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Old 6th September 2014, 06:42 PM   #1
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Default Really basic question...

Hi guys,

I used a DIY I found floating around online to build a basic power amp (first time), but don't know how to figure out what kind of power supply to use, and the schematics for it aren't complete on the page. So this is basically just a "what wall wart should I get" question (Whether I actually built the amp correctly is a whole other issue).

Here's the little description with a few numbers:

"TheTDA2050 is intended for use as an audio
class AB audio amplifier. Thanks to its high power capability theTDA2050 is able to provide unto 35W true rms power into 4ohm load @ THD= 10%,VS = 18V,f=1KHz and up to 32W into 8ohm load @ THD=10%,VS = 22V,f=1KHz. Moreover ,theTDA2050 delivers typically 50W music power into 4ohm load over 1sec at Vs= 22.5V,f=1KHz.

This is a power amplifier and requires 200mV RMS for full output. Voltage gain is 30.5dB with resistor values shown. Closed loop gain is set by Ratio R1/R2. Increase R2 for less gain and vice versa. Power bandwidth is 20Hz to 80KHz. R3, C3 and R6, C11 form a zobel network to prevent high frequency instability.

you can take any 2 power supplies from the market they can be 20V each or a single that can supply +-20V ..
The schematics are attached. "

I read a few formulas that help figure this out but they don't seem to use the same variables.

Thanks guys, as always any help is appreciated.
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Old 6th September 2014, 07:26 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Yuo can't use a wall wart, you need to build a proper supply.

You could try two 19V laptop supplies + a couple of caps.

rgds, sreten.
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Old 6th September 2014, 07:53 PM   #3
Mooly is online now Mooly  United Kingdom
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The power supply you need is one that produces what is called a split or dual supply.

Best solution is to make a simple power supply from a suitable transformer, a bridge rectifier and two capacitors. And as this is an untested project that would also allow you to power it up safely with a "bulb tester" (search the forum if you don't know what that is) and save any mishaps.

You need a 12-0 -12 or a 15-0-15 transformer of around 60 to 100va rating. That would deliver rails of around 17 to 21 volts DC depending which option you went for. The lower voltage could be the best bet because at light loads the voltage from a transformer rises a little anyway.

A bridge rectifier of say 200 volt 10 amp rating.

And two capacitors of around 3300uf and 35 volt or higher rating.

If you did use laptop supplies you would have to confirm first that they were suitable meaning that you could series connect them with no interaction via any ground leads in the mains input wiring. You would also lose the ability to safely power the project gently while confirming all was OK.

A mains transformer is by far the best.
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Old 6th September 2014, 07:55 PM   #4
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Quote:
Originally Posted by ocharlas View Post
So this is basically just a "what wall wart should I get" question
Unfortunately there isn't really a wall wart answer.
You see your documentation has a range of about 18V to 22.5V. The TDA2050 datasheet can give a fuller account of operating voltages.
Current is the square root of (power divided by resistance), √P/R. Using 4 ohms and 36 watts, that is 3 amps.
Higher currents can overcome unavoidable inefficiencies.
So a guesstimate for a minimum power supply could be 18V @ 3 amps for an 8Ω load.
6 amps for a stereo pair of course. Not something easily achievable with wall warts.
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Old 7th September 2014, 12:42 AM   #5
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Oh wow.

Thanks guys, I definitely didn't realize I needed something that substantial.

Unfortunately, while I can look at a schematic and solder parts together, I'll admit that I lack the knowledge to actually come up with anything on my own. So something like a capacitor I can find, but can could someone help me out with the transformer, and direction to go with them? I'm attaching the schematic that went with the amp diy. As you can see, pretty skimpy on the info. I have a few LM7812, don't know if they're useful or not.

Obvious query: Since I built a stereo amp with 2xTDA2050, I'll need to double everything right? Do I double the components, or simply their specs? Meaning 6600uF cap vs 2x3300uF caps. I realize it's not like the amp itself which is essentially two separate circuits, but again, I really don't know.

Thanks again!
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Old 7th September 2014, 06:58 AM   #6
Mooly is online now Mooly  United Kingdom
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This shows the typical power supply. The one power supply would power both channels. And we don't normally use voltage regulators on power amps. They work better without.

The transformer has two windings that are series connected and can either be in the form of a 12-0-12 which has three secondary leads or a 12-0, 12-0 which has four wires of two separate windings leaving the job of series connecting them up to you.

The caps in practice can be anywhere from as low as 2200uf to say 6800uf. The higher the value, the lower the ripple and noise on the power supply. Bigger isn't always better though and 3300uf or 4700uf would be ideal.

The "VA" rating of the transformer is open to debate. You can design for worst case continuous maximum power output of your amplifier where the chips are pushed to the limit under sine wave testing, but the practical reality of that is that the chips would probably have expired long before a lesser rated supply showed any distress. 60 va rating in a 12 volt tranny would build a very usable amp, 100va is a bit more conservative.

Typical tranny would look like this. (the traditional square type of tranny is even better if you can get one). Shop around and see whats available. We can always advise if you link to something.

MCTA060/12 - MULTICOMP - 60VA TOROIDAL 2X12V | CPC

Bridge rectifier, at least 100 volt "PIV" rating and at least 6 amp or so.
CP606 - MULTICOMP - BRIDGE RECTIFIER, 6A, 600V | CPC

or,
CM1504 - -- - BRIDGE RECTIFIER, 15A, 400V | CPC
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Old 7th September 2014, 04:56 PM   #7
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For the Multicomp transformer you linked, will the 2.5A rating be enough? As sofaspud mentioned above do I need double that since it's a stereo amp?
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Old 7th September 2014, 05:09 PM   #8
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Remember that is just an example, its a UK sourced part. You would have to look for similar from your local suppliers.

When specifying parts and such things as current ratings and so on you have to be practical...

The TDA 2050 will struggle to get rid of all the heat it produces under full output. To stand any chance of that it would need pretty large heatsinks.

A modestly rated transformer will easily supply anything the amp could realistically deliver music wise. For music transients and short term peaks it won't even blink. Under sustained full output the supply voltage will fall slightly (no bad thing on a chip amp) and if you kept that up for more than a few seconds you would find the TDA2050 starting to fry.

Have a look at this thread and try the test. It will give a very good idea of the kind of output you actually need.

A Test. How much Voltage (power) do your speakers need?
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Old 7th September 2014, 06:38 PM   #9
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Lets put some real numbers into the mix and see what we get... I will do it for both a 12-0-12 and a 15-0-15 transformer, values for that in red.

You have a 12 volt (12-0-12) transformer. That produces rails of around 16 volts (so 16-0-16 volts DC) (20-0-20)

The amp won't swing to those limits, it will loose a little due to internal volt drops and losses. Lets say it swings -/+ 13 volts (-/+17 volts). That's a peak value and we need RMS values, so we multiply the peak by 0.707 to get just over 9 volts (12 volts). Lets call it 9 volts RMS (12 volts RMS).

Power is volts squared divided by the load impedance which we assume for calculation to be typically 8 ohms. So that gives 10w watts RMS (18 watts RMS)

Peak current is Vpeak (13) (17)divided by 8 (load impedance). That gives 1.6 (2.1)amps peak.

The average current is I peak divided by pi so we get 0.5 (0.67)amps average current drawn from the supply.

That is one channel at full output into 8 ohms. Both channels will be pulling twice that of course.

Edit... transformers have in their specifications a "regulation figure". The secondary voltage figure is always specified as full load current. At light load the voltage is higher by that regulation figure percentage and for small transformers it can be a substantial value. So your rails will be a bit to quite a bit higher than you might think... another reason to be cautious on selecting the transformer voltage and another reason I favour the lower voltage option.
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Old 8th September 2014, 05:17 AM   #10
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Whew thanks for the read! Fairly simple for you I'm sure but it took a bit of poring over on my end. But I do believe things are starting to clear up. I'm going to shop around a bit and post up some of the things I find.
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