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Old 20th December 2001, 09:58 PM   #1
JoeBob is offline JoeBob  Canada
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Default Questions for my LM3386 based power-amp.

Alright, well I've got my LM3886T's now so I've started to aquire my other components. But I've got a couple questions. My amp will be based on the ESP Project 19, but instead of the LM3876 I'll use the LM3886, so I add pin 5 to the +ve... I'll also be using +/- 24v, and if my psu I'll build can give 6.5A per amp module, what value for the fuses should I use?

I won't be using a SIM so I can leave out the two resistors before the connect for the SIM, right?

Also, my amp will be to power a 4-6 ohm load, not 8ohm like ESP Project 19, so is there anything I'd have to change for this? I've gone over the LM3886 datasheet and it doesn't seem to sudgest this, but I just wanted to make sure.

Thanks for all your help so far, and I hope I'm not asking too many questions...
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Old 20th December 2001, 11:47 PM   #2
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Fuses aren't a particularly precise science. Best to start low and work up if they blow during normal use. 3A on each rail should be ample: though you may draw more than this with a 4R load, I'd be surprised if the fuse blew.

I'm using 2A fuses for 5R loads (30V rails).

The only calculation you may want to check if you've changed the supply rails is the biasing for the mute pin (details in data sheet).
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Old 21st December 2001, 12:02 AM   #3
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JoeBob if your using the transformers we discussed and the going ahead as per plan you will be using +/-28V .... just a reminder when you turn (rectify) AC into DC you get a higher voltage... the optimum is DC = AC x 1.4142 .... in reality dont plan on ever getting this depending on what type of power supply you use it is usually closer to DC = AC x 1.25 ... or less

You dont need to change the schematics ...

As for the fuse stick a slow blow fuse between the AC from the wall socket and the toroidal transformer and also fuses on the power supply rails.

You'll also need a solid state switch to turn the whole thing on ...
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Old 21st December 2001, 12:46 AM   #4
JoeBob is offline JoeBob  Canada
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Ya, I intend on using those trnasformers you sudgested in my previous post.

Ahh... now I see where the 28v came from, didn't know how that worked.

I guess I'll try 3A fuses on the psu rails, if it does blow I just use a higher rated one, right?

I'm going to be working on my fron5 channels first, and I'll be building my amp modules first, then the power suply when the transformers get here. But you said a slowblow fuse between the transformers and the AC outlet, now I do understand in a general sense what a slow blow fuse is (won't blow from spikes, just constant current, right?) but what would be a good value for it (they are rated in A's like normal fuses are they not?) for the psu containing the two 750VA transformers?

And you said a solid state switch to turn it on, I don't know what you mean by solid state switch. Are you telling me that to turn it on and off there's more I must do besides connecting the transformers to the AC line/cutting power off?

I'll just use 3A fuses on the rail line, and if there's more readings I must do for "solid state switches" or anything else involved for the powersuply I can read up on that while I build the amp modules and be ready when the transformers are here...
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Old 21st December 2001, 02:06 AM   #5
paulb is offline paulb  Canada
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I think "solid state switch" is what is normally called solid state relay. A small DC voltage controls a large AC supply.
They're not cheap (but neither are high-amperage switches), and you need a DC source to get the thing started in the first place. I use a battery and a couple of diodes so that the current is only drawn from the battery for a second or so, then the power supply itself runs the relay. I wanted to run low-voltage DC control wires to my preamp.
I haven't looked, but you can probably buy an SSR that is driven from AC so you can use a low-amperage switch to control the AC to the SSR, which controls the big whopping transformer that drives the amp that runs the speakers that drives your neighbors crazy.
But I digress...
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Old 21st December 2001, 02:09 AM   #6
JoeBob is offline JoeBob  Canada
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Ah, I'll have to read up on that...
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Old 21st December 2001, 03:16 AM   #7
palesha is offline palesha  India
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Just go to google.com & type LM3886. Many people have information about it. Parapix amplifier & dave site have lot of information regarding lm 3886.
Mahendra Palesha
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Old 21st December 2001, 10:48 AM   #8
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JoeBob

As suggested previously, you might want to consider a soft-start circuit, because the in-rush current to toroids of this size is likely to be huge (as smoothing caps short the secondaries on power up).

You can spot this by the mechanical tensioning (a diminishing buzz) that occurs with much equipment using large toroids. Providing some additional resistance during power-up gives the caps time to charge, thereby reducing the in-rush. The resistors must be switched out of circuit after this, otherwise they'll melt.

This stops fuses blowing, trips going, lights dimming, but most importantly, it places less stress on your expensive power supply components, so they will last longer. See Rod's Project 39 - and do heed his safety warnings.

My version of this adds another relay to switch the mains on. The first relay switches mains to the amp via the power-up resistors, then the second relay bypasses the resistors after 5 seconds. My rails come up to full tilt in about 3 seconds, so everything is nice and gentle. Both relays are powered from the same source, so if your relay control rail goes, mains is switched off. My version is safer than Rod's in this respect - but you still need to make it bomb-proof as you're dealing with mains.

I've used Al cased resistors which don't even get warm - more expensive but less hassle than Rod's approach.

If you power the relays from your pre-amp as I have done, it also means that the power-amp is always switched on after the pre-amp power up transients have gone.

Jamie
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Old 21st December 2001, 01:39 PM   #9
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By "solid state switch" i did infact mean solid state ... a relay is not solid state. I refer a simple circuit using a Triac which is controlled by a small current that is switched in the traditional manner.
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Old 21st December 2001, 02:38 PM   #10
paulb is offline paulb  Canada
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Yes, that's what I meant too. You can buy it as one package.
There are electromechanical relays and solid state relays:
http://relays.tycoelectronics.com/datasheets/solid.stm
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