I’ve just started to characterize my new QA451B and have noticed the “open” impedance is less than half the specified rating. In the specs it states 10kR for “open”. I figured that would have been fine for both amplifiers and for line stages (sources).
But that’s a differential input, so isn’t it a 8.7k differential load, which is approx 10k as in the specs?
(Compared to 4R or 8R that is open circuit(!)).
I think the intention is to be able to see the effect of removing the load from a power amplifier output, not for testing line stages which don’t need a programmable load!
Typical standard line stages are opamps which should all handle 2k loads and many handle 600R. Output impedances are typically 100 ohms, so 8.7k represents -0.1dB change if routing a single-ended line stage into the differential inputs.
It’s 4.35k from hot to cold. It only has a differential input. You can’t get to 8.7k - how do you figure that?
Not all tests are power amplifiers or line stages. Headphone amplifiers fit right in the ballpark of this QA451B and “unloaded” vs loaded determines OPT R at various frequencies.
The spec is ~10k. It’s not even close. Something must have changed, likely lower value resistors to reduce the input referred noise perhaps.
The specifications need to be updated going forward.
Ah, yes in that case it is off. But as I say its not at all critical when testing a power amplifier. ‘~’ means approximately, and 4.3k is approximately 3 orders of magnitude higher than the load resistances, all that matters is that there is no significant load to the power amp in “open”.
Actually I suspect it is more like 10k for a balanced differential signal in - the impedance is typically different for single-ended driving a balanced input than for a balanced differential signal driving it.
This style of differential amplifier presents different impedances on each input (but the input attenuator network will reduce this effect somewhat).
Hi @restorer-john, there hasn’t been a change, and so I think the spec sheet is in error. DC resistance is about 4.3k (as you’ve measured) instead of 10k. I think the QA451 is going to be end-of-lifed as the resistors went from being “active and recommended for new designs” to “obsolete” in a few weeks. In their PCN, they noted “issues with solderability”.
Erm the impedance at AC in the audio band is what is relevant, capacitors C27 and C28 couple the input to the active differential amplifier, so its more complex than a DC measurement, but yes I doubt its going to be higher than the DC measurement
[ in theory it is possible, but that would involve some exotic NIC or gyrator circuitry, not a differential stage ]
If you are considering discontinuing the QA451B, perhaps consider a programmable load for the future with user connectable and configurable loads. One where we could specify the load values in ohms and connect suitably rated resistors to the unit. Not necessarily limited to just two values.
Then the unit could be custom configured for amplifiers, headphone amplifiers, line stages whatever.
