Blackout Electronics | Category Archives: Utility

Passive cabinet merger

Posted on 1.4.2014 by Vesku Posted in Amps, Utility .

I have a situation where I have a guitar amp with just one speaker out, and a switch to select the impedance between 8 and 16 ohms. Now, I want to connect two speaker cabinets with 8 ohms speaker inside both of them. I ended up to build up a simple cabinet merger for future use also. When connecting speakers to amplifier, there is two ways to do it, in series or in parallel. Impedances go like this:

So basically, it goes like this:

– In series: Rt=R1+R2+Rn…

– In parallel: Rt=1/R1 + 1/R2 + 1/Rn…

Where Rt=Total impedance, Rx=Impedance of the single speaker

I built up a very simple wiring schematic for cabinet merger with switchable series/parallel -option:

So, no more than three ordinary jacks and a DPDT switch with some wiring.

NOTE!: If assembled inside conducting enclosure, you need to insulate Speaker out 1’s ground from the enclosure. Otherwise it won’t work correctly in series mode. Alternatively you can build this inside non-conductive plastic enclosure.

Always meet your amplifier’s impedance requirements, and don’t even try connect for example 4 ohms load for the amp which expects 8 ohms speaker load. You will most probably fry your output transformer.

Sound reactive LEDs

Posted on 3.11.2013 by Vesku Posted in Just for fun, Utility .

I’ve spent a lot of time inspecting how could I build up a system, which blinks LED lights in time with sound. Found some plans from the internet, but they all sucked. At first I tried to use a TIP31 -transistor, which in fact works somehow, but needs a lot of a amplification to work. My bass amp’s effect loop’s send’s signal wasn’t strong enough to blink the light. Added MXR’s Microamp to the line, and it worked just barely. TIp31 needs at least 0.7 V to conduct, which you don’t have in most cases. So, f**k the TIP31.

Instead, one good solution is a LM358, which is a low power dual operational amplifier. It can be used to amplify the DC signal generated by a microphone and act as a LED driver. It has DC voltage gain of 100db, and runs with DC-power from 3 volts to 32 volts. The circuit is as following:

I usually hate circuits containing connections between two sequential rows in vero board, but this time I tried to squeeze the circuit as small as possible. The driving signal can be almost anything. Signal from microphone, instrument level signal from guitar…
It’s very sensitive, so instead of amplifying the signal even more, we have to brake it down instead. The sensitivity can be adjusted with the value of R1 -resistor in the circuit. To make it easily adjustable, use appropriate potentiometer in place of the resistor. Something like 5K pot is worth to try. Everything below 1K makes the leds crazy even if you breath too loud.

R2 -resistor is for limiting the current going to the LED’s. If you plan to use several leds in your circuit, you can use a jumper instead, and connect appropriate resistors in led rows. How many led lights you can use in series depends on the input voltage used, and the colour of the leds. Blue and white are the brightest, and with 9 volts current you can run only 2 or 3 in series. It’s up to to the forward voltage of the led light, which is around 3 volts with white and blue. But that’s not a problem, just create several parallel led stripes.

In the video below I’ve built this system inside a 1590B -box. The LED stripes are built into mini sized guitar. I removed the pickup, and assembled the LED stripes into small piece of wood which I installed into humbucker frame. My bass amp’s direct out is used for the operating signal. So, basically it’s just instrument level signal, and 5K resistor pot is used for adjusting the sensitivity. This is our band’s on-stage decoration, and it takes its signal from the bass drum.

Lament of the Mouse

Posted on 6.8.2013 by Vesku Posted in Utility .

I’ve found it hard to debug my non-working circuits with my two hands, and simultaneously play the guitar with my other two hands. I needed some source of audio signal to get all my four hands free for probing. One good solution is an audio oscillator, which does nothing but produces audio signal in static frequency. Something like this:

This simple circuit produces audio signal in frequency of 1000hz. Works with 9V battery for ages, and has a volume control knob. It’s good that it has, because this little bastardo has the power. You really need to beware to keep this one’s nose clean. Otherwise missus’s porcelains will be broken.

I didn’t want to waste enclosure for this, so I built it inside broken mouse:

This is also a good tool to get somebody to confess something if forced to listen continuously. So, multitool this is.