Make a Mic Muter

Build a mat that switches your mic on and off when you step on it.

Enough theory for a while, let’s go build something! I always had a problem on stage because I didn’t have enough hands to play guitar and flip mic-muting switches, too. A microphone that sensed when I stepped up to it and activated it would’ve been perfect for me. I finally came up with an add-on that does exactly that.

This is based on burglar-alarm pressure mats: plastic mats with built-in switches that connect when you step on them. All you need to do is put the mat in front of the mic stand and wire up your microphone so that stepping on the mat un-mutes the mic and stepping off the mat mutes it. There is a minor problem in that the mat switches are open when you step off and closed when you step on them. If we used the mat switch directly, the mic would mute when we stepped on it. So we need to make the mat control a switch that works the opposite direction—on when the mat switch is off.

We also need a switch that will not impair sound quality by adding noise, distortion or hum. Relays might work fine, but they eat a lot of power, too much for such a simple device. Luckily, there is a suitable solid-state device: the Fairchild H11F1M. This LED-to-PhotoFET optical switch is almost ideal, having no effect on the audio when it’s open, and muting the signal when it’s closed so that any non-ideal characteristics cannot come through.

Tech Views This is the kind of project that is most useful to an experienced hardware hacker, so I’m not going to provide “solder wire C to contact 5” kind of instructions. You advanced hackers won’t need details except where to find an H11F1M. If you look at the schematic diagram, you’ll see how to hook it up. The box is primarily a pass-through box for your mic cable, and probably ought to sit right at the base of the mic stand. Ground the box to the signal ground on the mic cables.

Use an insulated 1/4” phone jack for the switch mat to plug into so that it cannot induce clicks and pops in the signal lines.

A 1/4” phone plug from the normally-open switch mat comes into the box at the jack, where the sleeve of the mono plug connects the battery to the rest of the circuit, saving battery life when the plug is not there. The battery provides 9V to a 47K resistor to the base of the 2N3904 transistor. The transistor is therefore fully on whenever the switch mat is open. When you step on the switch mat, it shunts the base current away from the transistor and the transistor turns off.

The battery also provides current through the 750 ohm resistor to the anode of the LED in the H11F1M. The cathode of the LED is tied to the collector of the transistor, so when the transistor is on, the LED emits light and causes a low resistance between pins 4 and 6 of the H11F1M. The H11F1M then shunts the two signal lines together, effectively muting the mic whenever the LED current is on. When you step on the mat, the mat turns off the transistor and LED, letting the signal lines pass signal freely.

This is probably best built on a 1”x1” bit of perfboard. It’s too simple to design up a PCB for, unless you’re making thousands of them. The resistors can be 1/4W or 1/2W, or even 5W if you want. Carbon composition mojo resistors will make absolutely no difference whatsoever here.

I’ve shown the setup for low impedance balanced mics. If you use high impedance unbalanced mics, wire the H11F1M to shunt the signal to ground. The connectors will be different, of course, to match the mic cables. Some low impedance mics will have a hot enough signal to bleed through even when the circuit is trying to mute them; however, they should have a very much reduced signal level even if they’re not fully muted.

Suitable pressure switches are available in most large cities from suppliers of burglar-alarm equipment. If you can’t find them where you live, here are some web addresses for suppliers of suitable mats. (Make your own switch mat)

Here’s the H11F1 datasheet online:

There are actually three parts that may be used interchangeably in this circuit: the H11F1M, H11F2M, and H11F3M. They differ in certain specifications, which don’t matter for this application. As of this writing, Mouser Electronics has the H11F1M for $1.88, the H11F2M for $2.00 and the H11F3M for $1.75.

Once you get used to stepping away from the mic to mute, I think you’ll find it’s really handy!

R.G. Keen
Cheif Engineer
Visual Sound

Although I’ve never repaired amps for a living, I have repaired a lot of amps through the years. I get a lot of questions e-mailed to me from amp-repair novices

Although I’ve never repaired amps for a living, I have repaired a lot of amps through the years. I get a lot of questions e-mailed to me from amp-repair novices about how to fix amplifiers, and this was one reason why I wrote the Tube Amplifier Debugging page at:

Through it, I hope to help the novices out and give them a good start at how to think about fixing amps. In addition, I have advised a lot of people who do make a living repairing amps. These guys don’t ask me about simple cases, so I get involved with a high percentage of the odd or tough cases, usually by email or telephone. It doesn’t always work, but it’s a real kick when I can help someone get to the bottom of an amp mystery.

One of our amps came in for warranty repair. The owner needed it for a gig in two days, and was very anxious. As you might guess, it had failed suddenly on him about two months earlier and only got to us when the gig was two days away.
Some of these are fun, like the time we found an amp from a major amp maker with intermittent crackling. It turned out that the output tube sockets had never been soldered. The connections to the output tube sockets had been made with the wires holding themselves in place in anticipation of soldering. They functioned for years that way, until finally vibrations caused them to lose connection. Another amp blew fuses when inside the cabinet, but not outside the cabinet. None of the usual suspects was a problem. It turned out that the choke had a short to its grounded core that opened up when the stress of the mounting screws was removed.

I just ran into a new-to-me problem; this may help you if you run into it. One of our amps came in for warranty repair. The owner needed it for a gig in two days, and was very anxious. As you might guess, it had failed suddenly on him about two months earlier and only got to us when the gig was two days away. Our repair tech called me when the obvious fixes had no effect at all. The tone stack on these amps is similar to the traditional tone stack, with bass, mid, and treble controls in the conventional arrangement. Turning the treble knob down all the way made it very loud and very bass-heavy. Turning the treble knob up produced lower volume, but almost no increase in treble until the sound suddenly went treble-y and thin most of the way up.

This one is easy, right? New treble pot, a couple of voltmeter checks and we’re done. Not so. Replacing the treble pot had no effect. Ah… one of the other pots, the bass or mid, is bad. You might get that behavior with an open on the bass pot wafer. And while we’re at it, let’s check the wires going from the PCB to the controls, and measure all of the pots for total resistance and wiper remaining connected from end to end of rotation. All of the pots and wiring checked out. This was becoming frustrating, even remotely by telephone. Ah-ha! Shorted/open cap. Nope, no leaking DC voltages, and all the parts looked to be of the right value. Meanwhile, the owner is standing there trying to look unconcerned, and it’s coming up on closing time.

With no other option, I asked our repair tech to start measuring resistances from lead to lead on parts to be sure that there was not a broken solder joint or some similar malady, even though he had previously re-melted all of the solder joints on the back of the board. All of the lead-to-lead connections checked out. In desperation, I asked for a check of all the resistances of every single part in the tone stack. When we got to the 100K slope resistor it measured zero ohms! Quickly removing it, we measured the errant slope resistor and found it measured—yes, you guessed, it, 100K. A quick measurement between the pads on the board measured zero ohms! Just to be sure I asked for one more measurement, and now it was open between the pads. We were never able to get a shorted measurement again.

Somehow, the glass-epoxy board had worked perfectly for several months and then developed an invisible short, which vanished as we worked on it. Like other shorts on circuit boards, this can be corrected with a few careful slices from an X-Acto knife around one of the shorted traces. I’ve seen open slope resistors, and drifted (or drifting) slope resistors, but never a shorted slope resistor. So in spite of my kit-bag of experiences with amps, I was a complete novice to this problem. I’ve seen them shorted by other stuff, like in the amp whose owner cleaned up the panel with a little steel wool, or by beer residue, but not by themselves in an otherwise clean amp.

In spite of our experiences, we are all novices when we hit that next funny kind of problem that we’ve never seen before. To solve these, you have to be open to dropping back to first principles, and testing for things on a much simpler, less sophisticated level than you usually do. Going back to basics is a very useful tool to keep handy in your mental toolbox.

R.G. Keen
Cheif Engineer
Visual Sound

R.G. discusses the importance of paying attention to your amp''s filter capacitors.

One element inside your amp that has a definite wear-out time whether you use the amp or not is the filter capacitors. If you keep an amplifier long enough, the filter capacitors will fail – it’s a question of when, not if. But there are capacitors available that will never wear out.

Immortality Mod #4: Replace electrolytic capacitors with plastic film types
Warning! These mods are located in the highest voltages in the amplifier. If you want to do this yourself, you must be absolutely certain before you start that you already know how to do it safely. If you have any doubts at all, take it to a tech who can do it safely. And always unplug your amp for at least several minutes before opening it!

Gather a list of the capacitance values and maximum voltages in your amplifier, then track down suitable film replacements. A suitable replacement for an electrolytic cap:

  • has a capacitance as high or higher than the replaced capacitor – or can be combined to make enough capacitance.
  • has a voltage rating as high or higher than the replaced capacitor.
  • is a metal can “motor run cap,” a “DC link cap” or a “Medium power film capacitor.” Do not use “motor start capacitors,” as they will not be durable.

For our example, we’ll look at replacing the caps in a Fender Super Reverb AB763 (see table). You can find the schematic online at

Fender made their first filter cap from two 70”F/350V caps in series to equal a 35”F/700V capacitor. The use of two caps and the higher voltages was probably to get higher reliability. But film caps are tougher than electros. The B+ voltage is normally 460V here, so we need a bit more voltage than that. Taking a clue from the rating of the other filter caps we’ll shoot for 525 VDC. That’s plenty for a normal 460V, plus maybe a 10 percent surge to 506V.

The next three caps are the replacements for the phase inverter and preamp B+ filters. In the catalog I consulted, there are two parts that work: one is 20”F/450-630V and the other is a larger 25”F/450-630V. Oddly, the 25”F part is cheaper than the exact replacement size of 20”F, so I picked that one.

Tech Views
The bias filter capacitor may be the most important filter cap in your amp. If this one shorts, you will buy at least new tubes, and possibly a new power transformer. I found a 10”F film cap that is also physically small. We can make a bigger capacitance at the same voltage by hooking capacitors in parallel. The particular cap I found was 10”F/250V for $2.23, so I specified five of them to be hooked up in parallel. This is equal to a 50”F/250V capacitor and costs $11.15. I could have used a 50”F/370VAC metal can capacitor, but this is quite large physically, and there is little ripple current stress, so I chose the smaller, plastic-cased 10”F units in parallel.

I listed prices because it is possible to spend a lot more on film capacitors if you don’t shop carefully. Our tab for film caps for the whole amplifier is nearly $60. To compare, Mouser Electronics sells a 20”F/500V film cap for $12.47 and Antique Electronics Supply sells a 20”F/500V electro cap for $6.35. The film capacitors are moderately more expensive for equal ratings. However, the amp tech’s time to put all new capacitors in will likely be $50-$100. Spending a little more for parts once means you’ll never have to replace them again just because they wear out.

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Preparing your amp for unexpected power surges and electrical spikes.

Any time you combine electricity and coils of wire, you have the potential to cause sharptoothed electrical spikes that can puncture insulation and cause shorts. Fortunately, we have the ability to stop them cold.

Immortality Mod #3: Stop transient spikes Our AC power lines get more polluted every day with the leftovers of all the electronics and mechanical equipment that run on them. There’s also the infrequent but real danger of a nearby lightning strike on the AC power lines putting a big spike into your amp.

Here’s a quick cure to make this a non-problem in your amp. Buy three “MOV” varistors for around $0.50 each and install one from each AC power line to the safety ground wire. Put the third one between the two AC power wires. Even if you pay a tech to do this for you, the cost is almost entirely for the tech’s time to do it correctly and safely.

Warning! These mods are in the highest voltages in the amplifier. If you want to do this yourself, you must be absolutely certain before you start that you already know how to do it safely. If you have any doubts at all, take it to a tech that can do it safely. Also, always unplug your amp for at least several minutes before opening it!

This is what is inside most surge protection power strips. Of course you could always use a protected power strip, but you have to remember it every time.

Electrical spikes are so common that there are special parts made to protect against them. The simplest of these is the metal oxide varistor, or MOV for short. A MOV is a chunk of powdered metal oxides pressed together and semi-melted into a solid disc, with leads on each side of the disk. The oxides don’t conduct very well until the voltage across the disc gets high enough, then it suddenly snaps into conduction and can eat huge amounts of electrical power. When it conducts, it eats the excess voltage until things get back down to normal, then it turns off.

For AC power line protection, use MOVs rated for more than the line voltage. In the U.S., the nominal AC voltage is 120V, but I often see it between 125 and 130VAC. There
are 130VAC-rated MOVs, but there is not enough margin between the 130VAC rating and the close-to-130VAC line – use MOVs rated for at least 150VAC. As an example, at
the time of this writing, Mouser Electronics lists the Littelfuse 14V150 MOV as 150VAC, 200VDC, and capable of clamping a peak current of 6000 amperes! It’s a tough little device and it sells for $0.35 as of deadline.

There’s another source of spikes in your amplifier – the output transformer. It’s an electrical coil of wire, too. It can make insulation-puncturing spikes very effectively under certain conditions. However, in this position in the amp, what it kills most often is itself.

You get output transformer spikes when current in the transformer is suddenly interrupted. It is unusual for the tubes themselves to be able to turn the transformer off fast
enough to cause spikes, even in failing, but a wire breaking or a speaker suddenly unplugged can do it.

Some amps have a string of diodes from signal ground to the plates of the output tubes for just this reason. It’s a clever idea, but it suffers from relying on the opposite side
of the output transformer to do the protection. The leakage between the two primary sections is one of the places that can generate spikes.

If we place a MOV from one output tube plate to the other, across the output transformer primary, the problem is solved directly. One MOV between each primary lead and the center tap of the output transformer clamps any transient that is larger than B+ and eats the excess energy. In normal operation, the MOV cannot conduct, so normal operation is not changed.

The skeptic would ask me, “Well, my vintage Fender-Marshall-Gibson is over 40 years old and still running. It doesn’t have all that fancy stuff. How come we need that now?” My answer is that there would not be companies making replacement power and output transformers if they were not needed; having been lucky before does not guarantee
you’ll always be lucky.

R.G. Keen
Chief Engineer
Visual Sound