Questions for my LM3386 based power-amp.

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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...
 

Jamie F

Member
2001-12-15 12:33 am
UK
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).
 
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 ...
 
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...
 

paulb

Member
2001-06-01 4:53 pm
Calgary
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...
 

Jamie F

Member
2001-12-15 12:33 am
UK
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
 

Jamie F

Member
2001-12-15 12:33 am
UK
JoeBob

Rod has used cheap resistors and provided the "heat removal" mechanics himself. Not being a great mechanic, I went for far higher rated resistors (metal cased) that saved me this hassle, but cost more.

I'll put some details together for you, and post tomorrow.

P.S. I don't know if you're following the thread on charging spikes, but this could become relevant to you. I feel am starting to get closer to an answer on this.
 
Well, my friends mothers boyfriend is an electrician so he said he'd explain solid state relays/switches to me, but my questions is as follows. I was reading as ESP about powersuplies, since I know know exactly how the amp modules are going to be (actually started work on them). And for example if I make a powersuply composing of a gloryfied version of ESP's project 04 with the two 750VA toroids that AudioFreak pointed out as well as a soft start circuit (haven't read up on it, was about to after this post, so correct me if there's such a thing in it already), either Robs of Jamie F's (depending on if his post explains enough for me to understand, would I need a solid state switch to turn it on?

Because I looked at the project 04, and it's composed mostly of the transformer, bridge rectifiers and caps, am I missing something (I obviously am)?

Just trying to understand more about the psu, I guess I'll read up on the soft-start circuit now, hopefully it'll reveal some answers to me...
 
Well, I read the esp pages o nthe soft start circuit, and they contain a relay, now I'm not clear on the differences between just relay as in the esp pages, and what you mean by solid state relay, but if I'm better off using both the solid state relay as well as the soft start circuit, I'll look into that, I'll ask my expert to explain it to me, not wanting to ask too many questions here.

As for power point, you mean actuall wall plug, correct? I understand what you mean by neutral to neutral, hot to hot, ground to ground, but something that I've been wondering (and probably need to know) is which one is hot and which is neutral?

Also the ESP pages explaining the soft-start circuit, and I was thinking I'd be best off with the first circuit encorperating the small transformer, but using an EI one here would be fine, no need for an expensive toroidal, right?

Another quick question to slip in, are chock input filters worth it? I know they're alot more expensive, but is worth the extra cash/effort, or is that same cash/effort better spent elsewhere (as I gather, considering I haven't heard much about them)?
 

Jamie F

Member
2001-12-15 12:33 am
UK
JoeBob

Project 4 describes a linear power supply (switches, fuses, transformer, bridge rectifiers, smoothing caps). This takes mains and makes it into two DC rails of the right value for your amps. It allows you to manually switch on and off. It will work without anything else.

However, there are two improvements you can make, which are in addition to the above:

1) Management of in-rush current on power-up to avoid lights dimming, etc and enhancing the life of your power supply. Rod's circuit does this, a triac will as well. Only needed because of the size of the power supply and use of toroids - only large commercial amps use these techniques.

2) Relay switching of power to the amp. Allows you to remote the amp power on. My very minor adaption of Rod's circuit does this, and has the side benefit of making Rod's circuit a bit safer.

As far as triacs are concerned, Rod provides the pro's and con's in his soft start documentation. AudioFreak is right to say that a triac will provide protection after power-up. The soft-start is a better way of protecting the amp every time you switch it on (often), the triac route comes into its own when things go horribly wrong or you overcook it (hopefully not often). My healthy respect for mains AC means that Rod's warnings are enough to put me off triacs, and the noise issues concern me.

If you want to do the soft start circuit, your decision is how to power the relay control circuit. Rod provides two options, but strongly recommends the former (separate transformer). You definitely shouldn't try the latter.

Having dismissed deriving the power from your main supply rails, you need to think where your 12Vdc is going to come from. One option is everything to the right of R3 (2k2) in Rod's Figure 2 circuit.

I have 12V easily available in my pre-amp, so I omit the components mentioned above. If you have an easy source of 12V, you don't need this either. The pre-amp is a good place to control the power-up of your power-amp, because it means you only have to turn everything on in one place, and the pre-amp output settles before the power amp comes on avoiding nasty pops. If your pre-amp is a commercial unit, you won't want to do this.

If you don't, an EI transformer will do the job, but I wouldn't put this in the same box as your amp modules, as they have strong stray fields which can lead to pickup. You could always just use a 12Vdc wall block (my circuit draws about 120mA) - very easy.

Because my adaptation of Rod's circuit was very basic, I didn't bother drawing it. However, I'll do this today with some block diagrams to make this easier for you to follow. I will also provide a parts list.

Let me stress the safety aspect again - if you haven't done much of this stuff before, heed all of Rod's warnings, and get your electrician friend to check particularly the mains wiring before you switch on. Always unplug before doing any work on this stuff.
 
Jamie F,

Rod's points regarding Triac's are valid.........

but.......

It should be no surprise that a triac adds some noise to the supply line..... all diodes add some noise and a triac is effectively 2 diodes connected in parallel with their terminals reversed relative to each other and controlled by a small current ... to this end a Triac will also cause a small drop in the supply voltage of 0.5% or less but when driven properly neither are of great concern.

The noise from the triac is the harmonics of the 50Hz or 60Hz mains supply so most of the noise will be at 100Hz and 150Hz for a 50Hz mains or 120Hz and 180Hz for a 60Hz mains....

The fundamental frequency of the noise from the bridge rectifier is twice that of the frequency of the mains supply so for 50Hz most of the noise will be @ 100Hz, 200Hz and 300Hz ..... for 60Hz mains the noise will mostly be @ 120Hz, 240Hz and 360Hz...

As one can see some of the noise is shared by both the Triac and the rectifier so will require more attenuation at these frequencies... but all said and done if the power supply is designed well then the little noise that does get thru will be taken care of by the PSRR of the amp circuit .... of course if you are building a Zen or SoZ amp which dont have great PSRR then more care must be taken in the power supply. In this case the PSRR of the amp is quite good so none of this is really of any concern and as far as the other issues go ...

1. The control circuit runs @ ac mains voltage.... well this is the whole point of it :)

2. There is a risk of insulation breakdown ... use a good quality insulation and a non metallic screw to attach it to the heatsink and it is highly unlikely that there will ever be a problem.
 
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