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Old 18th June 2007, 02:05 PM   #1
scaesic is offline scaesic  United Kingdom
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Default tda 2050 power supply/pcb

hi! i am planning on building a tda2050 to run on +/-20v split supply. First of all, is there any tried and tested or reccomended 2050 based projects around?

secondly, i am unfamiliar with diy ing power supplies, although i am confident in my assembling skills. What kind of transformer would i need to get +/-20v ?

iv read through rg's article but i'm finding the arithmetic a bit confusing (http://geofex.com/Article_Folders/Po...s/powersup.htm)

i think its just the terminology i dont quite understand for labelling transformers: i take it id need at least 50v ac(acounting for around 10v to be lost from the transformer/bridge rec) , but how does that relate to the terminology (ie 15-0-15 etc) and how much current consumption will the tda2050 take?
finally, wheres good place online to buy transformers?

i think an unregulated supply should be fine for the power amp, as long as i use a cleaner-regulated supply for the pre? anyone found anythign to the contrary?
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Old 18th June 2007, 02:13 PM   #2
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To get 20 volts DC you will need a 15 volt transformer:

AC = (DC / 1.414) + Vdiode

AC = (20 / 1.414) + 1.1

As you need this amount each side of ground you need to double this voltage for a centre-tapped transformer. i.e. 15-0-15.

Rapid Electronics is a good place to buy transformers.

moderator - please can you move this to Power Supply section where it belongs
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Old 19th June 2007, 09:12 PM   #3
scaesic is offline scaesic  United Kingdom
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what sort of va will a tda2050 require? i was thinking 50 or 75?
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Old 19th June 2007, 09:45 PM   #4
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Look in the data sheet to guesstimate what power it puts out at your rail voltage. Double the power to get the approx VA per channel.
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Old 20th June 2007, 03:50 AM   #5
JCM is offline JCM  United States
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Quote:
What kind of transformer would i need to get +/-20v ?
20/1.414 = 14 Volts - but allow for 1.2 V diode drops, so therefore : make it a 30 volt center tapped (CT) transformer. That will give +/- 20Vdc. However, that voltage will sag under load.

Output power is graphed at 32W @ 4 ohms, and listed at about 20W @ 8ohms at that supply voltage.

To deliver that power to a 4 ohm load, you'll need 16V peak. 16V/4 ohms = 4 amps peak. That is 4*.707 = 2.8 amps rms. You'll need 18V/8 ohms*.707 = 1.6 amps rms for 8 ohm loads.

So that calls for a VA rating of 90 for 4 ohm loads, 50 VA for 8 - assuming a transformer so rated could supply it - (which, it wouldn't.) Try a 180VA unit - meaning 30VCT @ 6 amps (might be ok with 5 Amps) for 4 ohms, 100VA for 8 ohm loads.

These are for ONE channel only. For stereo, double the current ratings, or course, or use two transformers (and for a Class AB amp, such as yours - I Highly recommend separate supplies for each cnannel.

If it were my project, I would avoid 4 ohm speakers - that thing is in a T0 220 package - it's got enough heat to get rid of without making it worse, and the extra transformer costs ... damn. But, that's my view. Also, we bear in mind that speakers with uniform impedance are hardly a reality.

But note this, please : A typical COMMERCIAL product would probably use a 100VA unit for BOTH channels. They can get by with it, and so they do. But what you get for spending more money on your transformers than they do in their ~ products ~ is more actual power output, greater dynamic range, and less distortion.

Quote:
i take it I'd need at least 50v ac (acounting for around 10v to be lost from the transformer/bridge rec)
The amount of voltage lost through the transformer depends on how robust it is. Obviously, the higher the VA rating, the better. Of course, the filter capacitors are also important here.

And the loss through the transformer shows up not at idle, it shows up in operation, playing music thru it.

BUT ! Notice the Absolute Maximum ratings : The supply voltage is not to exceed +/- 25 Volts ! In reality, it would take a couple or more volts and not blow up - I wouldn't give it more than 22 V. The difference in transformer output from a 115 V to a 120 V wall socket alone would raise the supply voltage over a volt.

The loss thru the bridge will be 1.4 volts.

Quote:
but how does that relate to the terminology (ie 15-0-15 etc)
Are you familiar with split supplies ? If not, it can be explained fairly quickly.

Quote:
how much current consumption will the tda2050 take?
Not sure I understand what you mean. It will take as much as it needs, no more, no less. For 4 ohm loads, it will require 2.8 amps DC, for 8, 1.6 amps DC. Transformers rated at those currents will do so only while giving far less than rated voltage, so we want to double the current rating.

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Old 20th June 2007, 11:04 AM   #6
scaesic is offline scaesic  United Kingdom
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ok, that post was extremely helpful, although the little violin man was a little uncalled for

i am familiar with split supplies, as i said i intend to use +/-20 for the tda (im in uk so its 230v mains here), itl be through a jensen c8r 8 ohms, iv been trying to find a suitable heatsink for the device, i came up with this:-kl100-1

if im using it under 8 ohm load at 20v+/-, thats 20watt output, i reckon i could use a 75va safely for this? the transformer cant be too big physically or it wont fit in the chasis.
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Old 20th June 2007, 11:14 AM   #7
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75VA would do for a stereo amp with 8 ohm load.
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Old 20th June 2007, 06:11 PM   #8
JCM is offline JCM  United States
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The violin emoticon is supposed to indicate impatience or sarcasm, I think (?) - but I use it only because I like it, kind of a personal signature. What it symbolizes to me is the initial groundwork and a little drudgery we have to go through before we get to the fun part of trying out our creations - going over things with a fine toothed comb, triple checking, etc., going over the basics.

75 VA will work. With class AB, the power demands are not so great as with class A, so that helps. I would consider a 32 VCT transformer. That would give you about +/- 22 V, and the voltage will sag substantially under high power demand. That extra voltage would help.

The heatsink looks like it ought to work well.
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Old 3rd July 2007, 05:59 PM   #9
scaesic is offline scaesic  United Kingdom
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iv been thinking about this again before i commit to buying a transformer,

surely that calculation is backwards?

you yourely cant get a 20v dc out of a 15 wind secondary? the 1.41 must be in the wrong place?

i.e the output of a 15 secondary is effectively 15*0707, giving 10.6V?? not the 20V i need.

really i need something more like 30-0-30 in order to get +/-20V ?
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Old 3rd July 2007, 06:53 PM   #10
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AC is specified in RMS which is the average. DC is rectified which uses capacitors to store charge upto the peak voltage.

I think you understand that peak is higher than average
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