Active Instrument Cables or Where The Amp Should Begin

[bbsailor]

See this link to a 2011 Masters Thesis about active instrument cables. http://drum.lib.umd.edu/bitstream/19...117N_12597.pdf

I labored at finding the most appropriate forum topic to place this new thread. I chose guitar tech forum as it sort of bridges the gap between the guitar and the amp the best.

Timothy Babich covers the history of guitars and how to this day how the high impedance guitar pickup and the high capacitance guitar coax cable do not play nice together. A guitar pickup at resonance has an impedance of several hundred thousand ohms. The combined impedance of the volume and tone controls with the 1 meg ohm amp input impedance places about 400K ohms load on the guitar pickup plus the 300+ pf of capacitance of the coax. This means that any load that is less than 5 times higher than the pickup resonant impedance is loading down the high frequency output before arriving at the amp. Tim's thesis outlines his development of an active guitar cable which buffers the pickup from coax, and amp loading. He covers many active device types with results of testing each type.

I built a few of these about 20 years ago with very noticable results in increases high frequency transfer to the amplifier. See Tim's recent conclusion graphs, in his thesis, verifying my old observations.

Why do we require a guitar amplifier to not include the coax cable with phantom power supplied by the amp manufacturer? Patent Hint!!!

When I explored the active cable years ago I places an early FET in a mono metal plug on the guitar end with a minimal of parts that would fit. See Tillmans active buffer with a web search. I placed the other parts in the amp end of a large jumbo plug with a Velcro mounted 9V battery sending power down one of the two conductors of a twin shielded mic cable. Conductor 1 was the audio out of the FET while conductor 2 supplied 9V power with the FET biased to a 4.5V mid supply voltage point.

Then I got to thinking how could I rewire my homemade Fender Twin Reverb to phantom supply power to the active cable. I rectified the 6.3 filliment supply and obtained a little shy of 9V DC. I changed one input jack to being a stereo type into the amp supply phantom power to the active cable by using the tip of a stereo plug for the audio, the ring for the 9V phantom supply and the sleeve for audio and DC ground. This may have been overkill as a 9V battery Velcroed to the jumbo plug lasted a few months of constant use.

However, if an amplifier manufacturer wanted to expand their notion of where the amp begins, they may want to patent an active pickup cable supplied with their amp supplying phantom power to say that "the amp starts at the plug end of the guitar cable". The added cost could just be a stereo input jack and a low voltage isolated power supply to power the guitar end FET plus the active cable. A player only needs one active cable to make all their guitars function like active guitars with no modifications.

So here is my question to the members/readers of this forum. Does extending the concept of the amplifier to include the buffer in the guitar end of the coax cable make any sense?
If any forum readers or members have any experience with this, please share your reaction to it?

Thanks

Joseph Rogowski

[David Schwab]

I've been a fan of active buffers in guitars and basses for years. I don't think I'd want it in my cable though. Cables break took often. Also I like the very sturdy Neutrik plugs, and they wouldn't have room inside for the buffer. I use the silent plug they make with the magnetic reed switch that mutes the signal when you unplug the cable from the guitar.

But here's what I notice, a lot of guitarist (and bassist) don't like that active tone. They seem to like the diminished high end of passive instruments. Neutrik even makes a plug with a rotary switch for adding capacitance! Even more dullness.

I did have a buffer in my Tele style guitar, and I have to admit it was too bright. But that's a bright sounding guitar to start with. But I have used them in most of my basses over the years. I used a circuit similar to the Tillman one, with a single JFET.

[Rick Turner]

Greg Gaultieri of Pendulum Audio used to market his "Smart Cable"...a guitar cable with a buffer built into the guitar end. And, as some of you know, I've been using on-board, usually fairly high voltage (high headroom) active buffers (sometimes with gain, sometimes without) since 1970 or so.

It's not the only way to go, but it sure opens up possibilities. And high impedance pickups can be mated very successfully to onboard preamps. Listen to Lowell George of Little Feat on the live album "Waiting for Columbus" or Lindsey Buckingham's lead guitar lines on "Rumours" for what an Alembic Stratoblaster can do to Strat output and tone. And there we're talking nearly 40 years ago...

I myself really like the combination of a well voiced high impedance pickup with a buffer that will deliver "the goods" down to the end of the cable. With this, you get one more level of predictability and consistency, and if you're winding for a particular tone, you get to load on as many turns of whatever gauge wire that it takes...giving you maximum "free gain", and then preserve that sound on down the line. For instance, you could overwind a PAF spec humbucker a bit, buffer it in the guitar, get killer output and deliver the same basic tone to the end of the cable as a non-buffered humbucker...and it wouldn't matter if you ran 10 feet of cable or 100 feet.