24' mute Sw Wire possible on lm3886?

[petemoore]

  Mute on off works.  [pin 8>39k>V- = mute]
  Mute switch works.
  *Mute switch with a long [12' each way to switch] wire...does this work ?...anyone try it...or is it supposed to be short and close to the chip or something?
  I don't really understand what's going on with the mute switch and the current etc. [apparantly >5ma. current turns the amp on].
   And I snapped some bridge diiodes pop pop...
   So I fixed them, and decided to use a 22ga. 'wirefuse' in the mute switch path, a 5'' stranded thin wire to the insulated plastic jack, that wire fries when I try to engage mute, through a 22' long cable [an electrolux vaccum AC line].
  So...after the diodes and such, I'm a bit hesitant to put a thick wire on the mute...I don't know how much current goes through that wire, gotta be more than 5ma to burn up 22ga. stranded wire I'd guess.
  I had this idea of using this as a 'cameo' lead break amp, set up just for distortion [whatever else] lead guitar, and de-muted mainly for lead breaks....cool concept I think'd work out great except I cant get it to work.

[R.G.]

I would put an NPN transistor near the chip to turn it on and feed the base of that from the power supply, possibly with some diodes and resistors to protect the base from reverse voltages and overcurrents, that kind of thing.

Then I would run a long wire to the base of the transistor. Grounding (or connection to V-?) then mutes the amp. You can put a R-C network on the long wire and ensure that it does not change too fast or too far and damage the NPN, which is less delicate than the chip.

[petemoore]

  I would put an NPN transistor near the chip to turn it on and feed the base of that from the power supply, possibly with some diodes and resistors to protect the base from reverse voltages and overcurrents, that kind of thing.
  I would too. I didn't know if that^ wasn't like..already in the chip [which has QAFew features.
  Perhaps there's an example of this or a more indepth explanation I could find to help me with the details...
  I like the amp. Great tones, now it's a really nice 12'' speaker, combo amp with 'legless lean back' feature.
  I would use that long cabled mute switch, using it as a 'lead guitar amp', or stereo...as one of two amps, presettable, distortable etc. ..that would make a great on-stage guitar sound option...needs figured out by me first though.
  Hmmm....what should I google for...google suggestions welcome !
  ..me fear is enough [and already have memory of debug/damage assessments/repairs/snap-pop-smoke] of the actual simple mute switch wiring to venture into experiment with Q switching a current load on the actual amp.

[grapefruit]

If you follow the formula for calculating Rmute that's in the datasheet I can't see there being a problem.
As RG said, a PNP transistor would be best for remote switching. Have emitter connected to V-, collector to Rmute, and base pulled down to V- via maybe a 10k resistor. Have a base series resistor. Then the remote switch only has to pull the base to ground to turn the transitor on.

You don't want the high current V- supply going down the cable and into a footswitch. Too much opportunity for sparks to fly.

Cheers,
Stew.

[petemoore]

You don't want the high current V- supply going down the cable and into a footswitch. Too much opportunity for sparks to fly.
  K, sounds pretty good, I happen to have some MPSA92 PNP darlingtons on me. I should be able to wire that right up and test it out, thanks RG and Grapefruit !

[petemoore]

  MPSA92's [darlington PNP] emitter to V-, collector to pin 8 [mute], the base has a 8k2 to V-,  10ohm resistor on a wire.
  Amp sounds constantly, with base having 10ohm series resistance
liftable/connectable to V-, ...perhaps I fried the mute function on this chip, it used to fade quickly between mute and sound [vice versa] now it clicksks on and off using the short mute wire to V-.
  The transistor is 'connecting' V- to the mute [must be if the amp is sounding] but not switching [off?,,,] or switching the mute.
  Still it seems that if the mute functions with the short wire, a transistor switched mute should work.

[petemoore]

http://www.kpsec.freeuk.com/trancirc.htm
  Looks quite simple, splice a transistor in there, since it's V- I'm trying to switch to... using a PNP...
  Strung with emitter to the lower voltage V-, base to do the switching, and collector being the other side of the switch contact...[through 39k to pin 8]
  That page suggested 100k for base protection R, I'd used 56k...
  The data sheet [IIReading correctly] states that currents between .1ma and 5ma. from pin 8 causes muting. Perhaps a high current gain darlington is not a good choice?
  Sometimes it is the simple things that are very difficult to make work,
  more I shouldn't think about it [I shidn't wind-er], with PNP the emitter is usually on the V+ side of things..more positive than the collector...opposite of what I'm doing here, but it's not an AC gain stage.

[petemoore]

  If Idda read the transistor pages only, Idda started with an NPN.
  Because the emitter to V- means the collector would have to be more positive..just seems like an NPN would be feeling more at home than a PNP, which generally sees it's emitter being connected to V+ or a more positive voltage...of course I dunno and am mixing what I do know...gain stages...into it.
  I'll call Parts Express monday and see if the tech dept. can help. BTW Parts Express has sent me so many of these chips I didn't even count them, I wanted 3, I have 15+ of them... so far, averaging ~5 chips recieved every ~three days for the last week...real fast on the first batch delivery too.

[petemoore]

  Daft..I just re-read RG's post...
  It does stand to reason using an NPN would make sense, switching between V- to a higher voltage on collector.
  But using 2n3904 emitter to V-, base to V- through ~10k W/56k series resistor, collector to pin 8.
  I must be missing something.
  Base w/10k to V-:
C -1.3v
B -18.5
E -18.5
  base with [10k + 1.4ohm resistors parallel]
E 20.8
B 19.2
C 1.4
  Mute function works but not with the arrangement w/transistor I have.
 
 

[petemoore]

Grounding (or connection to V-?)
  Grounding [through a 56k] the base of the NPN transistor switch, fixes the amp mute function to permanently on, creates an new circuit [I'm glad it fried to 'on', stinky/scary, but better than 'off'].
  I'm certain a relay would be easier, more foolproof [testable, easier/possible for me to figure out what it'll make the chip see] and more reliable in my case, I'll try for that on the next amp, parts for that should be arriving tomorrow.
  I guess it cooled off, and the mute function returned !
  I hardwired the pin8 >39k >V- connection,
  I might try a relay there...why shouldn't I...what should I look out for...?
 

[grapefruit]

Of course you'd use an NPN transistor. Sorry for the bum steer there. Normally the base is pulled to V- but if you ground it the base is more positive and the NPN transistor turns on. Not sure why it's not working for you.

Check the VCE rating of the transistor. 40V. Should be ok.

There's a bit of info in this app note about the mute pin. Says it will discussed further in another app note but I couldn't find it. It mentions using a voltage regulator as a current sink to avoid problems with power supply sag changing the mute pin current.
http://www.national.com/an/AN/AN-898.pdf#page=7

It would be good if you can get it working with a transistor. A relay seems like overkill.

Stew.

[petemoore]

It would be good if you can get it working with a transistor. A relay seems like overkill.
  No problem Grapefruit. I make mistakes. That's sometimes exactly what makes me think about them.
  Relay, 110ac plug [old 2 conductor from an old whatever..., and a WW from a grab bag of 'em...the relay itself, and..that's about it. Wiring is real straightforeward, expensive, less reliable, heavy...and...not too big but 'bigger than'. Oh also takes more juice.
   But...it's done, and it works, but pops a little at turn on, perhaps another cap to fill there would help, the 100uf [pin 8 to V-] is totally imbedded, I can't pull it but I can parallel it...we'll see.
  Relay's cool, just hook yer 12v [9v adapter is 11.8v unloaded] >30ma adapter up and it's clik clik time. Choose the Nc [normally connected] or No [normally open] side, through to common, and that's it. I put it outside the heatsink, something about a 'ma' coil next to the amp...it's about 2 x 4'' of wire away.
  Now that I got around the transistor circuit, I'll try again to get through it. I have another amp hopefully about to start really getting built tomorrow.

[R.G.]

I didn't know if you saw this from your other LM3886 mute thread.

OK, I read the rest of your LM3886 stuff.

This isn't rocket science. We just have to cobble up some way to have pin 8 go from 0V (that is, at the middle of the power supplies) to having 0.5mA pulled out of it. The amp is muted if pin 8 is open (read that as "no current is coming out of it") and is on if you pull 0.5mA out of it.  What the spec sheet does not say directly, only indirectly, is that you have to pull pin 8 more than 2.6V negative for any current to come out of it.

So let's design circuits.

An NPN sitting at the - rail makes a good pulldown. All we need to do is make sure that it can withstand the full -V power supply between collector and emitter and stay off, and that it can pull 0.5ma when on. The critical factor there is the Vce being more than -V. As long as your power supply is only +/-18.5V, you can use a 3904, which is rated at 40V. If you were using a higher power supply, like +/- 40V, you'd need a higher voltage transistor. The higher voltage MPSA part works too.

So the NPN is tied to -18.5V. According to the datasheet, we need a resistor of Rm = |-Vee|-2.6V/Im = (18.5V-2.6V)/0.5ma =
31.8k. That's a maximum. So let's use 22K. The current pulled is then Im = 0.0007227A, or a bit more than 0.7ma. The transistor's gain will be low down there, but let's assume it has a gain of 25, so the NPN needs a base current of 29uA to fully saturate with a 22K resistor. How do we get 29uA to the NPN base?

Let's try through a resistor from ground. We need (18.5V-0.7V)/29uA = 614k. So any resistor less than 613K will supply enough base current from 0V (ground) to the base of the NPN to turn it on, and turn the amp on.

By using this circuit (NPN +22K and, say, 470K base resistor) we have converted the problem from how to switch pin 8 from 0V to -V through a resistor into a problem of how to either source or not source 30uA from 0V. The nice thing is that if we get it wrong, it blows the NPN, which typically shorts under fault, and the amp will fault to playing, not muting.

So how to provide 30uA? How about - a PNP? We tie a PNP in a strange way. Tie its collector to the 470K base resistor of the lower NPN, and it base to ground through a 33K resistor. The emitter is an input. With the emitter below 0.5V, nothing happens with the PNP, the NPN is off, and the amp is off (that is, no current comes out of pin Cool. When we pull the emitter of the PNP above 0.5V, a current flows through the base-emitter of the PNP, limited by the 33K resistor to 30uA per volt for all volts over 0.5V. We only one of these to make the NPN work, so raising the input above 1.5V turns the amp on, lowering it below 0.5V turns the amp off.

What we have done is convert a switch closure to -V into a positive-on voltage above 0V to turn on the amp. And we've introduces some limiting resistors. What happens if we feed the PNP emitter input +20V? The base emitter goes up to 20V, the current that flows there is 20V/33K = 606uA. Not exactly a killing kind of current. The PNP is saturated HARD. So the collector goes up to +19.5V, maybe. That means that the 470K base resistor into the NPN is letting (19.5 +18V)/470K = 80uA flow into the NPN base. Again, the NPN is not going to die with that happening. It just switches the pin 8 0.5m harder.  So the switching setup is relatively impervious to input faults.

A final fillip is to tie the PNP emitter to +V with a 100K resistor instead of accepting a voltage into a 33k resistor. Now the PNP is on (and NPN is on, and amp is on) UNLESS you pull the PNP emitter to ground with a switch. The switch can be anywhere physically, and it can have a 0.1uF cap hung across it to keep transients from messing with the circuit.

I recommend you do something like the NPN/PNP setup to do your on/off muting. It's cheaper and more reliable than the relay, which is after all a mechanical part.

[petemoore]

  Yes, and I copied it to 'additions to the FAQ' so I can re-read and try the calcs.
   'l Vee l'  [those aren't 'I' letters, they are a math symbol, I cant' find what they mean...voltage emitter e___?

[R.G.]

Sorry - those upright lines on both sides of a mathematical term mean "absolute value of..." and that means to take the magnitude of the number and make it positive if it was negative, leave it positive if it was positive. It's a shorthand way of saying "take the size of the thing, whether it's positive or negative, and just use the size, not the sign."

So |5| = 5. and |-7| = 7.