Input protection diodes: More harm than good?

[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.

 

[DigitalJunkie]

Is this type of protection detrimental to sound quality?

Only if your signal goes above 0.7V or so. 😀

 

[AndrewT]

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]

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]

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

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]

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

Only if your signal goes above 0.7V or so. 😀

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]

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]

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

 

[myhrrhleine]

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]

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

 

[MJL21193]

Ok, I feel a little silly (nothing new there). This will only protect against reverse biasing of Q2, and does not limit input voltage.
I tried it in a real amp to see how it would work and found that it didn't do as I expected and did some more reading on the subject.

 

[lineup]

... isn't really needed too unless you want to severly overdrive the circuit, ...

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.

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

The above is the truth 😉

And this is why we do not see it in any Diy Audio amplifiers.
Not by Self, Not by Pass, Not by AKSA. Not by Curl.

If you intend to start making IC op-amps, for the market.
Well, then ....

 

[MJL21193]

The above is the truth 😉

And this is why we do not see it in any Diy Audio amplifiers.
Not by Self, Not by Pass, Not by AKSA. Not by Curl.

If you intend to start making IC op-amps, for the market.
Well, then ....

Well then...why didn't you reveal the exact purpose of this clamp? Anyone can say "it's not needed" without specifics.
Maybe you too didn't know exactly.

 

[Steve Dunlap]

The purpose of the diodes is to prevent the input transistors from being reverse biased and possibly damaged. I believe the diodes go from the input to the supply voltages. They only conduct if the input exceeds the supply voltage and should have no effect on the sound.

Steve

 

[Bonsai]

The diode s will limit th e differential voltage between the - and + input from exceeding 0.7V.

The diodes are particularly effective in protecting the input from differential and common mode overvoltage in the inverting mode.

However, in the non-inverting mode, they only offer differential signal overload protection. To offer commin mode overloa d protection, the diodes would have to clamp to some voltage within th e common mode input range.

My take on this:-

For normal audio power and pre-amp applications - not really needed.

If on the other hand, you ar e designing a gigh gain pre-amp for say a MC that is feeding some expensive dual transistor or FET's, then I'd clamp the input to 0V with two back to back diodes. This is what I've done on my current MC preamp - as a precatuion.

Hope this helps.

 

[lineup]

Well then...why didn't you reveal the exact purpose of this clamp? Anyone can say "it's not needed" without specifics. Maybe you too didn't know exactly.

It is so obvious to me, MJL21193
Why tell what everyone ought to be able to see and understand 😉


In audio amp the negative feedback will strive to
keep both input transistors at exactly same voltage.

This keeps all the time both input in good and close balance.

Suppose you use op-amp as a comparator. Without feedback.
You compare one voltage level with another level.
At a certain level, the output will shift from full positive to full negative.

Now if you have high voltage at one input
much higher than the level you compare against. At the other input.
Then the base-emitter voltage for one input transistor can be very high and for the other can even be a high reverse voltage.

Two diodes in opposite directions will give some protection against such things.
Overload and break-down or 'overdrive', as darkfenriz said.
So unless you really do some real foolish, stupid things with your audio amplifier,
you need not diodes.

 

[MJL21193]

It is so obvious to me, MJL21193
Why tell what everyone ought to be able to see and understand 😉

Of course. 😉

I do things differently: If I know the exact function and purpose of something, I'll say it. Once the person has an understanding of the matter, he is free to decide whether to use it or not.