Differences between HP 8903A and HP 8903B
at the moment someone from my town is offering a HP 8903A distortion analyzer on ebay. I consider to buy that device, because I save the shipping then. Heavy parcel like test equipment costs a lot (approx $100-$200) when sent from another European country!
While reading some posts here I found out that there are some differences between the 8903A and the 8903B: The B revision offers more possible filters, a true differential input etc.
Especially the fact that the HP 8903A has no true differential input makes me wonder, if I should buy the HP 8903A or if it's better to wait for a 8903B.
I want to measure THD in tube and opamp circuits.
What's the advantage of the true differential input of the 8903B when compared to the input of the 8903A? Or is there no difference between the quality of the results, when the input is floating?
Differential input is necessary when testing balanced amps. The 8903A can only handle a few volts on its -in before its no longer reading right.
While its not my favorite instrument its good and you don't need the balanced inputs that often, especially for what you are doing.
If the price is right, buy it!
The only filters I use are the HP (400 Hz) and LP filters (30 kHz, 80 kHz). There is a fourth filter, which on the B model can be swapped out. I see no use for this for the hobbyist.
If you are measuring single ended circuits, the A version will do just fine. And for differential measurements, as long as the signal amplitude with respect to ground on the (-) terminal is within a few volts you're fine with the A version. I've used my HP8903A for measurements on differential line level circuits with great results.
Should the pseudo-differential input of the A version bother you, just build a differential amp with an LME49710 and some precision resistors. As long as the four resistors in the diff amp are completely balanced, the performance of such a circuit will exceed that of the HP8903A/B.
It's a great instrument. I use mine all the time. The source is good to 0.002 % THD and the analyzer noise floor permits measurements down to 0.005 % or thereabout.
Note that OPT #001 moves the inputs to the rear panel. That's the only difference between the "straight" and OPT 001 versions.
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