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View Full Version : I-beam for nylon string doesn't work on other amps



onethumbpicker
11-26-2011, 04:06 AM
I added an I-beam nylon string pickup system(passive) to my Alvarez classical guitar. I installed this myself, and tested it through my Marshall AS50D amp. It works great-but whenever I have tried to use the guitar in someone else's amp/PA set-up, it doesn't work. I've tried this on about 3-4 other amp set-ups, and it has never worked on any of them. Yet, it always works on my set-up. I'd rather not continue to bring the amp along whenever I play with other players. Any ideas?

Bryan McManus
11-28-2011, 09:13 AM
Hi onethumbpicker,

I am certain the issue is with the input impedance of the other devices. The passive iBeam, like nearly any passive piezo sensor, has a very high signal impedance and requires the input impedance of the first active stage be nearly as high if not higher. The Marshall AS50D provides a 1MOhm(1 million Ohm) input impedance which is far higher than that of virtually any electric guitar amp or the line input of a PA channel. It is still not as high an impedance as the iBeam but higher than the PA input or electric guitar amp. We highly recommend that passive piezo signals be connected, for best results, to a device with an input impedance of 10MOhm or higher.

When you connect a high impedance passive piezo signal to a low impedance input, the first thing to go is low frequencies from the signal. It can make a pickup like the iBeam seem nearly non-existent due to lack of any significant bass response through the impedance mismatch. Add to that the fact that passive piezo signals attenuate(lose signal strength) down instrument cables and you have a recipe for weak or undetectable signal.

If you were to use a buffering preamp with a high impedance input between the passive iBeam and almost any amp or PA input, the iBeam signal would retain all its bottom end.

Any of these from LR Baggs are highly recommended:

http://www.lrbaggs.com/venue/
http://www.lrbaggs.com/paradi.htm
http://www.lrbaggs.com/gigpro.htm

onethumbpicker
02-07-2012, 05:24 AM
Thanks, Bryan. I picked up a LR Baggs Gigpro since posting this, and it solved the problem.