SMF-1: rackmount analog dry box

[Kazinator]

Hi all,

I recently made myself a rack unit called the SMF-1, inside the gutted chassis of an old DOD crossover.

http://www.kylheku.com/~kaz/smf-1/.

The SMF-1 provides an analog dry path around a digital effect box, which can then be run 100% wet. You can get the best possible signal quality this way from any box. Also, the SMF-1 has independent presence controls for the wet and dry path, which are variable-frequency -12db roll off active filters that sweep from about 3.8 Khz up to 20 Khz.

Basically I designed this to have all the features of the "last piece of the puzzle" in my guitar rig: analog dry, control over the amount of wet, and control over the high frequency content of wet and dry.

[isaac42]

Have you encountered any phasing issues due to the inevitable latency of the digital effects?

[Kazinator]

Originally Posted by isaac42

Have you encountered any phasing issues due to the inevitable latency of the digital effects?

Funnily enough, I was working on this right around the time that other thread was active about the phasing issues that guy was having when mixing an effected signal with dry and bypassing the effect.

I run the effect 100% wet. So only the time delayed signals (early reflections, tail of the reverb, or delays) appear from the effect box, and none of the dry signal. There are no phasing issues. Just a clearly articulated dry signal, with the effect smoothly blended in.

If I put the Yamaha box on bypass, though, it instantly reveals itself not to have true analog bypass. There is latency, and it results in a comb-filtering effect, similar to what you hear when you take an effects box and configure small delays in the sub-millisecond range, mixed 30 to 60 percent. The correct way to get bypass is to program a patch which mutes the effect, or, failing that, reduces the volume so many dB's that it's not audible.

One thing I did notice though is that the reverb quality somehow suffers if I set the reverb mix 100% with a level of 0, to try to get the maximum reverb output. Something is just not cool with the reverb level being that loud. Either the DAC in that old does not gracefully handle the larger numeric values, or the algorithm itself is affected by the unusual volume and mix settings, or maybe the analog side of that box's output is crappy. Whatever the reason, subjectively, cutting the digital volume of the reverb effect to at least -6dB (and then turning it up in the analog mix to compensate) results in a smoother, less "grainy" reverb that detracts less from the main signal.

I like being able to take vintage junk and optimize it this way; it's kind of cool. The effect unit is now the relish on the burger; it does not interfere with the beef. If we lose a few bits of sample width, who cares.