will overheat with a 12V supply and the output power will remain the same as with 9V.

 

[dudemcfly]

So audioguru, pardon my noobness but what does all that mean? Do I need a different amp then?

 

[audioguru]

So audioguru, pardon my noobness but what does all that mean? Do I need a different amp then?

The LM386 is designed to produce only 0.56W into an 8 ohm speaker when it has a 9V supply. 0.56W is not loud, it is like a cheap clock radio. You tried a Little Gem amp (which is the same as this one) and said it had a small amount of power. Was it enough power?

Its output power is much less if it drives two 8 ohm speakers in parallel and it heats a lot.

With a 12V supply and an 8 ohm speaker its output power is barely more than with a 9V supply but its heating is much more.

It overheats if its supply is more than 12V and it has an 8 ohm speaker.

If you want more power then use a TDA7240A amplifier IC. It can drive two 8 ohm speakers in parallel and produce a total of 11W with a 12V supply. A good pcb design is shown in its datasheet. The Jfet preamp of the Ruby amp can drive it.

 

[4pyros]

No.
The datasheet shows that with a 9V supply and an 8 ohm speaker it clips when its output is 0.56W and its heating is 0.52W.

It shows that with a 12V supply and an 8 ohm speaker it clips when its output is 0.66W (barely louder than 0.56W) but its heating is 0.82W and it will be very close to its max allowed temperature.

The datasheet lists it with a 32 ohm speaker when its supply is 16V so it cannot use an 8 ohm speaker with a 15V supply.
If it drives two 8 ohm speakers in parallel and the supply is 9V then each speaker gets only 0.15W and the IC heating is 0.8W. It will overheat with a 12V supply and the output power will remain the same as with 9V.

The OP never specified the impedance of the speakers. Andy

 

[dudemcfly]

Thanks audioguru, that's what I was wondering. One last question, what is a Jfet preamp?

 

[dudemcfly]

Thanks audioguru, that's what I wanted to know. One last question, what is a Jfet preamp?

 

[audioguru]

The Ruby amp has a Jfet preamp using an MPF102 Jfet. Its input impedance is very high so that the guitar pickup peaks at about 5kHz.

 

[dudemcfly]

Is this the right amp audioguru?

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Can I just use one speaker with this amp? Or are there others like this that are 30 watts?

 

[audioguru]

Why would anybody buy a Chinese copy of the TDA7240A European IC. The Chinese one is made by a manufacturer called "other"??
ST Micro in Italy invented it and Digikey in the US sells it for $.....

HEY! They don't have any and Newark also doesn't have any.
It is not listed at ST Micro today so maybe it is discontinued.

There are (was) over one hundred power amplifier ICs. A National Semi LM3886 amplifier IC can drive two 8 ohm speakers in parallel to 68W if the supply voltage is high enough. Its output can be limited to 30W if you want.