inductor values

csonics · May 31, 2005

Hi, I am putting together a pultec clone and am in the process of getting parts for the project. I am trying to figure out what the Q value stands for when looking at an inductor's specs and how it would affect the signal. Any help would be appreciated.

Thanks,

csonics

PRR · Jun 1, 2005

> I am trying to figure out what the Q value stands for when looking at an inductor's specs

It is the ratio of real losses (mostly resistance) to reactance impedance.

It is meaningless unless they tell the test frequency it was measured at!!! (The resistance is semi-constant with frequency; the reactance impedance rises directly as frequency.

Very few commodity inductors have Q measured in the audio band. Usually 50KHz, 1MHz, something like that. As a very rough guide, the Q at "your" frequency will be less than measured Q by about the ratio of frequencies.

If an inductor has Q=100 at 50KHz, Q will be about 10 at 5KHz and very roughly 1 at 500Hz.

Q for most music equalizers is in the range of 0.5 for "broad" to 5 for "sharp".

In most audio equalizers built with commodity inductors, you take any Q you can buy as long as it is more than you need, and add a series resistance to bring Q down to the desired value.

IIRC, the low-band of the Pultec uses values that you are not going to find mass-produced.

csonics · Jun 1, 2005

Thanks for explaining that for me PRR! So if I was useing say a 47mH inductor with a Q of 100 at 50Khz and another one that is 100mH with a Q of 50 at 25.2Khz on a filter circuit would there be a great discrepency in the resistance between the two?

PRR · Jun 1, 2005

> 47mH inductor with a Q of 100 at 50Khz

47mH at 50KHz is about 17K ohms. Q=100 implies series resistance of 17K/100= 170Ω, or shunt resistance of 17K*100= 1.7Meg, or some combination of both. But small coils are usually rated where their shunt resistance is negligible, so we can probably assume DCR is 150Ω-170Ω.

> and another one that is 100mH with a Q of 50 at 25.2Khz

100mH at 25KHz is again 17K reactance. Q=50 implies series resistance around 17K/50= 340Ω. So no, they are probably not the same series resistance. They also are not the same reactance (50mH versus 100mH) so not interchangable. However two of the 50mH in series (100mH) seems to be about the same series resistance as one of the 100mH.

CJ · Jun 1, 2005

Where's my cacalator?
0.047H x 6.3 x 50,000 cps = 14,800 ohms Q=100, sooo 14,800/100 is 150 ohms dcr.

Isn't this easy?
Fun? And educational!

0.1 H x 6.3 x 25,000 cps = 15,750. Hmmm, same freq as a horizontal osc.
Anyway, 15,750/Q = 315 dcr. A little high for a Pultec, so you might want to re-think this purchase if it isn't too late.

csonics · Jun 1, 2005

CJ and PRR, thanks so much for the help. That really shines some light on the situation.

CJ, what would you recommend for the Pultec? I am putting together the Gyraf version and it shows a 22mH, 47mH, and two 100mH inductors on the schematic. What would be a preferable q value for these?