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Old 23rd April 2006, 11:33 AM   #1
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Default Transformer Va choosing?

Hi

I'm building a new outstage for my CD player with separated PSU.
The transformer will be 24V or 18V regulated to15V.

The question is: How many Va transformer is needed to power it with 1 dual op amp?

thanks
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Old 23rd April 2006, 02:57 PM   #2
paulb is offline paulb  Canada
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1 op amp should draw no more than about 10 mA. So you could allow for about 50 mA. For an 18V transformer, that would be less than 1 VA. Pretty small.
Here are some ideas for you:
http://sound.westhost.com/project05a.htm
http://sound.westhost.com/project05.htm
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Old 23rd April 2006, 03:04 PM   #3
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Big thanks for yor reply.

I'm right now making the list of parts to order.The transformer was (hopefully) the last confusing part to solve...
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Old 24th April 2006, 08:32 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
check your opamp data sheet.
The Iq is the current draw when no signal is being sent through the opamp.

Check the maximum output currents (+ not = -).
Add these currents Iq +max *2 for a dual and you will come up with about 70mA to 100mA.

Some of this peak current comes from the smoothing caps and bypassing, so you could manage with a smaller average current transformer.

However, I recommend +- 15Vdc from a 2 times 15Vac @100mA as a minimum for a single dual opamp. = 3VA. But I would never use anything less than 20VA and you may find improvements in audio performance by going all the way up to 160VA.
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Old 24th April 2006, 08:58 PM   #5
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Hi AndrewT

I sent my order on Sunday evening.Hope I didn't order too small transformer .

I wanted the outer diameter to be as small as possible,so it will fit nicely in a plastick box I have.

It is 18-0,0-18V 2.3Va(encabsulated model), will be regulated to 15V.
The op-amp will "take" 90mA (4.8+40mA x2)

This transformer will be used only to power the dual op-amp in analog outstage of cheapo Sony DVD player.(the original op-amp receives uneven voltage to V+ and V- ).

If I have understood correctly this transformer is acceptable,but bigger would be better for the sound quality?(good to know for the future projects).
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Old 24th April 2006, 09:29 PM   #6
mzzj is offline mzzj  Finland
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offtopic: Pikkujöpö?
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Old 24th April 2006, 09:44 PM   #7
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Quote:
Originally posted by mzzj
offtopic: Pikkujöpö?

I'm actually from Finland,living in portugal for +15years....
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Old 25th April 2006, 08:39 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
if you find a problem with modulated rail voltage you can recover some of the situation by increasing the size of the smoothing and decoupling caps. Normally about 2mF/Apk i.e. about 180uF for 90mA
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