Hello, what do you think of tis PSU?

[Pato666]

Hello, This is my first post, after reading a lot in this great forum i want to share something with you and see if it is or not a good design.
I saw this design long time ago, i don't remember where, I think it was in a magazine (called "Electronica & Computadores"). See attached file

This consists on a simple 78XX and 79XX regulators, for V regulation and a common-base configuration transistor for current bypass (or amplification if you want).

I've built this PS, back then, but not for an Audio application but for a Stepper Motor driver/controller, and it worked AWESOME. In that case i used BDX66 and BDX67 transistor. The PSU had this features:

Input: 220Vac
Output: 15V + 15V @ 15A
of course, caps where different values, the transformer was 15Vac + 15Vac @ 15A, the 7815 (with no zener) didn't need a heat sink. Like I said worked excellent!

Now, for an audio application, I'm wondering if this can work or not, so I ask you for your help here, please

The transistor is a Darlington, to make use of their high hfe value, it could be any, in my example I'm using the MJ900 & MJ1000. I will explain the (+) part, the other is the same.

As you can see:

Ireg = Ir + Ib ; the current that flows thru the 7815 regulator.

Ie = hfe * Ib ; regardless the minus sign in this case as it's used to note the current flow direction only.

and Ic = (nearly) Ie ; This will be almost the same as IL (load)

When Ir * R, in this case R=27 ohms, reach Vbe, the Tr starts allowing Ie to flow, and Ib > 0mA.

Considering a hfe > 500, we can discard the effect of Ib and say that Ir = Ireg, that will reamin constant while Ib>0mA.

When the load starts to "take" current from the PSU and IL gets higher than Ireg the transistor will be move from cutoff to saturation, giving the extra current that IL requires from it, so:

IL = Ic + Ireg.

IL = Ic + Vbe/R

I have choosen R=27 ohms, because Vbe = 5V, so Ir * R must reach at least 5V in order to make the transistor conductive, and start taking current from Ie, which is like Ic.

I want to build this PSU, test it, and use it into a Gainclone using LM1875 (a single channel). The thing is what can you see according to your experience about this design?

My idea is to use:

Transformer: 20V + 20V x 4 or 5A
To make this PSU available for: 20V + 20V @ 4A

The Caps are chosen according to my experience, and for a little reading from this forum, I think I'll need more to enhance the system response for lower frequencies, although this will be used for a Guitar amp.

Anyway, I would like to know what do you think what's your advice. And maybe this is good for stepper motors drivers only LOL. So I'd have to redesign entirely my PSU, who knows

Thanks In Advance.
Pato.

[Pato666]

I'm reading more of this excellent forum, and i've found out that is better to leave the PSU non regulated instead of a regulated one.

here's a great discussion about how to calculate the whole PSU.

Power Supply Rating / Fuse calculation / Understanding ?

Thanks.

[h_a]

From a quick glance I see your circuit is a simple boosted 7x15 variant. However for sake of regulation I would go for e.g. the LT108x regulator family and drop the booster transistors.

In the end I see no reason why one should not use a regulated psu for audio.

All the best, Hannes