Amplifier idea..

[Dr.EM]

Hi. This is a schematic for a bridge amplifier of power 16w, from the web.

It doesn't actually say what the ICs are, but I couldnt help thinking they looked like 386s.

I know they're not, but could you use 2 386 amps in a bridge like this to output more power than one by itself?

 

[David Bridgen]

A bridge-connected pair of LM386s would work.

However, I've never seen or heard of this being done.

With twice the voltage swing across the load the current in the output devices will also increase by a factor of two.

Nevertheless I would have a go and see if you can get away with it.

You might like to consider gluing a token heat-sink on each i.c.

 

[audioguru]

The LM386 is a 1/2W into 8 ohms amplifier. With a 14.4V supply, it seems to be current-limited or just runs out of current gain at 1.1W into a 16 ohm load or 0.7W into an 8 ohm load where it also reaches its maximum temperature. With a 4 ohm load it reaches its maximum temperature producing 0.3W but only with a 10V supply since a higher supply will only cause more heating.

In a bridged amplifier with a 4 ohm load and a 14.4V supply both LM386's would try to operate far beyond their thermal limit but current-limiting would allow hardly any output power. They would also have the same problem with 8 and 16 ohm loads. But with a 32 ohm load they would be able to produce 2.2W and not get too hot.

 

[audioguru]

I Know!
Use a dozen of these little amplifier ICs.
Connect a 0.1 ohm current-sharing resistor in series with each one and connect 6 amp/resistors in parallel on each side of the bridge. You should be able to get about 15W into a 4 ohm load without a heatsink.

 

[samcheetah]

why not use a single IC solution that can output the required amount of power you need. a simple search on national and analog's website will help alot

 

[audioguru]

National have a few but Philips have many amplifier ICs for car radios. Most are bridged and provide about 16W ito 4 ohms, some provide about 28W into 2 ohms. They even have a 14.4V supplied IC that operates in class-H (supply is doubled on demand) that provides about 70 Whats :lol: (about 55 Watts) into 4 ohms.
**broken link removed**

 

[Dr.EM]

Cheers for the ideas!

That plan with the 12 sounds great, 15 watts is a lot for no heatsink. I am aware that you could do it with a single big one, but for novelty appeal, this has got to be good!

So, how exactly would you do it? A schematic would be great. Also, you say 12, could it be done with mabye 6 or 8 to get 9 watts or something?

Actually, just thought. These amps usually only need a low voltage to work (although this would yield less output power) but you could run a fairly high output amplifier of of a set of AA batteries?

 

[audioguru]

Hi Dr. EM,
You don't need a schematic to connect a 0.1 ohm resistor in series with a LM386 amp then connect 6 of them in parallel on each side of the bridge.
Actually, a schematic might be a good idea since most of the amp input capacitors aren't needed in your circuit when it uses LM386 amps, because their inputs operate at 0V. The output offset voltage adjustment pot also probably isn't needed.

That's right, with a 14.4V supply and an 8 ohm load only a total of 6 of them will provide about 9 Watts, but of course without a heatsink they will get just as hot as 12 of them providing 15 Watts into a 4 ohm load, but still within their thermal limit.

If you like novelty amps, you should try TI's surface-mounted (without a heatsink) 10W or 20W class-D amp IC. I don't like it, it has thirty some-odd tiny pins, and I think its bottom must be soldered to the board somehow (very hot air?). Operate the amp at about 50W and it will solder itself! Reminds me about when I soldered together a muffler for a model airplane's engine. It melted!

I have been taking a DIY bridged stereo amp to the beach for years. It is powered by only 6 AA Nicads. I'll replace them with much higher capacity Ni-MH cells for this summer. With my tiny but wideband speakers, my sound system blows away all those kids' boomboxes. :lol:

 

[David Bridgen]

Hi audioguru.

Yes, quite.

I must admit to not having taken a good look at the 386's data sheet for some considerable time.

The parallel/current sharing scheme sounds good and the idea of getting a good dose of power from a few of these is most satisfying!

 

[audioguru]

Hi David,
A dozen is "a few"? :lol:

 

[Dr.EM]

Cheers! I'll get back to you a bit later with a schematic of what I think it is needs doing. If it looks right, then I may be adding a lot of 386s to an order, they are only about 12p each, though the 0.1ohm resistors are 19p!

 

[audioguru]

Hi Dr. EM,
An amplifier that is cheaper than a resistor. Wow!
Ordinary 1/4W resistors are fine and maybe cheaper.
The amps and resistors are even cheaper by the dozen. You might get a baker's dozen with extra ones! :lol:

The bridged amps I built have a bootstrap capacitor which boosts the maximum output voltage about 2V. It was valuable with my low supply voltage but isn't necessary with your 14.4V.

If you are a real doctor then maybe you would be interested in my Electronic Stethoscope project:
https://www.electronics-lab.com/projects/science/019/index.html

 

[Dr.EM]

Can't seem to find such small values in any form other than high power wirewound (which are going to take up a whole lotta room coming to think of it).

Bootstrapping, have heard of that. I would like to be able to run from a resonably small power supply, so it may be useful. Its the big caps on the power supply, as seen in the stephoscope?

Can't figure out where everything is meant to go, btw. Tried to do a schematic, but got stumped striagt away :x

 

[Nigel Goodwin]

Can't seem to find such small values in any form other than high power wirewound (which are going to take up a whole lotta room coming to think of it).

Small wattage 0.1ohm are freely available, but as you don't have your location filled in there's not much point in me suggesting places though :cry:

Bootstrapping, have heard of that. I would like to be able to run from a resonably small power supply, so it may be useful. Its the big caps on the power supply, as seen in the stephoscope?

No, it's not a power supply function, and it doesn't really have anything to do with low voltage power supplies. Generally it's used around the output stages of push-pull amplifiers to provide additional base current on high level positive signals. It could be applied just as well in the negative side, but amplifier design 'usually' uses it on the positive side - it's only required on ONE side, the other side is taken care of by the driver transistor.

It's not absolutely required though, the same function can be performed by using a constant current source instead.

It's also been used to increase the input impedance of transistor amplifiers as well!.

 

[audioguru]

(About bootstrapping)
No, it's not a power supply function, and it doesn't really have anything to do with low voltage power supplies.

It's not absolutely required though, the same function can be performed by using a constant current source instead.

Hi Nigel,
My bridged amps were operating with a 7.2V battery and bootstrapping allowed about 20% more output power when compared to similar amps with current sources instead of bootstrapping.
I have compared the bridged LM2896 bootstrapped amp driving a 4 ohm load that I used, with the similar but non-bootstrapped and non-bridged LM383, but it is driving a 2 ohm load to give the same current as it would have if bridged into a 4 ohm load. Since the curve is for only a single LM383 amp, the output power is doubled when bridged:

 

[bmcculla]

If you want to drive a ton of power take a look at OPA549S - 8A continuous - not cheap though. I put 2 of these together in a circuit similar to yours for a quick and dirty TE cooler controller.

 

[Dr.EM]

That is a hefty op-amp :shock:

I had this other idea, with amps, and how to make some killer "portable" speakers using:

These little drivers (possibly 2 per box)
**broken link removed**

2 of these amp units
**broken link removed**

Most importantly, make the enclosures from rigid MDF. Power it off of one of those batteries from a 16v cordless drill. Possible?

 

[Nigel Goodwin]

That is a hefty op-amp :shock:

I had this other idea, with amps, and how to make some killer "portable" speakers using:

These little drivers (possibly 2 per box)
**broken link removed**

2 of these amp units
**broken link removed**

Most importantly, make the enclosures from rigid MDF. Power it off of one of those batteries from a 16v cordless drill. Possible?

Are you aware that those speakers are only mid-range drivers?, so no bass at all (or you would kill the speakers). It's hardly going to be a 'killer' system!.

I've not seen one of those Maplin amplifier modules, but I see no reason why it can't be fed from a drill battery.

 

[Dr.EM]

yeah, was just thinking. Well, perhaps these would be more suited:

**broken link removed**

 

[Nigel Goodwin]

yeah, was just thinking. Well, perhaps these would be more suited:

**broken link removed**

At least that's got a bit better range, and it's specified as usable as a full-range driver. However, the bass response is still rather lacking, as any small driver is really going to be?.

For good bass you need to move LOT'S of air, generally the bigger the cone the better - not to mention the high powers required :lol:

A lot depends what you are trying to do?, but if you're wanting a really good sounding 'earth moving' system, you need bigger speakers, and probably more power.