Author Topic: another BJT mute circuit  (Read 320 times)

bartimaeus

another BJT mute circuit
« on: January 21, 2021, 08:41:01 PM »
i'm trying to adapt one of rod elliott's mute circuits for guitar. the circuits there are all listed for +12V-15V, so i'd really appreciate if someone could tell me whether this'll work for +9V. i'm partly confused by the fact that he uses a reversed BJT in one example (https://sound-au.com/articles/muting.html) but not the other (https://sound-au.com/project147.htm).

i'm hoping this won't add noticeable distortion to a guitar signal at this voltage level. i know (from reading this old thread) that a jfet shunt would introduce less distortion when fully on/off, but i don't want to add a max1044 or similar just to get a negative control signal for the jfet.

i'm also curious if anyone has tips for selecting the cap values to get ~20ms smoothing (assuming i even put the caps in the right spot).

.

Rob Strand

Re: another BJT mute circuit
« Reply #1 on: January 21, 2021, 09:51:40 PM »
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i'm also curious if anyone has tips for selecting the cap values to get ~20ms smoothing (assuming i even put the caps in the right spot).
At Q3 and Q4 there's nothing to really *fully* discharge the caps.  You could add a pull-down resistor but that might give rise to distortion.  Perhaps a better place for the cap is across BE of Q2 since it has R5 to discharge the cap.   It also means you only need one cap.   
   
The load on Q2 is only the two 4.7k's so you could increase the values of R3, R4, R5, R6 to say 47k.     The higher resistance will let you use a smaller cap.     Very roughly you want 20ms = (47k/2) * C, which is about 850nF, still not small.

If you want to speed-up the mute-on time you would need to used a series diode + resistor across R6.

A spice sim would confirm all is good and the time constants are OK.
The internet:  answers without the need for understanding.

bartimaeus

Re: another BJT mute circuit
« Reply #2 on: January 21, 2021, 11:47:41 PM »
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i'm also curious if anyone has tips for selecting the cap values to get ~20ms smoothing (assuming i even put the caps in the right spot).
At Q3 and Q4 there's nothing to really *fully* discharge the caps.  You could add a pull-down resistor but that might give rise to distortion.  Perhaps a better place for the cap is across BE of Q2 since it has R5 to discharge the cap.   It also means you only need one cap.   
   
The load on Q2 is only the two 4.7k's so you could increase the values of R3, R4, R5, R6 to say 47k.     The higher resistance will let you use a smaller cap.     Very roughly you want 20ms = (47k/2) * C, which is about 850nF, still not small.

If you want to speed-up the mute-on time you would need to used a series diode + resistor across R6.

A spice sim would confirm all is good and the time constants are OK.


thank you very much for your advice!! you made me finally learn how to use ltspice haha!

here's the circuit and simulation. the red line is at the base of Q2. i guess the voltage divider of R5 and R6 didn't work the way i guessed.

looks like it smooths the mute-off more than the mute-on. maybe that's what the series diode would solve? though as long as it's clickless, i don't really mind the asymmetry to be honest.


Rob Strand

Re: another BJT mute circuit
« Reply #3 on: January 22, 2021, 02:26:29 AM »
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thank you very much for your advice!! you made me finally learn how to use ltspice haha!
You did well.   It's a good tool to have up your sleeve.


Quote
here's the circuit and simulation. the red line is at the base of Q2. i guess the voltage divider of R5 and R6 didn't work the way i guessed.

looks like it smooths the mute-off more than the mute-on. maybe that's what the series diode would solve? though as long as it's clickless, i don't really mind the asymmetry to be honest.
Because the Q1 transistor goes open it removes R6.

These circuits are a little delicate to get working.  A bit harder than having the caps on the mute transistors.
The diode+resistor thing won't work.   You need to increase R6 to say 100k, that slows down the turn on.
The thing you have to watch out for is to make sure Q2 has enough base current (I think it does).   However you don't want too much base overdrive. If you try to go for too much base overdrive you can get unnecessary delays in the switching.
It's a fine balance.
     
« Last Edit: January 22, 2021, 08:34:21 PM by Rob Strand »
The internet:  answers without the need for understanding.

bartimaeus

Re: another BJT mute circuit
« Reply #4 on: January 24, 2021, 05:43:38 PM »
thank you again for your help! adjusting R6 to 330k gave me a very good balance in simulation. haven't had a chance to see if it'll work in the real world, but i'm sure i'll be able to figure it out now :)