I am building 2x15W amplifier based on TA2024. I am also building small Munny speakers based on 1" full-range Tang Band drivers to go with the amp. The drivers are only 4W RMS and 8W max so the amp is way too powerful for them.
Is there an easy way to limit the output power of the amplifier so it does not exceed the speaker specs? This setup is going to my kids room so I am afraid that they may crank the volume all the way up destroying the speakers.
Thanks.
Is there an easy way to limit the output power of the amplifier so it does not exceed the speaker specs? This setup is going to my kids room so I am afraid that they may crank the volume all the way up destroying the speakers.
Thanks.
The dynamics of the sound is not controllabie, whay do you think that just may break the clip?
If the amp is more powerful, only output current you can limiter.
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At 4W per speaker unless you've greatly reduced sensitivity for bass extension the reason I'd be limiting the volume is ear protection instead of speaker damage. (Teach your kids to listen at 80dB max.)
A 10k resistor in front of the 500ohm potentiometer will reduce max output power by 26dB if you don't want to play with the gain resistors.
A 10k resistor in front of the 500ohm potentiometer will reduce max output power by 26dB if you don't want to play with the gain resistors.
To really protect a speaker, the two limiting factors are cone displacement and voice coil heating.
Limiting cone displacement involves calculating instantaneous cone displacement (2nd or 4th order filter), feding the signal to a peak hold circuit and using a controlled high pass filter for attenuating low frequencies progressively when a certain displacement threshold is exceeded.
Limiting voice coil heating involves measuring rms current through the coil, integrating it over time, comparing the result to a threshold (P=i^2*R) and using a controlled gain stage to reduce signal gain progressively when the threshold is exceeded.
Obviously, this is not worth the effort for a small driver. The light bulb solution is a good inexpensive approximation because filament resistance is low at low currents and high at high currents and it does not react instantaneously to overload, allowing for large but harmless audio transients but not for dangerous sustained overload.
The optimum type of lamp may be found by trial and error.
Limiting cone displacement involves calculating instantaneous cone displacement (2nd or 4th order filter), feding the signal to a peak hold circuit and using a controlled high pass filter for attenuating low frequencies progressively when a certain displacement threshold is exceeded.
Limiting voice coil heating involves measuring rms current through the coil, integrating it over time, comparing the result to a threshold (P=i^2*R) and using a controlled gain stage to reduce signal gain progressively when the threshold is exceeded.
Obviously, this is not worth the effort for a small driver. The light bulb solution is a good inexpensive approximation because filament resistance is low at low currents and high at high currents and it does not react instantaneously to overload, allowing for large but harmless audio transients but not for dangerous sustained overload.
The optimum type of lamp may be found by trial and error.
I agree with 454, input gain adjustment is the best option, since you won't be employing a "wasted current" solution; ie the others that use power unnecessarily (essentially, wasting it as heat). Also, whatever desirable characteristics the amplifier has will be preserved; ie it will still be able to provide clean transients, etc. A simple resistor of the appropriate value (easily implemented with a switch so that the "regular' speakers could also be used at any time), the input resistor does not have to dissipate high power vs a resistor at the output side, it's easy on the source, possibly lower system noise, simple and clean solution, etc.
That's pretty much a description of a true audio muting switch, found on so many products at one time, which drops level by, say, 20 dB, which is handy on any decent audio system, assuming you own a phone or need to hear during the party if that really is the cops at the door responding to the neighbour's complaints. The television remote has bastardized that control so that everyone now thinks "mute" on an audio component is supposed to mean "silent". Fine for TV, but wrong, so wrong, for music. Probably should put a *rant* warning in there.
Broadly speaking with Class D THD levels are improved at lower amplifier output as well.
Using the volume pot with the typical source (CD player will be higher in output level than most other devices) and connected to your "little" speakers, determine the appropriate level, turn the amplifier off, measure the resistance at the volume control, Bingo, there's your value that should provide for good use of the volume control below your threshold.
Having said that, the other solutions will work, including doing nothing and using common sense, same as you have to do with conventional speakers.
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My 2c worth.... get some polyswitches put them in between the speaker and the amp and forget about it 
Note you will need to work out what size polyswitch is appropriate (and if you undersize it will have the advantage that they can't play it too loud!
Having an amp that is rated at a higher wattage than your speakers in most cases will make it less likely that the speakers will blow IMO. Mainly because it is clipping (within reason) that is a speakers worst friend, and having a more powerful amp will mean it will take longer before it clips, occasional transients above the speakers rated power are much less likely to kill it than the distorted sound that you get when over driving an underpowered amp.
Using a polyswitch will allow the transients to be played but as soon as any clipping or continuous over current occurs the sound will just stop. The kids will learn very quickly what point they can turn it up to before it goes quiet
I used to get requests to take my amp and speakers to parties (20 odd years ago). There was invariably someone who wanted it louder than the mark on the volume control that was not to be passed... pop would go a tweeter or two. Once I found out about polyswitches never had the problem again Many years later I was testing the amp, and found that the magic line I had placed on the volume control was in fact the point where the amp would start to clip with my CD player as input.
Tony.
Note you will need to work out what size polyswitch is appropriate (and if you undersize it will have the advantage that they can't play it too loud!
Having an amp that is rated at a higher wattage than your speakers in most cases will make it less likely that the speakers will blow IMO. Mainly because it is clipping (within reason) that is a speakers worst friend, and having a more powerful amp will mean it will take longer before it clips, occasional transients above the speakers rated power are much less likely to kill it than the distorted sound that you get when over driving an underpowered amp.
Using a polyswitch will allow the transients to be played but as soon as any clipping or continuous over current occurs the sound will just stop. The kids will learn very quickly what point they can turn it up to before it goes quiet
I used to get requests to take my amp and speakers to parties (20 odd years ago). There was invariably someone who wanted it louder than the mark on the volume control that was not to be passed... pop would go a tweeter or two. Once I found out about polyswitches never had the problem again
Many years later I was testing the amp, and found that the magic line I had placed on the volume control was in fact the point where the amp would start to clip with my CD player as input. Tony.
Hi,
set the gain (volume) is not stable, the power will depend on the source and dynamics of sound. bulb instead provides excellent protection independent of the level of the source. Also, it is able to change the "R" automatically as a function of output current. the sound is perfect and you can exploit the dynamics of the amplifier.
Regards
Limiting the gain is useless. The extra gain is required because different recordings may vary in level as much as 10dB.
Automatic signal-dependent gain control is a must, and the light bulb is the simplest implementation I know.
The last time I blew a pair of speakers (woofers) due to overpowering was 6 months ago in a party near a river. The amplifier was capable of over 1200w and the speakers were rated 300W each and 15 inch size. I had it under control during hours, but at some point I raised the volume to play a MP3 folder with low signal level and later someone changed the music to another folder with loud songs when I was away. It happened very quickly, I wasnt able to reach the system and reduce volume in time.
Automatic signal-dependent gain control is a must, and the light bulb is the simplest implementation I know.
The last time I blew a pair of speakers (woofers) due to overpowering was 6 months ago in a party near a river. The amplifier was capable of over 1200w and the speakers were rated 300W each and 15 inch size. I had it under control during hours, but at some point I raised the volume to play a MP3 folder with low signal level and later someone changed the music to another folder with loud songs when I was away. It happened very quickly, I wasnt able to reach the system and reduce volume in time.
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You could also use a compressor (AGC) circuit before the output stage. Similar to this one < Automatic Gain Control – Volume AGC > (ex Elektor I think). You set it for max output and leave it. It will introduce a little distortion, but in its intended use that wont matter.
Tony.
Tony.
Good old Rod Elliott also has something that would work. See < Ultra Simple Bass Guitar Compressor >
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