My First Build

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Hi Guys

I thought I'd show off my first attempt at building an amp.

I acquired several old Coomber tape players which have TDA2040 chips inside them so I thought I'd build an amp based around those first and hopefully build a gainclone later on based on an LM3886.

I did make an attempt to use stripboard but that failed misserably so in the end I used the board layout from the datasheet.
I had to edit it on the PC and resize it before it could be used but that didn't take long to do.

I then etched my first circuit board using the toner transfer method and ferric chloride. I was very happy with the results for a first attempt.

Here is the pic of the etched board......
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I then started adding parts to the board. As you can see in the next image, I didn't have the correct value for some of the resistors so I had to improvise and add 2 resistors together to get the desired value

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Once it was finished, I used a small heatsink at first but then replaced it with a larger one.

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The next step after that was to throw together a PSU to run it.
I should note at this point that I opted for a single supply version of the circuit due to the fact that I only have single output transformers.

For the PSU I used a 20v transformer, then ran it through a KBU 6D bridge rectifier and then used a 4700uf capacitor before it connected to the amp board.

An externally hosted image should be here but it was not working when we last tested it.


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And there it is.
It's bloody loud. I haven't added a volume control yet.
I plan on building another one and casing the PSU so that I can use the system in my shed :)

I have noticed that without an input source, the speaker hums like mad but stops when I plug an audio cable in, even with no music being played.

Is this normal?

Is there anything I seem to have done wrong/badly/stupidly?
What changes would you all suggest?

One other thing I don't understand.....

Should I be using a single power supply or the split type power supply that supplies both a plus and minus voltage?

What are the advantages to the split type?

Sorry if I sound thick but I'm still learning :)

-- edit --

I'd also like to mention that one of the most usefull resources I used was Nuuk's Decibel Dungeon website.
I've have read his site more times than I care to remember but each time I managed to absorb something new.

Thank you Nuuk for such a brilliant website :D
 
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Check the amplifier alone.

Short the input hot to input ground.
Leave the output without any external load.
Power up and check the output voltages, Vac and Vdc.
Both should be near 0mVdc & ac.
power down.
Connect your source.
Repeat the measurements. Have they changed?
Report back.
 
Andrew T - I did as you suggested.
I am getting 0vac but I am getting 11vdc with and without the input shorted.

I have now built an identical circuit and have found the same results on both so it sounds like a circuit design issue rather than a faulty part issue...... or am I just being silly

Thanks
 
An AC coupled amplifier using a single polarity supply and DC blocking cap in the output should have ZERO mVdc into the speaker/terminal.
I'm sorry, I really am quite thick, please excuse my ignorance but what do you mean by that?

I have the amp powered from an AC transformer but then it's rectified into DC before being fed into the amp.

.....or am I missing the point here?

On a positive note, when the amps are working, they sound absolutly brilliant. Even to the point that I've been going round listening to several other systems in my house and have found that they don't seem to sound quite as good.
I suppose the speakers make a difference but I'm only using some old ones I found

I've even started planning to case it up soon :)
 
I've done some googling.

Am I correct in thinking that it's AC coupling bit relates to the input source?
i.e we're inputting an analogue source so it needs to be AC.
If we were inputting a digital source, it would be DC coupled ????

Or have I missed the point again?
 
I've done some googling.

Am I correct in thinking that it's AC coupling bit relates to the input source?
i.e we're inputting an analogue source so it needs to be AC.
If we were inputting a digital source, it would be DC coupled ????

Or have I missed the point again?
The MP3 player, CD and PC all have onboard DAC and are all analog at the headphone jack. However, an actually digital source fed into an audio amp sounds like a fax machine except louder and even worse. Please use the analog output jack, not the digital output jack. :)

DC is 0Hz, no signal on clean DC, although there may be plenty of current. Basically, you're doing AC coupling to avoid melting the speaker's voice coil. C7 is blocking the speaker fire, blocking DC offset. Apparently, C7 is a bit shorted and needs replaced.

That DC offset can happen from flopping about with all that duct tape or it can happen with voltage overrun (overcharge damage). Here's an example of a 63v 2200uF Nichicon: Nichicon PW Series 63V 2200UF Low lmpedance Capacitor
 
Hi Nurbit.

You may find this a little more understandable.

Capacitors

Your "perfect" audio signal is only AC but we all know things aren't always perfect. If any DC is passed into the amplifier input it will be amplified along with the signal. Once you push the DC into your speaker coils they will be held away from their normal resting point upsetting your sound and possibly burning the speakers.

You see the line on the linked page that says "They (capacitors) are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals"

So, if you pass your signal through a capacitor only the AC signal will pass through. The DC will be blocked preventing it from being amplified and keeping the magic smoke inside your speaker coils, always a good thing. If no capacitor (or other means of dc removal) is present the amp is considered to be DC coupled.

John
 
AH-HA

I get it now :) Thanks Daniel and John.

I can't re-test the circuit at the minute because they're both getting measured up for their shiny new heatsinks.

I've got some different caps to try too but sadly none of them are new and I don't have an ESR meter to test them.

I will be using brand new parts for the next amp. I'm going to build a true gainclone next time using LM3875s and point to point soldering.... perhaps :)

Thanks for the help though guys, it's easy to get disheartened when learning this stuff, I appreciate all your answers :)
 
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