Variable phono input loading

I prefer to model a cartridge output pre-equalised.

A magnetic cartridge output is a rising response at +6dB per octave.

I developed a cartridge model to do this...



The voltage controlled voltage source (VCVS) provides a zero impedance signal source to build the cartridge model to. The input circuit of the VCVS provides the rising response. Probe 2 measures the rising response. The 570mH inductor, 660 Ohm resistor and 1pf interwinding capacitance model the Goldring 1042. The interwinding capacitance isn't quoted by Goldring but even so there will be some - I chose an arbitrary 1pf. The 75pf cap is the arm capacitance I use. The rest is the input load of the Genera. L2 models the input DC blocking cap inductance and values of 30nH (a typical good electrolytic) or thereabouts don't show until " much closer to the frequency of light"

The 220pf cap we've been talking about is depicted in red. All values can be played with and the resulting charts are as follows...



This one shows the response to 1 gig! The reason I do it is to see what the amp input sees. Most audio op-amps have a Gain Band Width (GBW) product of 4 - 10MHz and it's important to consider what's going in to their inputs in the form of circuit stimulus whether real or imaginary - the simulator sees them as real.

The peaky response (up nearly 1.4dB from the curve at about 8kHz) is the 220pf load cap. The lesser one is 100pf. The red line is the rising response.

You can probably see why I chose 220pf by examining the stimulus at 4-10MHz. When the interwinding capacitance of the cartridge is factored in, the curve which otherwise would be well down at 4-10MHz is kicking back up due to this other LC response. With 220pf it's 6dB down on the 100pf curve. This suggests the input slew rate is half of what it is with 100pf and although stating the obvious, it shows how these curves help indicate what's going on. If the input stimulus is trying to race its output it could muddy the soundstage. The trade-off being the slight treble peak. The audibility of the peak is going to vary with the listener's age and many other things.



Reducing the load capacitance further than 100pf would only exacerbate the slew rate "hunting", plus the bottom end of the available polypropylene capacitor range is 100pf. We've already heard that the PS caps don't have the same sound. Going higher than 220pf just emphasizes the peak further.

Therefore, from a capacitive standpoint it seems the choice is 100pf or 220pf. We could do a series parallel arrangement to obtain 150pf I guess, but just look at the additional "material" involved. Yes, I think it would be a case of splitting hairs.

The only other place to go could be resistive loading. I'll take a look at that next.


Edited by Graham Slee - 31 Aug 2010 at 12:39am

A follow on to the Evox PFR "little gems" note above.

Edited by Fatmangolf - 09 Oct 2010 at 11:01pm

This could be what you are looking for Bob  - at least the "how to" part.

Edited by tg - 08 Dec 2012 at 12:20pm

I had forgotten this thread but not Tony's helpful one about adjusting the loading with plugs. Thank you! Great examples of the range and depth of advice we share.