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Old 1st May 2011, 10:09 AM   #11
AndrewT is offline AndrewT  Scotland
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for a target maximum output of 12W+12W a transformer with a VA rating from 24VA to 48VA will work. But I recommend that you always use >100VA for power amplifiers and I prefer 160VA as the minimum for a toroid to obtain a reasonably low regulation of about 6% to 8%.

For your higher target of 20W+20W the transformer becomes a 40VA to 80VA (at a slightly higher secondary voltage). Again I would apply the minima of >100VA and preferably 160VA with lowish regulation. Small EI transformers have enormous regulation. An EI designed for specifically low regulation (instead of cost and temperature) is no longer small.
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Old 1st May 2011, 04:00 PM   #12
johnr66 is offline johnr66  United States
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I have a split bobbin EI core transformer out of a 250w class D Sub that blew up. The transformer is good. The actual core measures 76 x 63.5 x 40.5mm. It appears to be around 100 VA.

Secondary (CT) measures 100 volts. The open design should allow for easy unwinding and rewinding a secondary of appropriate gauge wire. Unloaded volts per winding is .392.

According to the TDA2050 datasheet, a +/- 18 volt rail should provide 30 watts of output pre clipping power into a 4 ohm load. +/- 15 volts would provide 20 watts, So I think a loaded PS of in the range of +/- 16 or 17 volts would be my target. Looks like I would need a 5Arms PS for two channels, but not sure of the transformer calcs.
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Old 1st May 2011, 04:34 PM   #13
AndrewT is offline AndrewT  Scotland
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Does your salvaged EI have a good 110/120Vac Primary?

Does the salvaged EI sound completely quiet when connected to mains with the secondary open circuit?

Does your salvaged EI run cool, or warm, or too warm when the secondary is open circuit?

If all these are OK, then you can wind on a pair of secondary windings for your low voltage supply.

I would allow 3Aac per sq mm of secondary wire cross sectional area.
If you target 100VA and 16Vac+16Vac, then your secondary current is ~3.1Aac and this requires ~1sqmm of wire. This is ~1.1mm diameter, Easy to wind.
If you can get in sufficient turns of a slightly thicker wire then go for that. It will reduce regulation, it will reduce heating of secondary, it will reduce the voltage sag under power.
Try for 1.2mm diam or 1.3mm diam.

If the only problem is a little bit of excess noise when driving the primary only, then this can be cured.
Tell us what you find and then we can guide you forwards.

Can you measure the size of the core area of the middle leg of the "E" ? This should be exactly double the area of the "I".

Output VA ~ 30 * core area^2, where core area is measured in sq inches.
Quote:
Looks like I would need a 5Arms PS for two channels
don't try to predict the output current requirements, neither DC nor AC. An amplifier replaying audio is a very non steady load and normal current limits do not apply.
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Last edited by AndrewT; 1st May 2011 at 04:40 PM.
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Old 1st May 2011, 06:11 PM   #14
johnr66 is offline johnr66  United States
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Andrew,
It is quiet. Runs up to about 95F after 15 minutes. Barely lights a 25 watt bulb when I did the inline dim bulb test. Seems fine to me.

Measuring the middle leg volume (Not the full height right? Pretty much the area under the bobbin, if I understand.), I get 2.44 sq. inches or ~178VA.

Thanks for your help so far!
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Old 1st May 2011, 07:55 PM   #15
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by johnr66 View Post
Barely lights a 25 watt bulb when I did the inline dim bulb test.
An open circuit transformer should not light a dim bulb.
If you have a clear glass, a 25W filament may just show a tiny bit of red/orange glow, but pretty dim. An opaque glass would hide this.


170VA to 180VA 18+18Vac would be equivalent to ~ 5Aac.
Use 1.4 or 1.5 or 1.6mm diameter wire for each of the secondaries.
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