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Class D Switching Power Amplifiers and Power D/A conversion

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Old 22nd January 2011, 12:22 AM   #11
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Originally Posted by AP2 View Post
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.
If you attenuate the input signal, don't you think that will restrict the amp's maximum output?

Adjusting input gain gets my vote too.
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Old 22nd January 2011, 05:17 AM   #12
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Originally Posted by 454Casull View Post
If you attenuate the input signal, don't you think that will restrict the amp's maximum output?

Adjusting input gain gets my vote too.
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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Last edited by Johnny2Bad; 22nd January 2011 at 05:46 AM.
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Old 22nd January 2011, 06:00 AM   #13
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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.

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Old 22nd January 2011, 07:49 AM   #14
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Quote:
Originally Posted by 454Casull View Post
If you attenuate the input signal, don't you think that will restrict the amp's maximum output?

Adjusting input gain gets my vote too.
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
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Old 22nd January 2011, 08:56 AM   #15
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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.
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Last edited by Eva; 22nd January 2011 at 09:13 AM.
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Old 22nd January 2011, 09:36 AM   #16
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I agree with Eva, the good-old lightbulb is perfect solution. I remember all the JBL Control speakers flashing from inside in the rythm of music
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Old 22nd January 2011, 09:59 AM   #17
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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.
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Old 22nd January 2011, 10:06 AM   #18
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Good old Rod Elliott also has something that would work. See < Ultra Simple Bass Guitar Compressor >
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Old 22nd January 2011, 11:26 AM   #19
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You don't need to limit the output at all. With music signals the average output of the amp is around 2W at highest possible output. The only thing you should be careful of is to turn down the volume if it starts sounding distorted.
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Old 22nd January 2011, 11:55 AM   #20
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When there's kids involved you always have to limit the power.
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