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get ready to pull you hair out, crazy idea inside

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Thunderchild

New Member
ok so Ive built a twin bridged TDA2009 circuit to act as a 18 W/ch stereo amp for a little event my rotaract club is holding, now I suddenly got the crazy notion to have a mic available too. so I'll be feeding an mp3 player into the amp what if I preamp a 600 ohm mic with a TL082 and put its output straight into the amp as well ? ok I know its unorthodox but really it would't do any harm would it ? and might even work ? perhaps put resistors in series with the TL082 preamp outputs just to take up any "slack"

ok now feel free to pull hair out and throw rotten tomatoes at me.
 

Chippie

Member
TC..

Do you have the characteristics of the mike?

They would go a long way to determining what the mike amp would look like...
 

Thunderchild

New Member
well again thats just going to be thrown together, the mic will be a 600 ohm standard I don't know anything else, i was planing on using the TL082 as a basic inverting or probably non inverting amp, nothing fancy just very basic... and set the gain up accordingly..
 

Nigel Goodwin

Super Moderator
Most Helpful Member
As long as you connect both signals together via resistors, you're effectively making a virtual earth mixer, nothing 'crazy' about it at all.

Connecting the outputs of two preamps together WOULD be a bad idea though, each would short the other out.
 

Thunderchild

New Member
The output of an MP3 player is a preamp output.
true, I'll try the plain resistor method.

only thing I'm worrying about is one TDA2009 gets hotter than the other and the output could be just slightly higher than the other one but I'm unsure as its two identical bridge circuits. i suppose running at 12 volts rather than the recomended 24 volts accounts for the slightly limited volume
 

audioguru

Well-Known Member
Most Helpful Member
With a 12V supply, a bridged TDA2009 amplifier IC produces only 7W at clipping into an 8 ohm speaker. It might be able to drive a 4 ohm speaker with a higher output power but it is not shown on the datasheet.

The amount of mismatch of the output voltage of both amplifiers is not spec'd on the datasheet which affects the idle current and the amount of idle heating.

When you use the microphone, how will you prevent acoustcal feedback howling?
 

Thunderchild

New Member
yes the amp is using 0.8 A at all times and heatsinks get quite hot,

by feedback howling do you mean the larson effect ? I was hoping that keeping the mic away from the speakers would do it but I never done it before, do i need particular circuitry ?
 
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Chippie

Member
yes the amp is using 0.8 A at all times and heatsinks get quite hot,
QUOTE]

There's something wrong then...

Do you have an oscilloscope? can you check the waveform at the output of that ic...There should be little or no noise...

The ic could be oscillating at a high frequency..

Is the circuit built on a properly designed pcb?
 

Thunderchild

New Member
no its wired because I can't make a pcb myself and its too exspensive to get someone else to do it even if I knew where to go.

yes its oscilating at 6 MHz with a 4.5 Vpp output, any way I can stop this ? I can't understand though why its getting hot with no load on it as there is nothing to draw power, or is it that the internal circuit is running faster than normal and generating the heat ?
 
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Nigel Goodwin

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Most Helpful Member
no its wired because I can't make a pcb myself and its too exspensive to get someone else to do it even if I knew where to go.

yes its oscilating at 6 MHz with a 4.5 Vpp output, any way I can stop this ? I can't understand though why its getting hot with no load on it as there is nothing to draw power, or is it that the internal circuit is running faster than normal and generating the heat ?
It's getting hot because it's oscillating - layout on many audio chips like this is really pretty critical in order to get it working.

Post a picture of your layout.
 

Thunderchild

New Member
well picrtures: yes its quite a mess....
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Yes - not surprising it's unstable :D

If you check the datasheet, you will notice it gives a suggested PCB layout, most datasheets don't do this - but when they do it's a good indication that layout is critical.

Mounting the chips like you have and extending their leads is a bad idea to start with, all connections need to be as short as possible, and the tangled leads underneath are a further source of positive feedback, which is what makes it oscillate.
 

Thunderchild

New Member
rightho I think is gonna be a scrappa, the matrix board I have does not have the correct hole pitch hence the flying method, think i need to get a decent soldering iron tip too so I can more easily do tighter layouts, I made a stereo amp once with two TDA2002 chips and made it pretty much like this but I suppose they were more robust. Think I'm going to settle on the car amp I've just been given until I am able to start making class D amps (yea you can hit me on the head if I just said something stupid) as they seem to be more efficient and hopfully less prone to oscilate
 

Nigel Goodwin

Super Moderator
Most Helpful Member
rightho I think is gonna be a scrappa, the matrix board I have does not have the correct hole pitch hence the flying method, think i need to get a decent soldering iron tip too so I can more easily do tighter layouts, I made a stereo amp once with two TDA2002 chips and made it pretty much like this but I suppose they were more robust.
Depends how lucky you are, pretty well all audio amplifier chips are prone to layout related oscillation. Really it's all down to been inside a single chip, because of that the connections are too close together, and it's this that gives layout problems. With a discrete amplifer the input and output are widely separated, so this makes them far less critical about layout.

Think I'm going to settle on the car amp I've just been given until I am able to start making class D amps (yea you can hit me on the head if I just said something stupid) as they seem to be more efficient and hopfully less prone to oscilate
They run at far higher frequencies, and are probably far more prone to layout problems - double-sided PCB's are commonly used.
 
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Thunderchild

New Member
Depends how lucky you are, pretty well all audio amplifier chips are prone to layout related oscillation. Really it's all down to been inside a single chip, because of that the connections are too close together, and it's this that gives layout problems. With a discrete amplifer the input and output are widely separated, so this makes them far less critical about layout.



They run at far higher frequencies, and are probably far more prone to layout problems - double-sided PCB's are commonly used.
well i was thinking as they are an on/off circuit they would be less prone to oscilate, problem with the class AB chips is that they effectively amplify and can have any value ouput, if I'm just comparing and switching an output on and off then surely its less prone to oscilate ? it would also be a more descrete build and so not have the problems of integrated amplifiers
 

Hero999

Banned
You can save lots of power by using a class-H amplfier.

This involves using many different power supply voltages so when the output power is low power is taken from the lower voltage rails and when the power is high it's taken from the higher voltage rails. This saves power by minimising the VI losses in the driver transistors.

The disadvantage is you'll need a transformer with many different taps or you could use a SMPs.
 
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Thunderchild

New Member
well considering that class D amplifiers are compared to a class AB amplifier like a SMPS is compared to a linear regulator then I can't see the advantage of having so many power supplies that will run all the time and so be an efficiency drag instead of a single output stage controlling the output directly cutting out a lot of power supply circuitry and so reducing losses, its probable that very high power amps may benefit from class H but at medium/low poer I don't think the involment is worth it
 

Nigel Goodwin

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Most Helpful Member
well considering that class D amplifiers are compared to a class AB amplifier like a SMPS is compared to a linear regulator then I can't see the advantage of having so many power supplies that will run all the time and so be an efficiency drag instead of a single output stage controlling the output directly cutting out a lot of power supply circuitry and so reducing losses, its probable that very high power amps may benefit from class H but at medium/low poer I don't think the involment is worth it
I would agree, class_D amps are already pretty efficient, and far less complicated than class-H.
 
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