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-   -   Input protection diodes: More harm than good? (http://www.diyaudio.com/forums/solid-state/133442-input-protection-diodes-more-harm-than-good.html)

MJL21193 19th November 2008 03:22 PM

Input protection diodes: More harm than good?
 
2 Attachment(s)
Is this type of protection detrimental to sound quality? I would like to employ it in a project to make it as "dummy" proof as possible.

DigitalJunkie 19th November 2008 03:37 PM

Re: Input protection diodes: More harm than good?
 
Quote:

Originally posted by MJL21193
Is this type of protection detrimental to sound quality?
Only if your signal goes above 0.7V or so. :D :spin:

AndrewT 19th November 2008 03:38 PM

if that's good enough for many opamps, then why should be a problem with a power amp?
What diode characteristics are required to give good audio performance?

Extra diodes to prevent the input signal, in common mode, going above a limiting window voltage might be even better protection. There are a few schematics on this Forum showing this.

darkfenriz 19th November 2008 03:40 PM

It should not really affect the sound, but isn't really needed too unless you want to severly overdrive the circuit, like having an input voltage around 7-10V or so, so that transistor b-e diode must work at zener range.
Such diodes are used in op-amps which is good, because an op-amp does not have to work as a linear device with negative feedback.

Adam

AndrewT 19th November 2008 03:41 PM

Re: Re: Input protection diodes: More harm than good?
 
Quote:

Originally posted by DigitalJunkie
Only if your signal goes above 0.7V or so.
unlikely except on very fast slope test signals.
This is the same as an opamp and controlled by the same rules.
The output tries to ensure that the difference in the two input signals is zero. This is usually <10mV for all audio signals.
The protection is there for non-audio signals and/or faults.

MJL21193 19th November 2008 04:31 PM

Re: Re: Input protection diodes: More harm than good?
 
Quote:

Originally posted by DigitalJunkie


Only if your signal goes above 0.7V or so. :D :spin:


Hi,
Putting 2 or more in series would increase the input threshold, no?

I want to guard against "hot swapping" of input cables and other silly things that can take place when my back is turned.

lineup 19th November 2008 04:46 PM

as darkfenriz tells
- they are only needed in very special situations
- for most normal audio amplifiers you can do as I do = no diodes
- for commercially sold op-amps, they never know what user will try them for
- so is more a very worst case thing for avoid customer conflicts, just in case

PMA 19th November 2008 04:46 PM

You should rather use reverse biased, connected to zener defined voltage e.g. This shown here is no good.

myhrrhleine 19th November 2008 06:22 PM

Re: Input protection diodes: More harm than good?
 
Quote:

Originally posted by MJL21193
Is this type of protection detrimental to sound quality? I would like to employ it in a project to make it as "dummy" proof as possible.

only detrimental to purists.
it won't hurt.
go for it

destroyer X 19th November 2008 07:07 PM

Has no harm...only special cases when you have strong broadcast stations into your
 

surroundings....your neighborhood.... magnetic field from RF will be detected and can overdrive your input transistor... input may turn near saturation...noises will be captured and may pass to second stage.... you may read AC voltage there with 1 volt or more... the transistor will show saturating voltage from colector to emitter...sometimes happens.

But this is very rare to happens...only when you have a Radio Amateur with a powerfull station near your home...or AM/FM broadcasting.... Television Stations or Police Radio Transceivers, Radars and those equipments that use Radio Frequency (cell phones are too much weak to bother...you have to touch the input diodes with your cell phone aerial (antenna) to obtain interferences.

Cell phones use to send signal from time to time to the nearest reception station ... this signal can be heard into the amplifier when using high speed transistors and protective diodes into the input.

But i cannot imagine the idiot will put the cell phone over the power amplifier board inside the enclosure...hard to imagine the crazy guy doing that.

Well.... they have capacitances.... diodes have inductances and resistances.... of course may tune something.

So...no real problems.... only rarities will bother the one is using diodes..... just in case... i have never used... and just in case will never use them.

Carlos


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