"Q" value of Carnhill inductors?

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rascalseven

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Does anyone know how the Q factor of the Carnhill inductors used in the Neve reissues and clones compares to the Q factor of the Sowter or Cinemag Pultec inductors? The Pultec inductors max out at 150mH, but are physically larger than the ones Carnhills makes for the 1081, which go up to 5H. I'm guessing this means the Pultec ones have higher Q??

Does anyone have specs on the Max RDC or Q factor for the Carnhill units? And, assuming the Carnhill's Q is much lower, what practical difference does this make for audio eq circuits?

Very interested in understanding all of this.

Thanks, all.

JC
 
Here is a resonant circuit kind of like the MEQ-5.
Let the resistor represents the dcr of the inductor.
We can change the resistor value to look at what different dcr's will do to the "Q", or quality of the inductor.
This will show us the "Q" or selectivity of the circuit. Notice two different definitions of q, which are related.

q_meq_5.jpg


Here are two curves, one was simulated with a 100 ohm resistor, the broader curve by a 500 ohm resistor:
x
q_meq_5_graph.jpg


The funny thing is, you will have a hard time hearing any difference in the two curves, so don't sweat the dcr and q of your inductor too much. Worry more about frequency response of the inductor due to stray capacitance (turns wound next to each other builds up stary capacitance).

It takes a really radical q to hear a difference, atleast in the testing i did with a series resistor on the wiper of the MEQ-5 mid boost pot.
cj
 
Thanks, CJ.

You are, of course, dealing with someone completely uneducated in inductors, so could you please go a little deeper into an explanation of the stray capacitance that I should be avoiding, and which design (Carnhill or the Sowter/Cinemag types) typically gives more trouble in this area?

Thanks so much!!

:thumb:

JC
 
Whenever you have two conductors seperated by an insulator, you have a capacitor. So all those wires going around the torroid act as tiny capaicitors.
If they add up to a significant value, your signal, depending on what frequency the inductor is filtering, can travel right past the inductor via the stray capacitance. This means the inductor is not blocking this frequency like it should. This will show up as half the bell shaped curve you want:

A is the curve of an inductor with negligible stray capicitance
B is the curve for an inductor with too much stray capacitance.
It is letting the frequencies that you want to block get through.
This will turn your Pultec into a Lowtec!
stray_capacitance.jpg


Those curves represent a LCR circuit as in the previous post.
The sad thing is you probably won't notice a good inductor from a bad one without testing it's response with a scope. This is because the inductor acts on the upper frequencies, not the lowers. The cap acts on the lower frequencies, so you will hear a boost or rolloff. But you won't be getting a true bell shaped curve. But you will still hear some tone shaping, so you will be fooled into thinking everything is normal. (the story of my life!)
cj
 

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