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Audio Amp

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There are 3 versions of the LM386 with the same pinout:

LM386-1 has 325 mW output @6V and 8ohms
LM386-3 has 700 mW output @9V and 8 ohms
LM386-4 has 1000 mW output @16V and 32 ohms (?!)

Im not sure aboutwhy they spec'd the -4 at 32 ohm
 
philba said:
Im not sure aboutwhy they spec'd the -4 at 32 ohm
The poor little thing would get too hot at full output with a 16V supply and a speaker less than 32 ohms.
 
I am currently using an LM386-3 which is rated to withstand a max of 12V - according to the datasheet. I think I might pick up a 386-4 (18V max) because I wanted to use an eight AA battery pack and I guess I'm pushing my luck if I do that.

Thank you all for the tips. I will look into the class D amplifiers/PWM chips. I've done OK soldering some surface mount chips so far. The bigger problem (for me) will be in making a PCB layout. I haven't used any PCB design software yet, so I will need to spend some time learning it. I like to use perf board most of the time ...or pre-existing PCB layouts.

*EDIT* Found an article on class D amplifiers... looks like I won't be building one anytime soon.
**broken link removed**

From the article:
From the DIY perspective, Class-D is rather unfortunate. Because of the extremely high switching speeds, a compact layout is essential, and SMD (surface mount devices) are a requirement to get the performance needed. The stray capacitance and inductance of conventional through-hole components is such that it is almost impossible to make a PWM amplifier using these parts. Indeed, the vast majority of all ICs used for this application are available only in surface mount, and a look at any PWM amplifier reveals that conventional components are barely used anywhere on the board. Since SMD parts are so hard to assemble by hand and the PCB design is so critical to final performance, DIY versions of PWM amps are very rare indeed (I don't know of any).

I at first thought I could use the surface mount IC in conjunction with through-hole components. Apparently this is not done.
 
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Interesting page on class Ds. I didn't know that PWM could be used for audio amplification. It looks to be a close cousin of a switch mode power supply which are pretty DIY buildable.

His case against DIY is based on some very flawed points. "a look at any PWM amplifier reveals that conventional components are barely used anywhere on the board" - the main reason that you see few through hole parts on class D amps is because the they are cheaper to build with (cheaper parts, pick and place machines, reflow soldering). You basically don't see many though-hole parts on anything electronic these days.

On top of that, he comments that "since SMD parts are so hard to assemble by hand and the PCB design is so critical to final performance, DIY versions of PWM amps are very rare indeed (I don't know of any)." There may not be any DIY class D amps out there but his premise about SMDs is flat out wrong. SMDs are fairly easy to hand solder.

Maybe part placement is critical but I have a hard time believing that a sophisticated hobbiest doesn't posses the skill level needed. I'd certainly like to understand what he is talking about on placement issues beyond good PCB design pactice.

I'm far from an expert on audio-amps but people hand build switching power supplies all the time with SMD components. I don't think there is that much difference here.
 
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Texas Instruments have many class-D audio amp ICs. They are all surface-mount with very small lead spacing and their power is up to 240 Whats (about 150 Watts). Some require the bottom to be soldered to the ground plane of a pcb while it is surrounded with pins going elsewhere. Try doing that by hand.
 
Found some thru-hole DIP, class D amplifier chips. Looks like something I can more easily deal with: **broken link removed** ...no heat sink needed either. That one's 10W mono. They also have 25W ones.
 
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audioguru said:
Texas Instruments have many class-D audio amp ICs. They are all surface-mount with very small lead spacing and their power is up to 240 Whats (about 150 Watts). Some require the bottom to be soldered to the ground plane of a pcb while it is surrounded with pins going elsewhere. Try doing that by hand.

You can hotplate or hot air reflow fine pitch stuff (.5mm pitch) but people also use a fine soldering iron and a little solder wick to clean up the bridges. Not that hard. especially on boards with solder mask. it even looks like BGAs can be done by DIYer. You can also use a hotplate to do reflow. check this out https://www.sparkfun.com/commerce/present.php?p=Reflow Skillet I got a hot air station and it does a very good job (great for removing chips as well).

By the way, a fairly common technique for the thermal pad chips is to put vias under the slug and use reflow. I recall reading one guy who claimed to do it with a soldering iron though hot air is a much easier way to go.

Frankly, I am amazed how well hobbyists track and take advantage of even the most sophisticated packages.
 
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Class D amplifiers don't have to be complicated at all. All you need is a triangle-wave oscillator, a comparator, and two transistors:

**broken link removed**
 
It is interesting that the TDA7480 "10W" class-D amp IC clips at 6W into 8 ohms with a dual 14V supply. Then it dissipates about 1.5W.
A "normal" linear amp uses a dual 12V supply to clip at 6W into 8 ohms and dissipates about 3W.
The TDA7480 class-D amp specs don't mention high frequency distortion, which is the worst thing about a poor class-D design.
 
The TDA7480 class-D amp specs don't mention high frequency distortion, which is the worst thing about a poor class-D design.

I'm surprised that an IC amp would have that problem. Looking at the data sheet, it only switches at 120kHz--too slow for full-range audio. These things really need to run at about 1MHz. Also, dissipating 1.5W while outputting 6W makes the thing about 80% efficient.

Maybe it's an old design or something?
 
i_build_stuff said:
Maybe it's an old design or something?
That is why it is in an old through-hole DIL case. New ones are all surface-mount.
 
Isn't a 1k voltage divider unsuitable for the non inverting insput? shouldn't it be 47k at least? That's only 2k between Vcc and GND
 
This thread is 6 years old.

Thanks for that...

what so that means I can't look at it, or use it as a reference or ask questions does it? Sorry didn't see that in the rules when I signed up.

I had better Chuck away my Art of Electronics book too because thats written twenty years ago!
 
Just thought 1k looked low... Was wondering if I had missed something. No drama, just ignore me.
 
Yes 1k is a bit on the low side, but from a biasing point of view it is no big deal whether 1 or 47k is used.
There could be issues with input offset currents and temperature drifts on an old 741 circuit, but probably makes no difference in this application.

Regarding the 6 year old business, there seems to be a thing on this forum where someone with one or two posts will dig up a thread which is years old and make some comment or add some snippet of information.
This is all well and good, but, the origiator of the thread is long gone from here and no one else is bothered about the old thread.
Even worst people start putting some effort into answers which are not required.
Been there done that and got the tee-shirt!:eek:
Don't feel bad about it, if you have a genuine question, which it looks like you have, all well and good, but the pavlovian response here is to point out the age of the thread.
Yes, I do that myself quite often.:)

JimB
 
Isn't a 1k voltage divider unsuitable for the non inverting insput? shouldn't it be 47k at least? That's only 2k between Vcc and GND
The 1k resistors are simply wasting battery power. Since the input current of an opamp is extremely low the resistors can be 100k or more. Then the filter capacitor between them will work better.

It is a stupid design because the very low input impedance of the inverting opamp shorts away a lot of the mic level. The opamp should be in a non-inverting design with an input impedance of at least 22k ohms, not only 1k ohms (R1).
 
The 1k resistors are simply wasting battery power. Since the input current of an opamp is extremely low the resistors can be 100k or more. Then the filter capacitor between them will work better.

It is a stupid design because the very low input impedance of the inverting opamp shorts away a lot of the mic level. The opamp should be in a non-inverting design with an input impedance of at least 22k ohms, not only 1k ohms (R1).
Not to mention the gain of 1000, which would require an op amp with at least 20MHz GBW to realize 20kHz audio bandwidth.
 
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