LM3886 transformer/bridged diode...

[epilot]

Hi guys,

I am designing an amplifier out of an LM3886. The load impedance is 4 ohms.
I want to use two filter caps (6700 0r 10000uF) for the dual power supply which supply the LM3886 chip.

I have got just 2 questions.
1: how to choice the best suited bridged diode (foe instance if a 10A bridged diode is good enough for this purpose (the large filtering caps are shorted at first when the amplifier starts to work, So I would like to know how to select the best bridged diode so that it works just right and does not fry due to shorted filtering caps at the start point of amplifier working)).

2: what is the best choice for the toroid and non toroid transformer for this job? I mean the transformer current delivering and so on.

Thanks for any help.

[Bazukaz]

Hi, in general I would recommend bridge of around 25 amps(better have safety due to surge current when caps are charged on turn-on).
Toroids are mostly better(usually lighther for the same power, less hum).

Regards,
Lukas.

[CanAm Man]

What voltages do you want to run, for your +/- rails??

[epilot]

Originally Posted by Bazukaz

Hi, in general I would recommend bridge of around 25 amps(better have safety due to surge current when caps are charged on turn-on).
Toroids are mostly better(usually lighther for the same power, less hum).

Regards,
Lukas.

Hi And thanks,
a 25A bridged diode is not a way to high plz?

[epilot]

Originally Posted by CanAm Man

What voltages do you want to run, for your +/- rails??

Something like +-20Vrms (56Vp-p) or a bit higher or lower is a good choice for the power supply I think.

I do not how to choice a transformer for this job so that it works 24 hours a day...?

[ByronInLawrence]

Coincidentally, about a year ago I built a 2-channel LM3886 amp, which I've been using with a pair of 4 ohm speaker for a while now (though not 24hrs/day). I'm very happy with it. The rectifier uses MUR860s by On semiconductor ON Semiconductor MUR860: 600 V, 8.0 A Ultrafast Rectifier.
I'm using the Antek AS-3218 Antek - AS-3218 which has built-in shielding and 18V secondaries.

Good luck and be sure to post photos of your build!

[epilot]

Originally Posted by ByronInLawrence

Coincidentally, about a year ago I built a 2-channel LM3886 amp, which I've been using with a pair of 4 ohm speaker for a while now (though not 24hrs/day). I'm very happy with it. The rectifier uses MUR860s by On semiconductor ON Semiconductor MUR860: 600 V, 8.0 A Ultrafast Rectifier.
I'm using the Antek AS-3218 Antek - AS-3218 which has built-in shielding and 18V secondaries.

Good luck and be sure to post photos of your build!

Hi and thanks for the replys.

2 questins:

isnt a 8A diode a way too low for 2 channeled power ampifier?
why you did use an ultrafast diode for the job? is it nessesery to use an ultrafast diode?

[gootee]

There is a sequence that you can follow, to arrive at the sizes and values for the components.

If you know the desired output power and the load impedance, then you can estimate the required supply voltage. That will also enable you to estimate the voltage and Volt-Amp (VA) rating for the transformer, and the maximum steady-state current required. The desired maximum ripple-voltage amplitude and max load current will give you the total smoothing capacitance needed. The capacitors' ESR (equivalent series resistance) spec will let you estimate the possible initial inrush current magnitude (and with cap values would give an estimate of its duration and profile vs time). If a regulator is to be used, then max ripple voltage amplitude, rectifier voltage drop, the regulator drop-out voltage spec, and the worst-case transformer regulation and worst-case line voltage excursion will also need to be accounted for, when the transformer voltage is selected, to make sure that the bottom of the ripple voltage waveform can't make the regulator go into dropout, which would be quite ugly (smile).

Note that the initial inrush current might be many tens of Amps, but its time duration might be short-enough that lower-rated rectifier diodes or bridges could handle it OK.

Some of the basics that you need are here: Unregulated Power Supply Design .

The Chipamps forum has a "sticky" thread (at the top), that's all about power supplies for chipamps.

[AndrewT]

Originally Posted by gootee

Some of the basics that you need are here: Unregulated Power Supply Design .

The Chipamps forum has a "sticky" thread (at the top), that's all about power supplies for chipamps.

I like/love your reply. It seems to remind me of what I advise.
Give some guidance and tell them/him/her to go and read.

[gootee]

Originally Posted by AndrewT

I like/love your reply. It seems to remind me of what I advise.
Give some guidance and tell them/him/her to go and read.

Thanks. It wasn't entirely consciously planned that way. (smile) I just thought that the process-level view was what was needed the most, at that moment. And, maybe like you, I thought that the details would probably be best-learned by just churning around in them for a while. Either that or I was in a hurry. (grin)