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Old 24th January 2011, 05:40 PM   #31
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Originally Posted by macboy View Post
the obvious (to me) answer is to lower the rail voltage to the chipamp. Reducing it to about 2/3 will give less than half the unclipped output power.

Another way is to put a power resistor in series with each speaker. This makes a voltage divider with the speaker. Half the voltage gives 1/4 the power. This will affect the frequency response of the speaker, if the speaker impedance is not flat across frequencies (it never is). Given the lo-fi nature of this sytem, it should not be a problem.
Lowering the voltage rails would work. The only reason to consider not doing so, and limiting the input sensitivity instead, is that most Class D amplifiers have better distortion characteristics at all output levels when supplied at near optimum voltage rail levels. With more conventional amplifiers, eg Class AB, that would be less of a problem in many cases.
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Old 24th January 2011, 06:20 PM   #32
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I think the series light bulb idea is nothing but a compressor with a lot of distortion and will seriously affect the frequency response by increasing the impedance driving the speakers and in a very non-linear way. A series power resistor is a much better way IMHO.

But nobody mentioned a parallel resistor. By placing about a 4 ohm resistor in parallel with the 8 ohm speaker the resistor will consume 2/3 of the power (the speaker and resistor will see the same voltage) and the amp will be forced to current limit if you attempt to exceed the rating of the amp, thus protecting the speakers. This assumes overcurrent protection or clipping of the amp to limit the power.

This method wastes power but it won't screw up the frequency response (assuming the amp can drive 3 ohm loads) and it won't distort like the light bulb.

A light bulb is good for a compressor, but even then sized too small and it will sound horrible. Sized too big you have no real protection. Finding the perfect bulb will be very difficult IMHO. If you want to make a limiter using a light bulb you really need an adjustable output to size the output power to the light bulb, not the other way around. I'll bet you either end up with distorted sound or no effective protection unless you're very lucky.

I've been playing around with incandescent compressor/limiters and it's tricky to get it right. Using only an op-amp and a flashlight battery can yield a useable compressor, but the signal amplitude has to be just right. There's also the problem of more bass compression than treble. The filament has a thermal time constant and won't respond to fast transients as well as it will to bass, so the sound can get harsh without a treble pre-emphasis/de-emphasis before and after the bulb so that the treble has enough energy to heat up the filament.

But for limiting speaker power, ditch the light bulb IMHO. A fuse would be your best bet if you want to keep it low tech, but you'll need spares. My experience is that the sound will degrade and distort well before the peak power in the speaker is reached if the bulb is sized to protect the speaker.

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Old 25th January 2011, 07:14 PM   #33
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Quote:
Originally Posted by sampleaccurate View Post
I think the series light bulb idea is nothing but a compressor with a lot of distortion and will seriously affect the frequency response by increasing the impedance driving the speakers and in a very non-linear way. A series power resistor is a much better way IMHO.

But nobody mentioned a parallel resistor. By placing about a 4 ohm resistor in parallel with the 8 ohm speaker the resistor will consume 2/3 of the power (the speaker and resistor will see the same voltage) and the amp will be forced to current limit if you attempt to exceed the rating of the amp, thus protecting the speakers. This assumes overcurrent protection or clipping of the amp to limit the power.

This method wastes power but it won't screw up the frequency response (assuming the amp can drive 3 ohm loads) and it won't distort like the light bulb.

A light bulb is good for a compressor, but even then sized too small and it will sound horrible. Sized too big you have no real protection. Finding the perfect bulb will be very difficult IMHO. If you want to make a limiter using a light bulb you really need an adjustable output to size the output power to the light bulb, not the other way around. I'll bet you either end up with distorted sound or no effective protection unless you're very lucky.

I've been playing around with incandescent compressor/limiters and it's tricky to get it right. Using only an op-amp and a flashlight battery can yield a useable compressor, but the signal amplitude has to be just right. There's also the problem of more bass compression than treble. The filament has a thermal time constant and won't respond to fast transients as well as it will to bass, so the sound can get harsh without a treble pre-emphasis/de-emphasis before and after the bulb so that the treble has enough energy to heat up the filament.

But for limiting speaker power, ditch the light bulb IMHO. A fuse would be your best bet if you want to keep it low tech, but you'll need spares. My experience is that the sound will degrade and distort well before the peak power in the speaker is reached if the bulb is sized to protect the speaker.

Hi at All,
I think you mix the use of the bulb (old school) can be used as compressor, with effective behavior of the bulb used in output.
yes, R can change,but between 1 and 2R max. Just the slow response, in this case to reduce the power to the speaker. (the peaks are not dangerous) but prolonged period of power. 1 or 2 R, does not cause any distortion, whereas it changes only when the power is still high.
(would be worse if he changes very quickly) because listening to the amplifier constantly has different characteristics from 4R. for small corrections to power or to protect the speaker, the equivalent would be a very complex circuit (if it is to limit the output current) with low distortion introduced). (Is very important to chose precise current of bulb)
for recording levels, ...today are very various.

Regards

Last edited by AP2; 25th January 2011 at 07:16 PM.
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Old 25th January 2011, 08:12 PM   #34
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Originally Posted by AP2 View Post
(Is very important to chose precise current of bulb)
for recording levels, ...today are very various.

Regards
That's my main point. Choosing the bulb for optimal performance is tricky - good luck.

I've tried this method and the results I got were poor. Perhaps I was too conservative in choosing the lamp rating and should have used a higher power lamp.

I've also experimented with line level devices that use a flashlight battery in the same way but with a purely resistive load. The drive can (and subsequently the amount of compression) be adjusted. It's an unusual type of compression. I find for mixes it doesn't sound too good but for some solo instruments it can sound good.

This is not the same as the "optical" compressors that use lights or an LED to illuminare an LDR. I'm talking about compressors that function solely by using the filament resistance of an incandescent bulb. They DO work!
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