This is part two of a two-part post. For the first part please click here
With both the master and slave boxes completed, it was time to test out the prototype and see what tweaks were needed.
With it all plugged in, everything was working as planned. The send and return signals through the XLR were clean and didn't have any noise issues that we initially worried about. (This was tested in a shop environment though, so to really test this, we'll have to use it at the upcoming show). The 9VDC worked just fine with a 1/4" mono cable, and the switchable buffer was performing as expected, except for a little tone coloration.
Here is the 'slave' box to this 'effects loop' system. It is mostly a passive piece of gear, save for an indicator light, and sending 9VDC to the two pedals that will be on the same board as this box. It has an XLR jack for the send and return signals to the 'master' box. Two standard 1/4" jacks are for send and return for the effects pedals, the isolated 1/4" jack for 9VDC and two 2.1mm jacks on the side to power two pedals.
Our line-driving buffer in the 'master' box was based on a simple op-amp design with minimal parts to affect the tone as little as possible. Nonetheless the tone was slightly affected - the effect was as many people say, 'lifting a blanket from the speaker' which is a nice coloration as it helps bring out the top end a little more, which is affected most by long cable lengths, but this was not the objective of this buffer. So in order to tame the treble frequencies, we added a simple low pass filter to the buffer with an adjustable amount of treble roll-off via a trimpot on the surface of the box.
The trimpot would act like a guitar tone pot, but to a lesser degree, with finer control. Having it on the surface would also allow for adjustments easily during set up for sound check at a show, as the amount of roll-off required may be different depending on the cable length and quality at each venue.
Although using a 1/4" mono cable for 9VDC can be seen as overkill, it is one of the cables that are always at abundance in a variety of lengths at any venue. This means that we don't have to bring any type of specialized cable (along with a backup) for shows. The slave box has two 9VDC outputs to power the wah and boost pedals on the down-stage board. Even though the two 9VDC outputs are not isolated, this may change in the future depending on whether noise becomes an issue at the upcoming show where we can really test the system in a real life environment. We also wired up the slave box with a '9V Check' indicator LED to show whether 9VDC was being transported correctly from the 'master' box through to the 1/4" jack on the 'slave' box.
As mentioned earlier, the biggest concern we had over sending two different mono signals parallel to each other, in a shielded 2-conductor XLR cable, was noise or tone loss. This didn't seem to be an issue when we tested it in the shop. We installed a backup solution to noise problems with a 1:1 audio transformer but at the end, decided to remove it as it was affecting the performance of the 'Wah' pedal on the down-stage board.
The XLR jack on the 'master' box also has a ground lift switch to help with noise issues, should they occur. We tested the system with the 'slave' box via a 50ft XLR cable (which means the signal travels 50ft each way, 100ft in total), and with the buffer on, the cable length had minimal effect on the tone of the signal. Since we have an effects loop bypass switch, it was easy to a/b the two sounds. The buffer seemed to be doing it's job very well!
Stay tuned for a bonus post to see where this system is all going to use in!