Input impedance

[Hartley118]

Thanks for the extra info on the cartridge, Mike.

The obvious thing that leaps out is the recommended load impedance of 47k. ............

Code:

16Hz  - 290k 31Hz  - 247k 62Hz  - 175k 125Hz - 102k 250Hz - 54.8k 500Hz - 27.5k 1kHz  - 13.8k 2kHz  - 6.9k 4kHz  - 4k 8kHz  - 2.88k 16kHz - 1.63k

Quite a range!
.................

So when poor Mike innocently asked the simple question about input impedance, who knew it would be this involved?

Hope this is of interest!

Mark

It's always good to test these things by experiment!

So that input impedance characteristic does, as expected,  look like a capacitor, though an imperfect one because of the limited gain of that first stage amplifier.

That specified cartridge optimum load impedance of 47k  is a surprise for a ceramic cartridge - one would expect something more like 1Mohm.  A 47k load would typically turn its usual constant amplitude frequency response curve into a constant velocity curve, approximating the characteristic of a magnetic cartridge.  That would then need an RIAA equaliser in the amp just like a magnetic cartridge.  Maybe that's the intention, but there's no sign of an RIAA EQ in this record player circuit.  I guess that it relies on that idiosyncratic capacitive input impedance to get a flat response from the cartridge.

Mark - if you set exam questions for your BBC students, this example would surely get them thinking! 

Martin