Suggestion: VI-box for bridged amps

[PEB]

A REAL improvement to the VI-box, in my view, is to make it handle bridged amplifiers.

It pains me to have to break down my setup, and switch from LMS to Audio Precision, only because AP can handle the input not being referenced to ground. I much prefer to have all the data in one 99-curve LMS library.

This occurs much more frequently than one might think. (Think internal subwoofer amps.)

For now, I will attempt to tie the VI-box red jack to one output terminal, and the black jack to amplifier ground. Then I will use 46dB as my scale factor for conversion.

[cstrahm]

Both the LMS and AP units have balanced inputs. That is not the issue. Common Mode Rejection is the issue. LMS only has about 40dB CMR while the AP analyzers have around 80dB. However that is still not sufficient to do what you describe.

Let's assume you are doing a high power Z measurement. You are using a 0.01 Ohm current shunt and the impedance peak of the speaker reaches 100 Ohms at resonance. The difference between the common mode speaker voltage and the differential current shunt voltage will be about 80dB.

If you now attemp to measure a differential signal 80dB below the common mode signal, you are going to have problems. Even with 80dB CMR both signals will appear the same size. Your current shunt measurement will have large errors.

The correct way to do this is to place the current shunt at the ground potential where there is no common mode signal. That's what the VI-Box does. Even using an analyzer with higher CMR does not ensure accurate measurements with a bridged type amplifier output.

The problem is even worse when the frequency range is considered. If you intend to obtain accurate measurements all the way to 10k-100k Hz, the CMR requirements become enormous. Even using a transformer would only get you 40dB CMR at these high frequencies.

The bottom line is, if you want accurate current measurements through a shunt you need to minimize the common mode voltage. Bridged amps do the opposite.

I suppose a very specialized unit could be made to perform such a feat, using full on-board electronics and opto coupling, but you would not like the price. Not many people would want to pay $500-$1000 for this kind of accessory.

[Russell Storey]

Interesting . The simple way to measure a bridging amplifier is to
float the earth of your D.U.T and test Equipment above ground . Add a High current rating 120vac to 120vac Isolation transformer into the common feed to ALL your equipment feeds (Test Bench ) this will enable you to test the bridging amplifier . This will
not change the ratio of the CMRR as Chris states but you will have less smoke signals
Regards
Russell Storey
Stones Sound Studio (Australia)