Metropoulos Forum

For those that may not know many amp makers are now adding relay muting circuit to send the signal to ground during the time a relay "pop" would happen (Whenever a switch is thrown). I've looked through the schematics of both the Peavey 5150 and the Mesa Boogie 3 channel rectifier heads. They seem to try and accomplish the same thing. Randall Smith has also written a patent on the subject. Interesting read about how he mutes the reverb.

The basic concept is you use the collapsing magnetic field that the relay's coil produces when it is switched to make a P-channel FET (J175 or J174) close thus connecting the signal to ground for a while (for the duration of relays' pops).

Typically the diodes used to help eliminate the transients created by the collapsing magnetic field of a relay now feeds a "muting bus" basically feeding a "trigger" to the base of a NPN Transistor. The NPN Transistor's emitter is connected to ground and the collector is connect to a voltage source via a resister / capacitor. The resistor / cap act as the timer for how long the FET is power for, and subsequently how long the signal is sent to ground.

I am not to sure the reason for the transient voltage suppression (TVS) diode (4744) but I'm guessing to help stabilize the quick voltage change.

I was hoping to get some input as to the effectiveness and the implementation into a Marshall amp with a lot of relay switching.

Thanks
Rip

Perhaps something like this

Hi,

maybe SDM is willing to develop, produce and sell such a small PCB mute unit (like his outstanding FX loop)?
That would be fantastic, because a complete relay channel switching is much better than the usual pop free switching via LDRs (Vactrols). Vactrols are very expensive and have many drawbacks (e.g. time delay/fading, max. signal voltage only 100 Volts etc.)

I'm shure SDM could do that prefectly!

Regards

Robert

I use relays in my 2204 clone for channel switching and I honestly don't find the pop excessive enough to warrant using a mute circuit. I'd personally rather have a slight pop than to have the signal cut out for some fraction of a second when switching channels. That's what turned me off from the current production Marshall amps.

I'm not sure how Fender does their switching, but they seem to have it down much better than Marshall. They have 4 button footswitches that connect to the amp via a single mono 1/4" guitar cord and the switching is silent and seamless. I've been trying to figure it out, but I'm just not that smart!

I'm working on a 2-in-1 type footswitch circuit and getting the same pop. I added the 10M resistor to ground as suggested by Flemingmaras on the coupler going into the PI circuit. This seems to get rid of the pop for 2203 mode, but I'm still getting pop switching back to 1959 mode.

I'd LOVE some more insight on this, as I'm SOO close. The pop is tolderable, but I'd love to get to the next step where it's completely gone.

Got a schematic of the 2 in 1?

Rip

Wait I just re read your post, you mention a coupling cap into the PI, do you have an fx loop? Or just talking about the 0.022uf into the PI grid?

If I remeber the mod you talking about, you need to have all the plate caps and subsequent grids referenced (10M) to ground. If you did one of SDM's 2 in 1 circuits let me know which one and I'll try to help.

Rip

Hey Rip,

Does that mute circuit (in the schematic you posted) work?

Also, what voltage relays are being used with it?

xenrelic wrote:Hey Rip,

Does that mute circuit (in the schematic you posted) work?

Also, what voltage relays are being used with it?

http://forum.metroamp.com/viewtopic.php ... 00&start=0" onclick="window.open(this.href);return false;

If you are using relay coils for the pulse, you can get away with just two discrete devices and a cap. Peavey has been doing it that way for nearly 20+ years.

That being said you don't really need a mute circuit if everything is done correctly. I never have popping issues, and I have built many channel switchers with relay's.

The muting circuit is needed when you change the bias of a tube or switch a capacitor. If you're goin' to switch a whole stage in/out, you can do it using a pulldown resistor. Take a look at the JVM schematic.

THChrist wrote:The muting circuit is needed when you change the bias of a tube or switch a capacitor. If you're goin' to switch a whole stage in/out, you can do it using a pulldown resistor. Take a look at the JVM schematic.

You don't really need a mute circuit when switching a cap in and out, all you need is to keep it charged/keep it referenced to a common dc potential *ground* with a largish resistor, and then just shunt it. Again that covers doing things correctly, and if you are just switching between stages for channel switching all you have to do is make sure all contacts of the switching element are dc referenced in the same way so you are not switching between two points that are at a different "bias"

Mute circuits however are useful to kill both reverb, and fx "trail off" when switching channels though if you keep the timing of the mute reasonable.