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Geofex A/B/Y SplitterBox

10/7/1999 11:45 AM
Geofex A/B/Y SplitterBox
Has anyone tried the GeoFex splitter-A/B/C Box? In which way does the LM833 circuit in the signal path alter the sound of the guitar? Are there other( more passive) ways to switching humfree between two or more amps?  
Thanx for replies, Frank
10/7/1999 2:12 PM

The most-passive way is with only a transformer.  
Unfortunately, the design and construction of a transformer that gives wide, smooth frequency response at the impedance levels that make a guitar sound best is complicated. Jensen has several such items. They cost from $35 to over $50.  
You can use a fairly simple dual opamp setup that will null out up to about 2V of hum. Anything over that much and you are pretty much set up for an isolation transformer.  
The LM833 alters the sound not at all compared to the rest of the components. The transformers recommended in the GEOFEX splitter box are the biggest tone changers. So far, all the testing I've been able to run shows the GeoFex box flat to -6db (half power, voltage) between 60Hz and about 24kHz, which isn't good enough for hifi, but is pretty good for most musical instrument products. The low end is limited by the primary inductance of the transformers as driven by the output impedance of the LM833. There is a resonant peak at about 43kHz on the proto that I built.  
The GEOFEX box is a compromise, like all designs. It's a most-bang-for-the-buck design.  
The best way to switch hum free is probably to sit the amps next to one another and run a large grounding braid between the chassis grounds of each one and to break the cable shield ground at the amp end of one of the cords leading into that amp. Most people don't want to do that.
10/7/1999 3:28 PM
Dave H.

The GeoFex splitter will connect your guitar to amp A, amp B or amp A + amp B. If you can live without the A + B option a simple hum free passive option would be to use TPDT switch to switch the guitar signal and grounds.  
10/7/1999 5:36 PM

If you switch the signl AND the grounds, ain't there a risk of getting some switching pops, or even a shock?
10/8/1999 10:09 AM
Dave H.

Possibly pops but no shocks. I was only thinking of switching the signal grounds not the power grounds. The chassis of both amps will be grounded at all times by their power cord ground wires.  
When amp A is selected the guitar is connected between amp A input and amp A ground. Amp B input is shorted to amp B ground. When amp B is selected the guitar is connected between amp B input and amp B ground. Amp A input is shorted to amp A ground. The only common ground connection between the two amps is through the power cords so there is no ground loop. If you put the switch in a die-cast box you would need isolated jacks for the guitar and one amp. The other amp would have the ground side of itís jack connected to the box for screening.  
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