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Old 3rd November 2009, 05:03 PM   #1
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Default protection/limmit with ldr

ok got my hands on a few ldr ( first time been playing with this )

been reading rods idea about that and as it comes i have two options one is voltage sense and the other is current sense

voltage sense

i presume that this should be fairly simple a rectifier with filter a trimmer and a resistor a led and an ldr that will monitor how much voltage will be delivered from the output of the amplifier to the load and led .the trimmer will set the acting point of the led and then the limmit to the input .

Question is what happens if your load is not 8 ohms ? and or sudenly becomes lower or very lower like a short circuit ....

i presume that no led is going to lit and your amp goes boooom probably together with the speakers if no dc protection or crow bar is there .
then again if your load is 4 ohms and supposing that you have a realy rowbust psu then if you expect or wait the led to lit it might be too late for your speakers and your amplifier also

so this a serious drawback

curent sense

ok let us suppose that we monitor curent from any of the emmiter resistors of the output tranistors if set properly at a certain curent the circuit will apply limit to the input

that means that will also protect the amp from
A) very low load
B) short circuit
C) excesive curents if for some reason speaker is too reactive

now how fast will be the attack and release at peaks and how much of the "bench " power you will loose to keep the system in a safe operation area is another story

From the view that this is too be applyied to semiprofessional amplifier and not really hi end amplifier sounds way too perfect and too easy to be truth

Is there any other drawbacks involved that i ma not aweare off??? is there any more information that somebody can give me regarding this topology ???

regards sakis
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Last edited by east electronics; 3rd November 2009 at 05:08 PM.
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Old 4th November 2009, 08:13 AM   #2
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any ideas ????
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Old 4th November 2009, 10:52 AM   #3
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If the amplifier clips, there will be a difference in voltage between the input of the amplifier and the feedback node of the amplifier. It doesn't matter if the amplifier runs out of voltage (high impedance clip) or current (low impedance clip), there will be a difference in voltage between the input of the amplifier and the feedback node of the amplifier.

Use this to run the LRD as a voltage divider on the input to the amplifier. This is the way many amplifiers work, see schematics from McIntosh, QSC, Carver, etc.
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Old 4th November 2009, 12:00 PM   #4
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Is this for a party PA amplifier or one for home HiFi? For the former some kind of average signal regulating AGC circuit may be useful, but for HiFi amplifiers the idea has never made any sense to me.

The occasional clipping that may occur is not going to be avoided with an AGC circuit as they can’t respond quickly enough. This would be especially so with one implemented with and LDR, and more so again for an AGC loop frequency compensated such to be a contributor of minimal distortion.

A fast, active soft clipping circuit of some kind may plausibly be of benefit, but I’m not entirely convinced on that as here you are just sacrificing a degree of linear headroom for a less abrupt clip.

If you can hear an objectionable degree of clipping when listening to your favourite tunes, you have two real options as far as I can see:
1) turn down the volume
2) buy a more powerful amp
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Old 4th November 2009, 12:20 PM   #5
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Quote:
Originally Posted by djk View Post
If the amplifier clips, there will be a difference in voltage between the input of the amplifier and the feedback node of the amplifier. It doesn't matter if the amplifier runs out of voltage (high impedance clip) or current (low impedance clip), there will be a difference in voltage between the input of the amplifier and the feedback node of the amplifier.

Use this to run the LRD as a voltage divider on the input to the amplifier. This is the way many amplifiers work, see schematics from McIntosh, QSC, Carver, etc.


to be hones i dont reaky get it bet let us see one more time what i had in mind

this was the idea any way that the photo resistor is applying ground to signal in order to reduce the input signal ....

the question is if this photo resistor takes sense from any emmiter resistor of the amplifier and actually "watching " the current flow through this resistor ....

if you are watching current from this resistor gives you the bennefit ( i presume ) that will apply signal reduction in any case that current is exesive in this resistor no matter the cause

either overdrive
either too low load
either too reactive
either short circuit

the way i ve seen it if an amp is short circuited and there is no drive in the input or very low drive then may be outs are getting a bit warm but other than thna that nothing else is going to happen
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Old 4th November 2009, 05:21 PM   #6
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Default Here is what come up with

please take a look at this schematic ....give me a couple of secs to explain also the results

its applyied to an amplifier that produces arround 220 W @ 8 R probably almost double @4 ohm

probably it will not work with lower power and also with higer power there is also a chance that the led will need a resistor ....

this uses as a "pick up" my smps pick up coil ....yeap the one that has totally non linear behaviour versus frecuency

in this circuit its used to monitor how much current flows from the amplifier and apply limit to the input versus how many amps are drawn from the psu ....

as is this how it works

----- to drive the amp in a dummy load 8ohms and 100% of power you need drive in the input of about 1.2 volts

----just before you reach 800 millivolt you get a full bright led which means that you have a usable area to work with. Power at this point is arround 165w so before you end up to the 220 that the amplifier produces before cliping still you have a usable area to play with ....

----- then again if you make your load 4ohms you actually get a full bright led at arround 450 millivolts of drive which also provides a usable area to work with but also seems that the "limmiter" actually is auto adjusted to 8 or 4 or short circuit conditions ( makes sense since it monitors current draw from the amp )

----- and now the big issue !!!!! since it is very well prooven in previous related post and advices i was given by many forum members the blody smps coil is not linear ....
means that reacts like the above to a frequency of 200-1000-4000 hz and then after that becomes quiet sensitive which means also that if you manage to produce peaks at 10khz then of course this type of limmiter will shut you down ....

this is only bench tests with a dummy load and a signal generator ... i will produce one of this to use in small PA set to see how it works

of course the question remains how fast will this attack the input , how fast or slow will release it , and then how well will react in real life conditions , and music sources

one thing is for sure that if you power a three way active system and your high amp is equiped with this type of limmiter it will probably result that your high is going to be well ....limited ....

lets see
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Last edited by east electronics; 4th November 2009 at 05:26 PM.
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Old 4th November 2009, 09:07 PM   #7
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"The occasional clipping that may occur is not going to be avoided with an AGC circuit as they can’t respond quickly enough. "

Really?

The McIntosh Power Guard can limit a one cycle overdrive at 20Khz, isn't that fast enough?
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Old 4th November 2009, 09:29 PM   #8
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Well to throttle back a 20kHz transient of appreciable amplitude before it has a chance to actually clip you need a pretty fast attack time. Does that “Power Guard” circuit use an LDR? Got a schematic?
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Old 5th November 2009, 03:18 AM   #9
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"Superior, fuse-less short circuit protection is provided by the McIntosh Sentry Monitor circuit. Power Guard, an input/output comparator, responds within 1/1000 of a second to eliminate amplifier clipping."



Marketing mumbo jumbo. This is just a plain AGC circuit with an LDR/LED combo. Clipping has to actually occur before it can respond by reducing the average signal level. And that gives you amplitude modulation along with your occasional clipping - a cure worse than the disease for any HiFi amplifier.

Last edited by GK; 5th November 2009 at 03:31 AM.
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Old 5th November 2009, 07:11 AM   #10
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well gents ...beyond the titan fight on this or the other method of protection this generic design of course might have speed or distortion drawbacks but what can you expect from a "thingy" that is made of 3 -4 parts ....

regarding my above calculations though there is a very nice thing taken wrong :

my tests where done in a pure resistive load ..... in these tests it is prooven that the smps coil yet again has a non linear behaviour and acts better/faster in high frequency

in this case if you keep in mind that the real load of amplifiers tends to increase resistance versus frecuency but also the limmiter tends to increase sensitivity versus frecuency....

then after all i may end with a very fuc****ing linear application ...

will work on this an come up with a chart
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Last edited by east electronics; 5th November 2009 at 07:17 AM.
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