Help with cartridge loading.

[bcarso]

actual measurements of cartridge reciprocity?

Does anyone know if this has been done? The consensus is, the effect is very small but can't be nonexistent. There are some diverging opinions but the evidence presented sounds anecdotal at best.

Also, why is the particular preamp venerable? Voltage noise may be low, but it has a lot of current noise for MM/MI owing to the bipolars' base currents. Despite their significant drain-gate capacitance, which circuit techniques can alleviate, JFETs are your friends here.

Brad

 

[bcarso]

I have built the venerable Elektor Supra Phono stage..(circuit shown here...http://www.diyaudio.com/forums/group-buys/266887-supa-phono-preamplifier-pcb.html)

I have the original data and build info and it quotes input impedance as 49k/280pf then says this is adjustable via C1 and R1.

C1 is 4p7 so how can loading be both 280pf and 4p7 ?
If the input capacitance is 280 pf then adding any cable capacitance would result in a value almost too high for any modern cartridge..
If it is 4p7 then ???

Any illumination would be appreciated.🙂

According to simulation the input capacitance before the termination cap C1 is about 15.5pF. It may be that 280pF was an estimate from open-loop considerations, but because of series feedback to the input devices and closed-loop behavior lowering the voltage gain at the collectors, thus reducing Miller effect, the actual audio-band number is much lower.

The input voltage noise is going to be dominated by R14, that at about 2nV/sq rt Hz. Transistor voltage noise from r sub e and rbb' will contribute as well. The current noise from the uncorrelated base currents will dominate the noise at high frequencies for typical high-inductance MM/MI cartridges, although A weighting will reduce this a good deal.

All in all, not a bad design given the desire to use bipolars and a modest number of obtainable parts. I'd probably buffer the output and use a stiffer RIAA feedback network, but that's just me.