Quick Pick: Metasonix TM-7 Scrotum Smasher

Eric Barbour of Metasonix has a colorful approach to design, employing an all-tube audio path in his quest for unusual and sonically extreme products.

Eric Barbour of Metasonix has a colorful approach to design, employing an all-tube audio path in his quest for unusual and sonically extreme products. The TM-7 ($449), code-named Scrotum Smasher, is certainly both unusual and extreme.

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Like other Metasonix products, the TM-7 is housed in a taxicab yellow box. In this case, it is silk-screened with cartoon characters that, along with the manual, are for a mature audience and not for the easily offended (as the product's name suggests).

The TM-7 has ¼-inch input and output audio jacks and a ¼-inch CV (control voltage) input. The Scrotum knob controls the input volume to the first tube, a 6AK5, which was designed for military radios and makes “a bad preamp,” according to the Metasonix Web site. The Mega Scrotum pot controls the gain of a second 6AK5 and works somewhat like a drive control. I say “somewhat” because nothing on this box works as expected — every parameter is highly interdependent on the positions of all other pots and switches.

The Double Scrotum switch engages a feedback loop into the circuit, which can get the TM-7 to self-oscillate, creating sound on its own. The SUYA knob determines the saturation of the third tube, a 6BN6, which gets switched into the circuit with the Smash switch. The 6BN6 wasn't designed to have audio pass through it, and it sounds like it.

It's important to note that the wall-wart power supply gives 12V AC to the box, not DC like other wall warts. DC voltage will damage the TM-7, so it's essential to keep this supply separated from all your others. The one thing noticeably lacking on the TM-7 is a bypass switch, which I'll also address shortly.


My first application was to test the TM-7 on various tracks during a few mix sessions. I enjoy using different colors of distortion to make instruments stand out — from adding a subtle electricity to a vocal to making a drum set sound like it's ruining your speakers — and I employ many stompboxes and rack effects to this end. Because the TM-7 wants an instrument-level input, attenuation from line level is necessary to take full advantage of the range of controls. Once the gain staging was right, the TM-7 performed extraordinarily well in this setting (see Web Clip 1).

Although it seems the TM-7 is being marketed primarily as a “guitar preamp,” the aforementioned lack of a bypass switch makes it impractical for your average guitarist, at least in a live setup. In the studio, that lack of a bypass switch isn't a problem, but I found that guitarists trying the TM-7 for the first time were put off by having to turn their amp gain down enough to accommodate its high output. Also, in certain switch positions (notably when the 6BN6 gets switched on), the box becomes highly microphonic, and feeds back in front of a loud amp even without an instrument plugged into it. This could be a cool effect for a noise musician, especially because the feedback changes quality with different knob positions. I recorded some pretty extreme guitar sounds using the TM-7 (see Web Clips 2a and 2b).


The CV input, which controls the screen grid of the second 6AK5 tube, doesn't respond to a standard expression pedal but requires a stronger signal, such as that from an analog synth. Connecting it to modules by Doepfer, Livewire, and Plan B allowed me to create interesting effects, such as syncing timbral changes in the distortion to the tempo of a sequenced pulse.

In one patch, the speed and depth of an LFO rhythmically modulated the saturation amount — something you can't do with your average distortion box (see Web Clip 3). Metasonix offers an adapter kit if you want to mount the TM-7 in a Eurorack or MOTM modular system or in a standard 19-inch rack.


The TM-7 is a highly versatile, wonderfully erratic, and gorgeously raunchy distortion device. When you hear what it can do once you take advantage of the CV input, it's definitely worth the price.

The TM-7's design may seem crude at first glance, but there's an elegance to this processor that the cartoon characters belie. As a guitar effect, a synth module, or even a sound source, the TM-7 should be heard by anyone with a penchant for crunchy sounds.

Value (1 through 5): 4