Mute Point

[Anthony Dockril]

Hi,

I’ve been trawling through the archives looking at posts about the mute resistor and if it had any bearing on sound quality. There is not much here even though this resistor supplies current to the input stage so you would think this could be an area to improve all Lm chips that have a mute function.

One post that caught my eye was by Xelb.

“After trying a few resistors I tried the -Vcc connected directly to the mute pin
And yes, this is another step forward in the LM3886 implementation ”

Has anyone else tried this? At first this sounds wacky but if the mute function has a built in voltage/current protection then this might not only work but also may be step forward removing one more unnecessary component from the system.

Does any one else have any experience with this idea?

Cheers,
Anthony

[Jay]

After reading Xelb, I really wanted to calculate the exact (or maximum) current drawn from the mute pin, but priority went to other projects.

Xelb uses too high a supply rail but that doesn't trigger the protection. So may be the bias for input is reduced due to the removing of mute resistor and affects the allowed maximum supply rail??? Just a curiousity.

[sangram]

HI

The way I see it is that the mute resistor does not affect the bias of the input stage.

Far as I can tell it switches off a transistor that sinks the input stage curent, thus switching off the input stage amplification and attenuating the output. I'm sure more experienced people will have a better technical explanation.

As for connecting the pin directly to the rail, yes there is a 1K resistor internally connected to this transistor, which I believe will limit the current passing through the sink.

That said, National would have considered connecting it directly to rail and I'm sure there is a good reason for the resistor, however one can go as low as possible, I would consider about 10 mA the upper limit, about 5mA is OK as a base calculation for the resistor.

Again, feel free to try it. The worst is the chip will fail (not likely IMO) and you'll need a new one.

I don't know if the protection is at all linked to the input stage, only the output transistors. I am using too high a supply rail in almost all my projects (over-zealous transformer specs, + poor regulation in transformers) and I don't manage to get the protection to kick in. Those tolerances are accounted for by manufacturers.

[wintermute]

My guess is that the external resistor is there to slow down the charging of Cm so that the time delay for turn on is maintained

Tony.

[Anthony Dockril]

My guess is that the external resistor is there to slow down the charging of Cm so that the time delay for turn on is maintained

Yes the resistor is there to slow down the charging and thus give a delay but I guess the question is that does this configuration have some impact on the sound of the chip. I think many aspect of Gainclones have been thoroughly explored but is this one area that some improvement may be made?

[peranders]

Originally posted by wintermute
My guess is that the external resistor is there to slow down the charging of Cm so that the time delay for turn on is maintained

You ought to read the datasheet first This resistor is for NOT muting the amp. In other words: You must always have it. As a side effect you can get a delay by adding a cap. If it's OK by shorting the National would surely have mentioned it, now they haven't. I would not do as XELB if this design was to be commercial unless National garanteed a reliable operation. Remember also that National only shows a simplied circuit of the design.

If you choose 10k you don't have to worry. I have taken a bit more because I wanted more delay with 100 uF.

[wintermute]

I know that the resistor is not for muting the amp without it the amp (or at least a connection to the +ve rail) the amp will allways be muted.

Think about what the capacitor Cm (as stated in the datasheet "to create a large time constant for turn on and turn off muting) does. While it is charging there is no voltage getting to pin 8 to turn off the mute, if you don't have the resistor this cap will charge very quickly.

It is my understanding that this cap is there to ensure the amp remains muted until it reaches a stable operating state so you don't get any nasty noises on your speakers when you first turn on the amp.

Now do you still think I haven't read the datasheet peranders

Tony.

[peranders]

I'll believe you but the mute is more for eliminating other things that might casue thumps and/or clicks. The chip itself has a silent turn on even without the delay. It might be good to have a delay if you have a single power supply ( not plus minus).

[sangram]

Cm is also optional (I don't use one, but that's more because I use P2P and every added component is a pain...

BTW I'm using LM4766 and 4780, so less components are better as those pins are too tight otherwise...