SSR03 - Toroid transformer considerations for +/- 15V output

[tk-don]

Hi,

I''ll be doing a SSR03 in the near future based on the LM431 / 15V BOM and am considering which transformer would be suitable, i.e. not over-kill for this purpose. By that I mean, I want to minimize excessive voltage drops/heat dissipation in the LM317/337 pre-regulators. It will be used to power a QSXM2 phono amplifier (~200 mA in total) and my vinyl deck which draws around 150 mA) at +15V.

Taking the positive voltage side as example: I know that the voltage drop of the LM317 and Q3 transistor is approx 5-6V. Using the full-wave rectifier would introduce another 1.4 V drop. So the DC input voltage must be at least 7.4V+15V = 22.4V. I assume that this would mean I need 22.4V AC peak voltages, or a ~16V AC RMS rated transformer, or 18V which is very common.
I have estimated the required VA rating to be around 10-12 from this page http://www.mcitransformer.com/i_notes.html , where the AC current would be estimated by 0.350 A*1.8= 0.63 A.


My questions are:
- Are all my calculations and assumptions reasonable, and would one normally take into account mains power drifting also?
- When the SSR03 power supply is powered up, wouldn't the capacitors charge through the resistors R1-R12 (which are very low ohms) and hence still cause the current draw from the toroid go vastly beyond specs for most 18VAC/12 VA toroids? Or is this the reason why it seems to be the norm always to use "beefy" toroids with high VA ratings.

- Any recommendations of good toroids at 18VAC and 12-15 VAC? I have found this one, which comes with complete insulation sheets and bolt, but it's 30VA: http://uk.rs-online.com/web/p/toroidal-transformers/2574985/ .

Thanks!

[tk-don]

My above post must have been somewhat confusing. I should have paid more attention to this earlier. The QSXM2 is +/- 18V and not +/- 15V, right?

I think I got set on 15V from the mention of 7815 and 7915 voltage regulators here: http://sjostromaudio.com/pages/index.php/hifi-projects/132-qsxm2-the-extreme-phono-riaa-amplifier?showall=&start=5 .
Anyways, a 18VAC/12-15VA would still fit the bill it seems.

[peranders]

I'll recommend that you use ±15 V. Those extra volts will only produce extra heat. 2 x 18 VAC will be perfect. 2 x 15 VAC will work if you can make sure that you have 20 VDC as the lowest. At 19 V the regulator stops regulating.

The LM317 make a constant voltage across the series transistor and the voltage is set to 2.5 V. The LM317 it self stops working with less than 2.0-2.5 V across IN and OUT. In this case it means 4.5-5.0 V more in than you have set as output voltage.

R1-R12 is only for reducing harmonics. The transformer can take the inrush current without problems. Those resistors shall have a low value unless you have lots of overvoltage and a constant known load (which you have here).

When it comes to dimensioning the transformer size, take the actual power consumption (regulator losses + amp) and multiply it with 1.5 to 2.

[tk-don]

Thanks for the explanations, P-A.
I'll go for 2x18VAC just to be safe.

Having a look at the QSXM2 diagram, it is of course not the easiest thing to follow but it looks as if running it at +/- 15V would not require any change of component values compared to +/- 18V - am I right?

[peranders]

You don't have to change any values for 15 V supply voltage.