Add master mute switch to amp

[dreamert]

Hi,

Hi have a nice SR Technology Jam 150 Plus acoustic/voice amp with a small mixer on top (6 channels).

Unfortunately it doesn't have a master mute or on/off footswitch on it.

I'd like to add those functionnalities.

My first idea was to cut into one wire going to the speaker with a switch...would that make a loud pop when engaged/disengaged? Any better idea to accomplish that?

But as for the effect loop I have no idea what to look for or how to to it. What can you tell me to be on the look out for when I open the amp?

Thanks a lot in advance.

[sumgai]

dreamer,

Are you asking for one thing or two?

And are we to assume that this is a solid-state unit? If so, opening the speaker lead will work, but it will very likely pop, loudly. There are better ways to get the job done.

Effects loop?




sumgai

[thetragichero]

opening the speaker lead would cause no load or an infinite load to be seen by the power amp? (at first glance i thought no load... but now i'm thinking open circuit = infinite load?)

do we have a schematic for this amp?

[dreamert]

I guess I'm asking for 2 things in the end.

Yes this is solid state. It is Italian stuff and not well known, it is distributed through Godin (Canadian guitar manufacturer), really nice stuff but I couldn't find any schematic for it.

So if you know the 'generics' about how to achieve such switches I'd be interested.....

So far, cutting the lead to the speaker seems like a not so wise thing to do... any other suggestion?

[ashcatlt]

May 12, 2011 19:36:45 GMT -5 thetragichero said:

opening the speaker lead would cause no load or an infinite load to be seen by the power amp? (at first glance i thought no load... but now i'm thinking open circuit = infinite load?)

do we have a schematic for this amp?

Generally when we use the word load we mean the current demand. An open circuit is the same as infinite resistance. I = V/R.as R approaches infinity I approaches 0. No current demand. No load. Tube amps hate this, but solid state amps can usually handle it.

I think the switch wants to be between the mixer output and the power amp input, all of which seems to be internal to this amp. A schematic would be great. We might be able to sort it out with good photos of the circuit board(s).

The FX loop thing was a good idea. On most guitar amps the Return jack is switched so that plugging into it breaks the connection between pre- and power amp. Unfortunately, this is more like a mixer. The return from the FX is mixed in with the rest of the signal rather than pr replacing it.

[JFrankParnell]

wait, you just want to mute it? why not just a switch on the input that grounds it? Like a kill switch on the gtar, but on the other end.

edit: oops, 6 inputs. well, same theory, but find out where the master mix goes into the preamp?

[thetragichero]

he said it's like a mixer, so i'm guessing there's more than one input?
grounding the output before the power amp should do it, though

[ashcatlt]

Probably don't want to short the preamp/mixer output. Remember that I = V/R thing? What happens when R approaches 0? The opamp which is most likely driving that section tries to deliver infinte current. It might not explode...

We'll want to disconnect the preamp output and ground the power amp input.

[thetragichero]

damn you, ohms law!

[sumgai]

dreamer,

See that Effects Return jack? Plug a cord into it, and leave the other end unconnected to anything. Now plug in a normal input (meaning, your guitar and/or voice), and see what happens. Does the unit work (give out sound) when there's an "empty" cord plugged into that return jack? If not, then we have a solution.

First, find a footswitch that you can go Nutz over. Make sure that it has a physical switch inside, and not some circuit board with a lot of parts - that won't work for our needs here.

Next, find and prepare either two cords (standard guitar cords will do) or a stereo cable (as in, a standard mic cable, that has two conductors plus ground, which is what we need). Since we're not dealing with a long run of unamplified sound here, the cord's impedance rating (high versus low) is not a concern.

If you do opt for the stereo cord, you'll need a splitter/adapter in order to deal with the two separate Effects Send and Return jacks. For this reason no one would blame you if you stuck with just two simple regular guitar cords.

The idea here is that you're gonna imitate an external effects unit. Only instead, you're actually just gonnna select where the power amp will get it's input - the Send signal, or nada. You'll ground the input, via the switch, when you wish to mute the thing. Otherwise, all of the mixer's signal will pass through from the Effects Send and on to the Effects Return, thanks to your switch.

Fortunately, as noted above, you can safely leave the output of any solid state device open, it won't care. That means that we don't need to deal with controlling where that goes, we need to concern ourselves only with the power amp's input.

So. All that's required here is a simple SPDT unit. Many older footswitches came with such a goodie, but not all of them by any means. If you've procured a unit with a SPST, then you'll need to replace it. If it has a DPDT, then you're good to go, you need only one side, or one pole, to do the job.

Without breaking out the schematic diagram generator, let me describe the connections - the center terminal will go to the Effects Return wire (the center conductor of one of the mono cords), one of the outer terminals will go to the other center conductor, which will go to the Effects Send jack, and the remaining outer switch terminal will go to the ground braid of the two cords (or the stereo cord, of course).

That's it! It really is that simple. And it shouldn't pop as you switch back and forth, but if it does, we can cure that, no sweat. But for now, I'm opting to keep it simple for you. ;D

HTH




sumgai

[ashcatlt]

sg - I googles the manual (probably should have linked it when I had it) and, as I said above, this particular FX loop works like that on a mixer with sends from each channel and a knob to mix in the return. That is, it's a parallel loop, not a series one like you'd find on a typical guitar amp. In fact, it's a mono send stereo return setup.

Your solution is great for other amps, but won't work here.

[sumgai]

ash,

May 13, 2011 16:38:01 GMT -5 ashcatlt said:

.....
Your solution is great for other amps, but won't work here.

Which is why I said up front:

May 13, 2011 14:50:37 GMT -5 sumgai said:

Does the unit work (give out sound) when there's an "empty" cord plugged into that return jack?

Perhaps I should have withheld the solution until after we'd received the answer to that question.... sorry 'bout that.




sumgai

[ashcatlt]

Well, I suspect the answer to the question would have been something like "Which jack? And what shold I do with the Master FX knob?"

Like I said, though, great solution if somebody needed a mute switch for some other amp!

So here's the manual. I don't see anything we can use to accomplish the desired muting aside from the master volume knob.

Wait! That's a great idea! Use a switch to disconnect whatever's hitting the ungrounded outside lug and instead connect that lug to the grounded outside lug. Done. If it's a PCB mounted pot that might be easier said...but not by much.

I'm seeing a power switch on the back, though.