Ways to fry an Amplifier?

[Peter_wadley]

Was just wondering what is the most common ways to ruin an amplifier (lets say a car head)

If you have an amp that can produce 50 watt at 8 OHM and you attach a 100 watt subwoofer at 4 OHM what will happen?

Are speakers the only things that can really get damaged?

 

[Nigel Goodwin]

Was just wondering what is the most common ways to ruin an amplifier (lets say a car head)

If you have an amp that can produce 50 watt at 8 OHM and you attach a 100 watt subwoofer at 4 OHM what will happen?

Are speakers the only things that can really get damaged?

No, amplifiers are more easily damaged.

If the amplifier is only rated at 8 ohms, then connecting a 4 ohm speaker will seriously overload it, and probably kill the amplifier.

 

[Optikon]

No, amplifiers are more easily damaged.

If the amplifier is only rated at 8 ohms, then connecting a 4 ohm speaker will seriously overload it, and probably kill the amplifier.

Good designs should have overload protection for this kind of thing. Wiring & speaker mistakes happen all the time.

I've seen car amplifiers die because they got too hot internally. Again, the good designs would have a thermal shutdown feature.

 

[audioguru]

Car speakers are 4 ohms. Some are 2 ohms. Most car amplifiers can drive a 4 ohm speaker.

Most car amplifiers drive both wires of a speaker so grounding one wire might blow up the amplifier.

Your 50W into 8 ohms amplifier needs to have an output voltage that is 56.6V peak-to-peak. It needs a power supply that is 62V or positive and negative 31V. If it does anything with a supply of only 13V from the car's battery then its output into 8 ohms will be only 1.6W like a cheap little clock radio.

Did it make a big BANG when it blew up?

 

[Nigel Goodwin]

Good designs should have overload protection for this kind of thing.

Overload protection is a LONG way short of 100% effective, it helps, but it's not something to rely on.

 

[rjvh]

speakers are in 95% of all cases damaged by a incorect or damaged amplifier

if an amplifier clips (dc on the output) your tweeters will blown out inmediatly but your subwoofer can manage a few clips (he don't likes it)

subwoofers are more damaged by long time overload situations so the coil gets to hot and get misaligned in the magnet

if that hapens you here a scroffing sound out of your speakers on top of your music
then it's time to recoil your speaker if it is an expensive one otherwise change the whole unit

Robert-Jan

 

[Gayan Soyza]

Burned many speaker sets due to output short circuitry for the power AMP.At that time I was no using any speaker protections.After that I'm using high sensitive speaker protection circuits for higher voltage amplifiers.

 

[Peter_wadley]

I see..

I'd rather just be safe and not kill the cars sound system!

BTW the rear speakers are 8ohm (its a '98 lumina)

Im trying to attach a small subwoofer (it was originally for a DVD 5.1 surround sound system) to the back of the drivers seat..

I really just want the lower frequencies to be felt by the driver rather than have loud, annoying bass which will piss neighbors off.

I have made a small lm386 amp...

Powering it with ~13v from the cigaretter lighter and feeding it audio signal by branching off one of the rear speakers (the sub is mono)

It obviously not powerful enought - as I wanted to start small...

I have used an RC low-pass filter on the output .. I dont really know if its worthwhile .. or what cutoff frequ a subwoofer should be used at...

My question is .. how can I calculate the wattage of this little amp?

Gain is default (20)
Voltage (13v)
Spkr (8ohm)

and my other question is .. Could I use power transistors to increase the wattage of the lm386??

Thanks alot

 

[audioguru]

Hi Peter,
Simply look at the datasheet for the LM386 or for any amplifier IC to see its max output power vs supply voltage vs load impedance.
It shows a graph of Peak-to-Peak Output Voltage vs Supply Voltage. Then the distortion is pretty high at 10%. When the power is reduced 20% then the distortion is low.
With an 8 ohm speaker the graph shows 6Vp-p with a 9V supply and shows 6.5Vp-p with a 12v supply. Then the max power is only 450mW and 528mW at low distortion. The power into the speaker increases 17% but the heating in the tiny amplifier increases 60% and it might fry.

Car amplifier ICs for ordinary car radios deliver 14W into a 4 ohm speaker at low distortion. Car radios that advertise 200W deliver 14W into 4 ohm speakers for each of the four channels for a total output of 56W.
So 1/2W from an LM386 is nothing.

The TDA7240A is the simplest car radio amplifier IC. It produces 14W at low distortion into a 4 ohm speaker or 8W into an 8 ohm speaker. A pcb layout is shown in its datasheet.

A subwoofer should be a large speaker. The subwoofer in my car is 10" in a pretty big ported enclosure and is driven with 200W. The enclosure uses one side of the trunk.

I made a lttle portable sound system with a subwoofer that had a 4" woofer in a ported enclosure and driven with 6W. I heard bass but could not feel it.

I don't know where you will find space for a subwoofer enclosure at the back of a car seat.

 

[ronsimpson]

How to fry an amplifier?

I like mine lightly breaded with salt and pepper.

When I was doing auto design, I found that people put their batteries in backwards too often. In big trucks some people jump start them on 24 volts. In the north it is too common to jump start using an electric welder. (I have done that.)

We had a saying in auto electronics. “Design the power stage, so that when it is plugged into 110VAC only the fuse will blow.” We actually tested for that.

 

[cdstahl]

a more common theme for novice DIY amps would be contol loop instability. this can cause the amp to get very hot very fast, or otherwise exceed SOA.

professional, extensively tested, amps should be more resiliant to this.

 

[audioguru]

a more common theme for novice DIY amps would be contol loop instability. this can cause the amp to get very hot very fast, or otherwise exceed SOA.

Like when they build them all spread out on a breadboard with lots of stray capacitance and no supply bypass capacitor. Many times they leave out the RC circuit on the output that stops oscillation.
NOOBS.

 

[Dean Huster]

Back in 1972, we had built a really simple and very different power amplifier (a design idea in "Electronics" magazine that used a pair of 2N3055 NPNs in the output stage and tapped off the power pins of a 741 to create a differential signal to drive the output) that was capable of 125 watts RMS, DC to 100KHz. We drove it with a function generator and connected an 8" speaker to the output (it was only rated at maybe 10 watts max). Most of us had never seen a DC-coupled audio power amp before and were fascinated to watch the cone of that speaker "breathe" in and out when the generator was set to 0.1Hz. After a couple of minutes on that amp, you couldn't pick up that speaker by the yoke because it was red-hot. Warped the crap out of the voice coil.

Dean

 

[rjvh]

BTW the rear speakers are 8ohm (its a '98 lumina)

this is not the impendance over the whole spectrum and it depents also on the housing but you will have frequenties that the impendance is much lower and higher

I really just want the lower frequencies to be felt by the driver rather than have loud, annoying bass which will piss neighbors off.

to be felt by the driver means you have to work with PA equipment and a lot of power (extra bateries in your car and an amp and suply that is expensive)

pissing neighbours off is basicly the frequency you lisening to
the lisening room from car speakers (cabin) is normaly 2 a 2.5 meter that means that the lowest frequency to be heard in that room is 132 Hz all tones lower you dont hear in the cabin but only feel a 33 Hz tone you will hear 11 meters or further from the speaker



I dont really know if its worthwhile .. or what cutoff frequ a subwoofer should be used at...

it is worth wille to do it if you want to have a good stereo picture

tones are not direction sensetive under the 400 Hz
personaly if take the subwoofer frequency not higher than 250Hz
voices sound beter on your mid tones withoud that part of it is represented in the subwoofer (this is speaker depending)

Robert-Jan

 

[Peter_wadley]

Hmmmm ... OK I guess I cant just swap one of the stock speakers with the subwoofer...

Right now I have an 8in 100watt 4ohm subwoofer (its in a wooden box)

I think im just gonna build an amp to driver it.

Here are the audio ICs I have access to..

TDA2030A
LM3886TF
TDA7294
TA7222AP

I will buy an inductor / capacitor crossover to filter the low freq

Can someone recommend an IC and an accompanying schematic?

If you know off a better IC than those listed above, im all ears!

thanks

 

[Nigel Goodwin]

Back in 1972, we had built a really simple and very different power amplifier (a design idea in "Electronics" magazine that used a pair of 2N3055 NPNs in the output stage and tapped off the power pins of a 741 to create a differential signal to drive the output) that was capable of 125 watts RMS, DC to 100KHz.

I think you probably know better now?

125W is a bit over ambitious for only a single pair of 2N3055's, and 100KHz a bit hopeful for a 741?. But back in 72 you used what was available.

As I remember, feeding the output stage from loads in the opamps power rails was a design in a manufacturers application note - but I can't remember who it was now?.

 

[rjvh]

I will buy an inductor / capacitor crossover to filter the low freq

if you want to build your own amplifier only for you sub than i would advice you to use an active filter before the amp to filter the high and mid tones out

it's a easy, good, (cost wise probably cheaper) and it gives you less load problems on the back end off the amplifier

also the phase problems you can get with a pasive filter are not there

Robert-Jan

 

[cdstahl]

specifically, what phase problems. the active filter will also affect phase, so that can't be the issue.

 

[Nigel Goodwin]

specifically, what phase problems. the active filter will also affect phase, so that can't be the issue.

No sort of problem with a sub anyway, and certainly not a sub in a car!

 

[audioguru]

this is not the impedance over the whole spectrum and it depents also on the housing but you will have frequenties that the impendance is much lower and higher

No.
an 8 ohm speaker has a minimum impedance of 8 ohms just above its low frequency resonant frequency. Its impedance rises at high frequencies due to its inductance. Its resistance is about 6 ohms at DC.

to be felt by the driver means you have to work with PA equipment and a lot of power (extra bateries in your car and an amp and suply that is expensive)

No and no.
PA equipment is made to be efficient (loud with low power) not to produce very low bass frequencies.

Powerful car amplifiers don't need extra car batteries, they use a switching power supply to boost the supply voltage.

pissing neighbours off is basicly the frequency you lisening to
the lisening room from car speakers (cabin) is normaly 2 a 2.5 meter that means that the lowest frequency to be heard in that room is 132 Hz all tones lower you dont hear in the cabin but only feel a 33 Hz tone you will hear 11 meters or further from the speaker

No.
You can hear better bass in the cabin of a car due to the cabin effect that makes low frequencies louder down to the lowest frequency of the subwoofer which is about 16Hz. of course you can hear them and feel them.
Standing waves add as well as cancel at some frequencies. Most low frequencies have the same phase all over the cabin of a car so they do not cancel.