Liberty Instruments, Inc.
Making Differential Probes
(for sensing audio signal voltages across devices which are not ground referenced)
(Transformer: Mouser Part number 42TM018, audio transformer, 10K:10K
impedance, cost about US$3.00.)
Click here for Mouser's Web Site
These "differential" type probes can be used in the same ways as the normal (resistor) probes, with the following exceptions:
1) Both probe clip leads of this probe must be connected, as neither probe lead is internally connected to system ground. This is so you can measure signals applied to devices which do not have a ground connection, by clipping probe clips to both sides.
2) There must never be a DC voltage applied across the probe clips (from one side to the other). This will be no problem for speaker work (where signals are AC only), but care must be taken if using the leads with any electronic circuit which uses a power supply (such as working within a preamp or power amp).
3) These probes should not be used for measuring distortion, as their linearity is worse than for the resistor probes (as might be expected).
4) It is best not to mix probe types if you are using a differential probe for your "Meas" channel (1), use a differential probe for the "Cal" channel (2), too. That way most transformer response variations will cancel out in the setup.
5) The differential probes will have reduced accuracy when measuring at low frequencies (under 80Hz) when the impedances of the circuit are also above about 200 ohms. This again will not be a factor in speaker crossover work, but can affect measurements on some higher-impedance applications, or in some cases when measuring Thiele/Small parameters of woofers. For these, the more precise resistor probes should instead be used.
****The M/PP version differential probes will also work with IMP units.*****
We are not currently planning on making these probe types for sale (unless sufficient demand arises), but they can be easily assembled in an hour or two by anyone with basic soldering skills.