Single power supply - need help

[842]

Hi,


I want to build an amp with continous 150w and max 300 watt power, and +/-30v. I have some circuits for it. My problem is the power supply because I have a 400va transformer, but it isn't symmetric, so it has not center tapped secondaries. It has only one secondary, so i have to make a virtual ground .



Could you give me a good, complete circuit between the secondary and the amp?

[amplifierguru]

Hi 842,

need more information -

To drive what? Impedance?
Transformer secondary voltage?
150W rms continuous and 300W rms max, is that right?

Where does +/- 30V come in?

Cheers,

Greg

[LukasLouw]

OK for 150 watt rms into 8 ohm you need 34 Vrms output. Do-able with a bridged amp running off +-30v.

For 300 watt rms you need 49 Vrms, or 69.3v peak, add 5v for losses, and make it 75 v rail in practice, for a bridged amp, or +- 37.5VDC, and that's assuming perfetc regulation, so your'e out of luck right out of the gate with +-30v.

Into 4 ohm You'll make it IF the transformer regulation holds up for 300 wat peak. For 150 watt continious, you should be good into 4 ohm. All this assumes a brided amplifier.........

Lukas

[842]

Ok.

I want to make an amp with 2 lm3886. (I know it is a chip amp, and not a solid state, but my problem is the psu)


Here is my transformer:

the front side, with the primaries


the back, with the one usable secondary


I created a little circuit to provide the 'virtual ground'.
circ

Is this circuit good? Will be Unequal Virtual Ground Splits a problem???
How can I solve this problem- if it is a problem??

[Jennice]

Your schematic for symmetrical supply (+ and - supply) is ok. It just needs double the storage capacitors for the same ripple as a full bridge set would.

However, your transformer seems less than perfect. It has a lot of secondaries, but I can't see any current rating for each secondary. If you have a 400 VA transformer, the rating is split across all these secondaries. You can't choose for yourself, which secondary you want 400 VA from.

Based on the (little) available info, I don't have a good feeling about this project. sorry.

Jennice

[jaycee]

If you look at the data sheet for the LM3886 there is an example of how to use the chip with a single rail supply. This would be far better than a virtual earth arrangement. The catch is the need for a high quality, large capacitor in the speaker output.

[842]

yes, i know this transformer is not the best, but I don't want to spend more money on this project so i will try it out.

There are 2 10,5V outputs on the transformator, with 3 and 0,5 amp rating. Maybe the 43V output can deliver 3 amp. And if it can, it is enough for the 2 lm3886 chips.


I will double the capacitance of those caps.


I'm afraid the output volts, the (+) and the (-) won't be equal (maybe it can be 2 to 5 volts difference). But tell me i'm not right.


The original circuit That I modified has voltage regulator ic's on each rail.




original circuit

[842]

I saw the single supply example on the datasheet.

But I read somewhere the single supply is not the best for this ic. Maybe it's a stupid question but I'm a newbie so i ask: Why do you think the single supply is better?

[Dirk]

Hi,

The PS schematic is essentially a voltage doubler, which will give you something like 2 x 60 Volts. This should blow the chip amps to pieces!

If you want to use this particular transformer and these chips, single supply and bridging seems the way to go

Regards,
Dirk

[842]

Thank you for your reply.

I corrected the 2x60V mistake so it will be 2x30V with the circuits used below.

Those 2 circuit below are use batteries, I change them to bridge rectifier.


Here is the simplest circuit :



simplest

In this circuit the transistors are not appropriate, so i change them to 2n3055 and mj2955.
maybe the better circuit



Which circuit is better? Are these modified designs good?? if not , can somebody post me a virtual ground ps circuit?

Are the voltages on the output equal? +30 :: -30 or something like this: -34 :: +26 ???