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Disapointing result

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tblo163

New Member
I've just spent a couple of days building an Amplifier for MP3 Playback,I used the Vellerman's circuits for the pre-amp & two mono 7W Amps.Pre-amp uses a TL072 IC,& the Amps TDA 2003 ICs.All the components are new & I thought I'd made a good job of building it (it looks verry pretty with the rainbow LED)The problem is,it doesn't WORK;All I get from the speakers is a HUM.The connections between the Volume control,pre-amp & amps are all screened wires.The gain adjustment & volume control make no difference,so I suspect the pre amp? because whatever the problem is,it affects both channels.Is there any way to test the pre-amp,without ripping the whole thing apart? I might not have music,but I've got a pretty light to watch while I'm listening to the HUM;
 

dknguyen

Well-Known Member
Most Helpful Member
You can probe your circuit from the input of the amp and follows the input signal up until you no longer see it. However...this might require a function generator so you can input a steady input tone that is easy to search for in the circuit, as well as an oscilloscope to search for it with.

You can also probe from the output backwards searching for any places where the voltage matches the the power supply rails when it shouldn't.
 
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audioguru

Well-Known Member
Most Helpful Member
Can you post the schematic of the preamp, power amp and power supply? Show where and describe how the volume control is connected.

If the preamp is for a microphone and the power amp is driving a speaker in the same room then maybe you have acoustical feedback howling.
 

tblo163

New Member
O.K.Thanks,I probed around the circuits,and they checked out good.I connected a wire from the volume control to one of the amp inputs.& it nearly deafened me(I'd left the volume turned UP)I disconected the pre-amp & just used the Amps,it was perfect.So it proves that you don't need a pre-amp to play MP3s.I have not investigated the pre-amp yet,but I'm not too bothered now that it's working without it .Thanks again Terry
 
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tblo163

New Member
amp & pre-amp

I've done away with the pre-amp,it was not working.it works o.k. without it.I still can't figure out why it didn't work,but I don't need it now .
Thanks for the reply Terry
 

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audioguru

Well-Known Member
Most Helpful Member
The preamp and power amp circuits are fine.
Maybe you mixed up the parts.
Maybe your power supply has lots of hum.

Please attach the schematic of your power supply.
 

tblo163

New Member
The power supply I use for testing is a 12volt jump start power pack.I don't get the hum,
now that I've disconnected the pre-amp.The two mono amplifiers work O.K.& are controled by a twin gang 47K Pot.for volume control,the sound quality is very good,allthough I intend to install Bass & Treble pots later.cheers TERRY
 

audioguru

Well-Known Member
Most Helpful Member
I don't know what a "Jump Pack" is. It might be a poorly filtered battery charger for a car. it might have a lot of hum on its output.

The preamp has a gain of 35 and the power amp has a gain of 100. The resulting total gain of 3500 is much too high for anything.

The very simple power supply filter in the preamp is almost useless so no wonder your gain of 3500 produced hum.
 

tblo163

New Member
Thanks for the help,the power supply I use is simply a portable 12v battery,which I carry in my car,in case of a flat battery,with crocadile clips for "jump starting"any way to show my appreciation to you and the other members of the forum I have uploaded a video on my YOUTUBE Channel.go to terryblount163 (my channel) it's titled audioguru.
Thanks again Terry;
 

Sceadwian

Banned
Jump Packs are ultra high surface area batteries (I"m not sure what the construction is I'm guessing lead acid) to provide jump currents for a car in a small package. Morons in the US buy them by the bushel rather than replace a failing battery.
 

audioguru

Well-Known Member
Most Helpful Member
to show my appreciation to you and the other members of the forum I have uploaded a video on my YOUTUBE Channel.go to terryblount163 (my channel) it's titled audioguru.
Thanks again Terry;
Um, gee, thanks Terry.
I never had a video dedicated to me before.

The sound quality is horrible with no bass frequencies. Maybe computer fans sound bad when they are used as speakers.
 

tblo163

New Member
Your're right about the speakers,Bass & treble are my next addition to the circuit,I was only playing it in mono,(my mistake),I was just showing that a pre-amp was not needed.It can only get BETTER. Terry
 

audioguru

Well-Known Member
Most Helpful Member
You should not need bass and treble controls if your speakers are half decent.
Stereo has nothing to do with missing bass frequencies.
If you have two speakers and they are wired out-of-phase then the bass frequencies cancel.
 

tblo163

New Member
I appreciate that the speakers are not as good as they should be,when the unit is finished,I intend to fit it in my car & run it through two of the door speakers in my Volvo.I think these will be suitably rated for the output of the amp.At the moment I've got a new problem,one of the channels has stopped working,the IC on this is getting very hot,while the other is cool.The voltage readings are the same on both channels,& there are no shorts in the rails so I guess the chip has had it;I'll replace it as soon as they are delivered.So I'll put the project on hold for now.good luck Terry
 
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