38-0-38 smps

[Wizardmaxx]

Hi

Been looking through the forums, particularly for the LM3886 projects and would like to test a theory with you guys.

I was most intrigued by the following article

SMPS for the Gainclone chip amp

Now my limited knowledge on SMPS's is where you pros come in.

Can I connect four 19V laptop power supplies using the same topology as the above link to create a 38-0-38 power supply for a LM3886 amp.

Each laptop SMPS is rated at 3.5A output so in all practical terms would deliver around 266W of power maximum. Considering that each LM3886 is rated at 68W the SMPS's would be running at around 50% usage.

The other option is to use two SMPS' to create a 19-0-19 supply, but what is the calculation of output power for the LM3886 at this voltage. If I had to run two LM's in bridge I am assuming 4x the power from this figure...is that correct?

Also would I have the same mains earth issue as mentioned in the article.

Any advice would be appreciated.

Cheers

Wizard

[h_a]

Can I connect four 19V laptop power supplies

Probably not as their grounds are tied to safety earth and you would just create a dead short between one rail and ground.*

Anyway, I can't see how a gainclone could benefit from a SMPS? Of course I understand if you like to play around with SMPS a little.

It's a ClassAB amp with little current draw most of the time and SMPS are usefull for high power applications.

Don't forget also that nearly all SMPS create tons of noise and that's not a nice thing to have.

Have fun, Hannes

*If you have the SMPS already you could connect them separately and measure the resistance between their grounds. If they're connected to safety earth you will find a short. But that's not a bullet proof method for checking, as there could also be diodes used to connect to safety earth or thermistors or resistors... many things to check, not a good idea for beginners. SMPS carry lethal voltages!

[AndrewT]

Hi,
read the 3886 datasheet.
The maximum output is 7A to 11.5A
This will vary with temperature (drops to a lower current @ >25degC) and that is one of the biggest limitations of the 3886. Too low a current limit on transients.
Your PSU must be able to supply at least 7Apk and preferably >11.5Apk to ensure the 3886 can do it's job.

In bridged mode, the problem of meeting speaker current demand is even more difficult.

[Wizardmaxx]

Hi Hannes

Thanks for the reply.

I tried to measure the resistance between -ve and safety earth and got a reading of 1000kOhm / 1m Ohm. This albeit being a £6.99 multimeter and taking into consideration the diode, thermistor, etc. detail.

All I can do I suppose is to try it and if she blows....she blows.

I also had a thought of the implementation of the LM338 in a standard regulated PSU and wondered if sticking the 19v supplies through two of these as the earth on the output of a PSU of this arrangement is not true earth (or am I wrong???).

The 38-0-38 may prove to be unfeasable, but I sould be able to achieve a decent power output with 19-0-19. Again any idea on output power ?

Cheers

Wizard

[star882]

Modify a PC power supply. 13.8 V / 15 A from a PC Power Supply It should be trivial to get at least 24v with a bridge rectifier.

[Wizardmaxx]

Just a quick update

Hooked up two laptop chargers with output voltage of 22V a piece.

22-0-22 SMPS

And no smoke !!!

So it does work and most of all the mids and highs seem more accurate. Bass is also tight and controlled.

Now for a 44-0-44 regulated down to 35-0-35

[pheonix358]

Hi all,

Does anyone know if split rail smps are being manufactured anywhere. Seems a pity that they aren't.

Terry

[danspy]

yes!

Switching Power - dexicon.co.th == Sound Systems & Audio Parts

I Have the 600VA whit self oscillator and the Oscillator gets very Hot, but the newer Models haf an Active control.

Regards
Dan

[mikje]

Originally Posted by Wizardmaxx

Just a quick update

Hooked up two laptop chargers with output voltage of 22V a piece.

22-0-22 SMPS

And no smoke !!!

So it does work and most of all the mids and highs seem more accurate. Bass is also tight and controlled.

Now for a 44-0-44 regulated down to 35-0-35

Awesome. Thinking outside the box! Way 2 Go!!!!!!!!!!!!
Can you share a diagram of your works?
Thanks,
Mike

[stoc005]

Connexelectronic

Connexelectronic has 3 SMPS for audio amps.