Mute Circuit

[anti-idiot]

Hi everybody

I was analizing some Mesa/Boogie schematics (Dual Rectifier and Lonestar) and I came across this circuit:

http://i20.photobucket.com/albums/b228/Anti-Idiot/Mute-Circuit.jpg

I know what it does: it mutes the preamp for a fraction of time while the relays are switching. It uses P-Channel FETs for muting the preamp (J175 in strategical locations: reverb, fx loop, preamp, etc). basically, it's a "pull-down resistor". That's what it does.

Now, what i wanna know is how does it work? The reason is, my "Multiple Preamplifier" switches Gain and Tonestacks, and wanna get rid of the pops, but don't wanna add some 2M2 or 10M resistors everywhere (considering that i'm using 6 relays).

Can anybody explain me how does it works? I know its a pulse mute.


Here's the schn. of a Dual Rectifier w/3 channels. It has "mute pulse" @ preamp, fx-loop and PA input.

http://www.schematicheaven.com/boogieamps/boogie_dualrectifier_3ch_solo_head.pdf

[GibsonGM]

Looks to me that when the relays become energized (when the diodes exceed a threshold of conduction), the transistor in the mute circuit shunts the signal to ground, silencing any switching noise. Since the semiconductor will respond faster than the mechanical relay, the muting occurs faster than the "pop" can happen. 

[anti-idiot]

but how does it "un-mutes" the signal so quickly? 'cos this muting only takes a fraction of a second

thanks for your answer.

[GibsonGM]

Once the transistors aren't 'turned on' anymore, the signal is free to travel its normal path.  I'm not seeing where the signal comes in/out of that schematic, but the instant (speed of light) that it's "free", the signal can just go where it is supposed to go normally...

[anti-idiot]

Once the transistors aren't 'turned on' anymore, the signal is free to travel its normal path.  I'm not seeing where the signal comes in/out of that schematic, but the instant (speed of light) that it's "free", the signal can just go where it is supposed to go normally...

http://www.schematicheaven.com/boogieamps/boogie_lonestar.pdf

even when the switch stays on, the mute only works for an instant. can you take a look at the schematic?

[MetalGuy]

The collapsing magnetic field in the relay's coil produces and impulse which pulls transistor's collector load (1M) to ground which makes the P-channel FET (J175 or J174) close thus connecting the signal to ground for a while (usually for the duration of relays' pops and clicks).
Note however that for this trick to work both ways there always must be at least one relay switching ON and one switching OFF.
Most of the amps nowadays especially high gain ones have mute circuits because in certain locations relay pops are practically unavoidable.

[anti-idiot]

The collapsing magnetic field in the relay's coil produces and impulse which pulls transistor's collector load (1M) to ground which makes the P-channel FET (J175 or J174) close thus connecting the signal to ground for a while (usually for the duration of relays' pops and clicks).
Note however that for this trick to work both ways there always must be at least one relay switching ON and one switching OFF.
Most of the amps nowadays especially high gain ones have mute circuits because in certain locations relay pops are practically unavoidable.

now i get it. during the quicky change of voltage, the transistor shunts the signal to ground. i'll try to adapt it to the switching circuit i have (from Groove Tubes Trio)

[chi_boy]

I can't offer any real help, but I do have a Mesa Subway Rocket Reverb that has a similar feature.  I'm pretty sure there is an LDR involved somewhere too.  At least on the Rocket.  If there is an LDR involved you should see it in the "Switching Matrix" schematic.

[anti-idiot]

I can't offer any real help, but I do have a Mesa Subway Rocket Reverb that has a similar feature.  I'm pretty sure there is an LDR involved somewhere too.  At least on the Rocket.  If there is an LDR involved you should see it in the "Switching Matrix" schematic.

well, some amps used LDR and others used relays. i think the dual rectifier (2 channels) used LDR while the "3 channels" used relays. the reason behind my selection was that i will use relays for the internal switching (3 channels, 2 bypassable stages, indepent gains-tonestacks-volumes). M/B switching matrix is pretty much complex for an amateur like me.

tell me more about your amp, i guess it doesn't pop like crazy. how many channels?

[MetalGuy]

I can't offer any real help, but I do have a Mesa Subway Rocket Reverb that has a similar feature.  I'm pretty sure there is an LDR involved somewhere too.  At least on the Rocket.  If there is an LDR involved you should see it in the "Switching Matrix" schematic.

This mute circuit works only with relays and there is no LDR involved. You can use however and LDR as muting element (after minor modification) instead of J175  in places where the signal amplitude is higher otherwise it doesn't make sense because of the price difference.

[anti-idiot]

Found this:

http://www.google.com/patents/about?id=e9sOAAAAEBAJ&dq=6,621,907

[Nasse]

http://www1.silonex.com/audiohm/softswitch.html

Some years ago I did two ldr shunt attenuators in series and it was ok for communications, smooth as baby´s ass, used just cheap ldrs

[anti-idiot]

http://www1.silonex.com/audiohm/softswitch.html

Some years ago I did two ldr shunt attenuators in series and it was ok for communications, smooth as baby´s ass, used just cheap ldrs

but, do they work for a fraction of a second? b'cos the purpose of the muting device is to mute the circuit just for a fraction of a second to avoid pops. nothing else.

[xenrelic]

Does anyone have a working link to this schematic?

http://i20.photobucket.com/albums/b228/Anti-Idiot/Mute-Circuit.jpg

[anti-idiot]

Does anyone have a working link to this schematic?

http://i20.photobucket.com/albums/b228/Anti-Idiot/Mute-Circuit.jpg

Sorry dude. That schematic is a portion of Lonestar schematic. I uploaded it @ http://forum.metroamp.com/viewtopic.php?f=4&t=36800

[xenrelic]

Ahh, thanks. I just wanted to check out what it was. I'm in the middle of figuring out a decent mute circuit for a relay switching SLO/1959 2 channel hybrid. I've got everything figured out except for the mute pulse buss.

[anti-idiot]

Ahh, thanks. I just wanted to check out what it was. I'm in the middle of figuring out a decent mute circuit for a relay switching SLO/1959 2 channel hybrid. I've got everything figured out except for the mute pulse buss.

in order to make the mute-ckt work, you need at least 2 relays, one on, the other off.

[xenrelic]

Ahh, thanks. I just wanted to check out what it was. I'm in the middle of figuring out a decent mute circuit for a relay switching SLO/1959 2 channel hybrid. I've got everything figured out except for the mute pulse buss.

in order to make the mute-ckt work, you need at least 2 relays, one on, the other off.

So one relay is switching on while the other is switching off? 

[anti-idiot]

So one relay is switching on while the other is switching off? 

Yes. The mute-circuit activates only when a relay goes off. If you look closely, the diodes are connected to the ground-leg of the relays. when the relay goes off, it produces voodoo that sends a positive pulse thru the diodes. that's what i think...

[xenrelic]

In the LSS mute-ckt, it looks to me like both relays are switching on and that one is just slightly delayed in turning on.